Aircraft pitch-axis stability and command augmentation system

Abstract: Disclosed is a pitch-axis stability and command augmentation system (19) in which a pilot column input ( δ C ) is provided to a pitch command processor (26), the output of which is a C*U feedforward command ( C*U FFC ) that is supplied as an additive input to a combining unit (20). The combining unit (20) receives a second additive input of an augmented feedback command signal ( AFB COM ). The resulting output is filtered and generates an elevator command signal ( δ e,FILT ). The command processor (26) additionally supplies a corrected column position signal ( δ C,COR ) to a pitch command C*U processor (26) that converts the corrected column position signal ( δ C,COR ) into a C*U pitch command ( C*U PilotCmd ), representative of the pilot's requested elevator pitch which is generated by movement of the control column. A computed C*U processor (32) forms a computed C*U signal ( C*U Computed ) that is based on the current state of the aircraft. The C*U processor (32) includes a speed stability processor (150) that computes the difference between a reference speed and calibrated airspeed The speed stability processor (150) includes phugoid damping (400) in generating a speed error signal (E ), and providing a means by which the pilot may set a reference airspeed by using a trim device. An underspeed protection signal ( C*U ) is supplied by a stall protection processor (416). A flare compensation signal (C*U ) is supplied by a flare compensation processor (414). An overspeed protection processor (412) supplies an overspeed protection signal ( C*U ). A combining unit (424) is provided to combine the stall protection signal ( C*U ), flare compensation signal (C*U ), and overspeed protection signal ( C*U ) to form a compensation and protection signal A combining unit (22) combines the pilot-requested C*U command the computed C*U signal and the compensation and protection signal The resulting signal is an error signal which is supplied to an integrator (28) that outputs a C*U integral command signal ( IC*U COM ). Another combining unit (23) receives the C*U integral command signal ( IC*U COM ) and combines it with a pitch rate damping command signal ( Q COM ). The output of the combining unit (23) is the augmented feedback command signal ( AFB COM ).

Description: Field of the Invention This invention relates generally to flight control systems for aircraft, and more particularly, to an apparatus and method for controlling aircraft elevator commands using a pitch-axis stability and command augmentation system. Background of the Invention On some new airplanes, the static stability of the plane has been purposefully relaxed to include instances of flight wherein the plane has neutral static stability. A significant benefit in fuel-efficiency, weight-savings, and drag-reduction can be realized with such a system, due to an allowable reduction in required tail size. An airplane with relaxed static stability may have some undesirable and unacceptable handling quality characteristics, though. One characteristic is that a neutrally stable airplane will not naturally return to its previous airspeed after a change in pitch attitude. On a trimmed positive static stability airplane, the pitch attitude changes according to the pilot pushing forward or pulling back on the control column. When the column is released, the airplane will continue to fly at its new pitch for a short time. As the airspeed of the plane reduces, the airplane will have a natural tendency to nose up or nose down, returning to its previous trim speed. This tendency is an expected handling quality of conventional commercial aircraft, and one to which pilots virtually automatically respond in flying their craft. Neutrally stable planes have no tendency to return to their trim position and airspeed after an adjustment of pitch. For these aircraft, changes in pitch are maintained and airspeed varies accordingly. During pitch increases, airspeed decreases. During pitch decreases, airspeed increases. To return the aircraft to its trim position and speed, the pilot must return the plane to its trim pitch, by repositioning the elevators with the control column. A characteristic that is common to planes with or without relaxed static stability is that the amount of elevator input required to accomplish a certain pitch is sensitive to changes in weight and location of center of gravity relative to the mean aerodynamic center. An aircraft that is light in weight and has a center of gravity near the mean aerodynamic center, requires only a small change in elevator angle of attack to produce a large change in airplane pitch. Conversely, a heavy aircraft having a forward center of gravity requires much more elevator deflection to produce a like change in airplane pitch. Without control augmentation, the pilot must move the control column much farther in the heavy case in order to get the same pitch response as in the light case. This can be an undesirable characteristic because it requires the pilot to adjust his or her column input based on the weight and location of the plane's center of gravity--pieces of information not readily perceptible. A third characteristic, though not related to relaxed stability aircraft, concerns the manner in which pilots expect the airplane to respond when the control column is moved. It is fairly well known that at low airspeeds, pilots expect movement of the control column to produce a change in pitch rate. At high airspeeds, pilots expect movement of the control column to produce a change in normal acceleration. This situation was recognized in the 1960's and the C* criterion was developed as a way to express optimal airplane response taking it into consideration. The C* criterion is discussed in detail below. Thus, there exists a need for a superior pitch attitude control system capable of improving the control and handling characteristics of an aircraft. Optimally, this pitch attitude control system should provide such an airplane with handling characteristics (from the pilot's point of view) similar to an ideal conventional airplane with positive static stability, in order to reduce the amount of variation in flying technique required. The ideal control system should accomplish this goal by meeting the pilot's expectation regarding the airplane's short and long term responses to elevator command; and by responding to pilot pitch commands in a similar manner, regardless of weight or center of gravity shifts. The ideal control system should further accommodate the pilot's expectations to control pitch rate during low airspeed maneuvers and control to normal acceleration during high airspeed maneuvers. As will be appreciated by the following description, the present invention is directed to providing such a superior pitch attitude control system. Summary of the Invention The present invention provides a method for producing an aircraft elevator command signal according to claim 1 and a pitch-axis stability and command system according to claim 6 for augmenting aircraft elevator commands. The present invention uses a criterion herein referred to as the C*U criterion to provide ideal pitch response airplane characteristics and long-term speed stability in the pitch axis. The C*U criterion allows the present invention to accomplish the previously mentioned goals by optimizing the pitch rate and normal acceleration response of the airplane and by providing the desirable speed stability characteristics of a conventional airplane. The present invention uses a feedback system configured to receive a control column input and convert the input into elevator command signals. The feedback portion of the system processes signal representative of current airplane data which is formed in response to a previous elevator command. Augmentation of the system is accomplished by converting the column input into a pilot-requested C*U command signal and comparing that command signal with a computed C*U command signal generated on the basis of the current airplane state. The error signal, which represents the difference between what the pilot is commanding and how the aircraft is responding, is integrated and the result is added to the elevator command signal. Therefore, the pitch-axis stability and command augmentation system of the present invention zeros any difference between what the pilot has requested and what the airplane is performing. In accordance with the present invention as specified in claim 6, the pitch-axis stability and command augmentation system includes a pitch command processor that converts a pilot column input into a feedforward command signal that is one of two signals used to generate an elevator pitch command. The second signal component is discussed below. The command processor additionally supplies a corrected column position signal to a commanded C*U processor that converts the corrected column position into a C*U pitch command signal representative of the movement of the control column that requests pitch change. In accordance with the present invention, a computed C*U processor forms a computed C*U signal that is based on the current state of the aircraft. The computed C*U signal of the currently preferred embodiments includes at least three components: a normal acceleration signal, a pitch rate signal, and a speed error signal. In the preferred embodiment, the normal acceleration and pitch rate signals are earth-referenced and are supplied by a normal acceleration complementary filter and a pitch rate complementary filter, respectively. The speed error signal is supplied by a speed stability processor. The speed stability processor includes phugoid damping in its formation of the speed error signal. The speed stability processor further implements a method that allows the pilot to set a reference airspeed by using a trim device. In accordance with other embodiments of the present invention, three C*U compensation and protection signals are supplied to further enhance the handling and response characteristics of the airplane. An underspeed protection signal is supplied by a stall protection processor which includes the additional features of establishing a minimum reference airspeed below which the pilot may not trim and enhances the airplane response and handling characteristics during underspeed operation. A flare compensation signal is supplied by a flare compensation processor to cause the aircraft to perform as if encountering ground effects during landing. An overspeed protection processor supplies an overspeed protection signal to command the elevator to nose-up whenever the aircraft is being operated at an overspeed condition. A combining unit is provided to combine the signals of stall protection, flare compensation, and overspeed protection to form a C*U compensation and protection signal. The underspeed and overspeed protection signal can be overridden by the pilot by moving the control column to supply counteracting pitch signals. Thus, by conscious choice, the airplane can remain at underspeed or overspeed condition. In addition, a configuration compensation feature can be included to reduce pilot workload during configuration change. In accordance with yet other embodiments of the invention, another combining unit is provided to combine the subtractive input of pilot-requested C*U command signal, the additive input of the computed C*U signal, and the subtractive input of C*U compensation and protection signal. The resulting error signal is supplied to an integrator that outputs an integrated error signal. Another combining unit receives the integrated error signal as an additive input and sums it with a signal representative of pitch rate damping command. The pitch rate damping command signal is provided to damp short period response and adjust the short period frequency. The output of the combining unit is an additive input to the combining unit having another additive input of feedforward command. The resulting signal is the C*U augmented elevator command signal. In accordance with still further embodiments of the present invention, structural mode filters are provided to remove the frequency content of the elevator command signal. A stabilizer off-load function is provided to adjust the stabilizer such that elevator authority is available throughout the flight. Brief Description of the Drawings The foregoing embodiments and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE 1 is a control diagram of an overall pitch-axis stability and command augmentation system formed according to the teachings of the present invention; FIGURE 2a is a control diagram of a pitch command processor of FIGURE 1; FIGURE 2b is a control diagram of a feel command system of FIGURE 2a; FIGURE 2c is a control diagram of column null bias remover of FIGURE 2a; FIGURE 3 is a control diagram of a commanded C*U processor of FIGURE 1; FIGURE 4a is a control diagram of a computed C*U processor of FIGURE 1; FIGURE 4b is a control diagram of a turn compensation processor; FIGURE 4c is a control diagram of a pitch rate complementary filter; FIGURE 4d is a control diagram of a normal acceleration complementary filter; FIGURE 4e is a control diagram of a speed stability processor of FIGURE 4a; FIGURE 5a is a control diagram of a compensation and protection processor of FIGURE 1; FIGURE 5b is a control diagram of a stall protection processor of FIGURE 5a; FIGURE 5c is a control diagram of a flare compensation processor of FIGURE 5a; FIGURE 5d is a control diagram of an overspeed protection processor of FIGURE 5a; FIGURE 6 is a control diagram of a pitch rate damping processor of FIGURE 1; and FIGURE 7 is a control diagram of a C*U integrator of FIGURE 1. Detailed Description of the Preferred Embodiment The present