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Aviation History
1949
1949 - 1290.PDF
54 FLIGHT JULY 14TH, 1949 Wheel Brakes friction members without producing excessively high temperatures in the rotors. Each rotor, on the other hand, is not a continuous ring of metal, but is made as a series of separate segments. These segments are flexibly attached to each other by special connecting members. The resulting segmented rotor allows higher working tempera- tures without the warping or cracking which result from heat expansion if one-piece construction is used. A self- aligning effect, in which the segment surfaces tend to lie flat against the face of the lined member during brake application, is another advantage of the segmented rotor. The segmented rotors are keyed to the wheel at their outer periphery, so that fhey turn with the wheel but have the normal limited free axial movement to permit application and release. The rotors are pressed between thinner discs called stators, which are faced with friction material. The stators do not turn with the wheel, but are keyed to the brake carrier; similarly to the rotors, they have limited free axial travel for application and release. The friction lining of the stators consists of a number of separate segments instead of a continuous ring, thus providing a ventilation channel between each segment which allows air to circulate and cool the rotor. An important result of this internal ventilation is the elimina- tion of "fading," or decreased brake effectiveness, toward the end of periods of severe application. In addition, lining dust resulting from lining wear is scavenged into the ventilating channels, promoting better friction contact between the stator and rotor surfaces and reducing the required pressure for a given friction effect. The third main structural element of the brake is the carrier, which is rigidly mounted to the support member of the strut. An annular chamber in the carrier is fitted with a piston ring and seal, providing means of hydrauli- cally forcing the rotors and stators into contact. Variable Dimensions The basic design can be adapted to wide variations in diameter and thickness to suit the available space within the wheel. For a given total braking area the brake may be a large-diameter assembly using possibly only one rotor, or it may be a small-diameter, thicker assembly, using several rotors. A result of the design is that water within the brake apparently has no adverse effect upon its operation; tests have indicated that even the pressure of oil or hydraulic fluid does not seriously impair its effective- ness or smoothness of operation. Sizes have been developed ranging from five inches in diameter to 31 inches, using in some designs only one rotor and in others—such as the high-capacity brakes fitted to the Lockheed Constellation—as many as four rotors. As an illustration of adaptability, the braking of an aircraft can be reduced in capacity and weight even after it is in service, should (for example) altered operating con- ditions reduce the braking requirements. This can be done by removing one or more rotors and their accompanying stators and substituting a light-alloy spacer, so effecting a useful saving in weight. The high efficiency now attained by brakes of all types is due in no small measure to the linings fitted. The aim of the brake-lining manufacturers is to produce a friction material which will give a consistent brake effort under all conditions required of it. Owing to the rapid advances and changes in aircraft design to-day, calling for ever higher landing speeds with reduction in wheel diameters, the temperature range over which braking consistency is (Left) Small British Messier "Multi-spot" brake, which is also suitable for use in a car. (Above) The " Multi-spot" brake and wheel assembly. required is an ever-widening one. A further continual aim is to avoid high-temperature fade and wear, and good progress has been made in this direction. The stability and wear rate of a lining is, generally, more important than the actual coefficient of friction obtainable. Well-known names amongst those manufacturing high- duty materials for aircraft are British Belting and Asbestos, Ferodo, George Angus, Richard Klinger, Duron and Ray- bestos and it is a remarkable achievement that they have been able to produce materials which will stand up to the arduous duties required and at the same time give con- trolled friction. From the lining-manufacturer's point of view the major difficulty arising from the trend towards the ventilated-disc type of brake is the varying rubbing speed from one side of the brake pad to the other, which causes uneven wear. An interesting process in the manufacture of expanding- tube brakes by Dunlop is that of "running in," which is the last operation to be done before delivery. The brake linings are mounted in specially adapted disc grinders and their outside diameters trued up. The unit is then trans- ferred to a running rig comprising a dummy brake drum driven through suitable gearing. The backplate is mounted on a rigid arbor and fed inside the drum, while compressed air is introduced to inflate the bag, and thus expand the shoes, The Tunning-in process is then continued until a well-bedded friction surface is obtained. This process has been adopted by Dunlop and some other firms to avoid the user having to wait until excessive taxying has been done before optimum results are obtained. It particularly applies to some of the very latest extreme- duty materials, which do not give their best until they are run-in against a red-hot surface. The distance which an aircraft runs after landing depends not only on the efficiency of the brakes, but also on the skill of the pilot, who has to try to maintain the braking at close-to-maximum throughout the stopping period—a time when his attention is probably concentrated in other directions. This being so, considerable thought has been given to the possibility of using the potential energy of an aircraft to effect its braking. The amount of kinetic energy available in an aircraft landing at high speed is phenomenal —in a machine of 20,000 lb weight, landing at 100 m.p.h... it is approximately 6£ million ft/lb. In 1934 an experi- mental Messier aircraft was fitted with brakes controlled by the pressure in the undercarriage shock absorbers. Later; that year tests were carried out with the tailwheel shock absorber acting hydraulically to apply the brakes. The results were excellent and enabled the aircraft to land in a distance 25 per cent less than normal, and this without- any risk of skidding. Such a scheme, however, has the disadvantage of com^ plicating the duty of the shock absorber, and normally; there will already be ample power sources available om board. In addition, as the brakes must be able to operate! at take-off conditions, it would be necessary to store energy B 26
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