GB1574618A - Rotatable jack for a servo control system of an aerodyne - Google Patents

Description

(54) ROTATABLE JACK FOR A SERVO CONTROL SYSTEM OF AN AERODYNE (71) We, SOCIETE FRANCAISE D'EQUIPEMENTS POUR LA NAVIGATION AERIENNE S.F.E.N.A. a Society' Anonyme organised and existing under the laws of France of Aerodrome de Villacoublay78140 Velizy-Villacoublay, France, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a rotatable jack with incorporated stress components for a servo control system of an aerodyne in which movement of control surfaces of the aerodyne is effected by means of gear actuated by a control lever and by a servo jack in series with the gear, the rotatable jack acting on the gear so that in opposition to the action of the servo jack the rotatable jack provides a resistance which is greater than that exerted on the servo jack by the control surfaces.

As is known the control surfaces are at the exterior of an aerodyne and are movable to change their attitude whereby the aerodyne can be manoeuvred in flight. In the case of aeroplanes the control surfaces comprise wing ailerons and flaps, and tailplane elevators and rudders, whereas for helicopters the control surfaces comprise variable pitch rotor blades.

In general, it is known that direct control over the manoeuvring of aerodynes such as airplanes and helicopters is achieved by the action of the pilot on at least one operating lever, such as a control or steering column which is connected to the control surfaces of the aerodyne through the intermediary of linkages, as in the instance of a system for varying the pitch of the rotor blades of a helicopter.

This control of the control surfaces by the operating lever has an accessory back-up provided by an automatic pilot system, or automatic pilot, which acts on a servo jack allowing for rapid piloting, but which jack has a limited extent of action.

One or a first end of the servo jack is connected to the operating lever and the opposite second end is connected to the linkages for moving the control surfaces. In order to prevent the servo jack, which is associated with the automatic pilot, from merely moving the operating lever, i.e.

control column, instead of causing the desired movement of the control surfaces it is necessary to provide a device that acts upon the operating lever so that at the first end of the servo jack the device will apply a greater resistance to movement of the jack than the resistance to movement of the control surfaces experienced by the jack at its second end.

Within the basic technology such a device may include a tension spring or a compression spring acting on the first end of the servo jack, said spring opposing stress, experienced by the second end of the servo jack, according to a predetermined principle usually referred to as the "principle of stress". The position of a mounting or anchorage of the spring can be varied, for example, by means of a jack, such as a power or stress jack.

The nower or stress jack may include, in the basic technology, a motorized speed reducer, which can, on occasion, be controlled by the automatic pilot, or by the pilot himself. This jack has an externally proiecting shaft which responds to the control lever, the spring being pivotably connected to the shaft.

A shift of the anchoring point of the spring thus permits the position of the control lever or control column to be readjusted, and the servo jack may be maintained in a position so as to have the greatest extent of travel (to either side of a zero mid point), which is important because, as is well known, servo jacks have a very limited degree of travel.

One of the advantages of this system, and, particularly when the stress principle is incorporated therein, is that it restores a sensation or feel to the hands of the pilot which allows him to judge how much a control lever need be moved for the aerodyne to attain a position corresponding to a particular flight pattern.

This system may include a device for detecting a shift of the external shaft with respect to the anchoring point. For example this detection device can be constituted by a micro contact breaker providing a signal to stop or to minimize the action of the automatic pilot, for example by halting operation of the servo jack or by limiting its movement, which would otherwise conflict with the action of the pilot when he shifts the control lever. The system may also include a device for disengaging the anchorage, thus permitting the spring which utilizes the stress principle to be recentered.

Currently, all of these functions are performed by modular elements which are connected to each other by mechanical means and which are usually exposed to the open air. As a result, these elements are not reliable, they are difficult to adjust and control, and they age very poorly, particularly in a salty atmosphere. As a result, the life expectancy of such a system frequently barely exceeds fifty hours of service.

Furthermore, these prior art systems are all subject ot a serious drawback in that, when the anchorage is disengaged, there occurs a recentering or shifting of the spring so as to cause a sharp change in the operation of the flight control system. This sharp change can cause dangerous breakdowns and accidents.

