RAE Farnborough - steeped in history

[Bobthirtytwo]

BP Delta-Feel Problems-Dec '51

Sorry to have been out of commission for the past couple of weeks but here as promised the paper by Jock E. It shows the state of knowledge at that particular time and the problems which were being raised. It would be interesting if anyone would like to tease it out and maybe think about other ways in which the probs might have been approached at that time.

CONFIDENTIAL

AIRCRAFT "FEEL"

by

Lt. J. Elliot, A.F.C., R.N.

Introduction
There are many different types and makes of aircraft flying in the world today, but it is doubtful whether any two of them feel exactly the same to the pilot; some of these aircraft are pleasant to fly whilst others are anything but. With the increasing trend towards fitting aircraft with irreversible power operated controls where synthetic "feel" must be provided it would appear logical to attempt to produce a "feel" which is universally liked and to make the feel of all aircraft of a given class as near identical as possible, thus reducing the problems of pilot conversion and training when changing from one type to another.
Although there are in existence at the moment only a few aircraft with purely synthetic feel, no two of them feel alike whilst at least three of them are reputedly unpleasant to fly.
Before more synthetic feel systems are introduced it is felt that nothing can be lost by having an informal discussion on this important feature amongst those most qualified to speak on the subject, in the hope that some concrete facts for the guidance of designers concerned with systems of this kind may emerge.
1. General
With high speed aircraft it is thought that the actual feel of the rudder is relatively unimportant and this note is devoted entirely to elevators and ailerons .
1.1 Definition of Feel
When the pilot of an aircraft moves his control column the aircraft responds in some way or other and eventually assumes (it is hoped) a steady condition; at the same time (with aerodynamically balanced controls) stick forces may vary as the aircraft responds and as a result the stick and control surface position may alter. The aircraft responds and the changing stick forces are noted by the pilot and are correlated to give him an impression which he calls the "feel" of the aircraft.
1.2 Aircraft
There is obviously a difference in providing "feel" for a large aircraft with high inertia and good damping as opposed to providing feel for a small aircraft with low inertia and little damping, and both have their problems. For the large aircraft 'g' restrictors etc. may provide the answer whilst for small aircraft other devices may be essential. There are, however, several pilots who are highly qualified to speak of large aircraft whilst the author's experience has been gained almost entirely on a small aircraft and the remainder of this note will be mainly concerned with this type only.
1.3 Transient Response
Whilst a suitable stick force per 'g' etc. must obviously be aimed at, it is not entirely on these steady conditions that the pilot will judge the feel of the aircraft but to a large extent on the transient responses of the aircraft to stick movements which for several reasons may differ very noticeably from the conditions. To-day these transient responses vary from type to type and although two fighter aircraft may have identical values of stick forces per 'g' at a given speed it will be most unusual if the elevators feel the same to the pilot.
1.4 Control Damping
In addition to the variation in transient response between different aircraft, different controls and control systems give different inertia and damping loads to the stick, and these also have a share in the pilot's impresion of feel and anyone who has flown with a small stick movement/high force spring feel system will appreciate that it bears little resemblance to the feel of an aerodynmically balanced control and in fact is exactly like a spring self-centring device, which is not surprising since that is what it is. If displaced and released the stick will fly back into the no force position with a suddenness which is totally unlike the centring of a good aerodynamic control whilst the general sensation of moving the stick is somehow different.
1.5 Present System
It appears that, with a few exceptions, the designers of synthetic feel systems have up to date concentrated on steady aircraft conditions and have provided reasonable stick force per 'g' and stick force per rate of roll (although some have not even got this far - no names, no pack drill,) coupled in some cases with q or V variations, but the results have not been very satisfactory.
1.6 What is Pleasant
Although perhaps slightly irrelevant, it is felt to be worth a mention that, in the author's opinion, the most pleasant form of control to be found today is the light spring or servo tab type balanced in such a way that centralising the stick gives a certain amount of momentary control surface overshoot (until the aircraft adopts the new steady condition) thus giving a positive control over the aircraft both on initiating and in stopping a movement (the latter being a point of some importance which is invariably overlooked).
2. The Small Delta Aircraft
This section describes some of the main snags in the feel system of the B.P. Delta research aircraft.

