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Aviation History
1919
1919 - 0856.PDF
is then very heavy on his shoulders, and on the one occasion on which I experienced the sensation my head was distinctly unstable, and an incautious moyement brought it forward " out of control." The first record occupied nearly half an hour in the taking, of which about 20 minutes was in the air. The instrument, designed by Dr. Searle, F.R.S., of the Royal Aircraft Estab lishment, was strapped to the knee of an observer, and at intervals the pilot interposed some of the trick evolutions of flying on the more normal course of the flight. The Urst clear deduction from the record is the uneventfulness of straight flying, especially at good heights. This is shown by the tendency of the black line to keep about the value 1, so that the pilot and passenger have their usual weight. Flying low down it will be noticed that the changes called " bumps " sometimes reduce a pilot's apparent weight to half its ordinary value, and at other times increase it to nearly one and a-half times. The sensations are very like those ex perienced on a switchback or mountain railway, and in a well-shielded cockpit this is the only noticeable effect of motion on the sense of feel. The earth, however, becomes far less impressive as the representative of solidity, and has its attributes transferred to the aeroplane. So much is this the case, that in a loop it is the earth which goes over your head, whilst in a banked S turn the waves of a high sea are less wonderful than the swaying of the earth. These impres sions are very real, and have a rational basis. We are so accustomed to living on the earth and to the use of such instruments as levels, that it needs a definite effort to realise that on an aeroplane a level does not indicate the horizontal. On the contrary, a level will consistently say horizontal when the aeroplane is banked to 45 degrees or more, and not only does the level say this, but the pilot's sensations of feel tell him he is sitting upright. In the absence of side-slipping the local level is given by the wings of an aeroplane. With a horizon visible a pilot can keep level relative to the earth by the use of his eyes, but this power is removed by immer sion in a cloud. By the use of instruments of a not very intricate nature straight flying can be conducted in a cloud, and fog is not by any means an insuperable bar to aerial navigation. Leaving the record of ordinary flight and coming to the " stunts," it will be found that on two occasions the pilot was in danger of leaving his seat, and the importance of a belt will be appreciated. Looping and spinning produce heaviness on the whole, the extreme value being nearly three times the normal. A rapid turn almost always increases the apparent weight, and in a mock fight it is very noticeable that the greater number of manoeuvres seat the pilot more solidly, and therefore help in the accurate control of the aeroplane's motion. In a few instances and for a few seconds the apparent weight is zero, whilst the extreme in the other direction is as great as four times the normal. Periods of high stress do not last long, but the air forces have been fully transmitted through the structure, which is then liable in fighting to be loaded to four or five times that in level flight. Although the conditions of fighting are very exacting, the demand for adequate structural strength has been met with considerable success. It would, therefore, appear that for civil transport structural design is far advanced towards complete safety. Controls Reference was made earlier in the evening to the pilot's wish to have an aeroplane wholly subordinate to his well, even at the risk of danger to himself. Control is difficult to define in numerical form, especially as the manoeuvres are rapid ; it takes only a few seconds to get from a level keel to a vertical bank, and the pilot is far too busy to observe a stopwatch. At the Royal Aircraft Establishment the idea was formed and developed of photographing one aeroplane from another. The cinema camera used was carried on one aeroplane, which flew steadily along a straight level path ; the film was driven by a small windmill. Directed towards a second aeroplane flying behind, any motion, such as turning, rolling or looping, could be photographed. The latest stage of the experiment is to use simultaneously a number of instru ments bearing the undignified title of " Rats," which record the movements of the controls during the manoeuvre. The scheme will on'y be outlined this evening, as a full descrip tion will shortly be given by the Royal Aircraft Establish ment. The work of reduction proved to be analytically difficult, and has been circumvented by the use of a globe, on which the solutions of spherical geometry present no difficulties. Given such a method of investigation it would be possible, even in a fighting scout, to assess the merits of any particular control in a definite