Abstract:
A system for reliably limiting the maximum positions to which the power control lever of an aircraft engine may be set in the thrust reversal mode corresponding to whether the aircraft is in the air or on the ground. A plurality of cable drums, one for each of the power control lines extending between the cockpit and the engines of a multi-engine aircraft, is associated with an adjustable stop mechanism which provides a first stop position for the cable drum when the aircraft is in the air with landing gear up and a second stop position permitting increased reverse thrust when the aircraft is on the ground. The system is coordinated with the landing gear position sensing system of the aircraft and works in conjunction with presently installed thrust reversing systems. The system includes an arrangement for moving the elements to a safe, non-jamming position in the event of linkage failure and incorporates circuitry with suitable instrument panel lights to give indications of readiness for use and system failure. The system is designed for installation without modification of engine thrust reversal mechanisms or cockpit controls.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to aircraft control systems and, more particularly, to systems for limiting the maximum amount of reverse thrust which may be made available in accordance with different operating conditions of the aircraft. 
     2. Description of the Prior Art 
     Thrust reversing systems are commonly used on passenger and the larger cargo carrying jet aircraft. These thrust reversing systems are provided in order to permit the aircraft to decelerate more rapidly without such heavy dependence upon the wheel braking systems, once the aircraft are on the ground. One such system which is in common use has a plurality of deflector buckets that are moved into position in the jet exhaust when operation in the thrust reversing mode is desired. Once the thrust reversing system is in the activated position, the engine power control levers are moved to an advanced power setting and the engines wind up accordingly with the thrust now being deflected forward by the deflector buckets so that the desired reverse thrust is developed. 
     Special aircraft are now being developed for transporting cargo and/or passengers into landing fields shorter in length than those used by conventional aircraft. Known as STOL (Short TakeOff and Landing) aircraft, these planes need to be able to reduce their speed in the air with greater deceleration than is possible with available flap braking systems. Thrust reversing from the engines is available, as it is for deceleration once the aircraft has landed, but it is necessary to limit such thrust reversal to a greater degree than is required or even desired for thrust reversing after landing. If thrust reversal is to be employed during the landing approach or at any other time when the aircraft is airborne, it is important that the pilot be able to set the engine power control levels in the maximum thrust reversal position without having to look to determine the proper maximum power setting in the thrust reversal position. This calls for some kind of a lever stop mechansim in the power control system. However, a higher power setting is called for in use of the thrust reversal system after landing, which means that a different stop position is needed for thrust reversal after the aircraft has landed. 
     In-flight thrust reversal requires a reliable means of limiting the engine power control setting to prevent excessive reverse thrust power with possible damage to the structure and reduced operational reliability. The reverse thrust limiter must be capable of engaging or disengaging the engine power control system in a manner which precludes jamming or unsafe operation at any position of the throttle. Variation of the reverse thrust limiter position with temperature or power control system friction should be minimized in order to insure sufficient, but not excessive, power in in-flight thrust reversal operation. 
     SUMMARY OF THE INVENTION 
     In brief, arrangements in accordance with the present invention meeting the above-recited criteria incorporate a cable drum, one for each engine power control cable system extending between the engine and the cockpit, and mechanical linkages interconnecting the plurality of cable drums to a single stop actuator. Each cable drum is individually rotatable about a pivot axis by its associated power control drive cable. Each cable drum has a fixed stop pin projecting laterally from a point near its periphery for engagement by a corresponding movable cable drum stop controlled by the actuator. Drive circuitry for the actuator is controlled by the selector lever for the landing gear positioning system. Thus, when the landing gear lever is in the &#34;gear down&#34; position, the actuator is driven to an extended position in which the cable drum stops are set to permit increased thrust reversing power. Similarly, when the landing gear lever is in the &#34;gear up&#34; position, the actuator moves to a retracted position in which the cable drum stops are located so as to limit reverse thrust at a reduced power setting. 
