GB2219775A - Control systems for aircraft - Google Patents

Abstract

An automatic rudder booster and bias system for a propeller aircraft includes booster means 36 which is responsive to any output pressure differential between the engines 13 to urge the rudder 16 in a compensating direction in the event of an engine failure. Rudder bias means 37 modify this pressure differential in the case of high symmetric power and rudder starboard so as to use the booster means to provide a force to increase the pedal force required from the pilot which would otherwise drop. <IMAGE>

Description

CONTROL SYSTEMS FOR AIRCRS,vT This invention relates to control systems for aircraft. More particularly it relates to rudder control systems and includes means to relieve or supplement pilots control input forces to counter yaw and sideslip after engine failure or with high symmetric power.

The system of the present invention provides rudder booster means to relieve pilot's foot forces to counter yaw and sideslip resulting from engine failure and rudder bias means associated with rudder booster means to overcome the characteristics of reducing pedal force with sideslip associated with high symmetric power. The need for this function arises, in the case of a propeller driven aircraft, from asymmetric propeller-slipstream effects giving a tendency for rudder hinge moment to lighten off as rudder angle and resulting sideslip approach their maximum values.

According to the present invention there is provided an automatic rudder booster and bias system for use in an aircraft powered by port and starboard turbine engine driven propellors disposed symmetrically to either side of the longitudinal plane of symnetry of the said aircraft, comprising rudder booster means responsive to compressor outlet pressure differential between said engines such that in the event of a single engine failure said rudder is deflected towards that side which is opposite the failing engine, and rudder bias means coupled to said rudder booster means for modulating said compressor outlet pressure differential in order to modify pilot's input control forces in response to high symmetric power coupled with asymmetric propellor-slipstream effects.

With this arrangement, in the event of an engine failure the rudder is automatically urged in a compensating direction, while during normal high power operation the rudder may be urged to compensate for undesirable propellor slipstream effects.

Preferably the rudder booster means comprises a chamber divided into two parts by a piston, respective sides of said piston being connected by pressure lines to respective engine compressor outlets for the supply of a pressure proportional to engine thrust to said respective sides, and wherein said bias means is coupled to said booster means by pressure bleed lines for bleeding said pressure from said sides of said piston to said bias means.

In a preferred embodiment the rudder bias means comprises first and second chambers each adapted to receive pressure bled from said rudder booster means via said bleed lines, said first chamber having a first piston therein with a first piston rod extending out of said first chamber, and said second chamber having a second piston therein with a second piston rod extending out of said second chamber, said first and second piston rods being pivotably connected to an operating lever at respective first and second pivot points, said second chamber further comprising vent means operable by movement of said second piston whereby pivotal movement of said operating lever about said first pivot point causes said vent means to open to vent pressure from one side of said rudder booster means, said operating lever being pivoted in response to movement of the rudder beyond a predetermined extent in one direction and said resulting pressure differential across said rudder booster means acting to urge the rudder in the other direction.

To ensure that the rudder bias means is only effective to modulate the pressure differential at high symmetric power, said second piston is spring-biassed into a vent closed position whereby said operating lever can pivot about said first pivot point only when the pressure to said first cylinder exceeds a predetermined value, the operating lever pivoting about said second pivot point, with no consequent movement of said second piston, when said pressure to said first chamber is below said predetermined value.

In one form the vent means may comprise a needle valve defined by the inlet to said second chamber for the pressure bled from said rudder booster means and a valve member formed on said second piston, and a vent aperture formed in the wall of said second chamber.

The pivotal movement of the operating lever may, preferably, be effected by engagement with a projection formed on a rudder control member.

With conventional propeller driven aircraft the problems discussed above with respect to high symmetric power sideslip occur with sideslip to port (rudder starboard) and thus in such a case the pressure bled to said second chamber is from the side of said rudder booster piston receiving pressure drawn from the starboard engine and the operating lever is pivoted in response to movement of the rudder beyond a predetermined degree to starboard.

