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Aviation History
1936
1936 - 2406.PDF
SEPTEMBER 3, 1936. FLIGHT. MORE FULL-SCALE STALLING Part IIL—A New Aspect : "Deep" and " Shallow" Stalls : Further Evidence and Summing Up By W. E. GRAY IN the first two articles of this study (see Flight of July 16 and 23) an outline was given of the preliminary investi gations made in attempting to check the theory that had been formulated, and some of the full-scale tests were described. Since then numerous further tests have been carried out, and in the present—and, it is hoped, the con cluding—article, the results will be set out briefly so that those interested in the subject may be able to form opinions for themselves regarding the validity or otherwise of the ideas put forward. In the first article, reference was made to the discovery, in tunnel tests, of what seemed to be a new peculiarity of flow associated with the sweeping of a wing, and a promise was given to deal with it later if it were found to be real at full scale; this it does seem to be, and what will be termed " deep " and "shallow " stalling will be described. The earlier points that have been raised in the Correspondence columns of Flight have already been replied to briefly there, and do not seem to call for further comment, but the letters from Mr. Lock, of the N.P.L., and from Mr. Needham printed in Flight of August 27 will be dealt with here. Then, again, doubts had been expressed to the writer regarding the validity of the "theory" advanced, and further model and full-scale tests have therefore been carried out in an attempt to reach the truth; a description of these, too, will be given. Sivept'back Wings First of all, to link up with the second article, I promised to give photographs of the airflow on the Biicker biplane with its swept-back wings; unfortunately Herr Pabelick had apparently to return to Germany in too much of a hurry to be able to get in touch with me again before departing; an attempt was made later to secure a similar machine, but that, too, fell through. To take the place of this a test was there fore carried out on a D.H. Tiger Moth; this might have been done at the start, but, frankly, it seemed a little doubtful if the relatively small sweep-back of the Tiger would show suffi cient bending to give clear photographic reproduction. The result, however, was quite definite and is shown in Fig. i, the flows at 50 and 60 m.p.h. being superimposed on one print; it should be noted that the zeros are parallel to the wing ribs and in straight, fast flight the yaw-meter should and did read 6 deg., the approximate angle of the sweepback, for the ribs are at right angles to the L.E. and T.E. Between no and 60 m.p.h. there is little change in the direction of flow near the T.E., but when the speed is reduced to 50 m.p.h., or just before the stall, the flow l>ends out as shown by the wool and finally breaks down in an outward direction; there was a slight sideslip accentuating the bending at 50 m.p.h.—the yaw-meters " do not quite coincide—but the error is not great. In Fig. 2 is shown the flow over the lower wing of a tapered D-H. Hornet Moth, the flows at 70 m.p.h. and 46 m.p.h. being superimposed—46 m.p.h. on the A.S.I, being very close indeed to the beginning of the stall. It should be noted that the inner yaw-meter '' is influenced by the flow past the wide fuselage and always read 4 deg. in straight flight; the flow over the mner part of the wing, where the T.E. is at 90 deg. to the airechon of flight, remains unbent right down to the stall; °ver the aileron, where the T.E. is swept forward increasinglv. tne bending inward of the flow with reduction of speed can « seen to be quite marked; this is in agreement with all the Irevious evidence collected and with the theory advanced, as this plan-form combined straight and swept sections of • •. and had a normal wing section (instead of a reflexed be w rth°n ^e MonosI'ar). i1: was felt that a test on it would while. Reference was previously made to a brief (Top to bottom) Fig. 1 : Superimposed photographs of the tufted Tiger Moth wing at 50 and 60 m.p.h. Fig. 2 : The Hornet Moth wing at 46 and 70 m.p.h. (superimposed). Fig. 3 : The Swallow, with fins to prevent bending of the surface layer flow. test on a Dragonfly, and it was thought that the Hornet would be even more informative. Let us now turn to the question of the " theory " and con sider some further points of the'problem. It was suggested to me in connection with the Swallow tests that it may be the stall at the root that causes the bending, instead of the bend ing causing the stall there; this appeared to be a valid objec tion, but it seems to be fully answered by the Tiger Moth photo, for there we have marked bending without any stall to cause it, and, further, the Hornet flow confirms this. It was thought advisable, however, so as to put it beyond doubt, to
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