invention is directed to providing a pitch-axis stability and command system that provides desirable control surface responses for any aircraft, including relaxed stability planes, and that has desirable handling characteristics for pilots in flying the aircraft. The present invention accomplishes these goals by incorporating a new concept called the C*U (pronounced "C star U") criterion. The C*U criterion defined herein is based on the C* criterion developed in the mid-1960's and described briefly below. C* Criterion Generally Prior to the mid-1960's, the handling characteristics of a plane for any particular longitudinal maneuver were generally assessed by comparing the plane's pitch rate response produced by a control column input, with a pilot-derived definition of acceptable pitch rate responses. If the response was within a certain envelope, it was deemed acceptable. This type of analysis was done to assess airplane performance at both high and low speeds. It was established that at high speeds, however, the most significant motion cue to pilots was the normal acceleration of the plane. Because of this, it was desirable to find a way to express acceptable airplane performance that considered pitch rate and normal acceleration response, as well. During the 1960's, the method of C* analysis was developed as a way to assess the total longitudinal airplane response and handling characteristics that considered the primary pilot motion cues for both low and high speeds. C* is a measure for short period ride handling quality. The C* variable is defined as a blend of pitch rate and normal acceleration. C* = N Z + K q q where N z is body normal acceleration, K q is blending gain, and q is body pitch rate. The C* variable was formed as an index to assess the goodness of longitudinal dynamic response characteristics of an airplane. The pitch rate and normal acceleration were selected because they are the most significant motion cues to pilots at low and high speed respectively. In using the C* criterion, an analyst would compare the value of C* as calculated for any particular high or low speed longitudinal maneuver, with a pilot-derived definition of acceptable C* values. If the response was within this envelope, it was thought to have acceptable handling characteristics. Since the development of the C* criterion, a few commercial and military airplanes have used the C* criterion as a basis in their longitudinal control systems. A control system based solely on the C* concept converts the pilot command to a C* command. Therefore, at low airspeeds, the pilot command is viewed as a request for a different pitch rate, and at high airspeeds, the pilot command is viewed as a request for a different normal acceleration. A control system based solely on a C* variable provides neutral speed stability characteristics to an airplane (i.e., if airplane speed changes , there would be no tendency for the airplane to return to its trim speed). Neutral speed stability is a characteristic that is present in many previous airplanes. While pilot workload is reduced in an airplane with neutral speed stability, the pilot's awareness of airplane airspeed based on column force has diminished. The present invention utilizes a new variable, C*U, which is defined to be a combination of the C* criterion described above and a speed stability term, hence the name C*U (U referring to speed stability). The C*U variable may be determined as follows: C*U ≡ C* -K V U ERROR where C* is defined as in equation (1), Kv is the speed stability gain, and Uerror is the error between the airplane longitudinal speed and the C*U reference speed. Uerror is defined as: U ERROR ≡ V CAS -V REF where VCAS is the airplane calibrated airspeed and VREF is the C*U reference speed. Related Uerror may be computed from equivalent airspeed, true airspeed, indicated airspeed, and/or Mach, in which cases the numerical values for Kv will be different from that described herein. In simplified terms, the control law of the present invention accepts a column input and generates a feedforward C*U elevator command. In addition, a pilot-inputted C*U command is calculated according to the airplane response. A computed C*U signal is calculated according to the terms of equation (2), based on the current dynamics of the plane. The computed C*U signal is compared with the pilot-inputted command and an error signal is formed. This term is integrated and combined with the feedforward C*U elevator command and the pitch rate damping command to form the output of an elevator pitch command. This is explained in detail in section 1 below. As will be better understood from the following description, while the invention was designed for use in a relaxed stability airplane control system, and is described in connection with such a system, it is to be understood the invention can be incorporated in other types of airplane control systems, if desired. Further, as is conventional, while the invention is illustrated and described in the form of a control law containing discrete blocks designed to accomplish specific functions, it is to be understood that the invention can be actually implemented in various ways. For example, the various functions of the illustrated control law can be carried out by a suitably programmed digital computer system. Alternatively, the functions can be carried out by digital or analog circuits. The following detailed description is divided into seven sections. The first section discusses the overall augmented pitch control system and its major components. The remaining sections detail each of those major components. 1. Discussion of Pitch-Axis Stability and Command Augmentation System Shown in FIGURE 1 is a control diagram of a pitch-axis stability and command augmentation system 19 formed according to the teachings of the present invention. The output of the pitch-axis stability and command augmentation system 19 is an elevator command δ e,FILT , which is eventually sent to an elevator servo-mechanism instructing it to adjust the elevator. FIGURE 1 is a simplified diagram and therefore does not include all inputs. The remaining figures show further information. Prior to being sent from the pitch-axis stability and command augmentation system 19, the elevator command signal δ e,FILT , is preferably filtered by a structural mode filter 24 whose input is an unfiltered elevator command signal δ e,UNF formed by a first combining unit 20. The structural mode filter 24 attempts to remove the frequency content of the unfiltered elevator command signal δ e,UNF that could interfere with the structural modes of the aircraft. This is done to avoid undesirable and/or unstable flight characteristics and to avoid causing the aircraft to vibrate or resonate during certain control maneuvers. The preferred structural mode filter 24 depends on the airplane's particular structural frequencies. There are a wide variety of ways that this filter may be included in the command system 19. It may be located elsewhere in the command system 19 or may be separated into parts and located in various positions. The first combining unit 20 combines a C*U feedforward command C*U FFC with an augmented feedback command AFB COM . The feedforward command C*U FFC is based on a pilot column input signal δ C as modified by a C*U pitch command processor 26. The C*U feedforward command C*U FFC is preferably related motion of the control column, but the present invention encompasses using a commanded elevator position input from an autopilot or other controlling source instead of a pilot column input δ C . The pitch command processor 26 preferably operates to accomplish multiple tasks including managing the "feel" of the control column, interfacing with an external handling system, removing any column null bias, shaping the C*U feedforward command C*U FFC , and switching to an autopilot input mode when appropriate. The pitch command processor 26 also calculates a corrected column position signal δ C,COR for use in other parts of the pitch-axis stability and command augmentation system 19. Of course, not all of these function may be appropriate to a particular application. The primary task of the pitch command processor 26 is that of converting a pilot column input signal δ C into a C*U elevator position (here, C*U FFC , preferably in units of degrees) and into a corrected column position signal δ C,COR for use in a commanded C*U processor 30. The pitch command processor 26 is discussed in detail in section 2 below, and is shown in FIGURES 2a, 2b, and 2c. The commanded C*U processor 30 is discussed in detail in section 3, and is shown in FIGURE 3. The augmented feedback comman d AFB COM is generally formed by modifying the combination of a C*U integral command IC*U COM and a pitch rate damping command Q COM , by an elevator loop gain factor 29, at multiplier 25. The C*U integral command IC*U COM represents the integral of the C*U error signal, with C*U being defined in equation (2) above. The pitch rate damping command Q COM provides damping for the short period response and adjustment of the short period frequency. The pitch rate damping command Q COM is discussed in detail in section 6 below and is shown in FIGURE 6. The combination of IC*U with Q COM is accomplished by combining unit 23. The modification of the combination is done by a multiplier 25. The purpose of the elevator loop gain factor 29 is to provide sufficient stability margins for flutter modes. It is preferably a function of airspeed (e.g. Mach or calibrated airspeed). Depending on the precise application and embodiment to which the present invention is being used, all such elements discussed herein may not be necessary. Likewise, modifications of elements may be accomplished to shape the elevator command signal δ e,FILT in any desired manner. As will be observed from FIGURE 1, the preferred embodiment of the present invention also uses the augmented feedback command AFB COM to produce a trim negation signal TNS via a stabilizer off-load processor 21. The stabilizer off-load processor 21 adjusts the stabilizer such that elevator authority is available throughout the flight. Once the augmentation feedback signal command AFB cmd exceeds a certain threshold position, the stabilizer is moved. As the stabilizer is moved, the stabilizer position sensor (not shown) senses the position of the stabilizer off-load processor 21 where it is multiplied by a gain and differentiated to form the trim negation signal TNS supplied to the integrator 28 to be added to the integral C*U command. In this manner, the stabilizer movement gives the elevator additional authority. Off-loading the elevator also helps to reduce drag penalty on the aircraft. Suitable stabilizer off-load processors 21 are known and may be used in the present invention. For example, trim negation commands are often provided by auto flight systems. The embodiment of the present invention includes positioning the stabilizer off-load processor 21 in that portion of the pitch-axis stability and command augmentation system 19 that represents the augmentation portion of the elevator pitch command signal δ e,FILT only. This is important, since it is not desirable to move the stabilizers in short term maneuvers. The C*U integral command IC*U COM represents the portion of the feedback of the pitch-axis stability and command augmentation system 19 that commands the elevators to return to their trimmed state in the absence of an input from the pilot. In the presence of a pilot column input signal δ C , the C*U integral command IC*U COM ensures that the airplane response meets the pilot request. The C*U integral command IC*U COM is formed by the integration of a C*U error signal E C*U in the C*U integrator 28. The C*U integrator 28 is discussed in detail in section 7 below. Integration of the error signal E C*U operationally results in the pitch-axis stability and command augmentation system 19 continuously attempting to zero out any difference between a pilot-requested C*U command C*U PilotCmd and the sum of a computed C*U signal C*U Computed with a compensation and protection C*U signal C*U C&P . The pilot-requested C*U command C*U PilotCmd is formed in the commanded C*U processor 30 whose basic function is to convert the corrected column position signal δ C,COR to a C*U form, namely pilot-requested C*U command C*U PilotCmd . The commanded C*U processor 30 is discussed in section 3 below. The computed C*U signal C*U Computed is formed in the computed C*U processor 32 according to equation (2) and is discussed in section 4 below. The C*U compensation and protection signal C*U C&P is formed in the compensation and protection processor 34 and is discussed in section 5 below. A wide variety of control loop variations are possible in the present invention in order to accomplish its goals. The overall structure explained herein should therefore be taken an exemplary, and not restrictive. Although the presently preferred embodiment is disclosed herein, it is also to be understood that certain applications may not benefit from the inclusion of all elements. In a like manner, certain known control elements, not shown or described, could be added to obtain a specific desired result. As a side note, the concepts of gains and of transfer functions are used at several points throughout the system and method of the present invention as depicted in the preferred embodiments. The concept of a transfer function as used herein is to be interpreted broadly to include time lags or leads, amplification (linear or non-linear), attenuation, integration, and the like. Thus, the definition includes the concept of gain. These transfer functions in their various forms often are expressed mathematically in Laplace transform notation and can be realized in analog or