Posted 12 July 2011 - 21:02
THE INFLUENCE OF MILITARY AND CIVIL REQUIREMENTS ON THE FLYING QUALITIES OF AEROPLANES
By SQUADRON LEADER R. M. HILL, M.C., A.F.C.,
OF THE EXPERIMENTAL SECTION, R.A.E.
Presented by THE DIRECTOR OF RESEARCH.
Reports and Memoranda, No. 678. June 1920.
(1) SUMMARY.-In §§ (2) and (3) is discussed the relation of the pilot's senses to the behaviour of an aeroplane in flight. The reason for this discussion is that the consciousness of the flying qualities had to be developed and their nature realised before a coherent idea could be formed of them.
§§ (3) and (4) deal with these qualities as a definite set of ideas, and the influence of military and civil flying requirements on them is discussed. § (3) is based on past experience. § (4) is an application of this experience to problems of the present and future. Finally, suggestions are made giving the writer's view of the most important directions in which experiments should be carried on.
(2) The Nature of the Flying Qualities of an Aeroplane.-It is difficult as yet to express "manoeuvrability" and "stability" quantitatively, or to define the inter-reaction of the personal factor with control. Of what certain aeroplanes feel like there is a prevailing conception that is used as a criterion by which definitions of these elusive properties are made. It is a conception moulded from a blend of qualities, various in kind and in degree, the result of the designer's attempt to give the pilot the most liberal scope for handling the aeroplane by means of his senses of sight and touch under all conditions. When the qualities which underlie the conception come to be analysed a multitude of obscurities is found; the attempt to investigate, define and classify them is involving years of research.
For a long time it was left to the pilot to translate his conception of the feel of an aeroplane into terms generally intelligible; his description was coloured by his own predilections and as such was inimical to scientific handling. More recently, however, attempts have been made by means of recording instruments, or, failing these, accurate scientific observations, to apply a quantitative analysis in the light of which the conception can be examined. It is hoped that obscure differences of opinion among pilots may be made intelligible, and a large mass of evidence accumulated which should relate more closely the practice of flying as an art to the theory of aerodynamics.
By a process of evolution this point of view has developed Itself. In the early days of ~flying there must have come a growing consciousness of specific flying qualities; once they were appreciated, came the desirable to improve those which furthered practical objects. In. short, flying, once a practical proposition, was shaped bytwo main considerations, military and civil.
(3) The Realisation of the Flying Qualities. - During the Infancy of mechanical flight the sheer hazard of remaining aloft must have to obsessed the pilot that he had little opportunity for speculation about the flying qualities of his aeroplane, much less about what was desirable and its attainment. So long as he was on an even keel and in no imminent danger of losing control he was satisfied. Mechanical flight was in itself the end. Not until it was applied to some purpose beyond the mere demonstration of its existence were qualities sought particularly favourable to any purpose.
The demand that aeroplanes should fulfil certain conditions started to breed particular flying qualitiesl but to bear any meaning, or for their need to be brought home to him, these flying qualities had actually first to be experienced by the pilot - as It were by chance. He had to do a vertical bank before he knew what he desired of a rudder. His senses were the only medium for the expression of the qualities; their attainment was neither possible to any extent by scientific prediction, nor by accumulated experience, since there was none. Although the pilot must have realised vaguely that certain qualities were desirable they were as yet outside the range of his experience.
Early aeroplanes, such as the Wright biplane, must have required an equilibrist of the first order to fly them. Their qualities as flying machines must have taxed human skill to the uttermost in satisfying the elementary requirements of flight; in other words, while Just rendering the maintenance of equilibrium possible, they were bad.
At the outbreak of war aeroplanes were beginning to be used for commercial purposes, and a small amount of military development was in progress. A Cody biplane was not more favourable to military requirements than a Grahame White. The influence of practical requirements on their flying qualities had not as yet been very marked.
(4) The Influence of Military Requirements on Flying Qualities. - As soon as the pilot found himself on service the consideration, of taking off, maintaining equilibrium, manoeuvring and landing, that very consideration which had so sharply focussed his attention vanished from his mind. He had a machine which would take him into the air and give him a striking position or a view of enemy country, and the realisation of battle was far more impressive' than that of the flying risk. That the disappearance of preoccupation with the flying risk should make room for a truer appreciation of flying qualities seems strange, yet for the first time the pilot could review the qualities of his aeroplane in a detached mood, for his attitude was completely altered.
Military requirements changed so rapidly that those who flew found it difficult to express coherent requests for improvement, but small improvements yielded apparently disproportionate results, due to the operation of the human element. Although the limitation of the normal human senses is a fixed coefficient, the increasing efficiency of the mechanical contrivances gave the pilot behind the machine increased confidence; not in direct proportion to their increase in mechanical efficiency, but in proportion to that increase multiplied by the confidence it bred. A trimming tailplane may make all the difference in the world. Fresh departures in design introduced new tactics, and in those tactics was born the stimulus to new design.
Improvement would have been still more rapid had not a steady flow of production to be maintained, a break in which would have meant disaster at the front. If the requirements were diverse and subject to rapid change, at least some of the broad issues were clear.
