BP Delta-Feel Problems-Dec '51 (Part 2)
3. Remedies
At low A.S.I. with control power on it appears that as well as being slightly overcontrolled the machine lacks some form of damping on control column movement and it is hoped, by decreasing the available control angle slightly for the same stick movement and by adding a damper to the stick, to make the aircraft pleasant at those speeds.
It is thought that if stick movements and forces experienced for a given aircraft response at low A.S.I. could be retained at high A.S.I. then the aircraft would be pleasant at all speeds.
In order to provide sufficient control angle for all conditions of flight with the stick movement available, the stick movements actually required at high airspeeds are very small (¼" per 'g' at 500 knots), thus accentuating any faults in the spring feel system. It is felt that the provision of an entirely satisfactory spring system to cope with these tiny movements would be almost impossible. Thus to provide variations of spring force with airspeed (q or V feel) does not entirely solve the problem.
It has long been contended that although pilots have to date flown chiefly withreference to stick forces, there is no reason why this should continue, and if stick movements for aircraft response are made large enough it seems reasonable to supposethat a pilotwould fly quite happily with no stick forces at all. (This has in fact been confirmed on two power operated aileron installations.)
The provision of sufficient stick movement to meet this requirement is not likely to be desireable in aircraft with anything but very limited uses but there is no reason why a half way measure should not be adopted by using light self-centring springs and altering the stick to control surface gearing as speed is varied, thus keeping not only stick forces for steady conditions the same but it is hoped keeping transient forces constant as well, thus making the feel of the aircraft constant throughout the speed range (if desired of course the aircraft can be heavied up slightly with speed or otherwise).
This idea of variable stick/control gearing is not a new one and was considered in some detail when the first high speed monoplanes were designed and has been employed to a certain extent in some aircraft by using a fixed variation, i.e. a change in gearing with stick position. This latter may still offer an answer for some aircraft but with the trim changes which may occur at high Mach number a continuously variable gearing would probably be preferable.
By use of variable gearing not only will the high speed case under power control be catered for but in manual the forces at high speed will be reduced also.
4. Variable Gearing
A rough outline of the system which is being considered for the B.P. Delta is given. (With the use of elevons the mechanics of the system are somewhat more complicated than when dealing with a split control system and for ease of explanation the system is outlined here with reference to a single control.)
Firstly the basic (present) stick/control surface gearing is to be reduced and a damper is to be fitted to the control column; the stick to control surface gearing is then to be continuously variable from a ratio of 1:1 (basic) to 4:1 ( i.e. full stick movement = full control movement, to full stick movement = ¼ control movement) whilst in addition the control datum position (position of control with stick central) is to be made variable as the pilot wills.
At low speed the aircraft will be flown almost as at present but as speed is increased the amount of control movement for a given stick movement will be decreased. This necessarily involves a reduction of the total control angle available byuse of the stick but byoperating the control datum trimmer any range of control angle can be selected at will.
No spring trimming will be fitted and at any speed the pilot will trim the stick central (no force) by altering the datum of the control.
In this aircraft it is proposed that the gearing will be altered manually by the pilot (although if satisfactory a "q" system may be fitted) whilst the datum trimmer will be operated by means of an electrical actuator controlled from a button on the control column.
One of the more attractive features of this scheme is that it caters for both "power" and "manual" conditions whilst should either the gearing column or the datumn trimmer fail it should be possible to fly the aircraft quite satisfactorily by use of the remaining system.
5. Summary
5.1 It appears evident that the provision of satisfactory "feel" for aircraft with irreversible power operated controls is a problem of some magnitude, particularly in the case of the small aircraft. Since the "feel" of an aircraft plays a large part in whether the machine is liked (and hence used to it's best advantage) it is considered essential that this problem be solved.
5.2 With few exceptions, existing systems are unsatisfactory; they do, on the whole, provide suitable stick forces for any steady aircraft confition but do not cater for the transient responses which contribute largely to the pilot's impression of the "feel".
5.3 It is perfectly obvious that for an aircraft with a large speed range some variation in stick movement must be adopted (q or V variation etc.). This has been amply proved in the Boulton Paul Delta where stick movements (and hence forces) required to manoeuvre the aircraft have fallen to such a low figure at high speed that the aircraft is unsafe to fly.
5.4 In the case of the B.P.Delta it has been found that not only is the machine unsafe to fly at high airspeeds but that the control column movements have become so tiny as to make the provision of a suitable spring force system almost a mechanicalimpossibility; it is,however, hoped to achieve good "feel" by first fitting a damper to the control column (to reduce the rate of control application and thus overcorrections) and then, byvarying the stick tocontrol surface gearing with speed, to keep the aircraft response to a given stick movement and force constant throughout the speed range.
5.5 For the large aircraft, control column movements may not become so small as to make the provision of spring forces difficult, whilst in addition the sheer inertia of the machine will help to damp reponse; variation of stick force alone plus perhaps a "g" restrictor system may prove perfectly satisfactory; even so the adoption of variable gearing for aircraft of this type has many attractive features.
5.6 In conclusion it is pointed out that, except by accident, good aircraft "feel" will only be achieve if the pilot can express intelligibly to the technician exactly what he wants, for this reason the responsibility for the "feel" of future aircraft, be it good or bad, rests squarely on the shoulders of the pilots of today.
CONFIDENTIAL
(Delivered by Jock at RAE. 14/12/51)