digital form. Examples of analog means include linear amplifiers, capacitors, inductors, resistors and networks including some or all of these devices. Examples of digital means include binary adder-subtractors, comparators, inverters, controlled counters, and digital processors that sequentially process digitally encoded data. As used herein, the term "combining means" is to be broadly interpreted, and includes signal combination in the digital or binary sense so that it includes addition, subtraction, multiplication, and division. These combining means may comprise analog devices, such as summing amplifiers or transistors, or they may comprise digital devices, such as binary adder-subtractors, comparators, or shift registers in an arithmetic logic unit of a central processing unit. 2. Discussion of Pitch Command Processor 26 2a. Pitch Command Processor 26 FIGURE 2a is a block diagram of the pitch command processor 26, which was briefly described relative to the overall system arrangement of FIGURE 1. As previously noted, the primary functions of the pitch command processor 26 are to convert a pilot column input δ C into the feedforward command C*U FFC portion of the elevator command δ e,FILT , and to generate the corrected column position signal δ C,COR for use in the commanded C*U processor 30 of FIGURE 1. In the arrangement depicted in FIGURE 2a, the pilot column input signal δ C is coupled to a feel system 56. As is known in the art, a feel system establishes mechanical force on the control column that must be overcome in moving the control column forward or aft of its neutral (detent) position. As is also known in the art, the forces established by a feel system increase with increasing control column deflection and, further, are established so that the column force characteristics vary with aircraft speed (less force being required at low-speed flights and more force being required for high-speed flight). Although the present invention employs force relationships common to the prior art (provided by feel system 56 in FIGURE 2a), those force relationships are supplemented by the invention to change the control column force characteristics in a manner that: (1) alerts the pilot to overspeed and underspeed conditions; (2) provides pilot expected command column flare response during landings; and (3) produces an abrupt, significant increase in column force in the event that aircraft angle of attack reaches and/or exceeds the stall value (α ss ). The manner in which the invention is structured and operates during underspeed, overspeed, and landing flare conditions will be described relative to FIGURES 5a-5d. The provision of the invention that abruptly increases the command column force characteristic when the aircraft is near, or at stall, is indicated in FIGURE 2a as a feel command system 52 and will be described relative to FIGURE 2b. Continuing with the description of FIGURE 2a, feel system 56 supplies a command column displacement signal 58 that is coupled to a column null bias remover 60. The column null bias remover 60 receives the output signal of a linear variable differential transformer or other transducer that indicates the deflection of the command column from neutral (detent) (indicated by arrow 58 in FIGURE 2a). As shall be described relative to FIGURES 2c, column null bias remover 60 provides compensation for signal offset or bias terms that may be present in the output signal of the column position transducer (i.e., any non-zero signal level that is provided by the transducer when the command column is at detent). The column null bias removal feature of the invention ensures that the aircraft will remain at the trim condition (i.e., at airspeed equal to C*U reference speed V REF ) when the command column is in the detent position. The corrected column position signal δ C,COR formed by the column null bias remover 60 in FIGURE 2a is supplied to a transfer unit 64 of FIGURE 2a and, in addition, as indicated in FIGURE 1, is supplied to commanded C*U processor 30, which shall be described relative to FIGURE 3. Transfer unit 64 of FIGURE 2a converts the corrected column position signal δ C,COR provided by the column null bias remover 60 into the signal C*U FFC , which is a portion of the elevator command signal δ e,UNF . As is known in the art, the signal transformation effected by transfer unit 64 is dictated by the particular feel system 56 being employed and other characteristics of the aircraft. As also is known in the art, the signal transformation effected by transfer unit 64 may be scheduled as a function of airspeed and is to be arranged to exhibit any required or desired frequency response As is indicated by switch 66 of FIGURE 2a, the signal supplied by transfer unit 64 is coupled to summing unit 20 of FIGURE 1 as the C*U feedforward command signal C*U FFC , when the aircraft autopilot is not engaged. When an autopilot engage signal is supplied to switch 66, the autopilot 68 supplies its own version of a C*U feedforward command, C*U A/P,FFC . Alternatively, as described previously, the autopilot can generate an equivalent δ C or move the control column. Under these conditions, the switch 66 remains in the condition shown in FIGURE 2a. 2b. Feel Command System 52 The feel command system 52, depicted in FIGURE 2b combines two signal components that are used by the feel system 56 of FIGURE 2a in generating appropriate command column force gradients. The first feel command signal component corresponds to feel commands of the type used in conventional aircraft feel systems. Specifically, in the arrangement of FIGURE 2b, signals (indicated by arrow 102) representative of one or more airdata parameters are supplied to a look-up table 100 or other arrangement that converts the airdata signals to feel command signals that exhibit desired force gradients as a function of aircraft airspeed and other parameters such as aircraft pitch rate. In the arrangement of FIGURE 2b, the feel command signal produced by look-up table 100 is not supplied directly to the aircraft feel system (e.g., feel system 56 in FIGURE 1), but instead is supplied to a combining unit 98 where it is combined with a second feel command signal component. In accordance with the invention, and as described below, the second feel command signal component significantly alters the force required for the pilot to maintain or move the control column aft of detent (nose up command) for low airspeeds (or high angle of attack). In the arrangement of FIGURE 2b, a signal representative of aircraft filtered angle of attack (or, alternatively, filtered airspeed) (indicated by arrow 50) is supplied to an additive input to a combining unit 90. Supplied to a subtractive input of combining unit 90 is a signal (indicated by arrow 104 in FIGURE 2b) representative of an angle of attack that is slightly beyond aircraft stick shaker angle of attack (or a signal representative of an airspeed slightly lower than stick shaker airspeed). As is known in the art, signals representative of stick shaker angle of attack and stick shaker airspeed are available from systems such as a warning electronics system or other systems that provide stall warning. The output of combining unit 90 is supplied to a scaling unit 92, which has a gain factor that may be constant or may be scheduled in accordance with aircraft flap setting and/or one or more airdata signals indicative of aircraft airspeed. The scheduling unit 92 establishes the force gradient that will be added to the nominal feel command force gradient when the aircraft is operating beyond angle of attack stick shaker (or below stick shaker airspeed). Scheduling the gain of scaling unit 92 in accordance with flap and/or airdata is not necessary in most situations, but can be included to obtain precise control over variations in control column force as a function of airspeed. The signal supplied by scaling unit 92, which represents the difference between filtered angle of attack and a suitable value slightly beyond stick shaker angle of attack (or the difference between filtered airspeed and a suitable airspeed slightly less than stick shaker speed) is supplied to a limiter 94. Limiter 94 has an upper limit value of zero to eliminate signals produced when the filtered angle of attack is not above stick shaker angle of attack (minus the small margin) - or when filtered airspeed is not less than the stick shaker airspeed (plus the small margin). The lower limit of limiter 92 is selected to establish the maximum force that the arrangement of FIGURE 2b will assert on the control column if it is aft of detent when the aircraft is beyond stick shaker angle of attack (or below stick shaker airspeed). Continuing with the description of the arrangement of FIGURE 2b, the signal supplied by limiter 94 is supplied to the second additive input of combining unit 98 via a rate limiter 96. Rate limiter 96 filters or smoothes the signal supplied to combining unit 98 to eliminate abrupt changes in signal level that may occur in aircraft signals representing aircraft angle of attack and filtered airspeed. As is known in the art, various other arrangements, such as a lag filter, can be employed to "smooth" a signal in a manner similar to rate limiter 96 of FIGURE 2b. As previously noted, the signal supplied by combining unit 98 of FIGURE 2b is supplied as the system feel command signal 54 to the aircraft feel system (56 in FIGURE 2a). 2c. Column Null Bias Remover 60 The currently preferred control law for column null bias remover 60 of FIGURE 2a is illustrated in FIGURE 2c. As was described relative to FIGURE 2a, the invention employs column position null bias correction to compensate for offset or bias components that may be present in the signal supplied by the command column position transducers when the associated command columns are in the neutral (detent) position. In the arrangement depicted in FIGURE 2c, the signal (indicated by arrow 58) supplied by the command column linear variable differential transformer (or other transducer) is supplied to an additive input of a combining unit 82, the output of which is supplied as the corrected column position signal δ C,COR . The compensation or correction signal supplied in accordance with the invention is supplied to a subtractive input terminal of combining unit 82. As is indicated in FIGURE 2c, the correction signal supplied by the invention is provided by an integrator 80, the output of which also is connected to the subtractive input of a combining unit 72. The additive input of combining unit 72 is supplied with the command column position transducer signal (arrow 58) via a limiter 70. The output of combining unit 72 is scaled by a gain factor K 1 by a scaling unit 74 and supplied to one terminal of a switch 78. The wiper of the switch is connected to the input of integrator 80. As is indicated at block 76, when switch 78 is not activated (i.e., is in the position shown in FIGURE 2c), no input signal is supplied to integrator 80. The arrangement of FIGURE 2c operates as follows. Switch 76 is activated to connect the input of integrator 80 to the output of scaling unit 74 when the aircraft autopilot is not engaged and, in addition, there is little or no displacement of the control column from detent. Thus, when switch 78 is activated the signal supplied by the command column transducer is coupled to the input of integrator 80 via limiter 70, combining unit 72, and scaling unit 74, with the output signal that is provided by integrator 80 being fed back to the subtractive input terminal of combining unit 72. As will be recognized by those skilled in the art, this feedback arrangement corresponds to that of a conventional lag filter. Thus, except for a short time delay, the null bias correction signal supplied to the subtractive input of combining unit 82 is equal to the signal being supplied by the command column transducer (as long as the command column transducer signal is within the range of limiter 70). Thus, when the control column remains in detent or neutral position, and the column control transducer signal is inside the range of limiter 70, the corrected column control signal supplied by combining unit 82 is substantially equal to zero. In that regard, the range of limiter 70 is set so that typical command column transducer bias or offset signals will be within the limiter range, but signals of a greater magnitude will not. When the pilot applies force to the column to move the column out of detent, switch 78 deactivates so that no input signal is supplied to integrator 80 (indicated at block 76). With no signal supplied to the integrator 80, its output signal will remain constant. Thus, the signal supplied to the subtractive input of combining unit 82 will remain equal to the offset or bias component of the command column transducer. This means that the bias or offset component of the command column transducer signal will not be present in the corrected column position signal that is supplied by combining unit 82. The switch 78 is actuated when forces are low or the force sensors indicate that forces are invalid or at fault. This allows the null bias remover to function satisfactorily even if a column force transducer signal becomes invalid, causing switch 78 to remain activated when the pilot moves the command column from detent. In particular, under such a failed condition, the signal path that includes combining unit 72, scaling unit 74, and integrator 80 will continue to function as a lag filter. However, since the input signal supplied to the additive input of combining unit 72 cannot exceed the range of limiter 70, only a small amount of command column position signal will be lost. In particular, the offset or bias signal of linear variable differential transformers and other position sensors used with the invention are typically no greater than a few percent of the maximum column deflection signal. That being the case, there is little or no noticeable effect on the pilot command column required for pitch control of the aircraft should switch 78 remain activated when the command column is moved from detent. Moreover, the arrangement continues to provide substantially zero output when the command column is actually in detent. 