According to the invention there is provided a rotatable jack with incorporated stress means for a servo control system of an aerodyne in which movement of control surfaces of the aerodyne is effected by means of gear actuated by a control lever and by a servo jack in series with the gear, the rotatable jack acting on the gear so that in opposition to the action of the servo jack the rotatable jack provides a resistance which is greater than that exerted on the servo jack by the control surfaces, said rotatable jack comprising a) stress means comprising a tubular sleeve mounted on a stationary structure relative to which the sleeve is rotatable about the sleeve axis, a first shaft co-axial with the sleeve and disposed therein, said shaft being rotatable relative to the sleeve, a helical spring disposed in a gap between the sleeve and shaft, at its opposite ends said spring being connected to a respective part which is rotatable about the shaft, two abutments being mounted on the sleeve and extending radially inwardly said sleeve, two pins being mounted on the shaft, and said abutments and pins being so disposed that each of said parts can come into contact with and drive or be driven by a corresponding said abutment or pin, and b) means for rotatably driving the sleeve comprising a drive member which drives an output shaft, disengaging means having a driving element disengageably engaging a driven element, said output shaft being connected to drive the driving element, and said driven element being connected to the sleeve by a transmission system comprising brake means.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic view illustrating a servo control system effecting movement of control surfaces of an aerodyne including, purely for the purposes of explanation, a rectilinear acting jack operating on the "stress principle" and having certain features which are explained with a view to providing an understanding lof the corresponding features in a rotatable jack formed according to the invention; Figs. 2 and 3 are diagrammatic views of two embodiments of rotatable jack acting on the "stress principle" and formed according to the invention for substitution in the control system in Fig. 1 for the rectilinear acting jack operating on the "stress principle Fig. 4 is a longitudinal sectional view of the rotatable jack in Fig. 2; Fig. 5 is a perspective diagrammatic view, partly in section, showing the stress means in the rotatable jack in Fig. 4, and Figs. 6 and 7 are respectively, graphs illustrating counteracting torque C exerted by the helical spring in rotatable jacks formed according to the invention.

With reference to Fig. 1, a servo control system for moving control surfaces of an aerodyne, for example an airplane or helicopter includes a contol lever 1 or control column pivotally mounted for movement around an axis 2 which is integral with the fixed structure of the aerodyne.

The free end of the upper arm of the lever 1 is provided with a handle 3, which may incorporate many control devices, only two of which, components 4 and 5, will be mentioned in the following description.

The free end of lower arm 6 of lever 1 is adapted to execute, by means of its pivotal connection at 7 to a shaft 7a, command over control surfaces 8 (for example helicopter rotor blades) through the intermediary of a linkage comprising shafts 9, 9a and levers 10.

This pilot system is also operated by an automatic pilot 12, acting on a servo jack 13, which is rapid in operation but has limited action, and which is mounted at one end on shaft 7a and at the other end on shaft 9.

Thus, the jack is movable with respect to the fixed structure or frame of the airplane, and is supported, with the aid of shaft 9, on the end of lower arm 6 of operating lever 1.

In order to prevent the action of jack 13 from causing pivoting of the control lever 1 instead of acting on control devices 8, a jack 25 is provided for opposing action of the servo jack 13 towards lever 1. The jack 25 provides a resistance to the action of the jack 13 towards the lever 1 greater than the force exerted by the shaft 9 on the jack 13 due to resistance to movement exerted by the control surfaces and the linkage.

In order to obtain this result, an elastic element is used, generally referred to as a stress component and consisting, in the example shown in Fig. 1, of a precompressed or pretensioned spring 14 having one end connected to the shaft 7.

The other end of the spring 14 is connected to a movable anchorage point 15 which can be shifted longitudinally with the aid of a rack 16 and a reduction motor 17 provided with an output shaft driving a pinion 18 which interengages with the rack 16.

It should be noted that the anchored spring 14 anchorage proper is prestressed.

Without such prestressing, a shifting of the control lever 1 would be produced up to the stress limit of the spring, in effect, greater than the stress exerted on the jack 13 due to the resistance to movement of the linkage 9, 9a, 10 and the control surfaces 8.

The connection between anchorage 15 of the spring 14 and the rack 16 is by means of a clutch 19 which enables the spring 14 to be recentred while the clutch is disengaged.

Operation of the clutch is controlled manually by switch 5 in the handle 3 of the control lever 1.

In order to prevent a sharp change in the control system, which could occur upon the anchorage 15 being disengaged from the rack 16, the anchorage is connected to a damping system 22.

The reduction motor 17 can be operated either manually, with the aid of a switch 4 situated in the handle 3 of the control lever 1, or by the automatic pilot 12. In this regard, control over the position of anchorage 15 can be attained by means of a potentiometer 23 providing a signal, representative of the position of the anchorage, to the automatic pilot.

