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Aviation History
1935
1935 - 0013.PDF
JANUARY 3, 1935. FLIGHT. PISTON ALTERNATIVE THUMB PRESSURE TYPE CONTROL Section through the rudder-bar differential unit in the Palmer pneumatic system. (Left) General arrange ment of the components and controls in Palmer pneu matic-braking mechanism. (Right) This partly sec tioned view of the Palmer drum shows the rubber expansion chamber by means of which pressure is applied to the friction surfaces. system avoids mechanical complication, as it leaves the brake operation unaffected either by the normal move ment of the rudder bar or by adjustment of the bar for leg-room. A parking control, too, is easily installed. The various working parts are adequately lubricated during the assembly of the brake, and no further lubrication should be necessary be tween the periodic overhauls of the machine except in the case of the cables, for which a grease gun is em ployed. It is claimed that there is a definite progressive '' feel'' to the brakes, and that no difficulty is ex perienced in obtaining maximum brake power. " Pneudraulic " Types For a long time one of the main dis advantages of brakes operated by compressed air was leakage of air in the system. In the Dunlop pneumatic type this disadvantage has been vir tually overcome, and the makers state that their design may be safely depended on for use as a parking brake. The Dunlop unit (see p. 6) is com pact, and consists of an annular expansion chamber or air bag, which, when inflated, presses the brake blocks radially outwards against the brake drum of the wheel. These blocks are located by metal clips which pass through slots in the annular ring. When it is required to release the brake the blocks are positively re- General arrangement, in diagrammatic form, of the Palmer hydraulic system. turned to the " off " position by springs disposed at in tervals round the drum. One small lever, which can be mounted in any position on the aircraft to suit the designer, controls the brake gear. Usually, it is found most convenient to attach this lever to the control column, where it can be applied without affecting in any way the pilot's control of the machine during landing, and leaves his left hand free to operate the throttle. Differential control is obtained by directly linking a dual relay valve to the rudder bar or pedals. Thus, when the brakes are applied by the master control and the rudder is in the " straight - ahead " position, both wheel brakes are applied with equal pressure. Movement on the rudder bar then decreases the braking effect on one wheel and increases it on the other. This alteration of the braking effect is progressive and smooth, and does not interfere with the pilot's ability to release both brakes entirely, by means of the hand lever, should the necessity arise. The differential action of the brakes can be observed by the pilot on a neat triple pressure gauge mounted on the dashboard. One pointer indicates the pressure in the compressed-air con tainer, and the other two indicate the pressure in each wheel brake. Of exceptional simplicity, the Palmer fluid-pressure aeroplane wheel
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