Abstract:
A helicopter includes a rotor system having a rotor with an adjustable pitch that is controlled at least in part by a pilot using a collective control and which helicopter generates a Low RPM signal that is indicative of a threshold low rotational speed of the rotor. An actuator arrangement configured to move the collective control by exerting a force on the collective control such that the pilot is able to overcome the force but which otherwise moves the collective control from a current operational position toward a minimum pitch position. A control arrangement is configured for receiving the Low RPM signal and for responding to the Low RPM signal by activating the actuator arrangement for at least a predetermined period of time to apply the force to move the collective control from the current operational position to the minimum pitch position.

Description:
RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application Ser. No. 61/264,181 filed Nov. 24, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is related at least generally to helicopter control systems and, more particularly, to an emergency collective actuator and associated method for a helicopter. 
     It is recognized in the prior art such as is exemplified by U.S. Pat. No. 4,667,909 (hereinafter the &#39;909 patent) that a sudden power failure during the flight of a helicopter requires the immediate attention of the pilot to convert to autorotation by lowering the collective pitch of the main rotor blades of the helicopter. A failure to timely reduce the collective can result in stalling the rotor blades. Such stalling of the rotor blades will generally produce a catastrophic crash wherein the helicopter, quite literally, falls from the sky. One example of such an accident, which likely involved a rotor stall, occurred in the United Kingdom in March of 1998 and is the subject of AAIB Bulletin No. 11/98. Such an accident will generally be fatal to anyone onboard the aircraft. The particular helicopter that was involved in this accident was the Robinson R22, which is a lightweight helicopter having a low-inertia rotor system. It should be appreciated that a low-inertia rotor system can be stalled more easily than a rotor system having a greater level of inertia. The subject accident report outlines operational conditions for the Robinson R22 under which rotor speed will decay to an unrecoverable value in less than 1 second during a climb. 
     The prevalent teaching in the prior art with regard to avoiding rotor stall appears to be to simply instruct the pilot to lower the collective setting of the rotor immediately in the event of an engine failure to preserve inertia in the rotor system. In practice, Applicants believe that it is questionable how effective this advice might be relative to low rotor inertia helicopters since engine failure appears to be relatively uncommon. Hence, it is difficult for the pilot to immediately react to a situation that has never been fully experienced firsthand. Even during training, Applicants believe that few student pilots are provided with actual experience either in simulation or actual flight that would realistically duplicate an actual engine failure. The lack of such training is attributed to a certain enhanced level of danger that accompanies the training itself, since full down auto-rotation landings require considerable skill in low rotor inertia helicopters and might result in damage to the helicopter. In this regard, flight instructors are advised to warn a student pilot prior to initiating training exercises relating to power failure simulation, at least in the Robinson R22. 
     The &#39;909 patent appears to be consistent with the prior art in recommending that the pilot should react immediately and seeks to alleviate the problem by relocating the collective control. Applicants believe that this approach is of limited value since the collective control is traditionally located by the pilot&#39;s left hand. It is believed that most experienced pilots would object to relocating this critically important control, since reaction time could at least arguably be increased simply by moving the collective control to a non-traditional location. 
     The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. 
     In general, an apparatus and method are described for use with a helicopter including a rotor system having a rotor with an adjustable pitch that is controlled at least in part by a pilot using a collective control and which helicopter generates a Low RPM signal that is indicative of a threshold low rotational speed of the rotor. In one aspect, an actuator arrangement is configured for moving the collective control by exerting a force on the collective control such that the pilot is able to overcome the force but which otherwise moves the collective control from a current operational position toward a minimum pitch position. A control arrangement is configured for receiving the Low RPM signal and for responding to the Low RPM signal by activating the actuator arrangement for at least a predetermined period of time to apply the force to move the collective control from the current operational position to the minimum pitch position in an absence of a collective control input from the pilot. In one feature, the control arrangement is further configured for entering a lockout interval immediately following the predetermined period of time, during which lockout interval the Low RPM signal is disabled from activating the actuator arrangement. 
     In another aspect, the Low RPM signal is received and responded to by exerting a force to move the collective control from the current operational position to the minimum pitch position for a predetermined period of time in an absence of a collective control input from said pilot such that the pilot is able to overcome the force but which otherwise moves the collective control from a current operational position toward a minimum pitch position. In one feature, a lockout interval is entered immediately following the predetermined period of time during which lockout interval the Low RPM signal is disabled from causing the collective to move. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be illustrative rather than limiting. 