Finally, it is to be noted that any shifting of the shaft 7a with respect to anchorage 15 is detected by a microswitch 24 operatively connected to automatic pilot 12, in order to minimize the action of the servo jack 13 when the pilot shifts operating lever 1.

In the servo control system in Fig. 1, a rotatable jack which is the subject of this Application for Patent, is substituted for the rectilinear acting jack enclosed within the phantom lines 25 in which the anchorage 15 of the rectilinear acting spring 14 is movable rectilinearly. But in the rotatable jack the stress principle is effected by a torsion spring having a rotatable anchorage enabling the position of the anchorage to be shifted by rotation. This feature of the rotable jack is especially advantageous since it permits the jack to be composed of rotary elements of considerable structural precision and operational reliability, and which may be easily introduced into a small, water-tight receptacle or housing.

Fig. 2 shows an embodiment of a rotatable jack which will be described below in more detail with reference to Figs. 4 and 5, and intended as a substitute for the jack enclosed in phantom lines 25 in Fig. 1. The rotatable jack has a stress component comprising a torsion spring (not shown) with a rotatable anchorage. This spring acts on a rotatable exterior shaft 27. The jack is equipped with an anchorage potentiometer 28 which allows for detection of shifting of the point of anchorage with respect to the fixed structure of the aerodyne, and a microswitch 29 for detecting the rotational movements of the exterior shaft with respect to the point of anchorage.

Rotation of the anchorage is achieved by means of a reduction motor 30 having an output shaft connected to a clutch 31 which is joined, on the one hand, to a reducer 32 driving the anchorage and, on the other hand, to a brake or rotary damping device 33 which comes into operation when the clutch is disengaged to interrupt the drive from the reduction motor to the reducer 3.

In order that the rotatable jack can exert resistance on the control lever 1, the shaft 27 has a splined end keyed to a block to which is pivoted a rod which is also pivotably connected to the control lever.

Alternatively the shaft 27 can be connected to the axle 2 of the control lever 1 provided that the axle 2 is fast with the control lever 1.

The rotatable jack described allows the attainment of the following functions: application of stress or strain principle; mobility of the anchorage; motorization of the anchorage with a clutch; damping when the clutch is disengaged; detection of rotation of the shaft 27 with respect to the point of anchorage, and detection of shifting of the point of anchorage with respect to the fixed structure of the aerodyne.

It should be noted, however, that the invention is not limited to a jack which accomplished all of these functions. It also relates to simpler embodiments of the rotatable jack, such as shown, for example, in Fig. 3.

The rotatable jack in Fig. 3 includes all of the elements of the jack in Fig. 2, except the anchorage potentiometer 28 which has been eliminated.

The jack in Fig. 2 is shown in more detail in Fig. 4 in which all elements represented diagrammatically in Fig. 2 as well as their mechanical connections, are located within a water-tight housing 36. In the jack of Fig.

4 the stress component is constituted by a rotational system, shown diagrammatically, in Fig. 5, which essentially includes: 1) a tubular sleeve 37, rotatably mounted interiorly of housing 36, by means of ball bearings 38 and 39, and which includes, at one end, a cogged ring gear employed to drive the sleeve; 2) an outwardly projecting shaft 41 coaxially extending within sleeve 37, and rotatably mounted, at one end, to sleeve 37 by means of ball bearings 42, 43 and, at the other end, in water-tight relationship, to housing 36, from which it extends, by means of ball bearing 44; and 3) a helical spring 45 located in the space intermediate the projecting shaft 41 and sleeve 37, each end of the spring being fastened to plate in the shape of a sector of a circle 46, 47, and the spring being coaxial with projecting shaft 41 and adapted to rotate thereabout.

The mechanical connections between sleeve 37, spring 45, and projecting shaft 41 are effected, at one end, by two shoulders 48, 49 which are formed integrally with sleeve 37 and extend radially inwardly thereof, and by two pins 51, 52 which are fixedly fastened to the projecting shaft 41.

The two shoulders 48, 49 are located, respectively, at the generatrix of sleeve 37 and the two respective planes of sectors 46, 47, so that each of these shoulders 48, 49 can come into contact with the radial surfaces 53 and 54 of the sector that corresponds therewith.

In a similar manner, the two pins 51 and 52 are located on one generatrix of shaft 41 and in the respective planes of the two sectors 46 and 47.