     Although the reverse thrust limiting apparatus is ganged together by interconnecting linkages in order to permit movement and positioning thereof from a single actuator, the individual cable drums which provide limitation of movement of the associated power control systems in accordance with the settings of the stops are independently movable. This is necessary in order to permit each engine to be operated independently of the others. Such independent operation may be required under certain conditions, as for example in the event of the loss of power in one engine, in which case it is desired to be able to use the thrust reversing system with some of the remaining engines without using the engine on the other side which corresponds to the engine which has failed. Without this capability of independent engine control operation, a condition of asymmetric reverse thrust might develop with possibly disastrous consequences. 
     Electrical limit switches in the actuator drive circuitry are provided to interrupt power to the actuator when the limit of extension or retraction is reached. Annunciator lights are provided on the cockpit instrument panel to warn the pilot in the event of failure of the system. Thus, if the actuator has failed to move the limiting system to the reduced power setting position when the landing gear is up and locked, a warning light is illuminated to indicate that the thrust reversal system should not be used. Similarly, if the actuator has failed to move the limiting system out of the reduced power stop position when the landing gear is down and locked, another warning light is illuminated to indicate to the pilot that the thrust reversing system is limited at the reduced power setting. 
     The system of the invention includes a fail-safe mechanism to take care of the eventuality that one of the interconnecting links in the linkage train between the actuator and the last stop drive member might fail or become disconnected. This fail-safe mechanism incorporates a spring attached to the stop crank which is used to activate switches in the instrument warning light circuits and serves to move this crank away from the switches so that the occurrence of a failure in the system is indicated to the pilot. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A better understanding of the present invention may be had from a consideration of the following detailed description, taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a block diagram illustrative of the invention; 
     FIG. 2 is a graph showing the determination of power control limit points as related to engine operating parameters; 
     FIG. 3 is a perspective view, partially broken away, showing apparatus embodying the invention; 
     FIG. 4 is a schematic representation of a portion of the apparatus of FIG. 3 showing one position thereof; 
     FIG. 5 is a schematic representation of a portion of the apparatus of FIG. 3, showing an alternative position thereof; and 
     FIG. 6 is a schematic diagram showing circuitry included in apparatus of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates the relationship of apparatus of the present invention to salient elements of a conventional aircraft engine power control system. In such a system 10 as illustrated in FIG. 1, an engine power control element 12 (typically the engine cross shaft) at the individual engine is linked to a throttle element 14 in the pilot compartment by means of throttle bridle cables 16. The cables 16 extend over a variety of pulleys such as 18 to establish the appropriate path between the engine and pilot control elements 12, 14. Apparatus in accordance with the present invention is coupled to such a system by means of a cable drum 20 mounted along the path of the throttle bridle cable 16 with one or more wraps, as appropriate, of the cable 16 taken around the drum 20. In accordance with the present invention, and as described more particularly hereinbelow, the maximum rotation of the drum 20 is selectively limited in the reverse thrust direction, thus limiting the extent to which the cables 16 may direct the travel of the engine power control element 12. 
     FIG. 2 illustrates the plot of engine pressure ratio, which is proportional to thrust, as a function of engine cross shaft angle. The curve shown in FIG. 2 corresponds to a single ambient temperature and is representative of the family of such curves that are applicable for various ambient temperatures such as may be encountered in engine operation. FIG. 2 shows the engine idle position to be at a cross shaft angle of 38.4° with forward engine thrust being developed at larger cross shaft angles and reverse engine thrust being developed at cross shaft angles less than the engine idle position. 
     Reverse thrust systems presently in use for deceleration during landing roll limit cross shaft angle at somewhere between 0° and 10° by the mechanism employed at the pilot throttle control position. In the instant case, it is desired to limit the reverse thrust during in-flight thrust reversal at an engine pressure ratio of approximately 1.2, corresponding to a first cross shaft angle position of near 20° (point A of FIG. 2). This will provide the desired reverse thrust power for in-flight deceleration below a level which might cause damage to aircraft structure and possible reduced operational reliability. When the aircraft is on the ground after landing, the first position limitation is removed so that the substantially increased reverse thrust corresponding to an engine pressure ratio of approximately 1.8 (corresponding to the point B of FIG. 2) with a cross shaft angle of between 0° and 10° is available. 