Performance.- High performance compensates for many deficiencies in other respects and ranks first of the qualities in the confidence it gives. Many fighting aeroplanes, after being flown on service for some time with certain engines, were fitted with more powerful ones. In most cases the increase of power made them pleasanter to fly, and added a feeling of security in the air. Up to a point the controls were rendered more effective without an excessive increase in weight, but beyond this point the increased power simply made the controls heavier and the added weight discounted the value to the pilot of their enhanced effectiveness. The 110 h.p. Morane monoplane, the 90h.p. DH.6 and the 150 h.p. B.R.l Camel were generally considered an improvement over the same aeroplanes fitted with 80 h.p., 70 h.p., and 110 h.p. engines respectively; but when the 200 h.p. Arab Bristol Scout F (which serves as an example though it never saw service) was fitted with a 300 h.p. Cosmos Mercury, the weight on the controls was increased out of all proportion.
On service, performance is the prime means of attaining the effective striking position; it is the life of the fighting aeroplane, the capacity to manoeuvre, the power to outdistance an opponent, the extra two or three miles an hour which is everything to those who have experienced it. It gives height quickly and the possibility of very high speed in bursts that aerial combat requires. It should always be borne in mind how much lower is the ceiling of an average formation of aeroplanes than that of a single unit.
View -Its psychological effect on the pilot makes this quality most important for military purposes. In an aeroplane designed for every requirement of war if such a one can be imagined, the pilot wants the maximum view generally; because it inspires confidence, and particularly because It enables him to keep his opponent in sight. If the pilot feels boxed in with a sense of lurking danger his effective handling of the aeroplane is destroyed. The Fokker monoplane pilots, because they could only see below them in diving position made a practice of diving once only, and If the opponent was missed went right down out of range. Fine manoeuvrability is useless if unrelated to an opponent which the pilot can see.
View is attained by the position of the pilot in the fuselage relative to the wings and tail, by the absence of unnecessary obstruction caused by the fairing necessary to protect him and reduce resistance, and enhanced by manoeuvrability whereby he can quickly and adjust his arcs of vision. Manoeuvrability and view are thus closely interdependent; one is used by the pilot for the consecutive elimination of his blind arcs, the other to manoeuvre to good effect based on his increased arcs of vision. On an aeroplane with bad ,view, owing to his acute sense of ignorance of his opponent’s position, the pilot cannot manoeuvre effectively either to see better or to fight more vigorously. If he can see well initially he can use his manoeuvrability to advantage in both these ways.
© Manoeuvrability.-An aeroplane may be utterly dependent on the pilot for its control, such as the Spad and Sopwith Camel; or if trimmed correctly it may maintain a normal attitude of flight with the pilot's hands off the controls and possibly his feet, such as the S.E.5A. This, for which in practice the pilot has usually had to pay by a certain loss of manoeuvrability, should in the ideal aeroplane involve no such penalty. The Martynside F.4 is a big advance in the right direction.
Assuming his opponent to be flying an aeroplane of equal performance, the pilot can gain a favourable striking position only by out-manoeuvring him. The real fighting pilot manoeuvring for position is not often showy, but every turn is the result of exquisite judgment and foresight. To shoot effectlvely he must have a responsive but steady gun platform. Finally, If hi engine is crippled he must have manoeuvrability to make himself a difficult target.
Stability. - Suppose, even in a single seater scout the pilot wants to rectify a gun stoppage or look at maps, he will find his task an extremely difficult one if the moment he releases the longitudinal and lateral control the aeroplane nose dives or stalls. The ideal aeroplane which is to carry out the whole range of military duties should undoubtedly be stable. In such an aeroplane the pilot may require to examine detailed maps and compare them with the ground, adjust bombsights, care for his guns, work a wireless key, apart from concentration of the aeroplane instruments. If the pilot loses touch with the horizon by moving his head about, looking at objects inside the aeroplane or on the ground, or at aeroplanes above, and his aeroplane is unstable, it immediately runs away with him.
The late Captain Ball used to take advantage of this in breaking up hostile formations. Each pilot in the formation would be keeping station with his eyes fixed on the leader, using at the same time his sense of the horizon. Captain Ball found that by creeping above the formation unobserved and firing a few shots he could make the pilots look up instinctively, lose concentration of control and open out. These Albatross and Halberstadt Scouts were almost certainly unstable.
Loading. - This undoubtedly influences the control of an aeroplane. In general, a heavily loaded aeroplane is seldom "flicky" on the controls, if balanced elevators with no tailplanes are excluded. Heavy loading always makes landing more precarious, but within limits this is not serious in an aeroplane for war purposes. It appears to contribute in some measure towards a steady gun platform, in the Spad and Nieuport Scout, for example.
Steadiness.- There is a certain quality which is differentiated in the pilot's mind from stability, and which for lack of a better word may be called" steadiness." It is usually the result of large dimensions, and where the aeroplane is small is influenced by loading.
Comfort.- The comfort. of the pilot influences his whole appreciation of the flying qualities of the aeroplane. During the war the pilot's comfort often had to give place to considerations such as the complete accessibility of the guns and the convenience and position of the ammunition drums.
The qualities of a fighting aeroplane most obvious to the pilot have now been stated, but only incidental mention has been made of their interdependence. In actual fact they are very closely associated, and if a quality is taken, as it were, out of its context, it is frequently misappreciated. Some instances of this interdependence may now be mentioned.
Manoeuvrability, the kinds of responsiveness concurrent with light and heavy loading, stability and steadiness are flying qualities; view and comfort influence the pilot's appreciation of' them.