         FIG. 1  is a diagrammatic view, in elevation, of a helicopter collective control coupled with an emergency collective actuator of the present disclosure. 
         FIG. 2  is a diagrammatic view, in elevation and partially in perspective, showing one embodiment of an emergency collective control. 
         FIG. 3  is a block diagram showing one embodiment of a control section which can form part of the emergency collective control. 
         FIG. 4  is a timing diagram which illustrates timer output control signals and a motor drive signal produced by the control section. 
         FIG. 5  illustrates one embodiment of a method for operating the emergency collective actuator of the present disclosure. 
         FIGS. 6-9  are diagrammatic views, in elevation, showing details with respect to another embodiment of actuation components of the emergency collective actuator. 
         FIG. 10  is a diagrammatic, partially cut-away end view of another embodiment of actuation components of the emergency collective actuator. 
         FIG. 11  is a diagrammatic view, in elevation, showing another embodiment of the emergency collective actuator showing the collective control in a raised position. 
         FIG. 12  is a diagrammatic plan view, in partial cross-section, showing further details of the embodiment of the emergency collective actuator of  FIG. 11 . 
         FIG. 13  is a diagrammatic view, in elevation, showing the embodiment of the emergency collective actuator of  FIG. 11 , but with the emergency collective actuator in an engaged mode having lowered the collective control. 
         FIG. 14  is a partially cut-away diagrammatic view, in elevation, of the emergency collective actuator as shown in  FIG. 13 , but showing the condition of the mechanism when the pilot counteracts an actuation by the emergency collective actuator. 
         FIG. 15  is a block diagram showing another embodiment of a control section which can form part of the emergency collective actuator of the present disclosure. 
         FIG. 16  is a diagrammatic view, in elevation, showing a helicopter which includes an embodiment of the emergency collective actuator of the present disclosure in which at least ground proximity detection is used for purposes of controlling the emergency collective actuator of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles taught herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown, but is to be accorded the widest scope consistent with the principles and features described herein including modifications and equivalents, as defined within the scope of the appended claims. It is noted that the drawings are not to scale and are diagrammatic in nature in a way that is thought to best illustrate features of interest. Descriptive terminology such as, for example, upper/lower, uppermost/lowermost, vertically/horizontally and the like may be adopted for purposes of enhancing the reader&#39;s understanding, with respect to the various views provided in the figures, and is in no way intended as being limiting. 
     Attention is now directed to the figures wherein like reference numbers may refer to like items throughout the various views.  FIG. 1  is a diagrammatic view, in elevation, of a helicopter collective control that is generally indicated by the reference number  10  and which is representative of the collective control in a helicopter such as, for example, the Robinson R22. Collective control  10  is positioned on a pane  112  of the helicopter and is attached to a pivot  14  at one end. A handle end  20  of the collective control is configured for gripping by a pilot and includes a throttle control that is actuatable by twisting handle  20  as indicated by a double headed arrow  22 . The pilot can increase the collective by moving handle  20  pivotally in the direction indicated by an arrow  24  or reduce collective by moving handle  20  oppositely. 
     Still referring to  FIG. 1 , an emergency collective actuator arrangement is generally indicated by the reference number  100 . The emergency collective actuator arrangement includes a main unit  110  that is configured to receive a Low RPM signal that is generated by the helicopter. In the instance of the Robinson R22, the Low RPM signal is produced when the rotor speed falls to 97% of normal or less. Normally, the Low RPM signal is used to actuate a warning horn and light to draw the pilot&#39;s attention to the status of the rotor speed. The manner in which main unit  110  uses the Low RPM signal will become evident in the discussions which follow. 