     Apparatus 30 in accordance with the present invention is shown in FIG. 3 as comprising a plurality of the cable drums 20 interconnected and mounted to operate in unison. Four such cable drums 20 are shown in FIG. 3, each coupled to a separate throttle bridle cable 16 in the manner indicated in FIG. 1, as may be provided for reverse thrust limiting control of a four engine aircraft. As represented in FIG. 3, drums 20a and 20b are coupled into the power control systems of outboard engines Nos. 1 and 4 respectively, and are located in the figure forward of drums 20c and 20d which are coupled into the power control systems of inboard engines Nos. 2 and 3 respectively. The entire assembly of the drums 20 and interconnected actuator, levers, links, etc. is mounted on a support bracket or plate 32, portions of which are represented with the remainder broken away for clarity, which is adapted for mounting on the aircraft fuselage framework 34. Thus, the entire assembly may be readily installed by mounting it in the fashion shown and by leading the thrust bridle cables 16 over the respective cable drums 20 with sufficient lengthening of the cables 16 to accommodate the additional travel around the drums. Individual drum support brackets 36 and actuator support brackets 37 are provided for mounting the various elements of the apparatus 30 to the master support bracket 32, portions of the brackets 36 and 37 being broken away for clarity of illustration. 
     Each of the cable drums 20 is provided with a stop pin 38, such as the pin 38a as shown on cable drum 20a mounted near the periphery thereof for rotation with the associated drum. At the corresponding radial distance of the stop pin 38, there is provided for each cable drum 20 a stop pad 40 adjustably mounted in a drum stop arm 42. The drum stop arms 42 are mounted for rotation about the axes 44, 45 of the associated cable drums 20 as supported by the brackets 36, 37. The drum stop arms 42 of the power control systems for corresponding pairs of engines--for example, the arms 40a and 40b for the systems of Nos. 1 and 4 engines or the arms 40c and 40d for the control systems of the Nos. 2 and 3 engines--are secured together for movement in unison by torque tubes 46 and 47 respectively. The forward torque tube 46 has fixedly mounted thereon a pair of torque tube levers 48 and 50, while the stop arm 40d, affixed to the aft torque tube 47, is provided with a torque tube lever extension lever 52. The torque tube levers 50 and 52 are coupled together by means of an adjustable torque tube link 54 for driving the rotation of the torque tube 47 with its attached drum stop arms 42c and 42d in unison with the forward torque tube 46 with its attached drum stop arms 42a and 42b. Forward torque tube 46 is driven in turn by an actuator link 56 extending between the outward end of the torque tube lever 48 and an actuator lever 58 coupled to the drive rod 59 of a stop actuator 60. The actuator lever 58 is mounted for rotation about a pivot axis 62 established by the support brackets 37 as the actuator 60 drives the actuator rod 59 between respective extended and retracted positions. 
     The operation of the apparatus 30 of FIG. 3 may be explained more effectively by reference to FIGS. 4 and 5 which are schematic diagrams showing the cable drums 20a and 20c with associated interconnecting linkages as viewed from the plane of the cable drums 20b and 20d. 
     As shown in FIG. 4, the apparatus 30 is in the reverse thrust power limiting position (the first position A of FIG. 2) in which the actuator 60 has driven the actuator rod 59 to its extended position. In FIG. 5, the opposite is the case with the apparatus 30 arranged to correspond to the second position B of FIG. 2 with the actuator rod 59 being driven to the retracted position. It will be understood that this position as shown in FIG. 5 may be a second selected stop position for the apparatus 30 or, alternatively, it may be a non-limiting position in which the stop pins 38 attached to the respective cable drums 20 are not restricted by the apparatus 30 from travel to the limit of the associated systems to which the cable 16 are coupled. In both FIGS. 4 and 5, the nearside drum stop arms 42b and 42d are partially broken away in order to show the respective drum stop arms 42a and 42c in line therewith, consistent with the representation of the cable drums 20a and 20c and their respective stop pins 38a and 38c. 