Provided that the aeroplane is suitable to the engine, the pilot of' a high performance aeroplane is the gainer in everything. Compare, for instance, the 110 h.p. Nieuport Scout with the 80 h.p. Morane Scout, the S.E.5A. with the Sopwith Camel. The small dimensions of the Spad assisted its view and manoeuvrability, but for military purposes, such as bombing, a certain steadiness as apart from stability is required. This steadiness usually involves large dimensions. It can be present without, though it is often assisted by, stability. The load and fuel for bombing and long distance reconnaissance again involve large dimensions. For single seater and small two seater fighting aeroplanes manoeuvrability is essential. Small dimensions run parallel to manoeuvrability, and of the possible results good view is a great asset, while heavy loading usually improves the gun platform. The popularity of positive stability has been in the inverse ratio of its adverse effect on manoeuvrability.
The attempt to solve the problem of these requirements naturally resulted in many different types of aeroplane. Although difficult to embody flying qualities universally favourable in one type, it was found that where one quality was sacrificed a compensation of a different kind was gained. For instance, where manoeuvrability was lost owing to large dimensions, more guns could be carried for the same reason. Thus, in some cases, a fairly effective compromise was made; in others, however, certain qualities were definitely sacrificed.
The above remarks may be better illustrated if they are applied to a definite military type - for instance, the single seater fighting scout. A scout pilot requires the following :-a steady and responsive gun platform, and a highly manoeuvrable aeroplane, which will allow him to leave the controls to look after his guns without running away with him.
Assuming the striking position to have been gained by performance, the requirements of the platform and the flying qualities which make or mar as manoeuvrability, loading and stability, may now be examined.
The ideal aerial gun for a single seater scout should be susceptible to sensitive intentional movements, without appreciable time-lag. If rigid it is easy to keep the sight on the target, but not to bring there. It must, however, respond evenly to the pilot's hand, not so as to disturb his aim. In short, the pilot wants to nose of the aeroplane with one of his limbs, instead of to regard it as a separate agent, to be forced, coaxed, or juggled into the correct position.
Take the essence of manoeuvrability. The aeroplane must respond easily to rapid course movements, while resisting sufficiently for the pilot to feel it; it must be readily swung through 360º in almost any plane, and yet be delicate enough for the sight of the fixed guns to be registered. If "flickiness" which gives the impression of extreme manoeuvrability, is present in a high degree, then, especially at high speeds, the aeroplane will not be a steady gun platform. If its converse "stiffness" which gives the impression of extreme steadiness, is present, then at similar speed the aeroplane will not be a responsive gun platform. The gun platform must be responsive without being "flicky" and steady without being "stiff." It should not be necessary either to sacrifice the best qualities of manoeuvrability, if this "flickiness" or "superliveliness" in favour with so many pilots, can be dispensed with, or to impair them by "stiffness" in the guise of steadiness.
The steady gun platform should rather be enhanced than adversely affected by a kind of manoeuvrability which is at once an asset to the man in his ro1e of gunner and pilot.
It seems that "flickiness" is to some extent a function of loading, and "stiffness" of stability; that responsiveness is independent of loading, but obviously affected by "stiffness." Take the Sopwith Pup, which is about neutral with elevators fixed, and the Spad, which is just unstable with elevators fixed; the former lightly loaded, the latter heavily. The Spad goes into the dive at a touch, yet settles quickly at any desired speed and is a steady gun platform, whereas the Pup is inclined to be "flicky" and to hunt. Both are beautifully responsive.
In my experience heavily loaded scouts are rarely" flicky" and often very responsive. It may be for this reason. A lightly loaded aeroplane flies at a smaller angle of incidence (and consequently nearer the angle of no lift) than a heavily loaded one, and a small intentional change of incidence on the former may have a much greater effect on the actual loading of the wings in flight than an identical change on the latter, which at the same speed flies at a greater angle of incidence. Thus the lightly loaded aeroplane feels less steady than the heavily loaded one. Again, an aeroplane behaves so differently at its various speeds and angles of incidence, and may make a good gun platform at some and a bad one at others. The times when steady shooting is most vital are during a dive and a zoom. Heavy loading is advantageous in every way except a zoom, where light loading gives a lower stalling speed. When considering the relative weight on the controls in diving and zooming, the increase of weight due to high air-speed must always be allowed for.
Over the effect of stability on manoeuvrability a controversy raged fiercely and passed through various phases. The early scouts were eminently unstable; those with fixed tailplanes, longitudinally unstable elevators free; those with no tailplanes and balanced elevators, violently unstable elevators free, and some stable with them fixed (Halberstadt), some unstable (Morane).
There was a reaction against the first unstable types, and a highly stable type was produced. Immediately a school arose which preached against a high degree of stability and maintained that really good manoeuvrability was spoilt by it. Yet if an aeroplane is to look after itself at all it must possess a certain amount of longitudinal and lateral stability, the latter involving all the questions of the rudder and fin. The fighting pilot, just as physical comfort in an aeroplane gives him a feeling of security and enhances his powers of endurance, needs assistance from the aeroplane rather than interference in his many duties, more especially If his physical reserve power is diminished owing to altitude. He wants assistance both when his hands are on the controls and when they are removed.
