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Aviation History
1934
1934 - 1478.PDF
SUPPLEMENT TOFLIGHT 92 THE AIRCRAFT ENGINEER DECEMBER 27, 1934 retractable flap and associated controls. The former has already been fitted to a R. A.F. 28 section wing and involves " Park bench " ailerons, the latter is shortly to be fitted to a R.A.F. 48 section wing ; both wing areas are 294 sq. ft. The Zap installation weighs 8 per cent, of the all-up weight of the aeroplane and, from Fig. 4, should give a 75 per cent, increase in Kh max. The retractable flap installation weighs 2 per cent, of the all-up weight and, from Fig. 4, should give a 40 per cent, increase in A*L max. The weight figures in both these cases are for experimental installations on existing wings and are probably unduly heavy on that account, but even so a device which itself weighs only a tenth of the increase in lift that it gives may be regarded as good. Speeds of operation of commercial and military aircraft are steadily rising owing to competition and in the U.S.A. there is hardly a passenger transport aeroplane not fitted with some form of flap, and some of their military machines are now fitted with slots and flaps. Where high top speed is essential, landing devices will be used, not to reduce the landing speed, but to keep it the same, increase the wing loading and so obtain a higher top speed. Calculations show that even with a split flap used for landing only (on account of its high drag) the performance of a typical two-seater fighter could be in- creased by at least 10 m.p.h. on top speed and a variable ' amount on climb according to how the aspect ratio is changed. A maximum lift coefficient of 1.1 which is really usable at a normal stalling angle now appears to be possible ; this permits a wing loading of 20 lb. per sq. ft. for a stalling speed of 60 m.p.h., and 30 lb. per sq. ft. at 74 m.p.h., which is not an impossible speed for a large flying boat. SOME NOTES on the PARIS AERO SHOW By H. J. POLLARD, Wh.Ex., A.F.R.Ae.Soc. Mr. Pollard, as many readers will know, is on the Technical Staff of the Bristol Aeroplane Co., Ltd. Below he comments on Metal Construction, Three-ply Construction, and several other Aspects of the Machines exhibited at the Paris Aero Show OF the seventy or so aircraft exhibited at the ParisShow, the majority were, of course, monoplanes, ofwhich less than half were " metal covered " in the usually accepted sense of the term; that is, while the main surfaces are metal-clad, the rudder, ailerons, etc., may be fabric covered, a convention which permits the use of the term " all metal," just as a biplane of which the general structure and the internal framework is of metal construction is referred to as " all metal " in spite of complete fabric covering. , A question at once arises that requires an answer. Why were most of the monoplanes shown three-ply covered? The answer most frequently given is that in small-quantity production the use of three-ply has an appreciable influence on cost. If, however, long service in all parts of the world is taken into account, then few will argue in favour of three-ply, particularly where the climate is moist and hot. At the same time large numbers of aircraft will be operating in more temperate and favourable climates; consequently the use of three-ply as a material for " stressed-skin structures " is worthy of further consideration. The fundamental expression for the critical stress at which plane panels buckle under shear or compressive forces acting in the plane of the panels is p = kE (t/b)». where p = Critical stress E = Young's Modulus t = thickness of sheet b«= width of sheet k =-- constant By working on a weight-for-weight basis, and using appropriate values of Young's Modulus and density, it is easy to show that three-ply in panel form is a more economical proposition than any sheet metal. Here, then, is one good argument for its use. Against it there is the very important consideration of water soakage. It is well known that the weight of a ply-covered boat hull can be very adversely affected through this cause. The same thing, to a much smaller degree, of course, happens with landplanes made from ply—with ultimate dis- integration of the material. In the case of large landplanes such as cannot readily be moved into hangars, or for the largest aircraft that would have to remain out in any weather, the use of three-ply is unthinkable. The comparatively large percentage of civil aircraft ex- hibited in Paris, made from three-ply, forces one to review all the arguments that can be urged in favour of the material. Cost for small production (which is another way of making allowance for factories ill-equipped for metal work) has already been mentioned. The funda- mentals in regard to the strength of sheeting under planar forces is a second favourable argument. A third argument often used relates to external appearance, i.e., freedom from buckles. This argument immediately loses its force when really thin ply, say rsin. thick, is used. Thick ply, as used on the Heinkel 70 body, has always a good appear- ance—so long as it is not exposed to moisture for a long time. Judging, however, from the appearance of the three-ply exhibits they were no whit better, on the average, than those made from sheet metal, and the best looking body on show—the Czechoslovakian Avia 534—was made of metal; the third argument, therefore, cannot be admitted. Strength and Cost A fourth and last point put up in favour of three-ply is that it is readily repairable with tools available anywhere in the world. A better way of stating the case appears to be that structures made from ply are less susceptible to damage, since if, for example, a bad landing is made, sufficient to cause damage, then the damage is not likely to be less than a complete "write-off," for if the forces are sufficient to buckle panels beyond repair, then the splinter- ing of the framework used for the attachment of the panels would be a certainty. All the available evidence appears to indicate that ordinary metal monocoque con- structions are adequately robust. Summing up on this point, it would appear that the main factors in favour of three-ply are low initial cost and high strength-weight ratios. As to the latter, conclusive data are difficult to obtain. From the material laws it follows that three-ply constructions should be
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