In the same way, the two pins 51 and 52 can thus come into contact with radial surfaces 53, 54 of the corresponding sectors 46 and 47.

It should be noted that, when at rest i.e. in the absence of any torque acting on projecting shaft 41, the spring is subjected to a prestressing force so that each of the sectors 46, 47 abuts one of its radial surfaces 53, 54 against the pin 51 or 52 and the shoulder 48 or 49 that corresponds therewith.

Thus, each rotation of the sleeve 37, in either direction, imparts a corresponding rotation to the shaft 41, providing that the opposing torque applied to the shaft is no greater than the torque of the prestressing force on the spring 45. Thus, there is obtained a shifting of the anchorage of the spring 45, which is analogous to the one described with regard to Fig. 1.

Moreover, whatever the fixed position of the sleeve 37, the projecting shaft 41 will be able to rotate in either direction, while opposing any torque greater than the prestress torque exerted on the shaft 41.

The angular clearance of the stress component, that is the maximum rotation of the projecting shaft 41 for a given anchorage position, is equal to the angle formed by the two radial surfaces 53, 54 of the circular sectors. In the example shown, this angle has a value of 100 .

Moreover, rotation of the anchorage can be limited by means of an abutment 56 which can come into contact, from one side thereof or the other, with two protrusions 57 and 58 located on the outer surface of sleeve 37. The protrusions 57 and 58 are approximately 1200 apart.

The shaft 41 is splined at its end 59 (Fig. 4) remote from the housing 36 so that the shaft can be connected to the servo control system of the aerodyne.

The shaft 41 moves an indicator of a rotary potentiometer 60 which is located within the housing 36. The shaft 41 is integral with a cam 61 on which there rests a roller carried by an actuating rod of microswitch 62. The microswitch 62 is, in turn, carried by a disc 63 which is integral with the sleeve 37. This microswitch 62 allows for the detection of relative rotations between the sleeve 37 and the shaft 41.

Driving of the sleeve 37 is accomplished by means of a disengageable motor device with a damper, and which includes a servomotor 64 followed by a reducer (pinions 65, 66 and 67). This reducer moves drive disc 68 of an electromagnetic disc clutch which functions in such a way that, when a winding 69 is fed (or, to the contrary, when it is not fed) with electric current the drive disc 68 and a driven disc 70 of the clutch are operatively joined together.

The driven disc 70 is drivingly mounted on one end of a shaft 71.

The shaft 71 is fast with a disc 73 carrying satellite pin ions 72 that engage respectively a stationary cogged ring gear 74 and a pinion 75 rotatably mounted on shaft 71.

The pinion 75 moves a rotor 76 of a classic Foucault current damper providing braking action and whose magnetic field is generated by a permanent magnet 77. It should be noted that rotor 76 could be moved by means of spur pinions. At its end remote from the disc 70, the shaft 71 has gear teeth driving a reducer system composed of gear trains 78 and 79 which has an escarpment pinion 80 engaging the ring gear 40 of the sleeve 37.

It should be noted that, in the engaged position, the anchorage point (position of the sleeve 37) is reversible due to a torque greater than 30 mN being applied to the shaft 41.

As for the application of stress, due to the fact that it is provided by a torsion sping, its characteristic C (a) (opposing torque C as a function of the angle a of the shaft 41 with respect to the anchorage of the spring) is of the type shown in the graphs of Figs. 6 and 7.

If the shoulders 48 and 49 are mounted on the thinner part of the sleeve 37 at a position mid-way between the protrusions 57 and 58, then for this arrangement Fig. 6 shows a graph representing variation of the torque C exerted by the torsion spring as a function of the angular position a of the shaft 41 with respect to the particular shoulder 48 or 49 acting as the spring anchorage. In this case a is assigned the value zero when the anchorage is directly alongside the abutment 56 so that rotation of the anchorage can be through a maximum angle aN (the sign being neglected) in either direction from the a=0 positon to bring protrusion 57 or 58 into contact with the abutment 56. When a=0, the torque C increases or decreases to a threshold +Cs which is the torque threshold of the prestressed spring 45. The graph then includes two linear portions defined by the function C=Ka+Cs, where K is a constant.

One linear portion extends between the points (+Cs,O) and (+CN, +aN) and the other extends between the points (-Cs,O) and (C,aN), +CN being the maximum torque developed. This graph also shows the angular position +am of the anchorage point with respect to the shaft 41 at which there is an operative change of state of the microswitch. The microswitch can be arranged so that this operative change of state occurs when the pilot shifts the operating lever approximately 1 to 6 per cent.