3. Discussion of Commanded C*U Processor 30 As was discussed relative to FIGURE 1, the corrected column position signal supplied by column null bias remover 60 in FIGURE 2 is supplied to a commanded C*U processor 30, which converts the corrected column position signal to a C*U pitch command signal that represents pilot input (C*U PilotCmd signal in FIGURE 1). As also was discussed relative to FIGURE 1, the C*U pitch command signal is supplied to combining unit 22 where it is combined with C*U compensation and protection signal C*U C&P (discussed relative to FIGURES 5a-d) and a computed value of C*U pitch command C*U Computed (which will be discussed relative to FIGURE 4). In the commanded C*U processor 30 that is shown in FIGURE 3, the corrected control column position signal δ C,COR is supplied to a filter 110 or an equivalent arrangement that provides signal shaping in a manner that converts the corrected column position signal δ C,COR to a pitch control signal having the desired characteristics (i.e., the desired relationship between control column displacement and pitch attitude command). A signal representative of impact pressure (or another airdata signal representative of airspeed) (indicated by arrow 120) is supplied to a scaling unit 112. The gain schedule of scaling unit 112 is established so that the output signal provided by the arrangement of FIGURE 3 will result in a C*U signal exhibiting relatively consistent stick force per g throughout the flight envelope. A multiplier 116 combines the signal provided by scheduling unit 112 and the signal provided by filter 110 to supply a pilot-requested C*U command C*U PilotCmd . As is indicated by switch 118, the signal supplied by multiplier 116 is supplied to summing unit 22 of FIGURE 1 whenever the autopilot (68 in FIGURE 3) is disengaged. If the autopilot is engaged, a C*U command signal generated by the autopilot, C*U A/P,CMD , is coupled to summing unit 22 of FIGURE 1 via switch 118. In situations in which the autopilot command is in terms of column command, switch 118 remains in the position depicted in FIGURE 3. 4. Discussion of Computed C*U Processor 32 4a. Computed C*U Processor 32 The computed C*U processor 32 is the portion of the pitch-axis stability and command system that determines C*U based on current (referred to herein as feedback) values. Combining equations (1) and (2) yields the following criterion: C * U = N z + K q q - K v ( V CAS - V REF ) The signal processing effected by the computed C*U processor 32 implements this equation. As shall be described in sections 4c and 4d, in the C*U implementation of the currently preferred embodiments, the normal acceleration N z and the airplane pitch rate q are referenced to earth, rather than the body of the airplane. The C* portion of equation (4) is produced at a combining unit 132 shown in FIGURE 4a, which has a signal of normal acceleration N z as an additive input. A pitch rate signal q is scaled by a gain facto K q by scaling unit 140. The scaling unit 140 output is supplied to the combining unit 132 as a second additive input. The combining unit 132 supplies the C* criterion. The input signals N z and q are preferably formed as described in FIGURES 4b, 4c, and 4d, where normal acceleration signal N z is preferably a turn compensated normal acceleration signal with respect to the earth reference frame (symbol in FIGURE 4c). Likewise, pitch rate signal q is preferably a turn compensated pitch rate signal with respect to the earth reference frame (symbol  ˙ EST in FIGURE 4d). The processors used to form and  ˙ EST are discussed in sections 4c and 4d below. The second part of equation (3), the speed stability signal K v ( V CAS - V REF ) , is formed in the speed stability processor 150. The C* criterion is an additive input to a combining unit 134 as is the speed stability signal K v ( V CAS - V REF ). Combining unit 134 supplies the entire computed C*U signal C*U Computed to combining unit 22 used in FIGURE 1 to create error signal E C*U as described in section 1 above. In the preferred embodiment of the computed C*U signal processor 34, a switch 154 allows the speed stability processor 150 to supply the speed stability signal K v ( V CAS - V REF ) unless the autopilot is in use. When the autopilot is in use, the switch activates to receive a zero input. The autopilot used with the currently preferred embodiments of the invention includes means to control airspeed. If a particular application of the present invention is used without an autopilot, or with one not having airspeed control, this particular switch 154 may not be necessary. The pitch rate gain K q is equal to 0.217 according to the classical C* criterion. A different value may be selected based on pilot preferences. This value, however, has the benefit of producing a C* that has contributions from N z and q that are equal at a true airspeed of 400 ft/sec. The values used to form N z , q , and K v ( V CAS - V REF ) are based on airplane normal acceleration, pitch rate, and velocity error. The forming of these terms is discussed below. 4b. Turn Compensation Processor 200 FIGURE 4b shows a turn compensation processor 200 used to generate the normal acceleration N z and pitch rate q signals used in the computed C*U processor 32 and the speed stability processor 150. Turn compensation processors 200 are known in the art of stability and control system design and thus, are only generally described herein. When an aircraft is banked, the lift acting on the plane is still normal to its wing surface. This causes the amount of lift acting normal to the earth's surface to decrease, further causing the plane to lose altitude. In order to continue flying at the same altitude as when not banked, the aircraft must increase its lift in order to compensate for the decrease in lift in the direction normal to the earth's surface. Turn compensation is therefore provided to add elevator pitch to compensate for the increased lift required to sustain level flight, without additional column input signal δ C required from the pilot. The inputs to the turn compensation processor 200 include signals of roll attitude 170, pitch attitude 172, body-referenced normal acceleration 174, body-referenced pitch rate 176, flight path vertical acceleration estimate 152, and pitch attitude rate estimate 150. The outputs of the turn compensation processor 200 include turn compensated signals of normal acceleration N z and pitch rate q . In current embodiments of the invention, turn compensation is only provided for bank angles less than 30 degrees and is gradually removed for high bank angles. The present invention improves upon previous C*U arrangements by using improved input signals representative of flight path vertical acceleration 150 and pitch attitude rate 152 that are referenced to the earth instead of an inertial reference. The formation of an earth-referenced flight path vertical acceleration signal 150 is discussed in section 4c and is shown in FIGURE 4c. The formation of an earth-referenced pitch attitude rate signal 152 is discussed in section 4d and is shown in FIGURE 4d. 4c. Normal Acceleration Complementary Filter 264 The normal acceleration complementary filter 264 produces an earth-referenced flight path vertical acceleration signal 150 which in turn results in an earth-referenced normal acceleration signal N z . Usually, normal acceleration N z is the acceleration of the airplane with respect to an inertial-reference frame. The pitch-axis stability and command system 19 of the present invention is attempting to control the 'airplane to a normal acceleration that is referenced to the earth instead of the plane. Using an earth-referenced normal acceleration instead of an inertia-referenced normal acceleration, results in the advantageous handling characteristic of being able to fly to a constant vertical speed (including zero vertical speed) about a round earth (i.e., the airplane will fly around the round earth in the absence of pitch input commands). Controlling to an inertia-referenced normal acceleration does not result in aircraft vertical speed that is constant with respect to the earth. The earth-referenced vertical speed z ˙ can be diffentiated to produce an earth-referenced normal acceleration , however, differentiating z ˙ can introduce a not insignificant amount of undesirable noise. As illustrated in FIGURE 4c, the present invention uses a complementary filter to combine a manufactured normal acceleration signal with z ˙ to form an earth-referenced flight path vertical acceleration estimate signal that may be used in lieu of the flight path vertical acceleration signal 150 of turn compensation processor 200. This results in the turn processor 200 supplying a turn compensated, earth-referenced, normal acceleration signal N z,e . This signal N z,e is superior to the usual normal acceleration signal N z because it allows the airplane to fly around the earth in the absence of pilot input. Earth-referenced normal acceleration N z,e may be substituted for the normal acceleration signal N z supplied to the computed C*U processor 30 as well as other portions of the pitch-axis stability and command augmentation system 19. The generation of a normal acceleration signal is effected by mapping body-referenced accelerations onto the earth-referenced z-axis. This process is known in the art of airplane stability and control system design and is indicated in FIGURE 4c by an acceleration processor 272. The earth-referenced vertical speed z is typically received from an airdata system having an inertial reference system. As shown in FIGURE 4c, the earth-referenced flight path vertical acceleration estimate signal is supplied by a combining unit 282 having two signals as additive inputs. The first signal is a portion of the manufactured normal acceleration signal in the z-y plane . The second signal is a vertical speed error supplied by a scaling unit 270 having a gain factor . The input to scaling unit 270 is supplied by a combining unit 254 having additive inputs of earth-referenced vertical speed z ˙ and the integral of a signal supplied by a combining unit 278. The integration is accomplished at an integrator 268 in FIGURE 4c. The signal supplied by combining unit 278 is formed by additive inputs of the earth-referenced flight path vertical acceleration estimate signal and a portion of the manufactured normal acceleration signal in the longitudianl axis of the airplane, . As can be appreciated, the normal acceleration complementary filter 264 uses the earth-referenced vertical speed z ˙ to drive the steady-state characteristics of the earth-referenced flight path vertical acceleration estimate signal . The earth-referenced flight path vertical acceleration estimate signal is used in the computed C*U processor, and in the C* and/or C*U process. The turn-compensated earth-referenced normal acceleration signal N z,e is preferably used in the computed C*U processor, although it is possible to use the turn compensated inertial referenced uncompensated normal acceleration estimate signal , or the uncompensated inertial referenced normal acceleration signal 150. 4d. Pitch Rate Complementary Filter The pitch rate complementary filter produces pitch attitude rate signal 152 with respect to the local horizon. Although, the pitch attitude rate normally is the angular rate of the airplane with respect to the inertia-reference frame, the pitch-axis stability and command augmentation system 19 of the present invention controls the airplane to a pitch attitude rate that is referenced to the earth. Using an earth-referenced pitch rate instead of an inertia-referenced pitch rate, is advantageous because it provides the ability to fly to a constant flight level about a round earth. Controlling to a body-referenced pitch rate does not make the airplane fly around the earth by itself and pilot needs to make correction. The earth-referenced pitch attitude  could be diffentiated to produce an earth-referenced pitch rate  ˙, however, differentiating  can introduce a not insignificant amount of undesirable noise. As illustrated in FIGURE 4d, the present invention uses a complementary filter to combine a manufactured pitch attitude rate signal  ˙ MFTD with  to form an earth-referenced pitch attitude rate estimate signal  ˙ EST that may be used in lieu of the pitch attitude rate signal 152 of turn compensation processor 200. This results in the turn processor 200 supplying a turn compensated, earth-referenced, pitch attitude rate signal q e . This signal q e is superior to the usual pitch attitude rate q because it provides advantageous long-term airplane response. Earth-referenced pitch attitude rate signal q e may be substituted for the pitch attitude rate q supplied to the computed C*U processor 30 as well as other portions of the pitch-axis stability and command augmentation system 19. The manufactured pitch attitude rate signal  ˙ MFTD is formed by mapping the inertia-referenced pitch rate q , roll rate y , and yaw rate p onto the earth-reference frame. This task is known in the art of airplane stability and control system design and is indicated in FIGURE 4d by a pitch rate processor 330. The earth-referenced pitch attitude  is typically received from an air data system having an inertial reference system. As shown in FIGURE 4d, the earth-referenced pitch attitude rate estimate signal  ˙ EST is supplied by a combining unit 332 having two signals as additive inputs. The first signal is the pitch attitude rate signal  ˙ MFTD supplied by pitch rate processor 330 The second signal is a pitch attitude error E  supplied by a scaling unit 324 having a gain factor K E  . The input to scaling unit 332 is supplied by a combining unit 314 having an additive input of earth-referenced pitch attitude signal  and a subtractive input representative of the integral of the earth-referenced pitch attitude rate estimate signal  ˙ EST . The integration is accomplished at an integrator 328 in FIGURE 4d. As can be appreciated, the pitch rate complementary filter uses the earth-referenced pitch attitude  to derive the steady-state characteristics of the earth-referenced pitch attitude rate estimate signal  ˙ EST . The turn compensated earth-referenced pitch attitude rate signal q e is preferably used in the computed C*U processor, although it is possible to use the turn-compensated inertial-referenced pitch rate q, the earth-referenced uncompensated pitch rate estimate  EST , or the uncompensated inertial referenced pitch attitude rate signal 152. 