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Aviation History
1936
1936 - 2065.PDF
FLIGHT. JULY 23, 1936. MORE FULL-SGALE STALLING Part II.—Confirmation of Theory on Other Machines By W. E. GRAY Fig. 1 : The test on the Monospar : superimposed photographs of the wool-tufts at 70 m.p.h. in (a) straight flight and (b) with 20 deg. yaw forward. Fig. 2 : Superimposed photographs at 75 m.p.h. showing sideslip in the opposite direction. IN last week's article (pages 94-96) an account was given of the preliminary wind-tunnel and full-scale work on the stalling of tapered wings, and an attempt will now be made to describe the further ground that has been covered in the past week. It may be said at once that the main evidence is now complete, and, in the writer's opinion, conclusive; it has been established that a straight T.E. gives a straight flow right up to the stall and that a back-swept T.E. gives an outward surface flow; the evidence is not pictorially available at the moment in support of the latter statement, but it is hoped to give it in the near future. The film of the Swallow tests, referred to last week, has now been viewed in motion and in fairly detailed exami nation : the ultimate '' front stall '' has not come out well, as had been hoped; the layer of deflected air is very clearly shown, with the upper flow passing over it; the side flow towards the tip when the wing is the down-wind one inayaw is also well recorded, the flow ultimately driving along the wing and stalling the tip, as had been visualised when the "theory" was first evolved. As the film was taken at some sixteen exposures per second, it is not possible to run it slowly, so it needs a quick eye to detect all the details, and there has not been time yet to study it fully as " stills." Sivept'back Wings The most important of the new tests is on a swept-back wing. An untwisted wing of this type is rare or unknown in this country, and the writer is indebted to the sportsman ship of Herr Pabelick, who studied the angle of the flow on his Bticker Jungmann, at Heston. The sweepback is 12 deg. approx. and the machine an untapered biplane with some 16 deg. stagger and rounded tips. At a speed of 70 km./hr.—about its stalling speed—the outflow near the T.E. at about one-third along the semi span of the lower wing was approximately 3.5 to 4 deg.; near the wing tip it was considerably more. This was a visual test by Herr Pabelick; the machine was carrying luggage and there was not time to unpack it to take photographs, but he has promised to take the writer up for photos and films on his return in a few days' time. The flow was also studied in an impromptu test on a D.H. Dragonfly, but the photographs are not good enough for reproduction. Wool strands on the lower wing (no sweeping of T.E.) showed the flow to be straight right up to the stall, but one strand near the tip where the T.E. begins to sweep forward showed, as expected, definite inflow, in- increasing with incidence. The Monospar A number of photographs and many cine shots have now been taken on a small Monospar. Although the sweepforward of the T.E. on the outer part of the wing is some 16 deg., the bending of the flow in straight flight is less noticeable than on the Swallow; this is probably due to the somewhat reflexed T.E. In Fig. 1 are shown, superimposed, the flows at 70 m.p.h for straight flight and with 20 deg. yaw towards the wing tip (tip forward); it is difficult to allow for perspec tive, and this method makes it clearer; it will be noticed that the flow near the T.E. has been bent inward some 50 or 60 deg. on the inner part of the wing by the 20 deg. yaw. Fig. 2 shows a corresponding pair of photos at 75 m.p.h. for sideslip the other way, and it will be seen that the alteration is rather less; had the sideslip exceeded the sweepforward of the T.E. by more than the 4 deg. the change would probably have been accentuated. It should be noted that there is a balance tab half-way out the aileron, which upsets the flow there. In another photograph, not reproduced, of the flow at 60 m.p.h. just before touching ground, the flow along the T.E. is more noticeably in ward than at 70 m.p.h. The wool used in these tests is much thicker than in the previous Swallow tests, so as to make the cine films clearer, but it is rather stiff. The two photographs of the Swallow wing, with 20 deg. yaw ngh and left, printed last week, are very interesting if super imposed; the forward strands diverge at 40 deg., but as the T.E. is approached the angle rapidly widens, especially over the inboard part of the wing. If the corresponding photographs of the Monospar are superimposed the widen ing is nearly as pronounced on the second row of stran
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