The invention furthermore relates to a method of regulating the force which, in order to achieve a desired trim angle, must be applied to a control element for controlling an aircraft's control surfaces, the said control element being operatively connected to a mechanical control system and comprising, for example, a wheel, a control column or a pair of pedals, as specified in the pre-characterising clause of claim 6.

Providing smaller and medium-sized aircraft with mechanical control systems in which, in manual flying, only the muscle power of the pilot acts on the control surface via cables and steering system, is already known. The aim in this case is to design the control surfaces so that the aerodynamic and mechanical forces acting on the control surfaces in flight do not become so great that it becomes difficult for the pilot to alter the control surfaces position. Some common, known solutions involve the use of various types of auxiliary control surfaces, so-called tabs, in combination with aerodynamic balancing of the control surfaces in order to achieve manageable levels of force.

With the known solutions, however, problems still remain. For example, it may be difficult to optimise the dynamic connection between torque applied and column/wheel position and it may, in particular, be difficult for a pilot to act upon the control surfaces in a critical situation, for example when the stability is suddenly lost in a roll-trimmed position and the aircraft, as a result of this, rolls strongly in either direction, which can occur as a result of heavy icing.

One object of the present invention is to produce an arrangement of the type initially referred to, which gives the pilot servo-assistance in manual flying. The object is achieved by an arrangement having the characteristics specified in claim 1.

A further object of the present invention is to produce a method of the type initially referred to, by means of which servo-assistance is obtained in manual flying. The object is achieved by a method having the characteristics specified in claim 6.

Preferred embodiments of the arrangement also have any or some of the characteristics specified in the subordinate claims.

The arrangement according to the invention has several advantages: By means of this arrangement a linear power gradient can be obtained. A desired value can be assigned to the static power gradient. Desired values can be assigned to viscous damping and friction of the wheel. A desired magnitude can be assigned to the influence of the mass of the system on the control column/the wheel.

The positional feedback also offers the advantage that it is possible to influence the eigenfrequency of the system, so that this is not simply determined by the external load and moment of inertia of the system. Without positional feedback it is not possible to linearise any non-linear force in the basic mechanical system. Positional feedback means that a linear correlation can be obtained between the torque applied to the control column/the wheel and the trim angle.

The invention will be explained in more detail below with the aid of an example of embodiments of the present invention and with reference to the attached drawings in which: Fig. 1 shows a simplified general diagram of a servo-assisted control surface system according to the invention Fig 2 shows a simplified control technology description of the servo-assisted control surface system In describing all figures the same reference designations are used for identical or similar parts.

The simplified general diagram shown in figure 1 shows an aileron system, which on each wing comprises an aileron 1, which is connected by means of aileron cables 2 to a control element 3, in the form, for example, of a wheel. The aileron system comprises two control elements 3, one of which is acted upon manually by the pilot 4 when in flight. Somewhere along the aileron cables 2 there is a servomotor 5, which by means of a first change-over switch 12 can either be connected up (as shown) or alternatively disengaged. A servo-driver 6 is connected to the servomotor 5. Both a torque sensor 7 and an angle transmitter 8, which supply information to a control device 9, which also receives information from an angular velocity sensor 10 on the servomotor 5, are arranged at each control element 3 (shown only for one control element). Alternatively the angle transmitter 8 can be located on the servomotor 5 and/or the angular velocity sensor 10 at the control element. From this information the control device 9 generates a control signal for the desired influencing of the servomotor 5, the latter being designed to regulate the torque that the pilot 4 has to apply to the control element 3 so that the ratio between the torque applied and the trim angle of the control element assumes a desired, largely constant value irrespective of the trim angle of the control element. The servomotor 5 can thus work with and against the torque applied by the pilot 4. It will be seen from the figure that the servomotor may be the same as that used when flying with an automatic control/autopilot 11 and a second change-over switch 13 allows a choice between manual or autopilot control.

Figure 2 shows a simplified control technology description of a servo-assisted aileron system according to figure 1, in which the reference number 14 denotes the mechanical aileron system and the reference number 5 denotes the servomotor. In the figure the torque that occurs on the control element as a result of the force applied by the pilot is denoted by M. The wheel angular position, which in the main is proportional to the control surface angular position, is here represented by the feedback signal 6,,, and the wheel angular velocity by 40. J denotes the moment of inertia of the entire mechanical aileron system 14 transmitted to the wheel. In figure 2 the aileron system 14 is represented by a system of the second order with a spring constant KyL and a damping DyL, in which the spring constant KE9L and the damping DYL relate to the control surface shaft torque and both are relative to the wheel. A wheel angle acceleration is denoted as .