4e. Speed Stability Processor 150 The speed stability processor 150 provides a speed stability feedback signal K v U error that includes a direct trim signal 372 and a phugoid damping feedback signal 400. In addition, the speed stability processor 150 uniquely determines the C*U reference speed V REF based on the current position of a pitch trim device 354. In the depicted arrangement, the reference speed V REF is established with a pitch trim device 354, such as a thumb switch or aisle stand lever, that has three positions, with the normal position (unactivated) of the trim device 354 producing a pitch trim signal 364 equal to zero. When the pilot adjusts the pitch trim device 354, the pitch trim signal 364 is generated that represents either -1 or +1, depending on whether the pilot is trimming nose-up or nose-down. The pitch trim signal 364 is multiplied at multiplier 364 by a gain K t , converting it into a rate of change of reference airspeed (signal 378). As long as the pilot is adjusting the pitch trim device 354, the rate of change of reference airspeed signal 378 will continue to be integrated at integrator 376, thus forming a new reference speed signal 382. If desired, the value of K t may be a function of airdata. Items 374, 375, and 380 of FIGURE 4e relate to the setting of an initial value for V REF . Item 374 is an initialization and synchronization events processor having an output trigger 375 that, when set to true, signals the integrator 376 to use, as its input, the current filtered airspeed Vcas, indicated in FIGURE 4e as dashed line 380. When the output trigger 375 is set to false, the integrator 376 uses the rate of change of reference airspeed signal 378 described above. The logic used by the initialization and synchronization events processor 374 preferably sets the output trigger 375 to true for either of the following conditions: (1) The output trigger 375 is set to true after takeoff for a predetermined period of time T unless the longitudinal aircraft acceleration has decreased to less than a preselected value for a given amount of time during the predetermined period T. (2) The output trigger 375 is set to true when the airspeed difference Uerror is within a predefined range at the time the trim device is returned to its zero value. The first condition is to provide an initial reference speed after take-off. The second condition is to reduce pilot workoad needed to trim the airplane. The initial reference speed signal 382 is limited by a limiter 384 having minimum and maximum speed limits. These limits are based on the flight envelope of the aircraft as determined by its particular configuration. The minimum limits are described in the stall protection processor portion of section 5. The maximum speed limits are applied so that beth the resulting reference speed Vref and its corresponding reference Mach, are at or below maximum operating velocity Vmo and maximum operating Mach Mmo. Although the, pilot can adjust the reference speed V REF by using the pitch trim device 354, it is not possible to trim the aircraft to speeds outside of the speed range established by limiter 384. This feature is desirable in that it requires the pilot to hold the column in a non-neutral position in order to overspeed or underspeed the airplane, thereby alerting the pilot to the overspeed or underspeed condition. The resulting reference speed V REF is then combined at combining unit 398 with current filtered calibrated airspeed Vcas to form the airspeed difference Uerror. Filtered calibrated airspeed based on Vcas, which is supplied to the speed stability processor 150 by an air data unit. It is preferable that Vcas be adequately filtered prior to being supplied to the processor 150 in order to eliminate signal content caused by turbulence. Other airspeed signals may be used if desired, e.g., Mach, or unfiltered calibrated airspeed, assuming the related gains and schedules are established appropriately. At a combining unit 396 U error is combined with a direct trim signal 372 to form an enhanced U error (signal 404). The direct trim signal 372 is computed in a direct trim signal processor 356. The function of the direct trim signal 373 is to provide additional elevator command so as to emulate the immediate response of the elevators in a conventional airplane relative to the activation of a trim device. Airspeed airdata or flap position is used to create the direct trim signal 372 which is given the appropriate sign depending on whether the airplane is being trimmed using pitch-up or pitch-down, which is indicated by input signal 364. The enhanced U error signal 404 is multiplied by an airspeed gain K v at multiplier 406. This results in a preliminary speed stability feedback signal 408. The airspeed gain K v is used to set the stick force required in pounds per knot of deviation of airplane speed from reference speed. If desired, the value of K v may be a function of airdata. Finally, combining unit 410 combines the preliminary speed stabiltiy feedback signal 408 with a phugoid damping feedback signal 400 received from a phugoid damping feedback processor 344. The purpose of including the phugoid damping signal in the speed stability feedback signal K v U error is to provide adequate damping to phugoid mode. Such processors 344 are known in the art of airplane stability and control system design. The resulting signal is the speed stability feedback signal KvUerror used in the computed C*U processor 32 described above. A dead zone may be placed on U error so that speed stability is only provided when U error is greater than a predetermined value. Such a dead band will provide neutral speed stability for a band of speeds that includes the trim speed. 5. Discussion of Compensation and Protection Processor 34 As was mentioned relative to FIGURE 1, the invention includes a compensation and protection processor (34 in FIGURE 1), which provides a compensation and protection signal C*U C&P which is combined with the difference between the computed C*U signal C*U Computed and the pilot commanded C*U pitch command C*U PilotCmd to produce the C*U error signal E C*U . 5a. Stall Protection Processor 416 With reference to FIGURE 5a, the compensation and protection processor 34 of the currently preferred embodiment of the invention includes a stall protection processor 416, which supplies a C*U underspeed protection signal (C*U US ); a flare compensation processor 414, which provides a C*U flare compensation signal (C*U FL ); and an overspeed protection processor 414, which provides a C*U overspeed protection signal (C*U OS ). The signals provided by stall protection processor 416, flare compensation processor 414 and overspeed protection 412 are supplied to input terminals of the combining unit via signal paths 422, 420 and 418 respectively. In the depicted arrangement, the output signal of combining unit 424 is supplied to a switch 428, which may or may not be activated by the aircraft autopilot engage signal depending upon system design choice. Stall protection processor 416 functions to: (1) establish a lower limit on the value of C*U V REF that can be set by the flight crew; and (2) increase the magnitude of the system speed stability feedback during underspeed operation, which provides greater nose down corrective command and pilot awareness of underspeed. The currently preferred embodiment of stall protection processor 416, which is depicted in FIGURE 5b, has two operational modes. The first operational mode is a take-off mode that provides an underspeed protection signal C*U Us and a lower limit for the C*U reference speed V REF(MIN) only during the initial phase of the aircraft take-off operation. Specifically, during the initial take-off phase, both the C*U underspeed protection signal and the minimum value of the C*U reference speed are established to provide pitch performance characteristics that allow a pilot to take appropriate action should an engine fail. The second mode of operation of the stall protection processor shown in FIGURE 5b is the post take-off mode of operation in which the processor supplies a C*U underspeed protection signal and a minimum value of C*U reference speed that are matched to operation of the aircraft in normal flight regions. In both modes of operation, the value of the underspeed protection signal and the minimum value of the C*U reference speed are set with respect to the low speed amber band value, which is commonly referred to as "the top of the amber band." As is known in the art, the top of the amber band corresponds to the speed at which the aircraft can execute a 40 degree bank turn without angle of attack reaching "stick shaker," or onset of buffet, i.e., without reaching a flight condition in which the flight crew is warned of impending stall or onset of buffet. The arrangement of FIGURE 5b automatically switches between the early take-off and the post-take-off mode, with the associated signal switching being indicated by conventional switches S1-A, S1-B, S1-C, each of which is depicted in the reduced trim limit mode. In the currently preferred embodiments of the invention, switching to the post take-off mode of operation occurs only if three conditions are met. In particular, switching occurs: (1) if a predetermined time has elapsed since take-off (e.g., 15 seconds); and , (2) aircraft airspeed is greater than the top of the amber band by a predetermined amount (e.g., four knots); and , in addition, (3) the C*U reference velocity, C*U V REF , set by the flight crew exceeds the top of the amber band by a predetermined amount (e.g., four knots). With switches S1-A, S1-B, S1-C in the position shown in FIGURE 5b (take-off mode), the lower limit for the C*U reference speed, V REF(MIN) is established at a value that is below the top of the amber band. Specifically, in FIGURE 5b, a signal representative of the stick shaker V SS is supplied to a terminal 434 by the aircraft warning electronic system or similar source. At block 436, V SS is scaled by a constant KV Sl to provide a signal that is below the top of the amber band by the desired amount (e.g., one-half of the amber band speed value) to one terminal of switch S1-A. Passing through switch S1-A, the signal is filtered (by a rate limiter or lag filter 438) to eliminate any abrupt changes in the stick shaker speed signal V SS , with the currently preferred embodiment of the invention employing rate limiting of approximately four knots per second. With the system in the reduced trim limit mode, the signal supplied by rate limiter (or lag filter) 436 is transferred through switch S1-B as the C*U minimum reference speed C*U V REF(MIN) . Thus, it can be noted that during the early phase of take-off, the minimum C*U reference speed is below the top of the amber band, a feature which allows the pilot to trim below the top of the amber band, i.e., set a C*U reference speed, C*U V REF , that is below the top of the amber band. This feature is only necessary if the approximate speed for an engine out condition falls within the amber band. With continued reference to FIGURE 5b, during operation of the depicted stall protection processor in the reduced trim limit mode, the signal supplied by rate limiter 438 also forms the stall protection underspeed reference signal, which is coupled to a subtractive input terminal of a combining unit 440 via switch S1-C. Combining unit 440 subtracts the stall protection underspeed reference signal from a signal representative of the aircraft filtered airspeed, which is supplied to a terminal 442 in FIGURE 5b and coupled to an additive input of combining unit 440. The output signal of combining unit 440 is thus representative of the difference between the filtered airspeed signal and the stall protection set speed for the underspeed signal. This speed signal provided by combining unit 440 is scaled by a gain factor KV S3 at block 444 and is supplied to an additive input of a combining unit 446. The gain factor KV S3 is selected to obtain a desired change in the force that is required to move or hold the command column aft of its neutral position detent when the aircraft airspeed signal is less than the underspeed reference signal, i.e., when aircraft airspeed is less than a selected underspeed reference value. For example, as was described relative to FIGURE 4e, the speed stability feedback feature of the invention establishes a predetermined control column force gradient that requires the pilot to assert additional column command in order to maintain airspeed that is above or below the C*U reference speed (C*U V REF ). In the currently preferred embodiments of the invention the column force gradient