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Aviation History
1934
1934 - 0969.PDF
SEPTEMBER 20, 1934. FLIGHT. two spars of a size which could be housed in the thin wing- section employed would not have provided sufficient strength, even if they were of solid wood. To obtain the requisite strength the wing covering had to be made of a form which would reinforce the spars. The system finally adopted was one in which certain members having the appearance of orthodox wing spars, and of similar con- struction, i.e., box-section, are used for taking the shear loads and for transmitting the loads to the covering. Bending and torsional loads are taken by the skin, which is in the form of a spruce planking laid on after the manner of " double diagonal " planking of many lifeboats. That is to say, there is an inner and an outer layer, each com- posed of spruce strips some two inches wide, the strips of inner and outer layers crossing each other at approximately right angles. The thickness of this skin or planking is reinforced where the stresses are high, such as on the upper surface near the root of the wing, by a third and even a fourth layer, reaching in places a thick- ness of more than half an inch. At other places, such as at the tips where the loads are small, the planking is only about one-eighth of an inch thick. Fuselage Construction For the fuselage a somewhat similar construction has been adopted. The shape is almost a perfect streamline, but not quite. The sides, which are not of great depth, are flat, and there would be no point in using the double- diagonal type of planking for them. Consequently they are covered with plywood in the ordinary way. The top and bottom of the fuselage, however, have a double curvature, and are planked with spruce strip in the same manner as the wing. This is necessitated by the fact that sheet material cannot be bent to a double curvature. It can, to put it in a different way, be bent into the form of a cylinder or cone, but not into the shape of a barrel. The use of diagonal strip planking makes it easy to get a smooth double curvature. The nose and tail fair- ings of the fuselage are of beaten Electron sheet. From an aerodynamic point of view the de Havilland " Comet " is interesting on account of the trouble taken to reduce drag to a minimum. The fuselage is, as already mentioned, of nearly perfect streamline form. Its maxi- mum cross-sectional area has been kept down to the mini- mum which would house the crew, and by placing the cockpit well aft, increasing the fuselage depth slightly behind the cockpit, and using a flatly-sloping windscreen, the break in the airflow caused by this necessary ex- crescence has probably been reduced to a minimum. The view obviously cannot be as good as if the cockpit were in the nose of the fuselage, but in a racing machine something must always be sacrificed for speed. The Retracting Undercarriage The fact that the machine is a twin-engined monoplane has brought with it the almost inevitable use of retractile undercarriages, the wheels of which draw up into the tail. fairings behind the engines. A mudguard over the front of each wheel prevents stones, etc., from being flung into the airscrews, and when the wheels are raised this same mudguard forms part of the bottom of the engine nacelle, leaving merely an opening large enough to let the air escape from inside the engine cowling. Sheet metal fillets are used where the wing joins the fuselage in order to reduce interference and keep the aii- flow as smooth as possible. The same system has been employed on the tail, fillets being used to run the fin surface gradually into that of the fuselage and tailplane. The rudder extends down to the top of the fuselage only so that in straight flight and for small rudder movements, there is an easy path for the air to follow and eddying is reduced to a minimum. By giving wings and fuselage several coats of paint, rubbing down between coats, and repeating the process, a remarkably smooth surface has been obtained, which by reducing skin friction in such a fast machine should add materially to the speed. A machine with the aerodynamic '' cleanness '' of the "Comet" will obviously have a very flat gliding ang*e, and when most of the fuel has been consumed, so that the wing loading is reduced, it might be a matter of
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