It is quite clear that extremes of stability should be avoided. The highly unstable aeroplane makes a big call on the pilot's energy the whole time, runs away with him if he leaves the controls, and renders him liable in the course of violent manoeuvring to get into extremely awkward positions in which he is very vulnerable.
The highly stable aeroplane, on the other hand, while it will fly itself at most speeds with a trimming tail, is usually less manoeuvrable, stiff and unresponsive in a dive, and generally more unwieldy.
Take the Sopwith Camel and the S.E.5A., two conceptions diametrically opposed. The S.E.5A is stable with.elevators free, the Camel unstable with them fixed. The Camel is more lightly loaded than the S.E.5A., and has, with the exception of the rudder, more powerful controls. In a dive the Camel is"flicky". probably due to its lighter loading and excessive longitudinal instability; the S.E.5A.is very steady, but dull to small intentional movements. In a zoom the Camel improves greatly owing to its lighter loading and instability; the S.E.5A. is inclined to become languid, and its stability near stalling speed draws down the nose, so that a large backward movement of the stick has to be made.
In flying a Camel, the pilot has always to be making small movements of the controls in the endeavour to pick up a steady speed, which it is difficult to maintain. Going into a dive, compared with the S.E.5A., the elevators of the Camel work the reverse way. The control stick, though initially pushed forward, has to be pulled right in again, while the S.E.5A. If the stick is pushed forward, will drop its nose, creep up to its trimming speed and stay there. The S.E.5A. is impossible to fly inverted; the Camel may remain so unintentionally.
The above remarks show up some great advantages of positive stability. At the same time there is no doubt that the conception of the Camel as a fighting aeroplane made an irresistible appeal to a certain class of pilot. The effectiveness of a fighting aeroplane must be taken as the product of its flying qualities and the confidence they inspire in the pilot. Considering its instability with all the attendant disadvantages, the Camel was a surprisingly good war aeroplane; but it never could be comparable with the S.E.5A. because, even assuming that it possessed certain qualities that were equal or even superior, the difference in performance from 7 to 10 m.p.h. at 15,000 feet - and in view, gave the S.E.5A. a Superiority that only flghting pilots really appreciated.
(5) The Influence of civil requirements on Flying qualities. The order in which the flying qualities of military aeroplanes were taken was intended as far as possible to bear reference to their importance to the pilot. They were so closely interdependent that in a given aeroplane the lack of a flying quality, insignificant in itself, might seriously detract from the value of a vital one. If; then, allowance is made for this, the qualities of civil aeroplanes will be treated in a similar way. In the following section the flying qualities discussed will be those for commercial aeroplanes, as that is assumed to be by far their most important civil use. The small single seater sporting type will be ignored.
If commercial aeroplanes are to compete successfully with other forms of transport, they must compete on grounds of speed, economy and reliability, but such an achievement will not be of the slightest value until a standard of safety nearer to that reached by the railway and steamship is attained. There is still a tendency to set up a false standard of safety; a cross-country flight is safe If thought of in terms of flying, but is it yet so compared with the present means of transport ? The commercial aeroplane must transport its freight of passengers, mails or cargo in a shorter time, including the time of collection from and distribution to business centres, than could be done in any other way; the service must pay and must attract and retain the confidence of the commercial and travelling world. Safety in war was almost synonymous with striking power; safety in peace is absolute. Every quality, every line of research, every effort at improvement should lead in this one direction. Just as the pilot's attitude changed when he went on service, now it must again revert, this time with a great fund of experience behind it.
The most pressing difficulties of the moment seem to be those of flying to a place and landing when there is mist down to the ground, of the comparative unreliability of the light aero engine, of the space which any aeroplane requires to land in, and of the Imperfect control of small aeroplanes at low speeds and large ones at any speed.
Safety.-Any quality that is observed after the engine or engines have been once started will be regarded as a flying quality. A ship, even before it gets under way, is already supported in its element; an aeroplane is not, so there are qualities desirable other than these confined essentially to the air. It is necessary to consider the aeroplane under two general sets of conditions; firstly, in flight, and secondly, leaving the ground, landing, and the subsequent run. The importance of the second set is frequently under-estimated, although from it probably come the greatest losses from a commercial point of view.
Even if the aeroplane's power plant were made as reliable as that of any other form of transport, the aeroplane would be incomparably worse off. The engines of a ship stop and there is a considerable period before any danger need usually be anticipated; the stoppage of an aeroplane's engines implies great danger from the absolute safety point of view. So to compete on a similar basis, the motive power of the aeroplane should theoretically be made more reliable than any others. The maintenance of an all-weather service increases the probability of crashes due to forced landings. It is not sufficient not to risk a crash oftener than every six months; humanly speaking, crashes must be avoided.
This report is not concerned with the development of the power plant itself, but it should be noted that the measure of its reliability is the equivalent to the pilot of mental freedom; and while he has anything to do with the control of it in flight the mechanism must be simple and easily understood. It seems that for some time the pilot will at least control the throttles of his engines; the time is not yet near when he can do so by telegraph. It has been suggested that his case is parallel to that of the submarine commander who has to make rapid use of his engines in shallow water; but no machinery-short of his own hand gives the pilot confidence when he himself is "in shallow water." The effect of his engines is so bound up with the effect of his aeroplane controls that any lag in response, even if small, would utterly upset his mental concentration.