     A control cable  112  extends from main unit  110  to a clamp arrangement  114  that is attached to an intermediate position  116  on collective control  10 . Any suitable control cable and clamp arrangement can be used so long as the attachment is reliable. The clamp arrangement should be located sufficiently away from the pivot end and the handle end of the collective control so as to avoid any interference with normal operation or with actuation by the pilot. In response to the Low RPM signal, main unit  110  retracts cable  112  so as to lower the collective. This operation proceeds automatically in response to reception of the Low RPM signal and can be initiated essentially instantaneously, at least from a practical standpoint, e.g. in 0.001 seconds (1 ms) or less in response to the Low RPM signal, but in any event significantly less than the reaction time of even an attentive pilot. In one embodiment, the reaction time of main unit  110  can be adjustable, and yet remain far less than the typical reaction time of the pilot to provide a significant safety enhancement. Typically, however, the circuitry will be allowed to react as quickly as it is capable of reacting without introducing any additional delay. In this regard, under certain flight conditions outlined in the AAIB accident report that is discussed in the Background Section, Applicants believe that the required reaction time may be so short as to present a virtually unrecoverable condition in the absence of the use of emergency collective control  110  and its associated method. As will be described in further detail, the retraction force that is applied to collective control  20  by emergency collective actuator  100  can be overcome by the pilot, although the force is sufficient to inform the pilot that the unit is attempting to lower the collective when the left hand of the pilot is holding collective handle  20 . In this regard, the collective control may have a friction setting that can be engaged by the pilot to introduce additional friction at pivot  14  so that the pilot is able to at least momentarily release his or her hand from the collective. The retraction force is therefore configured such that the collective can be lowered by the emergency collective control unit even if the highest setting of the friction control is in use. 
     Referring now to  FIG. 2 , a diagrammatic illustration, partially in a perspective view and partially in an elevational view is provided showing one embodiment of emergency collective control  100 . In this embodiment, as part of main unit  110 , a motor  200  includes an output shaft  202  that rotates a drum  204 . The latter can be received directly on output shaft  202 , for example, at each one of a pair of opposing end walls  210 , only one of which is visible. Drum  204  defines a spiral groove  212  that can include one or more turns around the periphery of the drum. Groove  212  can have a suitable profile in cross-section such as, for example, a V-shape or a U-shape. Cable  112  can be received around the drum to form at least one turn therearound. The drum may be formed from any suitable material such as, for example, a lightweight aluminum alloy. One end  214  of cable  112  extends to and is attached to a first end  218  of a spring  220 . In the present embodiment, the spring is a helical coil spring, although any suitable type of spring may be used so long as the spring is able to maintain some degree of tension on the cable irrespective of the position of collective control  20 . In this regard, the spring serves to maintain tension and accommodate movement of the collective without providing any noticeable resistance to actuations by the pilot. 
     Cable  112  may be attached to the spring in any suitable manner such as, for example, by using a crimping clamp  222 . An opposing end  224  of the spring is suitably fixedly attached to a convenient location on the helicopter such as, for example, the bottom of panel  12  (shown diagrammatically) such that resilient tension is continuously applied to cable  112 . An opposing end  230  of cable  112  extends for attachment to intermediate position  116  of collective control  20 . One arrangement for attaching cable  112  to the collective control is shown in an enlarged view within a dashed circle  240 . A clamping ring  250 , typically a metal band, is shaped to fit around the periphery of the collective control and is tightened about the periphery using a suitable fastener  252  such as, for example, a rivet. A cable end fitting  254  can be attached to end  230  of the cable, for example, by crimping/compressing and can have a bifurcated shape with two opposing tines  258  (only one of which is visible), each of which defines an opening  260 . A pin, which is not shown due to illustrative constraints, is receivable in openings  260  and through ears  262  of clamping ring  250  such that end fitting  254  can pivot about the pin. 
     In this embodiment, motor  200  is configured to rotate with the drum in the event that the pilot pulls the collective upward to overcome the retraction force that is provided from motor  200 . The number of turns of cable  112  around the drum in spiral groove  212  should provide sufficient frictional engagement between the drum and cable so as to avoid slipping of the cable relative to the drum in view of tension that is provided by spring  220 . An electronic control section  300  forms another part of main unit  110  and is used to provide electrical drive to motor  200  with power being provided from the helicopter on a line  302  and Low RPM signal being provided on a line  304 , as will be further described immediately hereinafter. 