numerical statement, and so lay the foundation for the final design of the best systems. Relay Control for Large Aeroplanes With the exception of a few flying boats, the control of all heavier-than-air craft has been by column or wheel and rudder-bar connected by cables to the elevator, ailerons and rudder. This is a partial relay, as the couples which move the aeroplane come from the air and are far greater than those exercised by the pilot's muscles. An aeroplane of a given size may be light or heavy on its controls, and those designs in which the control is light are greatly appreciated. Forces on a pilot's hand of 1 to 3 lbs. are described as light, 5 to 10 lbs. acceptable, and 30 to 40 lbs. heavy. Bigger forces can be taken through the legs from the rudder bar. The largest manually-controlled aeroplane weighs twenty times as much as the smallest, and other things being equal this would represent the increase in pilot's effort. A small aeroplane with light control may represent a heavy type of large aeroplane. Some relief can be obtained by the use of an adjustable tail plane, but this is too slow for such opera tions as landing. In meeting the new problems involved in the control of large aeroplanes, balanced surfaces were intro duced, and have sufficed to bridge the present range of size, at any rate for those large aeroplanes which have sufficient stability to fly themselves for the greater part of their time in the air. It might be thought that no limit existed to the possibilities of balancing, but it is found that the conditions required are not the same at all speeds of flight. It is important that the controls shall not " take charge " due to overbalance, and so a small reserve of stability of the control surface is necessary. A more important, but less obvious, limitation arises irom the fact that the atmosphere is gusts-, either naturally or because of the airscrews. This gustiness upsets all hope of accurate balance, as the irregular forces on the pilot's limbs lead to discomfort. Some appreciable degree of stability in the controls is thus indicated, and as a corollary it is necessary to look to new ideas. The most promising development is one which assists the pilot in the same way as the steering engine of a ship assists the helms man. The stage of finality is still distant, but one control has been developed and used for seaplanes which relieves the pilot of three-quarters of the effort required to move the ailerons. Stability The relay control just mentioned only assists the pilot, and does not remove the necessity for his constant attention The human being is a complex mechanism on to which aero nautics throws all its outstanding difficulties, and although the opinion is probably heretical, I believe that pilots are not anxious to do unnecessary work. At any rate, one of them had the normal human characteristic and fixed a plate in the wind which was to operate the relay control when one wing went down and side-slipping began. He discovered a natural period of the aeroplane. Every 6 sees, down came the right wing, the control put over the ailerons and brought it up as expected, but overshot the mark, so that hunting occurred until the pilot intervened. It is probable that that natural period had never been suspected, just as early aero planes were thought to be free from phugoid oscillations. These natural periods are indicators of stability, and every aeroplane has its own characteristics. Many records are now available from the Royal Aircraft Establishment and some from Martlesham Heath. In a lecture to this Society some years ago I had the pleasure of exhibiting a number of small models made to" illustrate the mathematical theory of stability. Most of the examples to-night are taken from flight records, and as such are devoid of any theory. It is. jjerhaps, necessary to say this clearly, otherwise the distinction might be missed in the exactness of the agreement. Two pieces of apparatus, one a simple block of wood, will draw attention to the mechanical ideas which in aviation constitute the basis of stability. Every one knows that a top-heavy body falls over. An aeroplane may have a property exactly analogous to top-heaviness and a tendency to turn" over on its back. The wooden block stands steadily on the table until disturbed, when it oscillates and gradually settles back to its initial state. This is one of the noticeable characteristics of the motion of a stable areoplane. The other way up, the block is stable against a small blow, but is upset by a larger one, and is typical of a limited range of stability. All these motions are familiar. A fourth type is far less common, and the model used to illustrate it is corre spondingly uncommon. If I spin the gyro and set the apparatus in a vertical position, it will oscillate slowly, the amplitude getting greater and greater until the apparatus falls over. This is the condition known to engineers as 856
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