required to fly at speeds above or below C*U reference speed is 3 pounds/knot. In these embodiments, gain factor KV S3 of the depicted underspeed protection arrangement adds an additional 12 pounds/knot force gradient which comes into effect when airspeed is less than a selected underspeed value (approximately 0.5 times the top of the amber band with the arrangement of FIGURE 5b operating in the take-off mode and the top of the amber band when in post take-off mode). The added column force requirement alerts the pilot to underspeed condition, but does not prevent conscious decision to maneuver the aircraft at low speed. Returning again to the arrangement of FIGURE Sb, combining unit 446 provides damping of the C*U underspeed signal C*U US as a function of the aircraft pitch rate turn compensation. In the arrangement of FIGURE 5b, a signal representative of pitch rate turn compensation is supplied to a terminal 448; scaled by an appropriate gain factor KV S2 (indicated at block 450); and supplied to a subtractive input terminal of combining unit 446. The signal supplied by combining unit 446 is then processed by a limiter 452 and supplied as the reduced trim limit mode C*U underspeed command signal. The upper limit of limiter 452 is preferably established at zero. That is, in the preferred embodiments of the invention, a C*U underspeed signal is not supplied (i.e., is equal to zero) whenever filtered airspeed is greater than the stall protection underspeed reference. Thus, the stall protection processor 416 will have no effect on the C*U protection and compensation signal when aircraft speed is above a predetermined value (approximately 0.5 times the top of the amber band for the currently preferred embodiment of the invention that is operating in take-off mode and top of the amber band when in the post take-off mode). The lower limit of limiter 452 establishes the maximum amount of nose down pitch signal that can be supplied by the underspeed protection processor. In accordance with the invention this upper limit is set at a value that prevents the generation of underspeed protection pitch down command signals that cannot be overridden by the pilot forcefully operating the command column. As previously noted, the stall protection processor in FIGURE 5b automatically changes its mode of operation during the take-off procedure when there is no longer a need to allow the pilot to assert speed trim (set the C*U reference speed below the top of the amber band. When the arrangement of FIGURE 5b is switched out of the reduced trim limit mode (i.e., to the post-take-off mode), a signal representative of the top of the amber band supplied to the input of rate limiter 438 via terminal 437 and switch S1-A. Like the stick shaker speed signal used by the system in the reduced trim limit mode, the signal representative of the top of the amber band can be obtained from the aircraft warning electronic system or a similar source. Moreover the signal representative of the top of the amber band can be derived from the stick shaker speed and vice versa, with little or no system performance loss. In this regard, the input to switch S1-A can be based on either or both top of the amber band ( which is compensated for load factor) or stck shaker speed. It can be noted that, when the arrangement of FIGURE 5b is operating in the post take-off mode, the underspeed reference signal that is supplied to the subtractive input of combining unit 440 is not identical to the signal supplied by rate limiter 438. That is, during operation in the post take-off mode, the signal supplied by rate limiter 438 is multiplied (indicated at block 454) by an underspeed feedback reference gain (indicated at block 456), with the output signal provided by multiplier 454 being supplied to the subtractive input terminal of combining unit 440 via switch S1-C. The underspeed feedback reference gain 456 is a function of aircraft speed (e.g., mach) and is established so that the underspeed reference signal is relatively constant (e.g., equal to unity) at relatively low airspeed and decreases at higher airspeed. The purpose of scheduling underspeed feedback reference gain as a function of airspeed is to partially offset the increased column force characteristics that established at increased speed by the aircraft feel system described relative to FIGURE 2a. The specific objective is to achieve a control column force requirement at sticker shaker angle of attack ( α SS ) that is relatively consistent for all flight conditions (desired range of 15-25 pounds) in the currently preferred embodiments of the invention. As was discussed relative to operation in the take-off mode, the combining unit 440 generates a signal representative of the difference between aircraft filtered airspeed and the underspeed reference signal. The speed difference signal is scaled by KV S3 at block 444 to achieve the desired increase in command column force gradient at speeds below the underspeed reference; pitch rate turn compensation damping is effected at combining unit 446; and the resultant signal is limited (by limiter 452) and supplied as a C*U underspeed protection system that, if not overridden by the pilot, will result in a nose down pitch command signal. When the underspeed protection arrangement of FIGURE 5b is in the post take-off mode of operation, the minimum value C*U reference speed signal, is not supplied directly from the output of rate limiter 438. Instead, as is indicated in FIGURE 5b, the signal supplied by rate limiter 438 is multiplied (at multiplier 458) by a mach speed trim inhibit gain (indicated at 460), with the product being supplied as the minimum V*U reference speed V REF(MIN) via switch S1-B. Like the previously described underspeed feedback reference gain, the mach speed trim inhibit gain is scheduled as a function of aircraft speed; exhibiting a substantially constant gain (e.g., unity) over a low speed range and decreasing for speeds above a predetermined mach value (e.g., mach 0.6). Thus, the minimum C*U reference speed established by the arrangement of FIGURE 5b during post take-off mode operation is approximately equal to the top of the amber band at aircraft speeds below a predetermined mach value and decreases at higher mach values. This relationship is established to supplement the gain variation in underspeed feedback gain to achieve a relatively consistent command column force requirement at α SS for substantially all flight conditions. 5c. Flare Compensation Processor 414 FIGURE 5c schematically illustrates the flare compensation arrangement of the currently preferred embodiment of the invention. In the arrangement of FIGURE 5c, a signal representative of the altitude of the landing gear above the ground is supplied to a look-up table 462 or other device that generates a flare command signal (pitch down) that varies (increases) as a function of airplane gear height. Various signals are known in the art for providing an indication of airplane gear height, generally being derived from the aircraft radioaltimeter. Preferably, the gear height representative signal is filtered and processed to correct for aircraft pitch attitude. Look-up table 462 establishes a relationship between the aircraft gear height and the outputted flare command signal that in effect emulates ground effects encountered by the aircraft while executing a landing. Emulation of the ground effects by the flare compensation processor enables the C*U system to provide flare handling characteristics similar to those of an unaugmented aircraft. With continued reference to FIGURE 5c. The flare compensation signal supplied by look-up table 462 is supplied to a limiter 467 via a switch 466, which supplies the flare compensation signal to limiter 467 as long as a valid gear altitude signal is being supplied to the flare compensation processor. The lower limit value of limiter 467 is zero (to insure that nose-up compensation signals are not provided by the flare compensation processor), with the upper limit (0.54g in the current embodiments of the invention) being established equal to the desired flare command at touchdown. The output signal provided by limiter 467 is supplied as the C*U flare compensation signal C*U FR via a switch 468. Switch 468 is activated to supply a C*U flare compensation signal FC*U FL identical to the signal provided by limiter 467 only when the aircraft is operating in the approach mode and reaches a gear height altitude at which the flare maneuver is initiated. In the currently preferred embodiments of the invention, the logic employed for activating switch 468 requires that: (1) the aircraft has been airborne for at least 60 seconds (to prevent flare compensation during take-off); (2) the aircraft flaps are deployed; (3) the gear height signal indicates gear altitude of less than 50 feet for at least one second; and (4) the gear height signal is valid (i.e., is being provided to terminal 460 the depicted flare compensation processor). (Optionally, the gear down switch may be added to the above conditions.) In the event that any one of these conditions are not meant, switch 468 will not activate and, as is indicated at block 470, the C*U flare compensation signal C*U FL is set equal to zero (no compensation). However, once switch 468 activates, it will remain activated if the gear height signal is lost or otherwise becomes invalid. As described below, this allows the flare compensation processor to operate in a manner that does not result in abrupt changes in the flare compensation signal if the gear height signal becomes invalid while a flare compensation is being generated. Also included in the arrangement of FIGURE 5c is a rate limiter 472 which controls the operation of the flare compensation processor if the radio altimeter or other source of the gear height signal fails while the flare compensation processor is providing a compensation signal. In that regard, rate limiter 472 serves as an alternate source of flare compensation signal with the output signal provided by rate limiter 472 being supplied to limiter 467 via switch 466 if a alternate flare compensation engage signal is supplied to the flare compensation processor (indicated at block 474 in FIGURE 5c). The alternate flare compensation trigger signal can be a signal indicating that the radio altimeter signal is invalid or some other available signal that indicates that the gear height signal is not considered reliable. During normal operation (i.e., switch 466 in the illustrated position), the C*U flare compensation signal provided by limiter 467 is coupled to rate limiter 472 establish an initial value or bias level If an alternate flare compensation system engages signal activates switch 466, the current value of the signal provided by limiter 472 is established as the input (initial condition) of rate limiter 467, assuming that the aircraft is not executing a go-around procedure, a signal representative of the flare compensation signal at touchdown is provided to limiter 472 via a switch 474. Since the output signal of limiter 472 will then increase linearly as a function of time until the maximum limit is reached (0.54gs in the current embodiment), rate limiter 472 in effect emulates the flare command signal normally provided by look-up table 462 (operating as a function of time, instead of gear height). In the event that a go-around was initiated prior to loss of the gear height signal, switch 476 is activated to establish the lower limit of rate limiter 472 at a value of zero (indicated at block 478). Establishing the lower limit of rate limiter 472 at zero during a go-around maneuver means that rate limiter 472 will supply a C*U flare compensation signal that linearly decrease to zero if the gear height signal becomes invalid while a flare compensation signal is being provided. Those skilled in the art will recognize that a satisfactory C*U flare compensation signal can be generated by arrangements other than the flare compensation processor shown in FIGURE 5c. For example, look-up table 462 of FIGURE 5c can be replaced with a circuit or other arrangement that is triggered when the airplane reaches an altitude of less than 50 feet and generates a flare command signal that is a function of time (instead of gear altitude). Further, rate limiter 472 can be replaced with other arrangements that smoothly reduce the signal supplied to limiter 467 to the appropriate limit value in the event that the signal normally supplied to limiter 467 is interrupted or otherwise becomes of questionable validity. 