This machine has large and potentially very poweful controls (rate of roll for full differential elevon movement - full stick - at 500 m.p.h. has been estimated as 500°/sec.).
2.1 Control System
The Boulton Paul P.111 has two large elevons operated by irreversible hydraulic power units controlled by the pilot's stick. Stick forces are provided by fixed trimmable springs, no force variation with air speed change being made.
The control surfaces themselves have 23% aerodynamic balance which enables the aircraft to be flown without the power units operating, whilst an automatic trimmer tab on each elevon should ensure that the machine is at all times approximately aerodynamically in trim.When flying in "manual" the automatic tabs can (if desired) be operated by hand and the aircraft trimmed normally.
Stick forces are :- with springs trimmed central: full aileron either way 15lb: full elevator either way 35lb.
The stick force build up is linear with stick displacement.
Up to date the performance of the actual power units has been extremely good and the slightest stick movement gives a corresponding movement of the control surface(s) whilst a backlash is almost entirely absent.
Trouble has been experienced with the spring force system and with the automatic trimmers.
2.2 Automatic Trimmers
Although not really a feel problem as such it was thought that the trouble experienced with the auto- trimmer was worth mentioning. These tabs are caused to operate by force sensing devices in the elevon operating rods and should normally keep stick forces (if power control fails) within reasonable limits. At above 250 knots A.S.I., however, it was found that the aircraft was so heavy that it could hardly be flown without causing the tabs to operate and that due to their high rate of movement considerable overshooting took place and in fact complete control was lost on one occasion for some considerable time. It is hoped to rectify this trouble by reducing the rate of operation of the tabs (4½°/sec. to ½°/sec).
The aircraft has been flown in manual at 400 knots A.S.I. with the automatic trim system switched off and the controls were found to be very heavy.
2.3 Spring Forces
At low A.S.I. the stick force for steady manoeuvre, i.e. stick force for rate of roll, are considered to be reasonable satisfactory. For normal flying at low speed, however, there has been a strong tendency towards overcorrections, these in some cases becoming out of phase with the aircraft response and proving dangerous.

It is thought that this overcorrection is due to the powerful controls and low inertia and damping of this aircraft with the lack of damping on stick movement.
Initially the spring system fitted was trimmed by normal hand wheels and a large amount of backlash was present between the stick and the springs, this meaning that the stick (and controls) could be moved slightly without any stick force. At lower A.S.I.(up to 250 knots) this was not too serious but at higher A.S.I. it became most unpleasant.
At a later date an improved spring system was fitted (trimmable to no force by electric actuators operated from a button on the stick) which initially had very little backlash and flight up to 500 knots A.S.I.was found to be satisfactory in this respect, although the tendency toward overcorrection was still very much in evidence. Due to the continual loading of the springs in flight, however, the backlash in this new system is on the increase.
At high A.S.I. since no "q" or other variation system is fitted, stick movements for aircraft response (with as a result stick forces) fall to a very low value and in fact the aircraft can be flown in the backlash region (no force) and the impression is gained that flying is achieved by slight bending as opposed to stick moving. The danger of this becomes very apparent if a sudden motion is required when the tendency to make a large control movement followed by the most appalling overcorrection has been noted.
(lb./g at 500 knots = 2 lb. Stick movement/g at 500 knots = ¼".
2.4 Impressions Gained
The following is a summary of the impressions gained regarding the feel of this aircraft: -
1. At low A.S.I. the aircraft is considered to be reasonably “flyable” but the feel
is unsatisfactory in that although forces for steady conditions are satisfactory the stick forces for transient responses appear to be too low and there is a tendency to overcorrection.
2. At high A.S.I. the aircraft is considered to be dangerous and unpleasant to fly due to the tremendous amount of control available and to the small stick movements and forces normally required, together with the tendency toward overcorrrection.
3. With control power off the aircraft is satisfactory at low A.S.I. but at high A.S.I. the elevons are very heavy.

(Part 2 will follow in a few days.)