In the event of an engine failure the rudder bias means should preferably be inoperative so as not to interfere with the operation of the rudder booster means. Accordingly in a preferred embodiment in the event of failure of a port engine the operating lever pivots about the second pivot point and has no effect on the pressure differential, and wherein in the event of failure of a starboard engine the pressure led to said second cylinder is substantially nil whereby any venting of said second bleed line has no effect on said rudder booster means.

One embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates pictorially a part general arrangement of a twin-engined transport aircraft with particular reference to the rudder control system.

Figure 2 illustrates diagrammatically a combined rudder booster/bias system in accordance with the present invention.

Figure 3 illustrates to a larger scale the rudder actuating mechanism indicated as Detail III in Figure 1.

Figure 4 is a side elevation, partly sectioned, on the rudder bias valve in accordance with the present invention.

Figures 5a - 5c inclusive illustrate diagrammatically the rudder bias valve in three functional positions.

Referring to the drawings, Figure 1 illustrates pictorially a part general arrangement of a turbo-prop powered transport aircraft 10 including a fuselage 11, wings 12, twin gas turbine engines 13 (the propellers of this turbo-prop arrangement omitted in this illustration) a horizontal stabiliser 14 and a vertical fin 15 incorporating a rudder 16. For reasons of clarity only the rudder control system 17 is illustrated in Figure 1 and includes duplicated pilot's rudder pedal assembly 18 interconnected by cable and push-pull rod mechanisms 19err hereafter referred to as the control circuit 19, supported by pulleys and roller fairlead assemblies to a rudder actuating mechanism assembly 20 located rearwards of a rear pressure bulkhead 21 by which means the rudder 16 and the rudder trim and spring tabs 22 are operated.The operation of the tabs 22 is not further discussed in this specification. As further illustrated in Figure 3 the control circuit 19 is connected via a quadrant pulley assembly 23 to a tension regulator 24 and a servo unit 25, the tension regulator maintaining the cable circuit at the desired tension. A lever 26 on the tension regulator provides the input for the autopilot servo unit 25, the tension regulator and the servo unit interconnected by a control rod 27. Pilots control demand is transmitted from the servo unit/tension regulator assembly by means of a twin push-pull rods 28 and 29 extending upwardly through the fin structure and interconnected to each other by means of an idler lever assembly 30 attached to a suitably positioned fin rib station 31. The upper end of the push-pull rod 29 is pivotally attached to a bell-crank 32 which engages a final push-pull rod 33 through which control demand is transmitted to the rudder actuating mechanism 34 (Figure 1). Mounted to the structure above the autopilot servo-unit 25 is a rudder booster unit 35 and comprises a rudder booster unit 36 and a rudder bias unit 37 as diagrammatically illustrated in Figure 2.

Referring to Figure 2, the rudder boost unit 36 includes a balanced area ram 38 having a pivotal attachment 40 to the aircraft and comprising a piston 39, a shaft 41 extending from the piston and pivotally located at 42 to a lever 43 projecting from the rudder 16.

The ram- 38 includes upper and lower bleed connections communicating with bleed pipes 44 and 45 respectfully connected to the high pressure stage of the port and starboard turbine engines 13 and by which means a supply of high pressure air is piped to the respective chamber 44a or 45a adjacent the piston 39. With both engines functioning normally the air pressure within the ram 38 will be equally balanced to either side of the piston such that it will have no influence on rudder movement. The basic function of the rudder booster, however, is to relieve pilot's foot forces to counter sideslip resulting from a single engine failure and the loss of bleed air pressure from any one engine resulting from such a failure causes a pressure imbalance across the piston 39 and the rudder to displace angularly to the same side as the surviving engine.

