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Aviation History
1928
1928 - 0061.PDF
JAMUABV 26, 1928 THE .AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT WHEEL BRAKES AND THEIR APPLICATION TO AIRCRAFT Brake Design and Control. By G. H. DOWTY, A.F.R Ae.S., M.I.Ae.E. (Continued from p. 80.) Although much can be learnt from automobile practice in wheel brake design and construction, yet the requirements of aircraft brakes are, in many ways, dissimilar. The follow- ing are some of the chief differences that should be noted :— Aviomobile Brake Requirements. 1. Equal application of braking on all braked wheels. 2. Ability to absorb power for long periods. 3. Adequate provision for cooling brake drums. 4. Braking demanded at frequent intervals. 5. Provision for renewal of liners. 6. Brake drums generally mounted external or partly external to wheel. 7. Operation of brakes from one control. Forms of Brake. Braking members generally used may be divided into two classes, each of these depending upon the property of frie- tional adhesion between surfaces held in contact by consider- able pressure. The usual construction is the attachment of a pressed steel drum to the wheels and the retarding effect can be produced by an internal expanding shoe or by an external contracting band. The internal expanding shoe brake is more popular on automobiles, possibly due to its neater arrangement and better facilities for cooling. It would appear that this type of brake is the better one for aircraft purposes. The whole unit can be totally enclosed within the wheel, therebj' gaining protection against oil, dust, etc., and the operating means can also be enclosed. The band brake has little to recommend it except cheapness, and perhaps a slight saving in weight. Against this there is the great disadvantage of the cumula- tive action of the brake, the band tending to wrap itself round the drum with ever-increasing tightness. While this may be Fig. 10. Aircraft Brake Requirements. 1. Independent operation to each wheel for provision of steering. 2. The longest period of braking will not exceed 20-30 seconds. 3. The temperature increase will not be great during the short period of operation, and cooling is of smaller importance. 4. Braking only required during limited period of ground operation. 5. This is not absolutely essential due to the restricted use of brakes. 6. Brake drums must be within the wheel or enclosed by rim to hub fairing . 7. Each wheel must be braked from a separate control. 8. Weight must be reduced to a minimum consistent with adequate rigidity. This can best be obtained by the use of light aluminium alloys. Essential features common to both classes of brakes are :— 1. Self-balancing, i.e., elimination of unbalanced forces when brake application takes place. 2. The brake should preferably be self-energising. 3. The complete brake system should be exceptionally rigid to withstand vibration and distortion, both being primary causes of brake inefficiency. tolerated on an automobile, it must not be permitted on an aeroplane wheel. With some forms of internal expanding shoes this cumulative action can be obtained, but discussion of this point will be deferred. Brake Linings. The coefficient of friction of Ferodo fibre or bonded asbestos ferodo is not appreciably affected by temperature, pressure or speed. For a temperature of 200 C, pressure 100 lbs. per sq. in. and a speed of 6,000 ft. per min., there is no diminution of the value below 0-3. The work absorbed by a Ferodo lining varies from 100,000 to 120,000 ft. lbs per sq. in. per min., when fx= 0 • 3, and the pressure varies from 50-80 lbs. per sq. in. Standard automobile practice is to design on 80 lbs. per sq. in. pressure for brake linings under normal load. The following table gives the coefficient of friction for brake linings. This information has been taken from the Practical Engineer Handbook. It will be noticed that the effect of lubrication is to reduce the efficiency of the brake, and care should therefore be taken to see that the brake is not exposed to oil from the engine. Preferably the complete brake unit should be enclosed. Dissipation of heat is not so important as in automobile work, and advantage can be taken of this point and the brake placed within the wheel.
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