But he could, with advantage, be relieved of many controls connected with the engines and fuel supply which are now a source rather of embarrassment than assistance to him. Even in fairly large aeroplanes an engineer is not usually considered worth his weight, although on a modern twin engined aeroplane the pilot has really too much to look after as well as fly.
The pilot may be almost up to the limit of his achievement in simultaneous concentration on the various elements of his control mechanism, and in exertion of physical energy in flying. Little more should be expected of him than he now gives in controlling a large aeroplane; indeed, every effort should be made, as the aeroplane grows larger and more complicated, to leave him freer both mentally and physically. That is what can be done for the pilot in the direction of safety.
For commercial use, the single-engined type of aeroplane similar to the D.H.9A in type may well persist, owing to its general handiness and straightforward control in the air.
It is probable/that there will be a more definite adherence to the multi-engined aeroplane of the Handley Page 0/400 and V /1500 types, which will bring them into common use. It is certain that larger aeroplanes still will be tried, bringing with them acute problems of control. There is something attractive from the commercial point of view in the giant type, and in order to make it a really safe proposition a long process of experiment will be necessary. Pilots have coped with most of the difficulties of control on the smaller aeroplanes because the control forces involved have been comparatively small. Similar difficulties in larger aeroplanes are liable to be under-estimated, because the effect on the pilot of the greatly increased forces is not fully enough appreciated. On a small aeroplane the pilot thinks nothing of suddenly opening the throttle with the tail adjusted for gliding; on a larger aeroplane variation of engine power without appropriate tail adjustment may introduce extremely large forces on the pilot's hand.
Apart from fire in the air, the risk of which is steadily diminishing, the phases of flying which embrace most of the sources of danger are those of leaving, flying near and approaching the ground; in the first and last cases the speed is frequently near to stalling speed, and in all three there is little time to rectify an error in control. Given enough height a pilot should be able to overcome almost any difficulty of control. At a safe altitude the sense of having plenty of time to overcome difficulties assists him to act calmly, which is nine-tenths of the battle. A pilot is exceptional if he has the coolness to hold the control stick forward steadily while attempting to recover from a spin very near the ground, although he knows that it is the only means of saving himself. The instinct to forget that he is stalled, to use his elevators as if this were not so, overcomes his reason, and precipitates many crashes that might possibly have been avoided at the last moment. -When just about to hit the ground in a spin there is sometimes a strange unaccountable sense of the controls being jammed, of which I have heard from others and had personal experience. This all goes to show that the pilot cannot be treated in any sense as a machine and that the instinct to use the controls in a direct way, when the situation demands the use of an indirect one, is powerful under the influence of emotional stress.
If the pilot of a large aeroplane, with a complicated control due to the effect of variously disposed thrust axes, were asked to control it without being given the feel of the controls, his success would depend rather on a display of his own skill than on the amenities of such control. It is not the feel that leads him to make mistakes with the controls, but a false instinct near the ground which operates against his better judgment; to the pilot the feel is one of his most valuable means of maintaining equilibrium. It is essential for some considerable time to come that he should be allowed his sense of touch as well as of sight to control the aeroplane. But feel is not enough; the pilot must have his sense of horizon or certain instruments to make threedimensional control possible.
It has now been proved that aeroplanes can be flown perfectly in continuous cloud or mist by the use of the turn-indicator, bubble, airspeed and compass, so long as the speed is kept, say, ten to fifteen miles an hour above stalling speed; but the decrease in power of the control, with the correspondingly rapid onset of large disturbing forces at speeds approaching stalling, at once make the aeroplane feel capricious. The use of instruments becomes increasingly difficult, and it is at these speeds that an aeroplane leaves or approaches the ground. Supposing that a device that would automatically land the aeroplane on reasonably smooth ground had been perfected, and that by some n1eans provision had been made, such as a system of captive balloons, to convey to a pilot approaching the position of the aerodrome that there were no obstructions within a reasonable distance, the control at low speeds would have to be greatly improved before he could, with confidence, manoeuvre the aeroplane to land it.
It is assumed that all commercial aircraft will fly above cloud and mist up to heights at least of 10,000 ft. There are, however, very few days on which the clouds are not in layers, so that it should be possible to observe the balloons at a reasonably low altitude. In short, it does not seem impossible that a pilot could be brought by means of wireless to the approximate position of approach to the landing ground obscured by mist or very low cloud, that the balloons could be arranged so as to give him its exact location, that he could set the aeroplane to a steady glide at a certain distance from the balloons according to their height, and that a device fixed to the aeroplane could flatten it out on to the ground in safety. It is even conceivable that in time this sequence of operations might be brought to a reasonable pitch of safety, though it is beyond doubt that the first experiments aiming at the solution of this difficult problem would not be free from danger.
Stability.-Commercial aircraft will have to be stable. It is practically impossible, without a great expenditure of the pilot's energy, to fly an unstable aeroplane through continuous cloud. The larger the aeroplane the worse it is for the pilot. The great achievements of the Vickers Vimy seem to detract from the positive value of stability. When trimmed to fly reasonably, it is only just stable at low speeds, and unstable throughout .the rest of the normal flying range. It can only be said that its achievements have been in spite of this handicap.
