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Aviation History
1935
1935 - 0014.PDF
6 FLIGHT. JANUARY 3, 1935. brake consists of only two parts, the brake frame and the brake liner. The brake constitutes an annular expansion chamber to which is attached a complete ring of brake blocks, which, when the brake is in operation, are forced into frictional contact with the brake drum. The pressure required for even a considerable braking effort is small, owing to the fact that there is nearly 100 per cent, contact between the brake blocks and the drum ; and, as the pres sure exerted is uniform over the whole area, the drum itself can be made lighter than those normally employed with the ordinary shoe-type brake. Wear on the brake blocks is negligible. The brake drum support is machined perfectly circular and concentric with the wheel before the drum is placed into position, so that the distortion of the drum during the process of wheel-building and trueing is impossible. Should small distortions occur as the result of shock while in use, the efficiency of the brakes is unimpaired, as the expansion chamber to which the brake blocks are attached "breathes" in sympathy with any small irregularities in the drum. The friction surfaces can be renewed by lift ing out the liner and dropping another in its place. Optional Methods of Control Operation of the Palmer brake may be either by pneu matic or hydraulic pressure, and in each case there are numerous optional methods of control. Generally speak ing, for small types of aircraft the hydraulic system has been found more suitable, while on large machines air is generally adopted, as it is usually available from the supply used for engine-starting purposes. The principal methods of operation are: (a) hydraulic or pneumatic, with foot- operated individual control to each wheel; (b) pneumatic, with hand-operated control to each wheel; (c) hydraulic or pneumatic, hand-operated, with differential rudder bar control; (d) dual-control, hydraulic or pneumatic, for in stallation in training machines. In all cases pressure is conveyed to the brakes through a small pipe-line of alu minium, copper, or rubber. Special Palmer fluid is used in the hydraulic system, supply being maintained from a small reservoir. When operated by fluid, the brakes can be applied by means of two small pumping cylinders mounted on the rudder bar in such a position as to allow easy operation with the toe or h' \. Although the pedals do not, upon release, recover in stantly, the pressure falls to zero at once, the slow recovery being due to the flow back of fluid used to take up clear ance between the brake lining and drum. A "parking" lever may be mounted in any convenient position. In another type of pedal control, known as the " sealed " system, the fluid is under a small initial pressure. With hydraulic hand-operation plus a rudder-bar dif ferential system (which provides directional control on the ground by moving the rudder bar in the normal way), if the pilot finds it necessary to land with a certain amount of " rudder " to prevent his machine swinging and then to apply his brakes, the latter will not be differentiated. They will be applied equally to both wheels, giving a straight run until the rudder bar is centralised, when the operat ing mechanism comes into action, thereafter varying the braking effort according to the rudder applied. Single-lever Differential Control Control of Palmer pneumatic brakes may be effected by a fcot-operated relay valve, making the application and release of the brake as gradual as desired. Another method is to mount two of these valves together and operate them by a common hand lever giving differential action. A straight pull on the lever applies both brakes, and move ment to one side or the other gives a differential action. Yet another method of control for pneumatic operation is hand application, with rudder-bar differential action for steering. Straight braking is applied by a twist-grip- operated relay valve on the control column, or through the medium of any suitable type of Bowden lever and control. Differential action for manoeuvring on the ground is pro vided by two pistons operated by the rudder bar; these are retracted when not in use. Again, remote control for dif ferential units by a dummy rudder bar linked to the main bar and similar to that for the hydraulic system can be supplied. Upon application of the brakes, the pistons of the rudder-bar units extend immediately, and then differen tiate the braking effort at the wheels, according to the Two well-known pneumatic systems : on the left is the Dunlop and on the right the Vickers.
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