     Turning to  FIG. 3 , electronic control section  300  is shown in block diagram form. As noted above, operational power is received from the helicopter on line  302  [shown in  FIG. 2 ] and the Low RPM signal is received on line  304 . Power is provided to each of a timer  310 , an AND gate  312  and a driver  314 . It is to be understood that a power supply section can be provided in the event that these components have varying power requirements with respect to one another and/or with respect to the power that is available directly from the helicopter. For purposes of descriptive convenience, it will be assumed that control section  300  employs active-high logic, although active-low logic can just as readily be used. Initially, the Low RPM signal is provided as a logic high signal to one input  316  of AND gate  312  and to timer  310 . It is considered that one having ordinary skill in the art may readily implement appropriate circuitry in a wide variety of forms with this overall disclosure in hand. 
     Having described  FIGS. 1-3  in detail above, attention will now be directed to operational details of one embodiment of emergency collective actuator  100  with additional reference to the timing diagram of  FIG. 4 . This timing diagram illustrates the Low RPM signal in a plot  400  showing the signal versus time. At a time t 1 , the Low RPM signal transitions to a high, alert status at which time the helicopter horn would sound and the Low RPM indicator light illuminates. It should be appreciated that various conditions may be encountered during a given autorotation based, at least in part, on the pilot&#39;s choice of a suitable landing site. For example, in order to achieve maximum range in the Robinson R22, a forward speed of 70 knots is recommended with 90% of normal rotor rotational rate. Of course, under these conditions, the Low RPM horn will sound continuously. Under other conditions, however, the pilot may achieve more than 97% of normal rotor rotational rate after the initial sounding of the Low RPM horn such the horn is at least temporarily extinguished. One example of an event that would extinguish the Low RPM signal would be for the pilot to initiate autorotation with aft cyclic which causes the main rotor speed to rise above 97% of normal. Another example of an event that may result in a change in the status of the Low RPM signal resides in what is typically referred to as a “flare” that is used to slow the speed of the helicopter during autorotation immediately prior to landing. During this flare, main rotor speed increases and then decreases as the pilot adds collective just before landing. Accordingly, it should be appreciated that the Low RPM signal may toggle between active and inactive conditions during autorotation depending on the changing rotational status of the helicopter main rotor. In the present example, which is not intended as being limiting, the initial low RPM event ends at a time t 2 . Subsequently, at t 3 , the Low RPM signal again becomes active until t 4 . 
     Timer  310  of  FIG. 3  may operate according to a timer output plot  402  of  FIG. 4  which is provided to one input  404  of AND gate  312 . In response to the Low RPM signal, timer  310  produces a timer pulse  410  that is present at input  404  of AND gate  312 . The timer pulse is indicated as having an overall duration of TP extending from time t 1  to t x . The initiation of the timer pulse interval can take place, for example within 1 millisecond of the Low RPM signal based on any time delay introduced by timer  310 . From a practical standpoint, and in terms of human perception, timer output  402  can become active at t 1 . A motor drive signal  420  is generated at an output  422  of AND gate  312 . So long as both inputs of AND gate  312  are active/high, output  422  of the AND gate will also be active/high. If either one of the inputs of the AND gate is low, however, its output will likewise be low. In the present example, Low RPM signal  400  and timer output  402  are both active for the duration TP of timer pulse  410  such that motor drive signal  420  includes a motor drive pulse  430  that is of a duration MD which corresponds to duration TP of the timer pulse, at least from a practical standpoint, although a time delay may be introduced by the circuitry that is imperceptible by human observation. The duration of timer pulse TP can be customized based on a particular helicopter application, however, TP should be long enough to allow for a relatively slow reaction time on the part of the pilot. In this regard, Applicants note that it is often desirable to accommodate potential reaction times of several seconds on the part of the pilot. For this reason, the predetermined interval of timer pulse, TP, and therefore MD may be of six seconds or longer duration, although this is not a requirement; the predetermined interval may be any length, including a length with no end-point. For purposes of driving motor  200 , motor drive signal  420  is provided to driver  314  which provides current to the motor if the electrical current requirements of the motor cannot be satisfied directly by AND gate  312 . 