In dealing with military aircraft emphasis was laid on the "stiffness" arising from extreme positive stability, which was directly opposed to responsiveness for the gun platform. If the degree of stability required for commercial aircraft results in a loss of responsiveness, the loss should not be so important, firstly, because a lower degree of responsiveness will serve for navigation, and, secondly, the larger aeroplane is naturally more sluggish, so that the effect of stability on responsiveness is less marked. Extremes of stability in large aeroplanes result not so much in lack of responsiveness as in the appearance of large forces on the control stick; in the case of positive stability in the direction of a return of the trimming speed, in the case of negative stability of a divergence. The Handley Page 0/400 would only feel nose heavy in a dive if it were badly out of trim; a Vickers Vimy feels so because it is trying to diverge from its unstable trimming speed. At 120 m.p.h. this force is so considerable as to cause the pilot great exertion in pulling the nose of the aeroplane up.
All commercial aeroplanes should be at least as stable laterally as the S.E.5A. With engine full on and rudder held to fly straight, the aeroplane describes a lateral oscillation which appears to continue indefinitely without increasing or decreasing. With propeller stopped and rudder free, a similar oscillation occurs, seemingly in tune with the logitudinal period. This amount of stability in a larger and steadier aeroplane would give the pilot a, very fair basis for navigation, but if it were increased to any extent it would probably render the aeroplane too stiff for sensitive control. The instability in this respect of the Rolls Bristol Fighter makes it extremely difficult to keep an accurate compass course even when not in cloud. Some examples of the Puma Bristol are really bad, and the nose will swing to either side if continual checking movements of the rudder are not made.
With a form of tail adjustment which can be used quickly an aeroplane which possesses a considerable amount or longitudinal stability should be responsive enough for commercial flying requirements. The various factors which affect longitudinal stability, such as the C.G. position and the relative size of the tail, have long been the subject of close mathematical investigation, but judging by present practice insufficient application of this work has been made In the analysis of certain characteristics in which the pilot is vitally interested. Some aeroplanes are extremely sensitive to changes in the position of the load, others can be loaded up in the most extraordinary way and yet can be quite comfortable to fly. The Avro training biplane and the Handley Page 0/400 are examples of the latter; the R.E.8 and Vickers Vimy of the former.
Highly unstable aeroplanes like the early D.H.6 and the Sopwith Camel were usually trimmed to have an unstable trimming speed (no force on the stick) somewhere near their comfortable cruising speed, engine at half throttle. - With engine full on the unstable trimming speed was increased and they felt tail heavy throughout the greater part of their normal speed range; with engine off the unstable trimming speed was lowered and they felt nose heavy throughout the greater part of their range. At the same time in a general way instability decreased towards stalling and increased at high speeds. If they were not trimmed so as to feel tail heavy engine full on over the lower part of their range, there was a large force to hold them up in a dive. The Sopwith Dolphin and the Sopwith Dragon normally trimmed, seem to have an unstable trimming speed with full engine at about 90 m.p.h. If the factors affecting stability are changed to make an aeroplane less unstable one of two things seems to happen; either it becomes in trim at nearly all of its speeds - a characteristic much appreciated by the pilot - or it becomes stable at low speeds and actually shows a stable trimming speed, while retaining the characteristics of great instability at high speeds, showing at some point an unstable trimming speed. The flying feel at speeds between the stable and unstable trimming speeds is of extreme interest, for the aeroplane hesitates as if it could not make up its mind what speed in this range it should settle at. This is a characteristic of the Vickers Vimy. If an aeroplane is made more stable still, it wants to return violently to its trimming speed, and considerable force comes on to the control stick. By the position in the speed range of its trimming speed a pilot says it is nose or tail heavy. The operation of this force is intelligible to him, but where two trimming speeds of an opposite character occur in the normal range, his justification can well be understood. He cannot then express nose or tail heaviness in simple terms.
Between the stable and unstable aeroplane at and near the stalling speed there is a great difference in behaviour which affects ease of landing. The stable aeroplane tries to put its nose down violently, and necessitates a vigorous upward movenent of the elevators at stalling. Unless the upward movement is correct in time and amount, the tailplane bumps badly on the ground. The unstable
aeroplane only needs a smaller and more deliberate upward movement. The amount of longitudinal stability should not be so great as to make landing really difficult.
There seem to be certain harmonious combinations of the factors mentioned above which should produce a pleasant amount of longitudinal stability without much force on the control stick at any speed, which should eliminate undue sensitiveness to small changes of C.G., and allow fully enough responsiveness for commercial flying.
Manoeuvrability. - In commercial flying, beyond a relatively low degree of responsiveness for other purposes, manoeuvrability is scarcely wanted by the pilot except to ensure safety. Although it obviously takes more time to carry out a given manoeuvre on a large aeroplane than on a small one, what must be ensured is a normal response to the controls. Time taken by the aeroplane to assume a desired bank is very different to time lag between the pilot's intention to bank, i.e., his initial movement of the controls, and the actual commencement of the banking. Whatever the size of the aeroplane, the pilot must be confident that it will 'answer at once, although it may take a considerable time to complete the manoeuvre. For instance, a severe drift to starboard near the ground may be neutralised by a sideslip to port. If the aeroplane is sluggish in answering to the controls put over to sideslip, the drift may have assumed such proportions as to make it impossible to sideslip the other way. If, on the other hand, it answers immediately, the drift can be kept under control.
