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Aviation History
1946
1946 - 0614.PDF
FLIGHT MARCH 28TH, 1946 Tyres for To-Morrow Introducing the Squat Dunlop t Brake Testing in the Qrand Manner : Qun Qear and Flame-proof Pipes ARTISTS who depict "air liners of the future" like to/-% give scale to their subject by representing undercarriages i. lof juggernaut aspect, towering above stupefied on-lookers. Now, with the introduction of the Dunlop "squat" tyre, or Compacta, to give it its ttade name, wheel diameteris no longer a true criterion of aircraft size and weight. This was proved last week at Honiley, near Coventry, where anexperimental flight manned by Dunlop personnel is doing final tests on the new tyre and on advanced types of aircraft brakes. The essential merit of the squat tyre is its greatly reduceddiameter by comparison with a normal pattern of equivalent pressure and 'capacity. This means easier stowage in new aircraft with thin wings and closelyspaced spars. In other designs it will preclude bulging the nacelle. A typical tyre of normal form at golbpressure has a diameter/width ratio above 3 25. For " wheel-less" low pres-sure tyres, as used for tail wheels, a ratio of 2.5 is common but disallows adequate merit in certain aspects of shock absorber performance. As with all jiew developments of this sort, incidental advan-tages and disadvantages accrue. The reduced rolling radius, for example, increases brake power for a given torque andreduces side ioads on the moving assembly; on the other hand, the projected area is slightly greater than for single or twinnormal tyres at the same pressure. The real advantages of the squat tyre are chiefly apparentto the aircraft designer and could not be perceived from the Honiley control tower when Mr. SutclifFe, the Dunlop test The "squat," orCompacta tyre (left) should provea real boon to designers of largeaircraft as sug- gested by the viewof the Lancaster used for tests. brakes. The use ot twin wheels on a common leg, favoured inAmerica and Germany, is an attractive solution if a single leg is permissible, but in any case the squat tyre offers importantadvantages. Although the wheel with this tyre is lighter than a normal wheel it is rather heavier than equivalent twin wheels(all wheels at the same pressure), but for a true comparison the whole undercarriage• must be considered and the squat-tyre undercarriage assembly is the lightest. A Lancaster is flying at Honiley with twoot the new tyres, only 41 in in diameter and weighing 4881b each. A normal pattern atoolb pressure, as used on the Lancaster, is 62in in diameter and weighs 6261b. TheBrabazon I is to have two pairs of 65-inch " squats ' on single legs. The diameter/width ratio of the new typeis *ess than 2. The characteristic flat-crown section is achieved by constraining thecrown, which would otherwise assume its natural torroidal shape, by binding with syn-thetic cord. For initial trials a 9olb pres- sure is being used and tyre life has beenfound to compare favourably with a normal tyre at the same pressure. Full deflectionof the normal tyre occurs at about three times the static load, whereas for the squattyre the figure is about 2.5. Due to this reduced deflection the total energy absorp-tion is less than normal and some extra leg travel will, no doubt, be necessary, but thisapparent disadvantage may permit improve- The " Seiectric'' gun firing controlwith its safety flap. The next development is a "streamlined"model for catapult-seat aircraft. pilot, landed a Lancaster to demonstrate the effectiveness 0;the latest brakes as applied to the new tyres. A cross wind of 20 m.p.h. did not prevent his pulling up in about 350 yardsfrom touch-down, though usually, he said, the figure is more like 300 yaids. He also flies an Albemarle which, with itstricycle undercarriage, lends itself well to demonstration; this he stops in 250 yards or less. A Meteor and a Lincoln willeventually complete the Honiley "stable." Brake testing on massive flywheels is one of the more spec-tacular sides of research, but the proving of the new flame- proof flexible pipes seems even more stringent. The companyput in a great deal ol work in developing the new pipes. Even now that fire extinguishers have been so highly developed it isnecessary, in the event ot an induction fire, for the engine to function for a short period during which the airscrew can befeathered, the petrol switched off and the fire extinguished. The usual type of flexible hose fails rapidlyif exposed to a petrol fire in a nacelle, but Dunlop's have now developed a pipe whichwill withstand an engine fire without failure for sufficient time to permit combativemeasures to be taken. For testing one of the new pipes it is filled with petrol or oilat the maximum pressure likely to be met and bent to its minimum bending radiu^"before being subjected to the flame of a blow-lamp at about 1,000 degrees C. Two types of hose have been developed;the more popular is a wire-reinforced high- pressure type comprising a synthetic lininglube over which is braided successive layers of cotton, high-tensile wire and cotton, thewhole being finished with an outer cover oi synthetic rubber to resist the action of theflame. Asbestos braiding can be incor- porated further to improve flame resistance.This type of hose is preferred to the second variety, which incorporates heat-resistantcotton and materials like glass fibre and asbestos, because it is possible to maintaina smaller outside diameter for a given bore size. It has the disadvantage of weighingmore.
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