     Continuing with a description of the operation of emergency collective actuator  100 , it should be appreciated that a pulse  440  which is present from t 3  to t 4  of Low RPM signal  400 , in the present example, is not reflected by motor drive signal  420  for the reason that pulse  440  occurs well after t x  which terminates timer pulse TP. In this regard, after issuing a timer pulse, timer  310  is configured to enter a Lockout Interval LI during which time the timer output is low such that any active signal events that might occur on the Low RPM signal cannot influence motor drive signal  420 . The Lockout Interval can be of a duration that is sufficient to ensure enough time for a full auto rotation to the ground such as, for example, 5 minutes or longer. In this way, emergency collective actuator  100  reacts to an initial low RPM event immediately and advantageously need not counter any subsequent actuation of the collective control by the pilot which is required during autorotation, for example, such as may occur during the flare, as discussed above. Overall circuitry delays can be managed to a degree that causes the collective to actuate immediately in terms of human perception responsive to the Low RPM signal, for example, the overall delay to start motor rotation may be 1 millisecond or less, although longer delays are also acceptable and still faster than the reaction time of a typical pilot. It should be appreciated, however, that the pilot can counter any actuations by the emergency collective control at any time if he or she so chooses, even during motor drive interval MD. In this regard, for purposes of low-level hovering and maneuvers, the pilot will have his or her hand on the collective. At such sufficiently low altitude, pilots are trained to increase the collective since it is not practical to try to increase the amount of energy stored in the rotor system using autorotation. In the event that the pilot feels an actuation that is attempting to lower the collective at such low altitude, the pilot can be trained to resist the actuation and increase the collective. 
     Turning now to  FIGS. 3 through 5 , the latter illustrates one embodiment of a method for the operation of the emergency collective actuator, generally indicated by the reference number  500 . The method starts at  502  when the emergency collective actuator system is powered up. At  504 , motor  200  is maintained in an OFF status. At  506 , timer  310  ( FIG. 3 ) is reset to zero and held ready for triggering. Step  508  then monitors the Low RPM signal and executes in a continuous loop so long as the Low RPM signal is low/inactive. When the Low RPM signal becomes high/active, step  510  starts timer  310  (at time t 1  of timer waveform  402  in  FIG. 4 ). The timer output remains high/active until t x , as shown in waveform  402  which can be, for example, six seconds as described above. Step  512  checks the timer signal following t 1 . If the timer output is high/active, execution enters step  514  which reconfirms that the Low RPM signal is active/high. If the Low RPM signal is high, step  516  turns on motor  200  or maintains the motor in an ON status so as to retract the collective. Operation then returns to step  512 . If at step  514 , the Low RPM signal is determined to be inactive, step  520  turns motor  200  off and execution returns to step  512 . When step  512  determines that the timer output is low, subsequent to t x  in timer plot  402 , step  522  turns motor  200  off. Operation then proceeds to step  526  which monitors timer output  402  for the expiration of the Lockout Interval, shown as LI in timer output plot  402 . One suitable value for the Lockout Interval is 5 minutes, as described above. Following the Lockout Interval, operation returns to step  504 . 
     Turning now to  FIG. 6 , another embodiment is illustrated, in a diagrammatic perspective view, showing motor  200  which rotates a pulley  600 . One suitable motor has been found to be the MFA 942D series geared motor that is available from Como Drills of the United Kingdom. It is noted that this motor is suitable for all of the embodiments described herein at least for the reason that the application of external torque to the output shaft can freely rotate the gear assembly of the motor and the motor itself. Other geared and non-geared motors, however, may also be found to be suitable. The pulley may be configured in any suitable manner such as, for example, defining a groove for receiving cable  112 , as illustrated. The pulley is mounted on motor shaft  202  to extend through a helical coil spring  602 . One end  604  of the helical coil spring is fixedly attached, for example, to motor  200  or other suitable structure such as a bracket (not shown) that supports the motor. It should be appreciated that any suitable form of spring may be used. For example, a planar clock-spring may be used employing a spiral winding. The attachment of end  604  may be performed in any suitable manner, for example by using a fastener  606 . An opposing end of the spring, which is not visible in the present view, is attached to pulley  600  in any suitable manner such as, for example, by using a fastener. Cable  212  can be attached to pulley  600 , for example, using a swaged fitting  610  at the end of the cable which receives a suitable fastener  612  that is fixedly received in the periphery of the pulley. It is noted that the illustrated position of pulley  600  corresponds to the full up position of collective control  10  ( FIG. 1 ). At this position, spring  602  is pre-tensioned so as to apply a force in a direction  620 , indicated by an arrow, such that the pulley takes up slack in cable  112  if the pilot lowers the collective. In this regard, spring  602  applies sufficient force to rotate the motor output shaft in taking up the slack in the cable. Pulley  600  is configured with a diameter such that less than one rotation of the pulley takes place from a full up position to a fully lowered position of the collective control so that there is no need for cable  112  to overlap on itself. In response to the Low RPM signal, motor  200  rotates pulley  600  in the direction of arrow  620  so as to lower the collective in accordance with the descriptions above. Motor  200  should have sufficient torque to apply additional tension to spring  604  as the motor lowers the collective. Since less than one turn of the pulley is needed, the added torque that is needed to further wind the spring is readily manageable. In another embodiment, motor  200  may be supplied with a relatively small current which causes the motor to rotate pulley  600  in the direction of arrow  610  so as to take up any slack in cable  112  but without applying enough torque to lower the collective during normal flight conditions. In this later embodiment, spring  602  is not needed. 