The average single-engined aeroplane is fairly satisfactory on the controls except at low speeds, and the control forces are not usually such as to embarrass the pilot. The present twin and four-engined aeroplanes are less satisfactory, mainly because the control forces are relatively larger, and also because of the introduction of more than one thrust axis. It would be a large step beyond this to ensure that the resultant thrust from more than one propeller passed always through one point, and thus simplified the aeroplane control. Therefore, apart from the necessity of improving the control of any aeroplane near stalling, it will be vital to investigate every means of keeping the control forces on the pilot's hand as small as possible consistent with adequate control surfaces.
One method, that of sub-dividing control surfaces, such as a rudder, into several rudders of smaller chord, has met with considerable success; for, although balanced control surfaces may be fairly safe on smaller aeroplanes if the proportion of balance is reasonable, they are not so on large ones. Though normally they feel light and easy, near stalling they may feel very heavy. Multiple unbalanced surfaces on a large aeroplane do not normally feel so light as an equivalent single balanced one, but they do feel normal at all speeds.
Another method is to employ some form of relay control to reduce the control forces while retaining the feel. A large aeroplane can never feel exactly like a small one because it is bound to move more slowly, but could the pilot be left with a certain proportion of the force on the controls he would be given some indication of what was happening: In the relay controls tested on the ailerons of a Handley Page 0/400 at the R.A.E. a step has been made towards the controlling of big aeroplanes, and at speeds considerably above stalling these controls might be employed with safety. But, although they greatly reduce the forces, the relation of their feel to that of the ordinary control is not close enough for a pilot to use them with confidence in taking off or landing. They are in the stage where they could well be used when at a height to reduce the fatigue of controlling an aeroplane that could be safely taken off and landed Without them, but not on an aeroplane which, in being taken off, might require more force than the pilot could safely be asked to exert. They do not introduce the violent caprice of a highly balanced control surface, but where they are to be relied on utterly any abnormality on their part is unsafe. When the Handley Page 0/400 was put on a bank with the relay control in gear, and the control stick put back to normal, there was a tendency to continue to bank even with the best type of relay control. That is, there was a tendency to overshoot in each case the normal movement of the controls, for which the pilot had to make an allowance. It will be seen that combined with the behaviour of the longitudinal control of a. stable aeroplane when getting off or landing, the effect of relay controls in their present state would be too much for the pilot, even though the forces were not large. The difficulties experienced with these relay controls should not be insurmountable if they are experimented with sufficiently, and in my opinion a limit to the size of the aeroplane has been reached even judged by, the present standard of safety in control, if relay controls are not developed.
Especially in considering the control of large aeroplanes, the set of conditions obtaining when the aeroplane is running over the ground at less than stalling speed and is only partially airborne is most important. An aeroplane may be made to balance and to be controlled safely in the air but may have an abnormally high C.G. with regard to the wheels. Such a structure is top-heavy on the ground and will occasion the pilot difficulty in getting it off. To ensure safety under these conditions the C.G. and thrust axes should be kept as low as possible consistent with air requirements. On a twin-engined aeroplane like the D.H.IO a considerable tipping tendency is felt when the engines are first opened out and before the tail plane and elevators become effective. Even if the engines are opened out with reasonable deliberation, this is a sudden effect, and the pilot has to pull up the elevators to their full extent to counteract it; it is one which might assume very large proportions in getting off a large aeroplane with C.G. and thrust axes high relative to the ground. The position and design of the undercarriage is also an important factor in taking off the ground. The present types of undercarriage, mostly the product of military requirements, should be capable of extensive improvement. If a pair of wheels, strong enough to take the whole weight of the aeroplane, could be arranged well forward, the tipping tendency when getting off and possibly landing, could be prevented from becoming serious. Undercarriages like this would involve more weight, but it might be weight well expended.
Large aeroplanes will never be so difficult to land as small ones because, irrespective of loading, they are bound to be more sluggish and give the pilot more time to think. However, a device which, hanging below the undercarriage, would flatten out the aeroplane from a steady glide by operating the elevators as it came in contact with the ground, would be a great asset if it could be perfected. The chief obstacle at present seems to be the uneven behaviour of the aeroplane just above stalling speed. As the incidence approaches stalling the speed drops so quickly that a slightly incorrect use of the elevators utterly spoils a good landing. To reproduce by an automatic device the use of the elevators while landing will necessitate the device being far from a simple one.
(d) Performance.- High performance ranks by no means first of the requirements of a commercial aeroplane. Its value is relatively low in the sense that other considerations may be allowed in some degree to interfere with it without sensibly impairing the value of the aeroplane. It is unnecessary here to emphasise what has been said about performance in war; perhaps pilots have come to look on an aeroplane with a performance less than that of the latest military ones with a certain contempt, but on a service it may be considerably more economical to use aeroplanes whose performance has not been pushed to a high figure. Provided that they can compete favourably with other means of transport, little more, for the moment, is required.
A comparatively low cruising speed would be adequate if it were not that the effect of an adverse wind may halve the ground speed. In order that a considerable reserve of speed may be in hand the figure of this cruising speed must still be kept high. Climb should not be so important as speed, its main value being to enable the aeroplane to rise from the ground more rapidly and hence with greater safety. Present-day aeroplanes of the twin-engined type take a considerable time to get off when fully loaded, and are compelled to climb at a low altitude over any obstacles in their line of flight. If the aeroplane feels definitely underpowered, the pilot suffers from lack of confidence, and getting off near the ground may have to manoeuvre at a low speed. Provided, then, that it has enough power to feel a safe flying machine when fully loaded, its actual speed and climb will almost certainly be settled by commercial requirements.