5d. Overspeed Protection Processor 412 FIGURE 5d illustrates the control law effected by the currently preferred overspeed protection processor (412 in FIGURE 5a). The arrangement of FIGURE 5d supplies a C*U overspeed protection signal C*U OS that, unless overridden, will result in a nose up pitch command signal whenever the aircraft is being operated at an overspeed condition. As a result of the C*U overspeed signal, the pilot must assert more than normal forward force on the command column to maintain or increase overspeed. In accordance with the invention, the value of the C*U overspeed pitch command signal is established so that the forward command column force required to maintain overspeed sufficiently alerts the pilot to the overspeed condition, but consciously can be overridden if the pilot chooses to do so. Basically, the arrangement of FIGURE 5d determines the existence and extent of overspeed in view of a first overprotection signal that is based upon aircraft calibrated airspeed and a second overspeed protection signal that is based upon mach speed. The two overspeed signals are then compared to determine the greater of the two signals, which is then limited to preclude generation of nose down attitude command signals and to maintain the signal at an appropriate value that is compensated for roll attitude. The limited signal then is provided to summing unit 424 of FIGURE 5a as the C*U overspeed protection signal C*U OS . More specifically, and with reference to FIGURE 5d, a signal representative of aircraft filtered airspeed is coupled to an additive terminal of a combining unit 500 via a terminal 502. Connected to the subtractive input of combining unit 500 is a signal representative of an airspeed that is slightly higher than the maximum operating airspeed for the aircraft (VMO). In the currently preferred embodiments of the invention, the signal supplied to the subtractive input of combining unit 500 representative of 6 knots above VMO. The output of combining unit 500, which represents the difference between aircraft filtered airspeed and VMO plus the noted margin, is scaled by a constant gain factor by a scaling unit 504 to control the magnitude of any resultant overspeed signals based upon exceeding VMO relative to the magnitude of overspeed signals representative over exceeding maximum mach speed. The scaled airspeed difference signal that is provided at block 504 is supplied to an additive terminal of a combining unit 506. Supplied to a second additive input terminal is a signal representative of the aircraft filtered airspeed rate (i.e., time rate of change of filtered airspeed), which is applied to a terminal 508 and scaled by a constant gain factor by a scaling unit 510. The gain factor of scaling unit 510 is set so as to suitably increase the overspeed protection computed airspeed command produced by summing unit 506 when the aircraft is approaching or operating above VMO and, in addition, airspeed is increasing. That is, the gain factor of scaling unit 510 is established in view of the gain factor of scaling unit 504 to control the relative contribution of the signal representing excess airspeed (supplied by combining unit 500) and the signal representative of airspeed rate, which is an acceleration term. In the arrangement of FIGURE 5d, the overspeed protection mach command signal is supplied by a combining unit 512 and is generated in a manner similar to the generation of the overspeed protection computed airspeed command signal. Specifically, a signal that represents filtered aircraft mach speed is supplied to an additive terminal of a summing unit 520 (via terminal 514), with the subtractive input terminal of the combining unit 520 receiving a signal representative of the aircraft maximum operating mach speed (MMO) plus a desired margin (e.g., 0.01) (indicated at block 516). The signal supplied by combining unit 520 is scaled (at block 518) and coupled to the additive input terminal of combining unit 512. Coupled to a second additive input of combining unit 512 is a signal representative of mach rate (supplied to terminal 524), which is scaled by an appropriate factor at block 522. The overspeed protection calibrated airspeed command signal produced by summing unit 506 and the overspeed protection mach command signal supplied by combining unit 512 are compared by a comparator 526 in FIGURE 5d. The greater of the two signals is then coupled to a limiter 528. As is indicated at block 530, limiter 528 has a minimum value of zero. The maximum value of limiter 528 is not a constant, however, but is established based upon aircraft roll attitude. More specifically, a signal representative of aircraft roll attitude (in degrees) is supplied to a terminal 532. The magnitude (absolute value) of the signal is determined at block 534 and the resultant signal is scaled by a scaling unit that exhibits a gain-roll attitude relationship of the type indicated at block 536. Specifically, the gain factor of scaling unit 536 is substantially equal to unity when the absolute value of the aircraft roll attitude is within the range of 0-30° and decreases linearly to 0 over the roll angle range of 30° to 60°. The output of scaling unit 536 is then scaled by a constant (at block 538) that establishes the maximum command column force gradient that can be developed by the depicted overspeed protection processor. That is, the scaling factor employed at block 538 sets an upper limit on the magnitude of the C*U overspeed protection signal supplied by the depicted arrangement, thus establishing the maximum amount of pitch up attitude command that may be overridden by the pilot if an election is made to fly the aircraft in overspeed condition. Establishing the lower limit of limiter 528 at zero ensures that the overspeed protection processor will not produce nose down attitude command signals that, unless overcome, would further increase the speed of the aircraft. Scaling the maximum limit value for limiter 528 in the above-described manner reduces the amount of overspeed protection command signal during high-speed banked turns in which the absolute value of the roll attitude is between 30° and 60°. If the aircraft roll attitude reaches or exceeds 60°, the signal supplied by scaling unit 536 becomes zero, in turn establishing the maximum limit value for limiter 528 at zero. Thus, no overspeed protection signal is supplied for high bank angles. During overspeed conditions with the aircraft operating within a typical roll range, the signal supplied by limiter 528 is scaled at block 540 to provide a C*U overspeed protection signal C*U OS at appropriate levels (e.g., convert a signal supplied by limiter 528 that represents C*U units per pound to a signal that represents C*U units per g. It can be noted that the above-described C*U overspeed protection system provides increased crew awareness of overspeed by increasing the amount of forward command column force required to maintain the aircraft in overspeed operation. Moreover, like the previously described protection and compensation processors, the depicted arrangement establishes command column force relationships that allow the pilot to assert command column pressure sufficient to overcome the protection and compensation signals that are provided by the invention. In that regard, and with respect to the currently preferred embodiments of the invention, a maximum force on the order of 40 pounds is required to override overspeed protection at or near aircraft maximum design speed (Vdive) and for airspeed at or near maximum design mach speed (Mdive). 6. Discussion of Pitch Rate Damping Processor 36 The pitch rate damping processor 36 functions to produce a pitch rate damping command Q COM that, when added to the C*U integral command IC*U COM will damp the elevator command short period response and adjust the short period frequency. There are two basic features of the preferred pitch rate damping processor 36. The first feature is a shut-off multiplier 631, the second is a combining unit 630 for combining a lagged pitch rate feedback signal 628 and a proportional pitch rate feedback signal 626. The shut-off multiplier 631 shuts off pitch rate damping augmentation when the airplane is on the ground. The shut-off multiplier 631 multiplies the pitch rate damping command Q COM by a value in the range of 0.0 to 1.0, as supplied by a computing unit 638. The computing unit 638 accepts an on-ground signal from an air/ground status determination processor (not shown). Starting from an on-ground state and transitioning to an in-air state, the on-ground signal will switch from false to true. This causes the computing unit 638 to switch from outputting a value equal to zero, to a value equal to one. The preferred computing unit 638, however, ramps the output value over a relatively short period of time (nominally less than 10 seconds) instead of stepping it. The reverse situation causes the computing unit 638 to switch from outputting a signal equal to one, to a signal equal to zero, in a ramped fashion. By ramping the output value, the pitch rate damping command Q COM is gradually reduced. This makes the transition from an augmented to an unaugmented system a smooth, unnoticeable change. The lagged pitch rate feedback signal 628 and proportional pitch rate feedback signal 626 are the additive inputs to a combining unit 630 The output signal of combining unit 630 corresponds to the pitch rate damping command Q COM before multiplication by the shut-off multiplier 631 described above. The two signals 626, 628 are combined to generate the pitch rate damping command Q COM . The lagged pitch rate feedback signal 628 is supplied by a lag filter 618 having a transfer function of the form K ql /(τs+1) , where K ql and τ are provided according to a schedule indicated at block 614. The input to filter 618 is the turn compensated pitch rate signal q . The values of K ql and τ preferably depend on flap position, or airspeed, where flaps-up establishes K ql equal to 1.0 and τ equal to 1.5, and flaps-down establishes K ql equal to 1.5 and τ equal to 1.0. This transition between flaps down and flaps up values is done gradually over a predetermined period of time (nominally 10 to 30 seconds). The lag filter 618 establishes functions to provide a lagged pitch rate feedback signal 628 based on the turn compensated pitch rate signal q that can be used as a feedback to adjust the short period frequency. The proportional pitch rate feedback signal 626 is supplied by a shaping filter 624. The shaping filter 624 has a transfer function that removes frequency content of the pitch rate signal that could interfere with the airplane natural modes. The input to the shaping filter 624 is the turn compensated pitch rate signal q as supplied by a gain 620 having a gain damping factor K q , provided as shown at block 619. As indicated in FIGURE 6, the term K q is preferably dependent on flap position, where flaps-up establishes K q equal to 1.0 and flaps-down establishes K q equal to 1.35. The gain damping factor K q provides adequate short period damping for high speed and low speed operations as dictated by flap setting. Again, the transition between flaps up and flaps down gain values is relatively gradual. In the preferred embodiment, the flap setting is used to determine K ql and K q , the flap down status being first confirmed by airspeed data. The use of flap setting to determine K ql and K q precludes gain changes based on erroneous airdata as might be the case during an ash cloud encounter, while using airspeed precludes increasing the gain if the flap data is erroneous. Trigger mechanisms other than flap position may be , used to set K ql and K q depending on the available data and the designer's preference (e.g., other combinations of logic using both flap position and/or airspeed). Basically, the turn compensated pitch rate signal q is supplied to the pitch rate damping processor 36 and is multiplied by a gain damping factor K q that is dependent on flap setting. The resulting signal is shaped by a shaping filter 624 and added to a lagged pitch rate feedback signal 628, which is supplied by the lag filter 618 which is connected for receiving the turn compensated pitch rate signal q . It may be noted that the pitch rate damping processor 36 may be based on a pitch attitude signal  instead of, or in conjunction with, the turn compensated pitch rate signal q . Such a change would require an appropriate revision of gains 620 and 618 and filter 624. The preferred input is the turn compensated pitch rate signal q as discussed in section 4 above and shown in FIGURE 4b. 7. C*U Integrator 28 The C*U integrator 28 integrates the error signal E C*U to continually add a portion of the error signal E C*U to the elevator pitch command δ e,FILT . Eventually, the C*U integrator 28 will wash out the error signal E C*U since the elevator pitch command δ e,FILT causes the elevator to move, which causes the inputs to the computed C*U signal C*U Computed to change, causing the difference between the pilot-requested C*U command C*U PilotCmd and the sum of the computed C*U signal C*U Computed with the compensation and protection C*U signal C*U C&P to go to zero. There are six features in the preferred form of the C*U integrator 28: a gain K, a shut-off multiplier 642, a windup-preventor 648, a wash-out gain 666, a combining unit 654, and an integrator 662. Each of these features provides a desirable modification to he pitch-axis stability and command system, although, the only essential feature is the integrator 662. The basic structure of the C*U integrator 28 is such that the error signal E C*U and the trim negation command TNS are the additive inputs to combining unit 654, with the output of the combining unit 654 being supplied to an integrator 662. The resulting signal is the integral C*U command IC*U COM discussed in section 1. Before the error signal E C*U is input to combining unit 654, a number of different conditions may exist that may cause another value to enter the combining unit 654 instead of the error signal E C*U . These conditions are discussed below. The error signal E C*U is scaled at gain 632 by an integral gain factor K according to the schedule shown in block 631. The scaling of the error signal E C*U by K provides desirable short period response for high speed and low speed conditions (as indicated by flap position, shown in block 631). Block 631 sets K = 5 for flaps-up and K = 8 for flaps-down with the transition between the flap down and flap up values being gradual. Other means, such as air data, may be used to determine high speed or low speed conditions, if desired. The error signal E C*U as modified by the gain K by gain 632 is shown in FIGURE 7 as signal 634. The second feature of the C*U integrator 28 is the shut-off multiplier 642 (or gain reducer) that shuts off C*U augmentation when the airplane is on the ground. The shut-off multiplier 642 multiplies the output of the gain 632 by a value in a range between 0.0 and 1.0, as supplied by a computing unit 638. The computing