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Aviation History
1920
1920 - 1190.PDF
NOVEMBER 18, 1920 CAMBRIDGE UNIVERSITY AERONAUTICAL SOCIETY (OFFICIAL ORGAN, "FLIGHT") ' . ' *" A Paper on "Spins" v _ , ' THE ninth meeting of the Society took place in the EngineeringLaboratory on November 3, at 8.30 p.m., the President being iirthe Chair.The President said that the lecturer of last week paid a graceful and a generous tribute to the work done by Cam-bridge men in aeronautics, and that the lecture that night w,ould be given by one to whom that tribute was so deservedlypaid. Mr. H. Glauert obtained a First Class in the Mathe- matical Tripos in 19*3, and went to Farnborough in 1916,aud since that time he had been doing some valuable work, of which he was going to tell the meeting something thatnight. He was sure that the Society would be pleased to hear of Mr. Glauert's election to a Fellowship at Trinity. He thencalled upon Mr. Glauert to give his lecture. Mr. H. Glauert, M.A., A.F.R.Ae.S., then delivered thefollowing lecture :— "Spins"Viewed from the ground, a spin can be described by saying that the aeroplane descends almost vertically and rotatingrapidly, and from the point of view of the pilot, the rotation is also observed by the peculiar behaviour of the ground, andthe force by which he is pressed into his seat. The one pecu- liar thing from the point of view of the pilot, however, is thatalthough the aeroplane is pointing nose down to the ground, there is no tendency for him to fall forwards in his seat, asthere is in an ordinary dive. In the early days of flying, the spin was known simply byvirtue of the number of accidents which it caused. It was known that at times an aeroplane became uncontrollable andfell into a spin, and the result was usually a bad, and often a fatal crash. I can quite well remember my own first ex-perience of a spin. There was a small pusher aeroplane coming down to land at Farnborough. At a height of a thousandfeet, the pilot made a bad turn, and the aeroplane fell into a spin, from which it made a partial recovery, and then fell intoanother spin. In those days, a spin was regarded as a form of instability. If an aeroplane got into a spin, the pilot mighttnake an attempt to get out of it, but it was more by good fortune than by good judgment that he succeeded in doing so.One of my objects to-night is to show you that the spin is not a form of instability, but one of the possible steady motionsof an aeroplane. Now I must first digress a little to talk about the stabilityof an aeroplane. Suppose we think only of motion in the plane of symmetry, ignoring banking and turning. There isonly one possible steady motion for an aeroplane under these conditions. It must fly in a straight line. Of course, it maybe climbing or diving down, it may even be coming down vertically or flying on its back. There is also a series ofperiodic motions, with the oscillation neither increasing or decreasing. You might imagine, for example, a series ofstalls and dives, or a succession of loops. The conditions at each- moment vary in a periodic manner. Well now, what-ever your aeroplane is, it must settle down eventually to one of two things. It must either settle down to a steady flightin a straight line, or to one of the periodic motions. But in talking of the problem of stability, we think of the straightline flight as a steady condition, and see what happens if there is a small divergence from these steady conditions. If thedisturbances die away, the aeroplane is said to be stable, if they increase with the time, it is unstable. That is the prob-lem of stability worked out by Professor Bryan and other writers. But the stability analysis does not say anythingof what the subsequent state of the aeroplane will be. It only considers the conditions in the first few seconds. Theaeroplane must eventually settle down to steady flight under some other conditions, either the straight line, or the steadyoscillating movement. An unstable aeroplane, for example, is known to be generally stable on its back. We have hadnumerous cases of that, where the aeroplane is unstable in flying in the ordinary manner. If released, the aeroplane willsettle down and glide to the ground upside down. In the general case there is another possible steady motion,the helix or steady turn. You will probably know this motion as the spiral glide, and I may seem pedantic to refer to thepath as a helix. It is necessary to do this, however, in order to distinguish the steady motion from the form of instabilitycalled spiral instability. In this general case there is another series of steady periodic motions which are really undampedoscillations about the helix. Very little work has been done on the stability of this helical motion. Our knowledge is confined mainly to the case of the straight line as the steadymotion. In this problem, instability arises in two ways ; one, whichia calleS spiral instability, occurs if the fin and rudder of the aeroplane are too large, when the aeroplane turns more andmore rapidly, banking up all the time, and eventually will settle down to some other condition. But the stabilityanalysis says nothing of that. The other form of instability has always been known as spinning instability, and occurswhen the fin and rudder are too small. The aeroplane then whips round suddenly instead of this slow gradual turn, andas it turns, its outer wing goes faster, the nose dtops and again you have got something similar to what happnes after thespiral instability. Well, the aeroplane eventually must settle down to something. It must fly in a helix, if not in a straightline, and that is why the spin was so often the result of instability. It is not the instability itself, but the steadymotion to which the aeroplane settles down. I have men- tioned the accident I saw to a pusher seout. At that time,there was no general knowledge of how to deal with an aeroplane when it got into a spin. Some pilots had recoveredfrom spins, but it was not known certainly what was the proper course of procedure if the aeroplane got into thatcondition. The only thing to do was to avoid getting into a spin. These accidents were unfortunately rather frequenton the F.E.8, but the chief experimental pilot at Farnborough, Major Goodden, was convinced that it was possible to get anaeroplane out of a spin, and to test his convictions he went up in one of the aeroplanes, put it into a spin, and brought it outsuccessfully. I will read you the report of that experiment in his own words. — "At a height of 3,500 ft., I put on gradually full rightrudder, and at the same time gradually pulled the control- stick over to the left. In this way, the aeroplane was turnedto the right without any bank. When I had turned the machine in this manner for about 1800, the speed had dropped,due to the turn and the resistance of the fully-over rudder and flaps, to nearly the stalling speed. The nose of theaeroplane then went gradually down, due to the loss of speed. I next pulled back the control-stick, and this increased theturning speed, and when I had completed 300° turn the spinning suddenly started. " I kept the control-stick in the position described above,and the spinning continued, and gradually got steeper. The whole aeroplane was then turning about a point mid-waybetween the right hand wing tip and the body. In this way I tried three spinning tests to the left andthree to the right, and whereas spinning starts after about 300° on the right-hand turn with control as described above,spinning to the left does not take place until a turn and a half had been completed. " In bumpy and disturbed air a turn attempted with thecontrols set in the manner described would undoubtedly start the aeroplane spinning much earlier than the turn." The following was the procedure adopted-to get the aero- plane out of the spin :—" (1) Switch off motor. (2) Control-stick put central and pushed forward." (3) Rudder put in centre. " This resulted in a nose dive, from which the aeroplane,having once got up speed, can easily be pulled out with the control-stick pulled back slightly." If the aeroplane is dived to bring it vertically downwards, spinning cannot continue, provided all the controls are central." This aeroplane is perfectly stable and is as safe from spinning as any aeroplane I have flown. There are largeelevator, rudder and wing-flap surfaces, and the controllability of the aeroplane is consequently very great. This con-trollability, which is so desirable in any aeroplane, and particularly in a fighting machine of this type, should it bemisused, must recruit in upsetting the whole stability of the aeroplane. Similarly, if the aeroplane is allowed to get out ofcontrol, then, if care is taken to make the correct movements required by the particular conditions, these powerful controlwill enable the aeroplane to be readily brought under control again. '' I could only succeed in making the aeroplane spin by th^-misuse of the controls I have described, and from reports I have of the spinning accidents to F.E.8 aeroplanes, thisseems to have been the cause." Well, there are two points in that report to which I want II92
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