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Aviation History
1920
1920 - 1132.PDF
. it. ',".:,.' OCTOBER 28, 1920 A COMPARISON OF THE FLYING QUALITIES OF SINGLE AND TWIN-ENGINED AEROPLANES* By Squadron-Leader R. H. HILL, M.C., A.F.C. THERE is no doubt that the twin-engine types involve greater concentration in flying generally, simplv because the pilot has more to look after ; he has to cope with the diffi- culties met with every day in the single-engined type, but in addition he may meet with certain extra ones involved in the twin, the cumulative effect of which has undoubtedly led in some cases to crashes which might have been avoided had these difficulties been justly appreciated. Undoubtedly, for pure flying, that is the pilot's part in the maintenance of an aeroplane in flight, control will be uppermost in the pilot's mind, and as such claim nearly all his attention in flight. Control may be roughly divided in two : control of the aeroplane, and control of the engine or engines. I think that the key to the problem may be said to be the correct appreciation of how the two parts of the control re-act on each other. A failure to appreciate this, an attempt to treat subconsciously the two parts as separate involves grave confusion. Firstly, when the power is completely shut off, the two elements of aeroplane and engine control are most nearly separate in the pilot's mind. Theoretically, with an aero- plane control complete in itself, a pilot might carry out any manoeuvre whatsoever, using gravity as a motor. Even -if the aeroplane control could be regarded as complete in itself, it has not up to the present been made complete enough to meet effectively the control requirements of the aeroplane between all engines on and no engines on. Secondly, in practice, especially with twin-engined aero~ planes, the aeroplane control is not complete in itself. Al- though recourse to it alone is necessitated in a forced landing with no engine or engines, in which case the best possible use has to be made of it, in actual fact most modern aeroplanes pass the accepted standard of controllability, when their control at low speeds is bolstered up by the use of engine to assist the rudder for instance. If the same standard were demanded without any engine, many aeroplanes would fail to come up to it. Even the most modern aeroplane engine may be controlled in two ways, firstly by the pilot, and secondly by mechanical faults, including faults in the petrol system. There will be two main considerations then : firstly, the effect of the engine on the aeroplane and the consequent movements of the controls as the pilot varies the power at will, or for some particular purpose, and secondly, the effect of sudden engine failure. If the pilot varies the power at will, he knows what he intends to do and therefore has a general idea of the effect on the aeroplane. The effect will be foreseen in the first case, may be violent and unforeseen in the second, and in general influences the aeroplane in four ways :— (a) The presence or absence of slipstream will, by affecting the tail, influence the longitudinal and lateral trim of the aeroplane. (6) Simultaneously the position of the thrust axis or axes relative to the centre of gravity will do the same when the thrust is varied, (c) The engine torque varies with the horse-power developed, and naturally affects small single-engined aeroplanes to a much greater extent than large aeroplanes. \d) The gyroscopic effect of a rotary engine may influence the aeroplane when it is manoeuvred. Before considering in detail the effect of engine on the control of multi-engined, it will be well to examine shortly that of single-engined aeroplanes. This must be done just so far as it affects the pilot and his power of control over his aeroplane. Take the four effects mentioned, the most important of course is that of the slipstream on the tail. Assuming that the tail portion consists of a fin and a rudder tail plane and elevators, and that these are partly in the .slipstream, various and sometimes unsymmetrical loads will be applied as the power varies. The air of the slipstream is washed downwards, and also given a rotary motion by the propeller. If it were possible to design the fin symmetrically in the slipstream, there would be no force on it. However, in practice, this is very difficult to achieve, owing to angle of incidence of the wings when landing and other considerations. Therefore in nearly all modern aeroplanes there is a tendency to turn one way with engine on ; in some aeroplanes violently, in others less violently. It may be noted in passing that it has been observed that the smaller the diameter of the propeller, the more violent is * Extracts from paper read before the Royal Aeronautical Society, on Oct. 21, 1920. the action of the slipstream on the fin. A Lion D.H.9A. was turned out with a propeller of a certain diameter, and exhi- bited a violent turning tendency. By increasing the diameter of the propeller, the turning tendency was reduced to reasonable proportions. Again, the slipstream acts on the tail plane. Most modern aeroplanes are provided with a movable tail, which is specially necessary on aeroplanes with high-powered engines, for the purpose of rendering the pilot's aeroplane control adequate to meet the control requirements from engine full on to engine off. It should be noted here that the tail cannot be operated very quickly, especially in the case of a wheel control. That is why, if possible, a lever should be provided apart from its other advantage of registering the position of the tail plane to the pilot. Slipstream acting on a tail set at a negative angle will render the aeroplane more tail heavy engine on than engine off. According to the design of the aeroplane this feature is in evidence to a greater or less degree. Pilots naturally like an aeroplane which has a small difference of trim engine on and off. In some aeroplanes the range of movement of the tail plane is not sufficient to provide adequately for this difference of trim, and elevator control, which should be avail- able for emergencies, has to be used up in trimming the aeroplane in what ought to be its normal range of flying speeds. With stable aeroplanes it is possible to ascertain immediately whether the tail plane has sufficient range for flying require- ments, as the aeroplane with elevators free will take up definite stable trimming speeds, if they exist; with unstable aero- planes, or aeroplanes only stable at low speeds, the problem is more complicated, and it is necessary to measure the forces on the control stick. (fi) In most single-engined aeroplanes, the position of the axis of thrust, unless the aeroplane be designed for some special purpose, does not affect the pilot nearly so seriously as the effect of the slipstream. Its actual effect cannot be dissociated from that of the slipstream as it occurs simul- taneously, but the axis of thrust is not usually at a great distance from the centre of gravity of the aeroplane. It should be noted that its effect may add to or subtract from that of the slipstream. (c) The engine torque is in general a small effect. It was noticeable on €he D.H.2 when the engine was switched on and off, and on other small span aeroplanes. If the engine is opened gradually the torque is difficult to detect, but with an engine like a Monosoupape Gnome, which is switched off and on without throttling, the effect on a small span aeroplane is at once apparent. (d) As far as I know the gyroscopic effect of a large rotary engine, with one or two exceptions,lonly arises nowadays with single-engined aeroplanes, and so consideration of it hardly assists a comparison with multi-engined aeroplanes. How- ever, on a single-engined aeroplane this effect is felt in almost every natural manoeuvre that a pilot can carry out. As the engine power is varied, its relations to the aeroplane control, as in (a), (b), (c) and (d) have been discussed as separate factors in a complex effect, such as the slipstream on the fin, or the position of the axis of thrust in its relation to the longitudinal control. But these in flight are so closely interrelated, at least to the feel of the pilot, that it may be well to consider the whole in relation to practical flying. It might first be mentioned that the pilot may be flying along steadily and using his controls to overcome unsym- metrical forces due to, say, slipstream. The slipstream may disappear due to engine failure, and cause the unsymmetrical forces to disappear also. Because for the moment the pilot has become accustomed to them, the effect is just as upsetting as if the aeroplane were subject initially to symmetrical forces, and unsymmetrical ones were suddenly introduced. Any sudden change of trim may necessitate the use of the aeroplane controls to maintain rectilinear flight, and the ease of doing this depends on whether the pilot is near to the margin of his control or not. If an inexperienced pilot flies an aeroplane with a strong turning tendency and a large alteration in longitudinal trim engine on and off, he may do something like this-—he takes off with the tail wheel adjusted for climbing. He flies round, and before gliding in to land he winds the wheel full back. He throttles right down, glides in, and finds that he is going to overshoot badly. He decides to open the throttle and go "34
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