unit 638 basically switches the value used by the multiplier 642 between 0.0 and 1.0 in a ramped fashion. The computing unit 638 is described above in section 6. By ramping the outputted value, C*U augmentation is gradually reduced. This makes the transition from an augmented to an unaugmented system a smooth, unnoticeable change. The third feature of the C*U integrator 28 is the windup-preventor logic 646 that prevents the integrator from adding elevator pitch command δ e,FILT augmentation when the elevators (or airplane) are not able, or required, to respond. A switch 648 is provided in the C*U integrator 28 that in its normal closed position does not disturb the output of multiplier 642. The switch is normally closed unless a specific set of criteria are met. The logic for determining when the switch 648 opens is shown in FIGURE 7 as being supplied by unit 646. The preferred logic of unit 646 opens the switch when any one of the following conditions is met: (1) the airplane is on ground, (2) the airplane tail is near the ground and the error signal E C*U is requesting more nose-up, (3) the airplane is at a high angle of attack and the error signal E C*U is requesting more nose-up, or (4) the elevator has reached its maximum limit of authority and error signal E C*U is requesting further elevator movement in the limited direction. Under any of these conditions, it is not desirable to allow the integrator to operate, since requesting further pitch does not produce, or should not be allowed to produce, a further change in pitch attitude. Opening the switch 648 results in a value of 0.0 being sent to the additive input of combining unit 662, instead of the output of multiplier 642. The fourth feature is the combining unit 662 for combining the additive inputs of error signal E C*U (or zero if switch 648 is open) and trim negation command TNC discussed in section 1 above and shown in FIGURE 1. The preferred trim negation signal is derived from the rate of change of stabilizer position as converted to degrees of elevator per second. The generation of a trim negation signal is included in the operation of currently available autopilot systems. Adding the trim negation command TNC to the output of the multiplier 642, during the off-load operation, moves the elevator toward its faired position with respect to the stabilizer, without generating a noticeable airplane response. The fifth feature of C*U integrator 28 is provision of wash-out gain 666 that includes a decay gain K d and that causes the output of integrator 662 gradually to decay to zero over a predefined period. The wash-out gain 666 gradually eliminates augmentation when the airplane is on the ground. This is a desirable feature, because the integral C*U command signal IC*U COM should be set to zero before the next flight. It is preferable to accomplish this decay in a short period of time and after a short time delay after airplane touch-down. In FIGURE 7, the wash-out gain unit 666 is supplied with integral C*U command IC*U COM signal and multiplies it by the decay gain K d . A switch 652 located in the output signal path of the wash-out gain unit 666 opens when the airplane is on ground to supply the output of the wash-out gain 666 to the combining unit 654. The preferred decay gain K d is less than one. The sixth feature is the integrator 662. The integrator 662 computes a portion of the error signal E C*U and supplies the integral C*U command IC*U COM which is used as discussed in section 1. Basically, the error signal E C*U , supplied to the C*U integrator 28, is modified by an integral gain K then combined with the trim negation command TNC ; and, integrated to generate the C*U integral command IC*U COM . While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the scope of the claimed invention. This C*U control law has been developed for statically relaxed airplanes. It is to be understood, however, that other non-statically relaxed airplanes, may benefit from the present invention as well.

Claims: A method for producing an aircraft elevator command signal comprising the steps of: generating a signal representative of the normal acceleration of the aircraft; generating a signal representative of the aircraft pitch rate; multiplying said signal representative of aircraft pitch rate by a constant, K, to form a blended pitch rate signal: summing said signal representative of said normal acceleration of the aircraft with said blended pitch rate signal to supply a pitch command signal; generating a signal representative of the airspeed of the aircraft; generating a signal representative of a desired airspeed; combining said signal representative of the desired airspeed of the aircraft with said signal representative of the airspeed of the aircraft to generate an aircraft airspeed error signal that represents the difference between the desired airspeed and the aircraft airspeed; multiplying said aircraft airspeed error signal by a second selected constant, K 1 , to form a speed augmentation signal; subtracting said speed augmentation signal from said pitch command signal to generate said elevator command signal; and repeatedly generating said pitch command signal and said airspeed augmentation signal to provide said elevator command signal at a periodic rate that allows continuous command of aircraft pitch attitude. The method of claim 1, wherein the steps for generating said speed augmentation signal further include the steps of: generating a signal representative of impending aircraft stall; generating a signal representative of the condition of the aircraft relative to stall; combining said signal representative of impending aircraft stall with said signal representative of the condition of the aircraft relative to stall to supply a stall protection error signal that represents the difference between said signal representative of impending aircraft stall and said signal representative of the condition of the aircraft relative to stall; detecting whether the magnitude of said stall protection error signal is less than a predetermined value and whether said stall protection error signal indicates that said representative of the condition of the aircraft relative to stall is less than said signal representative of impending aircraft stall, and, if so supplying said stall protection error signal as a stall protection signal; and combining said stall protection signal with said speed augmentation signal. The method of claim 2, wherein said signal representative of impending aircraft stall is a signal representative of aircraft stick shaker speed and wherein said signal representative of the condition of the aircraft relative to stall is said signal representative of said aircraft airspeed. The method of claim 2 or 3, wherein said signal representative of impending aircraft stall is a signal representative of stick shaker angle of attack and wherein said signal representative of condition of the aircraft relative to stall is a signal representative of the current aircraft angle of attack. The method of any of claims 1-4, wherein said steps for generating said speed augmentation signal further comprise the steps of: generating a first overspeed signal when the airspeed of the aircraft is above a predetermined value; generating a second overspeed signal when the mach speed of the aircraft exceeds a predetermined mach speed; comparing said first and second overspeed signals to supply a third overspeed signal that is substantially equal to the larger of said first and second overspeed signals: determining whether said third overspeed signal is greater than a lower limit and is less than an upper limit; multiplying said third overspeed signal by a predetermined value when said third overspeed signal is greater than said lower limit value and is less than said upper limit value to provide an overspeed protection signal; and including said overspeed protection signal as a component of said speed augmentation signal, wherein generation of said speed augmentation signal further comprises the step of determining whether the roll attitude of the aircraft is within a predetermined range and wherein said upper limit is established based upon said signal representative of whether the aircraft roll attitude is within a predetermined range. An aircraft elevator command system comprising: a control column moveable in the fore and aft direction from a detent position; a position sensor for generating a column position signal (δ c ) representative of the displacement of said control column from said detent position; a first signal combiner (20) having a first and second inputs and an output; a pitch command processor (26) connected for receiving said column position signal (δ c ), said pitch command processor supplying a feed-forward elevator command signal to said first input of said first signal combiner (20), said pitch command processor further supplying a signal (δ C,COR ) representative of the position of said control column relative to said detent; a commanded elevator signal processor (30) connected for receiving said signal (δ C,COR ) representative of the position of said column, said commanded elevator signal processor converting said signal representative of the displacement of said control column into a pilot commanded pitch control signal (C*U PILOT CMD ); a computed pitch control processor (32) receiving signals representative of the aircraft normal acceleration (N z ) and aircraft pitch rate (q) to produce a first pitch command signal (C*), said computed pitch control processor further processing signals representative of the airspeed (V CAS ) of the aircraft and a desired airspeed (V REF ) to produce an airspeed augmentation signal (U ERROR ), said computed pitch control processor including means (396) for supplying a computed pitch signal (C*U COMPUTED ) representative of the difference between said first pitch command signal (C*) and said airspeed augmentation signal (U ERROR ); a third signal combiner (22) connected for receiving said computed pitch signal (C*U COMPUTED ) and said pilot commanded pitch control signal (C*U PILOT CMD ) and for supplying an error signal (E C*U ) representative of the difference between said computed pitch signal and said pilot commanded pitch control signal; integrator means (28) connected for receiving said error signal (E C*U ) and for supplying an integrated error signal (I C*U COM ); a fourth signal combiner (23) connected for receiving said integrated error signal (I C*UCOM ) and a signal representative of the aircraft pitch rate (q), said fourth signal combiner supplying a feedback augmentation signal (AFB COM ) to said second input of said first signal combiner (20), said output of said signal combiner being supplied as an elevator control signal (δ e , FILT ) of said system. The system of claim 6 further comprising a compensation and protection processor for generating an underspeed signal when the airspeed of the aircraft is below a predetermined value, said underspeed signal being supplied to an input of said third signal combiner for inclusion as a component of said error signal. The system of claim 7, wherein said compensation and protection processor generates an overspeed signal when the aircraft is being operated outside a predetermined operating region, said overspeed being supplied to an input of said third signal combiner to contribute to said error signal. The system of claim 7, wherein said pitch command processor further includes bias signal correction means connected for receiving said column position signal and for supplying a corrected column position signal as said signal representative of the position of the control column to said commanded elevator signal processor, said bias signal correction means being configured and arranged for determining the amplitude of the column position signal when said command column is at detent and for subtracting a signal substantially equal to said amplitude of said column position signal when said control column is at detent from the column position signals that are supplied when said command column is moved away from detent. The system of any of claims 6-9, wherein said pitch command processor further includes bias signal correction means connected for receiving said column position signal and for supplying a corrected column position signal as said signal representative of the position of the control column to said commanded elevator signal processor, said bias signal correction means being configured and arranged for determining the amplitude of the column position signal when said command column is at detent and for subtracting a signal substantially equal to said amplitude of said column position signal when said control column is at detent from the column position signals that are supplied when said command column is moved away from detent, and further comprising a compensation and protection processor for generating an underspeed signal when the airspeed of the aircraft is below a predetermined value, said underspeed signal being supplied to an input of said third signal combiner for inclusion as component of said error signal, wherein said compensation and protection processor generates an overspeed signal when the aircraft is being operated outside a predetermined operating region, said overspeed signal being supplied to an input of said third signal combiner to contribute to said error signal.

Inventors: ANDERSON DORR MARSHALL, BLEEG ROBERT J, BREUHAUS RICHARD S, COLEMAN EDWARD E, EVAND MONTE R, NAJMABADI KIOUMARS, NELSON TIMOTHY A, ANDERSON, DORR MARSHALL, BLEEG, ROBERT J., BREUHAUS, RICHARD S., COLEMAN, EDWARD E., EVAND, MONTE R., NAJMABADI, KIOUMARS, NELSON, TIMOTHY A.
Applicants: BOEING CO, THE BOEING COMPANY
Application year: 1995
Main classification: B64C13
IPC codes: G05D1, B64C13