(e) Loading.-It is probable that commercial aeroplanes will be fairly heavily loaded. Up to a certain point a heavily loaded aeroplane feels steadier to fly than a lightly loaded one, and when it has to land after a journey it should have used up the greater part of its fuel so that its stalling speed will be lower than at the start. Excluding developments in wing design, on twin-engined aeroplanes it would be unsafe to exceed the present-day figures for loading. If they are to be exceeded, the first step is to see if the present fixed form of wing could be improved on, and by means of full-scale experiments to investigate the practical value, if any, of wing surfaces variable in form, without more regard to mechanical neatness and simplicity than is demanded by accurate working conditions. The ultimate value of such 'wing surfaces will always be relative to the extra weight they involve, so that a further step would be to see if they could be made light enough, consistent with the requisite strength, to be practicable.
(f) Steadiness.- Commercial aeroplanes flying for long distances will have to be navigated; in the case of large ones by a regular navigator. It has been advanced that for navigation a comparatively low degree of responsiveness is required, but the aeroplane must be steady and capable of being flown at a steady airspeed if reliable observations are to be taken. In large aeroplanes, which will probably be stable, this steadiness will come partly from size, partly from stability; but in the smaller types everything should be done to render the conditions favourable to the maintenance of a good compass course. The D.H.9A is good in this respect; the Rolls-Royce Bristol Fighter unsatisfactory.
(g) View. - For commercial flying the pilot wants a good view downwards and generally of the horizon to assist steady flying. His view upwards is not so important. I have not flown an aeroplane in which the pilot is completely enclosed, but should imagine that this would have an adverse effect on the handling of the aeroplane. The average pilot likes to be well screened, but at the same time dislikes a cramped, shut-in sensation when flying owing mainly to military considerations, where the best view was essential; it has been customary on multi-engined aeroplanes to place the pilot right in the nose of the aeroplane, in which position he undoubtedly has the best view. But if the aeroplane turns on to its nose, not necessarily involving a very serious crash, he is likely to sustain considerable injury. It might be worth while to see if the pilot could be placed just behind the wings in a large multi-engined aeroplane, without losing the essential arcs of vision that he requires. As far as the feel of the aeroplane is concerned he would be better off, for, provided that he is not unreasonably far behind the wings, the fact that he has them subconsciously in view assists his sense of balance. If, however, the pilot is too far behind the C.G. or too far in front, the aeroplane does not feel so responsive, owing to the reactions that the pilot experiences on his own body. If it is not essential for view that he should be seated in the nose, it would certainly be much safer to place him where he is less liable to risk of injury from a minor crash. This applies most forcibly in an experimental type of large aeroplane.
(h) Comfort.- There are no considerations which should prevent the pilot's being made as comfortable as possible. The fairing should be so arranged that it screens him perfectly, allows him a. perfect view of his instruments, and the best possible view outside. It should be brought up close to his head and slope away in an uninterrupted line from his eyes as far as possible in all directions. Some cockpit fairings are too low round the pilot, while their contours actually prevent him either seeing his instruments well or having a good view downwards without craning his head outside, so that it has actually been necessary for the pilot to duck his head to see the top instruments on the dashboard, while gaining nothing by the shape of the fairing. Its design should receive the closest attention.
It is perhaps unnecessary to emphasise the importance of arranging the control stick, rudder bar and throttle levers, so that the pilot may operate them without unnecessary effort. Although this point has often been brought forward there are aeroplanes still being built in which the pilot's comfort has not been sufficiently studied.
In the above paragraphs an attempt to analyse the flying qualities desirable in civil aircraft has been made, in which what seem to be the most fruitful lines of experiment have been suggested. I recognise that in nearly all these directions some experimental work has been carried out, and I do not claim that the suggestions are necessarily new. I simply wish to emphasise those which seem most important to me as a pilot. They may be summarised as follows :
(a) That new developments in large multi-engined aeroplanes should be rigidly examined in the light of the pilot's capacity to deal with them as a human being. That he should not be required to do too much.
(b) That the effort that is now being made to relate the pilot's flying requirements to the phenomena of stability should not be relaxed; that closer investigation of the undue sensitiveness of some aeroplanes to small movements of C.G. should be made, and that it should be a definite aim to produce a degree of positive stability which pilots have every reason to appreciate and none to criticise.
© That on large aeroplanes the gain to the pilot due to the sub-division of large controlling surfaces and the effect on his control should be further investigated.'
(d) That the question of relay controls should be followed up, and the maximum flying experience gained with them as soon as possible.
(e) That the control at low speeds of all aeroplanes should be investigated with a view to improvement, and that every means should be tried to improve the control of aeroplanes where they tend to turn or pitch due to variation of thrust.
(j) That further experiments should be carried out to solve the problem of an automatic landing device, attempts at which have been made at the R.A.E.
(g) That definite full-scale experiments should be proceeded with to ascertain the practical value of wing surfaces variable in form.
(h) That the question of seating the pilot behind the planes in a large multi-engined aeroplane should receive consideration.
(k) That small details which may increase the pilot's comfort should not be ignored.