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Aviation History
1943
1943 - 0958.PDF
- 390 FLIGHT APRIL TSTH, 1943 A CO-OPERATIVE CHALLENGER Cooling air entered the duct at the forward end, was ex-panded and slowed down in the duct, and allowed to escape at the stern. •• riiau " phetogravhs. At the wing roots the ratio of wing thickness to wing chord was 16.2 per cent, and at the undercarriage attachments it was 12.8 per cent. The thickness at the tips was 9 per cent. The wing as a whole was, aerodynamically , untwisted. The camber at the tips appears to have delayed tip stalling satisfactorily, for when the machine stalled and crashed it did not hurriedly "drop a wing." The basis of the fuselage outline was a good streamline shape of low drag, and in order to spoil it as little as possible the windscreen was as low as was considered com- patible with sufficient view, while the cooling system was, as previously mentioned, of a novel type in order to avoid the unsightly and drag-producing '' whiskers " of an under- slung radiator. Put briefly it was ducted cooling, with a special Gallay radiator housed in a tunnel in the bottom rear portion of the fuselage. All control surfaces were carefully mass-balanced and provided with mass balanced trimming tabs. It was thought that the ailerons in particular would need careful setting of the tabs, but the torque reaction was found to be damped out by the wings. " Compregnated " Wood So far as the structure is concerned, it is interesting to record that extensive use was made of Saro compressed and impregnated timber and plywood mouldings. In some places the timber was compressed to 62.5 1b./cu. ft. The main wing spars were of box type, the rear spar straight but the front spar having a backward rake which necessi- tated metal fittings at the sides of the fuselage. The wing covering was fairly thick birch plyjvhich, combined with closely spaced ribs, ensured freedom from waves and wrinkles. The leading-edge was a preformed plywood moulding. The fuselage was of birch plywood with spruce frames and stringers. The lower part formed the radiator duct and was divided by a horizontal panel into two compart- ments, the upper being intended for " skimming" the turbulent boundary layer and keeping it away from the radiator duct. The speed of the cooling air was gradually reduced by increasing the duct area, so'that by the time the air reached the radiator it was travelling at quite low speed, something like one-third of the flight speed. It was led into the narrowing angle between the two Gallay radia- tor portions, which were arranged in the form of a vee, and were of such large surface area that the flow through the radiator was reduced to about 30 m.p.h. The air then passed out through the open stern of the fuselage. Very special care was taken in connection with the con- BOUNDARY LAYER DUCT \ DUCT ENTRY RADIATOR EXIT DUCT EXIT ENTRANCE. The principle of the ducted Gallay radiator is shown inthe diagram, and the photograph shows the feed pipes on the outside of the duct. trol surfaces. For instance, it was realised that at the speeds contemplated delicacy of control would be essential, and a very neat and simple variable-speed device was in- corporated which gave very small movement of control surfaces near the neutral position for a fairly large move;.. ment of the control stick or rudder pedals, but the gearing increased rapidly as the angle of maximum control angle, was approached. Avoidance of backlash in control surface tabs is always a serious problem in high-speed aircraft. In the racer this was achieved in a very simple manner: a torque ro«l had its free end bent over at an angle of about 30 deg. and the free end engaged, without "play," in a slot in the tab. Control surfaces, by the way, were of metal con struction with fabric covering. Ailerons were of the Frise type, with the hinge line almost on the upper wing surface. The gap between aileron and wing was completely sealed on the upper surface. Mollart universal joints were used where angles in the landing flap spars had to be negotiated. Dowty hydraulic control was used for the wing flaps and undercarriage. The Dunlop wheels were mounted on fittings of the Dowty "levered suspension" type, and it will be noticed that the struts were raked forward at > rather unusual angle. This was due to the fact that they were pivoted to a stout universal member housed between the front and rear spars, i.e., farther aft than usual.
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