       FIG. 7  is a diagrammatic perspective view of another embodiment in which cable  112  extends through the pulley groove beyond a capture band  630  that is fixedly attached to the pulley periphery in any suitable manner such as, for example, by welding. Capture band  630  may be formed in any suitable configuration so long as cable  112  is able to freely move vertically in the pulley groove. In instances where the cable diameter is greater than the width of the pulley groove, capture band  630  may be in a loop configuration so as to extend outward from the periphery of the pulley. A distal end of cable  112  supports an endpiece  632  that can be attached to the cable end in any suitable manner such as, for example, by swaging. The illustrated position of the pulley and capture band corresponds to a home or idle position under normal flight conditions such that cable  112  can move freely in the vertical direction responsive to actuations of the collective by the pilot. The weight of endpiece  632  can serve to prevent binding of cable  112  between capture band  630  and the pulley and generally maintain the orientation of the cable. As shown, cable  112  and endpiece  632  represent the fully raised position of the collective. The cable and endpiece are shown in phantom at the fully lowered position of the collective as indicated by the reference numbers  112 ′ and  632 ′, respectively. 
     Referring to  FIG. 8  in conjunction with  FIG. 7 , motor  200  rotates pulley  600  in the direction of arrow  620  responsive to the Low RPM signal. At some point in the rotation of pulley  600 , endpiece  632  encounters capture band  630 . Since the endpiece is sized so as to be unable to pass between capture band  630  and the pulley, cable  112  is retracted onto the pulley and the collective is lowered in a manner that is consistent with the descriptions above.  FIG. 8  illustrates the arrangement of the components at the fully retracted or lowered position of the collective. The pilot may counteract the operation of motor  200  during the retraction period or following the retraction period by applying upward force to the collective which will counter-rotate the motor and pulley. 
       FIG. 9  is a diagrammatic end view, in elevation, of the embodiment of  FIGS. 7 and 8 , further including a shield  650  that is arranged to deflect cable  112  and endpiece  632  such that interference with components of the helicopter below the emergency collective actuator can be avoided, if such interference is a possibility. Shield  650  may be formed from any suitable material such as, for example, metal and in any suitable shape such as, for example, a trough shape. The shield may be supported in any suitable manner such as, for example, by using a bracket that is attached to the helicopter. 
     Attention is now directed to  FIG. 10  which is a diagrammatic partially cut-away end view of another embodiment which resembles the embodiment of  FIG. 6  with an exception that helical coil spring  602  is not used. Further, pulley  600  is shown as being partially cut-away to illustrate the presence of a micro-switch  680 . The micro-switch can be of the normally open type, although this is not a requirement, and includes an actuator  682  that extends through floor  684  of the pulley groove such that an open condition of the switch indicates that there is slack in cable  112 . On the other hand, when tension is applied to cable  112 , pulley  600  rotates and applies tension to the cable such that the cable causes the micro-switch to close. Indications of the status of the micro-switch can be provided on a pair of electrical leads  686 , for example, to control unit  300  ( FIG. 2 ). Based on the status of the micro-switch, control unit  300  can monitor the switch to ensure that there is no slack in cable  112 . Since the pulley rotates less than one turn in its travel, the use of simple electrical connections is facilitated such as, for example, a flexible wiring harness. Pulley  600  may be machined to support micro-switch  680 . The location of the micro-switch may be maintained in any suitable manner in the pulley, for example, using an adhesive and/or one or more suitable fasteners. 