     Referring again to FIG. 3, there is shown therein a pair of actuator limit switches 70 and 72 mounted for operation at the respective limits of travel of the actuator rod 59. As indicated in FIG. 3, which shows the actuator rod 59 in the extended position corresponding to the position of the apparatus 30 in FIG. 4, switch lever 74 of the extend limit switch 70 is contacted by the actuator lever 58, thereby serving to interrupt the circuit to the extension drive of the actuator 60 so that the actuator rod 59 remains in the extended position. When the retraction drive of the actuator 60 is energized to withdraw the rod 59 to its retracted position, as shown in FIG. 5, a switch lever 76 of the retract limit switch 72 is contacted by the actuator lever 58 so as to interrupt the circuit thereto. 
     With the apparatus 30 in the reverse thrust power limiting position illustrated in FIG. 4, the throttle bridle cables 16 may be moved forward only so far as is permitted by the counterclockwise rotation of the cable drums 20 to a point where the associated stop pin 38 contacts the stop pad 40 of the associated drum stop arm 42. This is the in-flight position in which the reverse thrust power must be limited to a relatively low level. This of course has no limiting effect with respect to forward thrust, since this is controlled by movement of the cables 16 in the opposite direction with corresponding rotation of the associated drums 20 in a clockwise direction. It will be noted that each of the cables may be operated independently to the limiting position established by the apparatus 30 and that all or any number of the cables 16 may be moved in unison with assurance that they will be stopped at the limiting position. 
     With the apparatus 30 in the position shown in FIG. 5, the drum stop arms 42 with associated stop pads 40 are withdrawn to a position which permits further counterclockwise rotation of the drums 20 and associated stop pins 38 so that the greater power levels for reverse thrust may be realized, as needed for on-the-ground deceleration. 
     Also shown in FIGS. 3, 4, and 5 is a pair of position sensing switches 80 and 82, each with an associated switch lever 84 or 86 mounted in position so that the levers 84 and 86 are engaged by the torque tube 47 at its respective limits of travel. Thus, sensing switch 80 is located to detect the position of the apparatus 30 in the unlimited or second position mode, corresponding to the retraction of the actuator rod 59, whereas the sensing switch 82 is located to detect the position of the apparatus 30 (see FIG. 4) in the thrust limiting or first position. As will be described hereinafter, the sense switches 80, 82 are used to provide annunciator indications to the pilot relative to the condition of the apparatus 30. 
     Also shown in FIGS. 4 and 5 is a safety spring 90 extending between spring mounts 92 and 94, mount 92 being affixed to a support bracket of the apparatus 30 and mount 94 being affixed for rotation with the torque tube 47 about the pivot axis 45. The purpose of the spring 90 is to provide an indication to the pilot in the event of any separation in the linkages extending between the actuator 60 and the farthest downstream torque tube lever 52. Such a separation might occur with the loss of a connecting pin or the actual failure of a connecting link. If such linkage separation occurs, the spring 90 causes rotation of the aft torque tube 47 away from contact with either of the switch levers 84 or 86, which condition with neither of the switches 80 or 82 being activated is interpreted as a failure signal in the pilot&#39;s compartment with respect to the reverse thrust power limiting system. 