This arrangement is well known; however, it was recognised that the directional characteristics in sideslip for example at low speed, take-off power and with certain wing flap settings give large differences in pedal force between port and starboard sideslips. When sideslipping to starboard pedal forces increase rapidly with increasing rudder angle and sideslip. When sideslipping to port the pedal force may peak between 100 and 150 sideslip and approximately 160 rudder, the pedal force reducing at larger rudder angles.

Thus in addition to assisting the pilot to suppress sideslip after engine failure, it would be desirable to increase the pedal forces required to apply maximum sideslip to port (rudder starboard) with high symmetric power. The need for this function arises from asymmetric propeller-slipstream effects causing a tendency for rudder hinge moment to lighten off as rudder angle and resulting sideslip approach their maximum values.

To overcome the characteristic of the reducing pedal force with sideslip, the amount of boost given by either engine bleed should ideally be proportioned to increase pedal force relative to rudder angle. This is achieved by the introduction of the rudder bias unit 37 of the present invention, which may alternatively and more descriptively be referred to as a bleed bias unit, modulating the bleed pressure from the starboard engine into the aft chamber 45a of the ram 38.

The rudder bias unit will now be described with reference to Figures 2 and 4 and is a twin chamber device 46, comprising an upper chamber 47 and a lower chamber 48. The upper chamber 47 is connected by means of a pipe 49 for the supply of high pressure air from the port HP bleed whereas the lower chamber 48 is supplied by means of a pipe 50 from the starboard HP bleed. As diagrammatically illustrated in Figure 2, these pipes may be branch lines off their respective HP bleed pipes 44 and 45. The upper chamber 47 includes a piston 51 incorporating a horizontal support rod 52 having an adjustable rod end 53 providing an upper pivotal attachment 54 to a substantially vertical operating lever 55.The lower chamber 48 includes a spring loaded piston assembly 56, comprising a piston portion 57, incorporating a concentric support rod 58 having an adjustable rod end 53 providing a lower pivotal attachment 59 to the operating lever 55 and an opposing concentric support rod 60 terminating in a pressure reducing needle valve assembly 61. A compression spring 62 is installed between the piston portion 57 and a retaining cap 63, the spring force maintaining the needle valve closed in its static state; an air vent 65 extends through the body of the lower chamber 48 such that the chamber can vent to atmosphere in the vicinity of the needle valve 61.The lever 55 extends vertically downwards to span the servo to tension regulator control rod 27 which incorporates an abuttment block 64 positioned lengthwise to strike the operating lever 55 when starboard rudder is applied, the lever being adjusted by means of the adjustable rod ends 52 and 53 such that its vertical face 66 only comes into contact with the block 64 at half starboard rudder.

Rudder movement to starboard above half travel causes the operating lever 55 to angularly pivot about pivot axis 54 which is held in position under port side bleed pressure. This results in an associated linear displacement of the piston 57 against the spring 62 lifting the needle valve 61 from its seating, leaking starboard bleed pressure to atmosphere via the vent 65. This controlled leak increasing the pressure differential between the chambers 44a and 45a of the ram 38 causes the booster unit 36 to oppose starboard rudder, thereby increasing pedal force.

In the engine failure case if the port engine fails or loses power, causing the pressure in the upper chamber 47 to fall below its minimum support pressure (typically 100 psi) then pivot 54 collapses due to a closing linear displacement of the piston 51 and the associated support rod 52 subjecting the operating lever 55 to angular displacement about the pivot centre 59, freely moving through the full starboard range of rudder movement without reacting the needle valve 61. In the starboard engine failure case the rudder will initially be urged to port and the operating lever will not be engaged. If subsequently, however, the pilot wishes to apply starboard rudder and the operating lever is engaged by the block 64, the needle valve 61 is displaced but, of course, no pressure exists. Thus the bias means will have no effect.In both single engine failure cases the booster unit 36 reverts to its original role of reducing pedal forces.

Three operational states of the rudder bias unit are schematically illustrated in Figures 5a - Sc inclusive.