     Turning now to  FIG. 11 , another embodiment of emergency collective actuator arrangement  100  is diagrammatically illustrated. In this embodiment, the actuator arrangement can be concealed below floorboard or deck  12  of the helicopter. In this regard, it should be appreciated that the deck can have a complex shape. Clamp  114  is pivotally connected to an upper counterbalance arm  700  via a pin  702 . A counterbalance spring arrangement  710  can be connected at an upper end to upper counterbalance arm  700 . A lower counterbalance arm  712  can be connected at one end to a lower end of spring arrangement  710  and pivotally connected to a suitable fixed location on the helicopter at a lower end via a pin  713 . A helical coil spring  714  can be captured between upper and lower disks  716   a  and  716   b  using three shafts  718  (one of which is indicated). It is noted that the upper and lower counterbalance arms as well as the counterbalance spring arrangement may be provided as original equipment in a particular helicopter. In view of the present example, however, it is considered that one having ordinary skill in the art can readily implement an installation for a helicopter that is equipped with a different collective control configuration. 
     Referring to  FIG. 12 , in conjunction with  FIG. 11 , the former is a diagrammatic view, in partial cross-section, taken from a line  12 - 12  that is shown in  FIG. 11 , and shown here to illustrate further details with respect to the components of emergency collective control arrangement  100 . First and second lever arms  720   a  and  720   b  are pivotally connected at a first end to a suitable fixed position in the helicopter, for example, using a bracket  722  and pivot pin  724  such that second ends  725  of the lever arms can rotate as indicated by a double headed arrow  726 . The lever arms are disposed at either side of upper counter balance arm  700  and can pivot against a crown member  730  that includes an arcuate head which defines an aperture for receiving the shaft of the upper counter balance arm. Output shaft  202  of motor  200  supports a disk  740  for selective rotation responsive to control section  300 . A lower actuator arm  742  is pivotally connected to disk  740  having a pivot point  743  off-center with respect to the motor shaft. An upper end of lower actuator arm  742  is connected to an actuator spring arrangement  746 . In the present example, the actuator spring arrangement includes a helical coil spring  748  having caps  750   a  and  750   b  mounted on its opposing ends with cap  750   b , in turn, attached to an upper end of the lower actuator arm. It should be appreciated that any suitable type of spring arrangement can be used. An upper actuator arm  760  can be pivotally received between lever arms  720   a  and  720   b  at their second ends  725 , for example, using a pin  762 . As will be further described, rotation of motor  200  can apply a downward biasing force on collective control  10 . Any suitable motor may be used such as, for example, a gear motor. 
     Referring to  FIG. 11 , collective control  10  is shown in an uppermost, fully raised position. Disk  740  is oriented having pivot point  743  at an uppermost position such that actuator spring arrangement  746  does not apply a downward biasing force to the collective control. If the pilot moves the collective downward, crown  730  moves downwardly away from lever arms  720   a  and  720   b , as counterbalance spring  714  is compressed by the pilot, such that the emergency collective actuator has no effect on the pilot&#39;s actuation. It is noted that the pilot can lower the collective from any given position above minimum with no influence from the emergency collective actuator, since crown  730  moves downward and away from lever arms  720   a  and  720   b . The emergency collective actuator arrangement may readily configured to overcome a collective friction setting that is intended to prevent inadvertent movement of the collective, for example, if it is necessary for the pilot to move his or her hand away from the collective control. 
       FIG. 13  diagrammatically illustrates the collective control and emergency collective actuator after having fully lowered the collective as a result of motor  200  rotating disk  740  such that pivot point  743  is at a lowermost position. As disk  740  is rotated, which can take place either clockwise or counter clockwise, actuator spring arrangement  746  pulls downward on ends  725  of lever arms  720   a  and  720   b . The lever arms, in turn, engage crown  730  so as to compress counterbalance spring  714  and thereby lower the collective. Motor  200  can rotate another 180 degrees in either direction to release the collective, for example, after a predetermined time interval. 
