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Aviation History
1936
1936 - 2352.PDF
AUGUST 27, 1936. FLIGHT. 229 CORRESPONDENCE The Editor does not hold himself responsible for the views expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases, accompany letters intended for publication in ihese columns WINDSCREENS IN your issue of July 30, under " Private Flying," your contributor "Indicator" discussed the feasibility of the rotating disc type of windscreen for transport machines, raising the point concerning their weight. Has the split or two-layer type of screen as used, I believe, bv Capt. Eyston on his record-breaking saloon car been tried by any manufacturer, and if so in what direction was it found wanting ? The screen to which I refer, and with which you are no doubt familiar, consists of a portion embodying a horizon tal gap of several inches partly covered by a separate sheet of glass mounted for ward of the main screen. A draught is induced between the two layers of glass, which carries all dust, rain, etc., with it. Vision is through a space left between the bottom edge of the main gap and the bottom edge of the outer sheet of glass, i.e., not through glass at all; hence it is non-fogging under all conditions. The possibility of designing the screen to be effective over a wide speed range would appear to be the most important point. A. E. TUPPEN. Rugby. POP RIVETING CONSIDERABLE publicity has been given by your paper to a system of " pop riveting " which claims as a special advantage that the broach used for expanding the rivet is not deformable and may be used repeatedly. Your paper implies that an advantage of this is that the cost of using a fresh broach for each rivet is obviated. We are manufacturers of rivets and supply these threaded on the broach or mandrel which is used in the British pop riveting process, and we believe our selling price for the com bined rivet and broach is considerably less than the selling price of the special rivet used in the alternative process. The use of the undeformable broach has long been known in this country, but we submit it does not give such good results as the deformable broach which has the advantage that its head is large when it expands the point of the rivet and becomes small before passing through the rivet bore. A more recent development in this country is the " break- head " mandrel or broach, the head of which is practically fial and compresses or squeezes the rivet in the same way as a pair of snaps, and this has been found to give a better job than is obtainable by other means. It has the advantage, for instance, that it draws the plates together and gives a tight job even when one length of rivet is used to connect plates of considerably different thicknesses. Moreover, squeezing the rivet expands it to fill the hole, even when this is consider ably over size, as is bound to occur when rivets are drilled by hand tools in flexible structures, such as stress skin wings and monocoques. With regard to speed of riveting, there appears to be little difference between the two processes. Pop riveting is a one- man, or rather, one-boy job, and there are several structures in which each boy is now setting rivets at the rate of 1,200 per hour, and over. GEO. TUCKER EYELET CO. LTD. Birmingham, R. RUDDUCK. THE STALLING OF YAWED AND TAPERED WINGS JN an article in Flight, July 16, 1936, Mr. W. E. Gray , suK"Rsts an explanation for certain observed features of he stalling of yawed and tapered wings. As a first step he suggests that there will be a tendency for a transverse flow n«ir the upper surface of the trailing edge of a yawed aerofoil a result of a transverse pressure gradient. This suggestion _j ™s to require rather more careful examination than he has nil r il In the stricfb/ two-dimensional ideal flow with cir- theo around an aerofoil, assumed as a basis for the Prandti eory. it ls evident that there will be absolutely no tendency to transverse flow if the aerofoil is yawed. For the flow may then be resolved into a two dimensional flow in planes normal to the aerofoil together with a uniform velocity component along the span which will not affect the equilibrium of the transverse flow. In the case of an actual aerofoil (still assumed to be of indefinite length) there will be a tendency for the boundary layer to be carried along by the transverse flow, and when the boundary layer breaks away at the stall, this effect will be intensified. The whole effect in the case of a yawed finite aerofoil when stalled, comprises transport of the dead air above the trailing edge towards the downstream wing tip, and this will tend to delay the stall at the upstream end and to accelerate the stall at the downstream end as actually observed. For an aerofoil with swept forward trail ing edge (not yawed), the dead air will be transported from the tips towards the centre, thus delaying the stalling of the tips and accelerating that of the centre by comparison with the corresponding aerofoil with straight trailing edge which would be expected to behave more nearly in accordance with the ordinary Prandti theory of a monoplane wing, on the assumption that all sections have the same maximum lift co efficient. There will, of course, be some transverse flow corresponding to the velocity field of the trailing vortices, and this will be additional to the transverse flow in the boundary layer when the trailing edge is yawed. All these conclusions appear to be in accordance with the recent observations of Mr. Grey and of Mr. Irving at th? N.P.L. As a further example of the fundamental fallacy of the sug gestion that the transverse flow in the boundary layer is due to pressure differences, the idea that a reverse effect will occur near the leading edge appears to be entirely without founda tion. Actually, a yawed leading edge may be expected to have little effect, because the boundary layer there is thin. Aerodynamics Department, N.P.L. C. N. H. LOCK. MR. W. E. GRAY'S comments in your issue of August 13, dealing with my letter of the previous issue, call for some replv. Mr. Grav has difficulty in understanding my reference to span wise flow and states that it is "a mere bend ing inward " of the flow, but, surely, if the flow bends in ward it has a lateral, or spanwise, component! One may as well eliminate the terms up-wash and down-wash. I do not know why Mr. Grav should think that 1 assume that there is no vortex on the clown-wind wing tip ; I merely suggested that the vortex effect was accentuated on the up wind tip, and decreased on the other, thus resulting in earlier stalling of the latter. I am sorry this was not understandable. One appreciates of course, that the mass of air set in motion bv the lateral pressure gradients may be relatively small, but all such flow must decrease lift to some extent, and, more over, may be sufficient to initiate or delay stalled conditions. Finally, experiments in the wind tunnel with a series of models, in which the sweep of leading and trailing edges are varied in turn, should not be difficult to carry out. By re taining the same span and area throughout, the aspect ratio would remain constant, and in this way the true effects of sweep would be accurately determined. R.A.F., Henlow, Beds. C. H. LATIMER-NEEDHAM. Going Abroad IN July the A.A. foreign air touring business transacted at Heston was more than double that of the previous month. Thirtv-eight carnets were issued and fifty-nine maps were hired out. There was a big exodus of private aeroplanes during the Bank Holiday week-end. Thirty-two left on July 30 and twentv-one on the following day. Most of these, however, were bound for English destinations. It is perhaps not generally known that owners of the Air Touring Card can hire maps, not only for the whole of Europe but for North Africa and for the East as far as Singapore. Maps of the London to Capetown air route will also soon be ready for circulation. A 30s. deposit is payable, in addition to the hire fee of 5s. per map, but in the case of hired aircraft Air Hire, Ltd., assume responsibility for the safe return of maps.
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