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Aviation History
1936
1936 - 2407.PDF
rig. 4 (top) : The Leopard Moth in straight flight tall just beginning at the wing roots. Fig. 5 : immediately before the stall. the tufts indicate a A sideslip of 14 deg. do a new test on the Swallow: If the bending of the surface layer of flow were to be prevented by some artificial means, and the stall developed as before, it would be clear that the bending was not causing the stall; some rough three-ply fins were therefore fitted, as shown in Fig. 3, to try the effect, and although they were not over so much of the span as they might have been, owing to its being a somewhat hurried preliminary test, they did show the nature of the spread of the stall to be altered, and it was not thought worth while to repeat the test for mere appearance. It will be seen that the flow breaks down in each section independently, the inboard part of each sec tion stalling first, instead of the stall spreading outward from the root as it did without the fins—as was shown in the photo graphs that were printed with the first article. A short film was also taken, ind the change in the manner of stalling can be clearly seen. It should be noted that the length of wool beyond the forward part of the outer fin is caught on a wing bolt, and is not stalled. The exposure was 1 /100 sec. The influence on maximum lift appears to be negligible, however, for there \Vas no noticeable tendency for the machine to drop one wing or the other at any incidence around the stall—fins only being fitted to one side. It seems to be quite clear, all the same, that once the root part of the wing has stalled it will suck away the " dead " air that would otherwise accumulate near the T.E. of the more outboard parts and thus delay the stall there to some extent. This was strikingly shown on the cinematograph film of the FLIGHT. SEPTEMBER 3, 1936. flow on the Monospar; the stall started somewhat inboard of the aileron, and immediately the surface flow over the aileron was suddenly bent inward so as to be about parallel to the tiailing edge, but the flow just ahead of the aileron was not noticeably altered; this would seem to be obviously due to the fact that the pressure over the inboard part near the T.E dropped suddenly to the stalled value, whereas the pressure over the aileron was more nearly atmospheric; the pressure diagram curves for stalled and unstalled conditions probably cross in the region of the aileron spar. This tendency of a stalled region to suck away the dead air near the T.E. in its vicinity probably accounts for the peculiarity found in some recent American tests where a tapered wing of high aspect ratio was found to stall in patches along the span, with unstalled regions between; apparently the stalling of the patches helped, in this way, to maintain the flow over the intermediate parts. The point was also raised as to whether a transverse surface flow really does interfere with the flow over it so as to encourage a stall, and a tunnel test was carried out to find what the effect was. Air was conducted from the under-surface of a wing and emitted from under a thin covering over the upper surface extending over the rear half of the chord; the aperture was about one per cent, of the chord in depth and extended chordwise. The flow of air over the surface caused stalling several degrees earlier in that region, the flow on either side being steady. The High-wing Monoplane Reference will now be made to two photographs of the flow over a D.H. Leopard Moth wing, shown in Figs. 4 and 5. These proved rather difficult to get, as careful co-operation was needed, and the camera imposed limitations of distance if a sharp image was to be obtained. Actually it had been desired to get a picture of the down-wind wing stalling first in a sideslip; a high-wing machine was chosen in the hope of eliminating body interference, and a wing with considerable taper was decided on as being of more general interest; although the phenomenon was observed visually to a fair extent it was not caught with the " still " camera. With too much sideslip the centre-section spoils the flow over the root on the down-wind side and it stalls before the outer part. Fig. 4 shows a straight stall just beginning at the wing roots, and from other photos this is seen to be closely followed by stalling over the inner parts of the ailerons; it will be seen that the flow there Is most bent, and therefore presumably most retarded. A number of thinner lengths of wool were sup ported 5m. above the surface, and although they may not be visible in the reproduction, they do show considerable bending also. Other photos show the flow over the wing tips to be quite good after a great part of the wing is stalled. Fig. 5 shows a sideslip of 14 deg. just before the stall, and in drawing any conclusions it should be remembered that the bending of the flow on the starboard aileron is accentuated by the fact that it is down in order to hold the sideslip, while on the port one it is reduced; the disturbed flow in the wake of the centre- section is clearly shown, but the convergence of the surface flow near the tip of the starboard aileron and divergence on the port one are not so evident as on some of the other photos taken. The exposures were i-3ooth sec. Attention will now be given to what I have termed "deep" and "shallow" stalling. It was found when studying the flow over a model tapered half-wing with varying degrees of sweep, in the early part of this work, that the breakdown of the flow at the stall varied very considerably with sweep Fig. 6 (left): A "shallow" stall on the B.A. Swallow, with the wing yawed forward 5 deg. Fig. 7 : A " deep " stall with the wing yawed back 10 de%.
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