The bias unit is designed to permit progressive bleed of air from the system via the needle valve 61 when both of the following conditions are met:i) air pressure above a specific value is supplied to chamber 47, ii) lever 55 is operated so as to open needle valve 61.

If condition i) is met then as the needle valve opens (by movement of lever 55) air is bled from the booster system. The amount of air bled is designed to be nominally proportional to the movement of the lever 55, ie, the more the needle valve opens the greater the leakage of air becomes (see Figure 5b).

If condition i) is not met, ie, should the air pressure supplied to chamber 47 drop below a specific value, the force acting on the piston 51 is reduced to a value below that of the spring force acting on the piston 57. Thus, if lever 55 is operated, the needle valve 61 remains closed and no leakage of air occurs (see Figure 5c).

If, in accordance with condition ii) lever 55 is not operated, then the spring 56 holds the valve 61 closed and no leakage of air occurs (see Figure 5a).

Claims (9)

1 An automatic rudder booster and bias system for use in an aircraft powered by port and starboard turbine engine driven propellers disposed symmetrically to either side of the longitudinal plane of symmetry of the said aircraft, comprising rudder booster means responsive to compressor outlet pressure differential between said engines such that in the event of a single engine failure said rudder is deflected towards that side which is opposite the failing engine, and rudder bias means coupled to said rudder booster means for modulating said compressor outlet pressure differential in order to modify pilot's input control forces in response to high symmetric power coupled with asymmetric propeller-slipstream effects.

2 A rudder system according to Claim 1 wherein said rudder booster means comprises a chamber divided into two parts by a piston, respective sides of said piston being connected by pressure lines to respective engine compressor outlets for the supply of a pressure proportional to engine thrust to said respective sides, and wherein said bias means is coupled to said booster means by pressure bleed lines for bleeding said pressure from said sides of said piston to said bias means.

3 A rudder system according to Claim 2 wherein said rudder bias means comprises first and second chambers each adapted to receive pressure bled from said rudder booster means via said bleed lines, said first chamber having a first piston therein with a first piston rod extending out of said first chamber, and said second chamber having a second piston therein with a second piston rod extending out of said second chamber, said first and second piston rods being pivotably connected to an operating lever at respective first and second pivot points, said second chamber further comprising vent means operable by movement of said second piston whereby pivotal movement of said operating lever about said first pivot point causes said vent means to open to vent pressure from one side of said rudder booster means, said operating lever being pivoted in response to movement of the rudder beyond a predetermined extent in one direction and said resulting pressure differential across said rudder booster means acting to urge the rudder in the other direction.

4 A rudder system according to Claim 3 wherein said second piston is springbiassed into a vent closed position whereby said operating lever can pivot about said first pivot point only when the pressure to said first cylinder exceeds a predetermined value, the operating lever pivoting about said second pivot point, with no consequent movement of said second piston, when said pressure to said first chamber is below said predetermined valve.

5 A rudder system according to Claims 3 or 4 wherein said vent means comprises, a needle valve defined by the inlet to said second chamber for the pressure bled from said rudder booster means and a valve member formed on said second piston, and a vent aperture formed in the wall of said second chamber.

6 A rudder system according to any of claims 3 to 5 wherein said operating lever is pivoted by engagement with a projection formed on a rudder control member.

7 A rudder system according to any of claims 3 to 6 wherein the pressure bled to said second chamber is from the side of said rudder booster piston receiving pressure drawn from the starboard engine and the operating lever is pivoted in response to movement of the rudder beyond a predetermined degree to starboard.

8 A rudder system according to Claim 7 wherein in the event of failure of a port engine the operating lever pivots about the second pivot point and has no affect on the pressure differential, and wherein in the event of failure of a starboard engine the pressure led to said second cylinder is substantially nil whereby any venting of said second bleed line has no effect on said rudder booster means.

9 An automatic rudder booster and bias system substantially as hereinbefore described with reference to the accompanying drawings.