The servomotor 5 has a gain KS and emf-constant KE, which are proportional to the angular velocity 4,. In order to eliminate the effect of the servomotor's counter-emf and in order to compensate for variations in the resistance of the motor due, for example, to temperature fluctuations, a current-controlled motor is normally used. The servo-assisted aileron system also has closed-loop gains KM KL and KD, where KM represents a gain of the torque signal M, KL represents a gain of the feedback signal (5. and KD represents a gain of the signal The values of the closed-loop gains are calculated beforehand from the desired values of the spring constant kf, eigen frequency co0 and relative damping z of the entire servo-assisted aileron system, measured on the wheel, where From these equations the closed-loop gains KM, KL and KD are obtained as: The control signal u to the servomotor is thus described by the equation: KMM-(KL##+(KD+KE)##+##_trim)u=

the signal being described in more detail later.

In order to arrive at a simple description of the system it has been assumed that the mechanical aileron system 14 can be treated as a linear system of the second order. In reality the aileron system 14 is considerably more complex. In particular there is significant friction and play, which together with the elasticity in the control surface cables 2 contributes to the occurrence of local resonances with local frequencies. These characteristics limit the scope for free selection of the closed-loop gains KM KL and KD and hence also of the dynamic characteristics, that is free selection of kf, o and z.

Furthermore the signals M, 6", and which form input signals to the mechanical aileron system 14 through the servomotor 5, ought to be filtered so that their frequency interval and amplitude are such that the approximation of the aileron system as a linear system of the second order is valid. In order to achieve this all input signals M, 6, 0 and 40, have been low-pass filtered. In one example the cut-off frequency of the filter which filters the wheel angle signal 6,,, and the wheel angular velocity signal is 50 Hz, whilst the cut-off frequency for the filter that filters the torque signal is 15 Hz. For reasons of strength, it is advisable to filter the torque signal M through a filter designed, at high torque signal values, to produce an output signal that decreases as the torque signal increases. It is also advisable to filter the torque signal M through a filter with a dead zone, in which the torque signal is close to zero in order to produce a torque threshold, which must be surmounted before the influence on the servomotor 5 occurs. It is also advantageous to filter the torque signal through a phase-advanced filter. In one example the amplitude of the wheel angular velocity signal i, is limited by a limiter at high angular velocities.

The servomotor 5 functions more or less linearly. Maximum torque is obtained when the angular velocity is zero and the torque is decreasing linearly with increasing revolutions.

The ailerons 1 on most aircraft with purely mechanical control systems have so-called trimming tabs, the position of which is adjusted by means of so-called trimming jacks.

Normally only the trimming system on one wing is used, for example the left wing, whilst the trimming system on the other wing constitutes a reserve system. The trimming system functions so that in trimming the trimming jack acts on the angle of the trimming

tab, the aileron 1 being turned out by a certain angle. In the mechanical system this will mean that the zero position of the wheel, that is the trimming position that the wheel assumes when the torque applied by the pilot 4 is zero, will be adjusted.

When trimming, problems might occur with a servo-assisted system if no compensation is made for the new zero position. This problem can be eliminated by applying an angular adjustment term 5., i., corresponding to the trimmed zero position of the wheel, to the wheel angle feedback signal. The angular adjustment term can be created from signals from position sensors located on the trimming jacks. Allowance must be made, however, for the fact that the efficiency of the trimming tab is greater when moving downward from a neutral position into the high pressure under the wing, than when moving upwards into the low pressure on the upper side of the wing. If the trimming jack position sensor is arranged so that the signal from this is positive when the trimming tab moves downwards and negative when the tab moves upwards, allowance must also be made for this. Allowance must also be made, depending on whether just the ordinary trimming system is used, the reserve trimming system or a combination of these.

It will be obvious to a person skilled in the art that the invention is not confined to the embodiments described above but rather lends itself to modifications within the framework of the idea of the invention defined in the following claims. The arrangement may, for example, be constructed so that an autopilot servo located in the control system is used and/or so that the ratio between the force applied to the control element and the trim angle of the control element is increased as the aircraft speed increases, and vice versa, in order thereby to give the pilot a natural sensation of speed in the control surfaces. The system can furthermore naturally also be used on elevators and rudders.