     Referring to  FIGS. 4 ,  11  and  13 , control section  300  can be configured in one embodiment with a microcontroller that is configured to generate a first pulse at t 1  having a duration which rotates disk  740  by 180 degrees from the position in  FIG. 11  to the position in  FIG. 13 . At t x , the microcontroller can generate a second pulse having a duration which rotates disk  740  by 180 degrees from the position in  FIG. 13  to the position in  FIG. 11 . Thus, the motor drive signal can be made up of these two pulses that are represented by dashed lines  764  which represent the trailing edges of the pulses, as shown in motor drive plot  420  of  FIG. 4 . As part of this embodiment, the flow diagram of  FIG. 5  may be modified such that step  516  drives motor  200  to move pivot point  743  to its lowermost position so that the emergency collective actuator is able to lower the collective in an engaged mode. Step  520 , on the other hand, rotates the motor to position pivot point  743  at its uppermost position to disengage the emergency collective actuator in a disengaged mode. Step  522  likewise drives the motor to position pivot point  743  at its uppermost position to disengage the emergency collective actuator, if necessary. 
       FIG. 14  diagrammatically illustrates the appearance of a relevant portion of emergency collective actuator  10  in the case where the pilot is moving the collective upward, thereby counteracting the emergency collective actuator which has previously lowered the collective, as evidenced by pivot point  743  being located in its lowermost position. The actuation by the pilot causes actuator spring  748  to extend such that ends  725  of lever arms  720   a  and  720   b  rotate in a clockwise direction. It is noted that the length of the collective arm provides the pilot with significant leverage for purposes of causing the extension of the actuator spring. It is considered that the configuration that has been shown by way of example may be modified by one having ordinary skill in the art in a wide variety of ways while remaining within the scope of these teachings. 
       FIG. 15  is a block diagram illustrating another embodiment of an emergency collective actuator that is generally indicated by the reference number  800 . Embodiment  800  shares many of the components described above with reference to embodiment  300  of  FIG. 3 . In this embodiment, however, timer  310 , AND gate  312  and driver  314  receive electrical power from a power controller  802  via a power line  804 . In one embodiment, power controller  802  can be an electrical switch such as, for example, a toggle switch that is mounted for actuation by the pilot. The pilot can therefore selectively use the switch to disable the emergency collective actuator, for example, when performing low altitude maneuvering or hovering at which there is insufficient altitude for purposes of autorotation. In such an instance, as discussed above, the pilot should react by increasing the collective and allow the helicopter to settle to the ground, using inertia that is present in the rotor system. 
     Turning to  FIG. 16  in conjunction with  FIG. 15 , the former is a diagrammatic plan view of a helicopter  900  using another embodiment of the emergency collective actuator in which power controller  802  is used with a ground proximity detection unit  910 . In one embodiment, the ground proximity detection unit transmits a radar signal  912  that is used to detect the immediate distance to a surface  914  of the ground based on a reflected signal  920 . Controller  802  can be configured to automatically disconnect output power from timer  310 , AND gate  312  and driver  314 , and/or one or any combination of these components, below some predetermined altitude such as, for example, twenty feet. Controller  802  can receive a signal from the ground proximity detection unit on an altitude input  930  (shown as a dashed line). In another embodiment, controller  802  can additionally be configured with an airspeed input  932  (shown as a dashed line) to receive the airspeed from an airspeed sensor  934  on the helicopter so as to use both altitude and airspeed to determine an appropriate combination of minimum altitude and minimum velocity below either of which the emergency collective actuator is automatically disabled. It should be appreciated that sufficient forward speed would increase the appropriate low altitude to some degree by providing the capability to contribute inertia to the rotor system. In one implementation, lookup tables based on combinations of airspeed and altitude can be formulated and stored in controller  802  based on the height-velocity diagram for a given helicopter in which the system is to be installed. Power controller  802  can then operate in accordance with altitude  930  and airspeed  932  inputs based on the lookup table(s). It should be appreciated that helicopter manufacturers routinely generate height-velocity diagrams for their helicopters. Such diagrams illustrate regions of safe and unsafe operation. Generally, as altitude increases, forward airspeed becomes relatively less critical. In one embodiment, the emergency collective actuator can be disabled from lowering the collective when detected altitude and airspeed indicate that the helicopter is operating within one or more predetermined unsafe regions of the height-velocity diagram for that helicopter. Generally, a predetermined unsafe region can be considered to include the border of that region although an additional safety margin could be included which would slightly expand one or more of the predetermined unsafe regions. In any embodiment for which controller  802  provides for automatic operation, input power for the controller can be provided via a switch that is inserted in a series connection in power line  804  such that the pilot can manually actuate the switch to disable or enable automatic operation of controller  802 . 
     While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.