     FIG. 6 is a schematic diagram illustrating the actuator and the annunciator circuitry associated with the apparatus of the present invention. In this circuitry, the limit switches 70, 72 and the position sensing switches 80, 82, are all of the normally closed type. The retract limit switch 72 is connected in series with the retract drive of the actuator 60 whereas the extend limit switch 70 is connected in series with the extend drive of the actuator 60. Both of these parallel circuits are connected in series with the landing gear selector switch 100, the retract circuit being connected to the down contact of the selector switch 100 and the extend circuit being connected in series with the up contact of the selector switch 100. Corresponding landing gear sensors 102 are activated when the landing gear is in one or the other of its two selected conditions, either down and locked or up and locked. The systems includes annunciators 104 and 105, preferably indicator lights, to indicate to the pilot the condition of the thrust reverse limiting apparatus. Annunciator 104, indicating &#34;Reverser Do Not Use&#34;, is in series with the extend sensing switch 82 and the gear-up sensor switch 102a. The annunciator 105, indicating &#34;Reverser Limited&#34;, is in series with the extend sensing switch 80 and the landing gear-down sensor switch 102b. The annunciators 104, 105 may be read by the pilot in conjunction with the conventional landing gear position annunciators 106 to govern his use of the thrust reversing system of the aircraft, and also to provide an indication of a failure of the reverse thrust power limiting position system. 
     As shown in FIG. 6, the landing gear selector is set to the down position and, in response thereto, the actuator 60 has retracted until the retract limit switch 72 is opened, thus removing power from actuator 60. The apparatus 30 is moved to the second position (as shown in FIG. 5) and the sense switch 80 is opened. The landing gear is down and locked as indicated by the closure of the switch 102b. In the situation shown in FIG. 6, the gear-down annunciator 106b is illuminated but neither of the reverser annunciators 104, 105 is activated. With the gear down and locked, if the switch 80 had not opened, thus indicating that the reverse thrust limiting apparatus was not in the second position, the Reverser Limited annunciator 105 would be illuminated, thus indicating to the pilot that he would not have full reverse thrust available for landing deceleration. 
     When the landing gear selector 100 is moved to the up position, the extend circuit of actuator 60 is energized until it is fully extended, at which point the extend limit switch 70 opens and deenergizes the actuator 60. This should result in the closure of position sensor switch 80 and the opening of switch 82. Switch 102a closes to indicate that the gear is up and locked. Under proper operating conditions, again neither of the reverser annunciators 104, 105 will be illuminated. However, if the position sensor switch 82 is not opened, corresponding to the condition where the reverse thrust limiting apparatus has not moved to the limiting position, the Reverser Do Not Use annunciator 104 will be energized, thus indicating to the pilot that the thrust reversing system should not be used in flight. 
     In the event of a failure of the linkages in the reverse thrust limiting apparatus as previously described, the safety spring 90 moves the downstream lever arm extension 52 and torque bar 47 to an intermediate position between the switch levers 84 and 86, so that both of the position sensor switches 80, 82 are closed. Therefore, one or the other of the reverser annunciators 104, 105 will be energized, depending upon whether the landing gear is in the up or down position. When cycling the landing gear from up to down position results in first one and then the other of the reverser annunciators 104, 105 being illuminated, this may be interpreted as an indicator of some failure in the reverse thrust limiting apparatus. 
     There has thus been shown and described hereinabove a reverse thrust limiting system which is particularly adapted for incorporating in existing aircraft thrust reversal systems in a manner which permits the use of such a thrust reversal system for safe deceleration in flight with a selected limited reverse thrust being available. The system described may be incorporated in existing power control systems with minimum modification. It should also be understood that apparatus of the type described may be incorporated in other mechanical systems with single or multiple control, wherein variable limited control is desired. Such variable limited control could be a function of landing gear control lever position, flap lever position, or any pilot control device or a device of the controlled mechanism. It thus may be used to limit thrust, flap position or other settings as a function of an independent variable, such as air speed or aircraft configuration, for example. The apparatus as described introduces mimimum effect on the systems to which it is added. The friction added to the cable systems is minimized by the use of the large diameter pulleys utilized as the cable drums. Over-center drive linkage actuation of the stop arms serves to reduce limitation sensitivity and hysteresis. The stop mechanism levers are concentric with the drive drum pivot axes to permit applying or disengaging the stops with the throttles in any position, thus providing a jam-proof mechanism. 
     Although there have been described above one particular arrangement of a reverse thrust limiting system in accordance with the present invention for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations or equivalent arrangements which may occur to those skilled in the art should be considered to be within the scope of the invention as defined in the appended claims.