In another modification the shoulders 48 and 49 can be positioned immediately adjacent the shoulder of protrusion 57 or 58.

The graph in Fig. 7 is appropriate to an embodiment of rotatable jack in which the shoulders 48 and 49 are positioned immediately adjacent the shoulder of protrusion 58. In this case a is assigned the value zero when the particular shoulder 48 or 49 acting as the spring anchorage is alongside the abutment 56 by reason that the protrusion 58 is in contact with the abutment 56. The maximum torque CM exerted by the spring is produced by rotating the sleeve 37 and therefore the anchorage, about shaft 41, through the angle between the protrusions 57 and 58 until the abutment 56 contacts protrusion 57, which angle is called the clearance and is equal to aM which is the maximum relative angular displacement of the anchorage relative to stationary shaft 41. aM--2aN and the maxirnum torque CM=2CNCE With regard to the safety of the rotatable jack shown in Fig. 4, in the hypothetical case of the locking of sleeve 37 to the projecting shaft 41, for example, in the event that one of the ball bearings becomes jammed, the pilot would be able to continue flying by disengaging the motor of the jack by disengaging the clutch.

In the event that the anchorage becomes locked, for example, because of the locking of a ball bearing 38 or of one of the various gears, the pilot can continue to fly within the limit of rotation of the shaft 41 while opposing the action of the spring 45.

WHAT WE CLAIM IS: 1. A rotatable jack with incorporated stress means for a servo control system of an aerodyne in which movement of control surfaces of the aerodyne is effected by means of gear actuated by a control lever and by a servo jack in series with the gear, the rotatable jack acting on the gear so that in opposition to the action of the servo jack the rotatable jack provides a resistance which is greater than that exerted on the servo jack by the control surfaces, said rotatable jack comprising a) stress means comprising a tubular sleeve mounted on a stationary structure relative to which the sleeve is rotatable about the sleeve axis, a first shaft co-axial with the sleeve and disposed therein, said shaft being rotatable relative to the sleeve, a helical spring disposed in a gap between the sleeve and shaft, at its opposite ends said spring being connected to a respective part which is rotatable about the shaft, two abutments being mounted on the sleeve and extending radially inwardly of said sleeve, two pins being mounted on the shaft, and said abutments and pins being so disposed that each of said parts can come into contact with and drive or be driven by a corresponding said abutment or pin, and b) means for rotatably driving the sleeve comprising a drive member which drives an output shaft, disengaging means having a driving element disengageably engaging a driven element, said output shaft being connected to drive the driving element, and said driven element being connected to the sleeve by a transmission system comprising brake means.

2. A rotatable jack as claimed in claim 1, in which each of said parts is a plate having the shape of a sector of a circle.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. composed of gear trains 78 and 79 which has an escarpment pinion 80 engaging the ring gear 40 of the sleeve 37. It should be noted that, in the engaged position, the anchorage point (position of the sleeve 37) is reversible due to a torque greater than 30 mN being applied to the shaft 41. As for the application of stress, due to the fact that it is provided by a torsion sping, its characteristic C (a) (opposing torque C as a function of the angle a of the shaft 41 with respect to the anchorage of the spring) is of the type shown in the graphs of Figs. 6 and 7. If the shoulders 48 and 49 are mounted on the thinner part of the sleeve 37 at a position mid-way between the protrusions 57 and 58, then for this arrangement Fig. 6 shows a graph representing variation of the torque C exerted by the torsion spring as a function of the angular position a of the shaft 41 with respect to the particular shoulder 48 or 49 acting as the spring anchorage. In this case a is assigned the value zero when the anchorage is directly alongside the abutment 56 so that rotation of the anchorage can be through a maximum angle aN (the sign being neglected) in either direction from the a=0 positon to bring protrusion 57 or 58 into contact with the abutment 56. When a=0, the torque C increases or decreases to a threshold +Cs which is the torque threshold of the prestressed spring 45. The graph then includes two linear portions defined by the function C=Ka+Cs, where K is a constant. One linear portion extends between the points (+Cs,O) and (+CN, +aN) and the other extends between the points (-Cs,O) and (C,aN), +CN being the maximum torque developed. This graph also shows the angular position +am of the anchorage point with respect to the shaft 41 at which there is an operative change of state of the microswitch. The microswitch can be arranged so that this operative change of state occurs when the pilot shifts the operating lever approximately 1 to 6 per cent. In another modification the shoulders 48 and 49 can be positioned immediately adjacent the shoulder of protrusion 57 or 58. The graph in Fig. 7 is appropriate to an embodiment of rotatable jack in which the shoulders 48 and 49 are positioned immediately adjacent the shoulder of protrusion 58. In this case a is assigned the value zero when the particular shoulder 48 or 49 acting as the spring anchorage is alongside the abutment 56 by reason that the protrusion 58 is in contact with the abutment 56. The maximum torque CM exerted by the spring is produced by rotating the sleeve 37 and therefore the anchorage, about shaft 41, through the angle between the protrusions 57 and 58 until the abutment 56 contacts protrusion 57, which angle is called the clearance and is equal to aM which is the maximum relative angular displacement of the anchorage relative to stationary shaft 41. aM--2aN and the maxirnum torque CM=2CNCE With regard to the safety of the rotatable jack shown in Fig. 4, in the hypothetical case of the locking of sleeve 37 to the projecting shaft 41, for example, in the event that one of the ball bearings becomes jammed, the pilot would be able to continue flying by disengaging the motor of the jack by disengaging the clutch. In the event that the anchorage becomes locked, for example, because of the locking of a ball bearing 38 or of one of the various gears, the pilot can continue to fly within the limit of rotation of the shaft 41 while opposing the action of the spring 45. WHAT WE CLAIM IS:

1. A rotatable jack with incorporated stress means for a servo control system of an aerodyne in which movement of control surfaces of the aerodyne is effected by means of gear actuated by a control lever and by a servo jack in series with the gear, the rotatable jack acting on the gear so that in opposition to the action of the servo jack the rotatable jack provides a resistance which is greater than that exerted on the servo jack by the control surfaces, said rotatable jack comprising a) stress means comprising a tubular sleeve mounted on a stationary structure relative to which the sleeve is rotatable about the sleeve axis, a first shaft co-axial with the sleeve and disposed therein, said shaft being rotatable relative to the sleeve, a helical spring disposed in a gap between the sleeve and shaft, at its opposite ends said spring being connected to a respective part which is rotatable about the shaft, two abutments being mounted on the sleeve and extending radially inwardly of said sleeve, two pins being mounted on the shaft, and said abutments and pins being so disposed that each of said parts can come into contact with and drive or be driven by a corresponding said abutment or pin, and b) means for rotatably driving the sleeve comprising a drive member which drives an output shaft, disengaging means having a driving element disengageably engaging a driven element, said output shaft being connected to drive the driving element, and said driven element being connected to the sleeve by a transmission system comprising brake means.

2. A rotatable jack as claimed in claim 1, in which each of said parts is a plate having the shape of a sector of a circle.

3. A rotatable jack as claimed in claim 1

or claim 2, in which the sleeve is fast with a cogged ring.

4. A rotatable jack as claimed in any preceding claim, in which the first shaft drives an indicator of a ' rotary potentiometer.

5. A rotatable jack as claimed in any preceding claim, in which a microswitch is mounted between the first shaft and the sleeve.

6. A rotatable jack as claimed in any preceding claim, in which the disengaging means comprises an electromagnetic disc clutch.

7. A rotatable jack as claimed in any preceding claim, in which the brake means comprises a Foucault current brake.

8. A rotablejack as claimed in claim 3 and claim 7 in which the drive member comprises a motor driven first reducer, and the transmission system comprises a second reducer having an escarpment pinion engaging the cogged ring.

9. A rotatable jack with incorporated stress means for a servo control system of an aerodyne in which movement of control surfaces of the aerodyne is effected by means of gear actuated by a. control lever and by a servo jack in series with the gear, the rotatable jack acting on the gear so that in opposition to the action of the servo jack the rotatable jack provides a resistance which is greater than that exerted on the servo jack by the control surfaces, said rotatable jack being substantially as hereinbefore described with reference to Figs. 2 to 8 of the accompanying drawings.

10. An aerodyne having a servo control system in which movement of control surfaces of the aerodyne is effected by means of gear actuated by a control lever and by a servo jack in series with the gear, and a rotatable jack as claimed in any preceding claim acting on the gear so that in opposition to the action of the servo jack the rotatable jack provides a resistance which is greater than that exerted on the servo jack by the control surfaces.