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Aviation History
1953
1953 - 1433.PDF
FLIGHT, 30 October 1953 NON-SKID BRAKING The Maxaret Automatic Control: What It Is and What It Does FOLLOWING extensive testing and development, the Maxaret automatic brake control has attained a high degree of efficiency and reliability, and we readily afford space for this concise explanation, prepared by the Dunlop Aviation Division technical staff responsible for the work, of the operation of the unit and the results obtained. The control permit is maximum braking effort to be applied, under any conditions, without danger of wheel-locking. ON a modern aircraft there is no accurate feed-back of "feel" from the wheels to warn the pilot of the onset of a skid, particularly where a multi-wheel or bogie undercarriage is employed. Skidding wheels reduce braking efficiency, and the damage done in a few seconds as a result of locked wheels can result in burst tyres and possibly in serious damage to the aircraft. Sensitive to that sudden drop in wheel revolutions which is the prelude to a skid, the Maxaret automatic control temporarily relieves the brake pressure at the wheel or wheels concerned. The improved braking efficiency thus achieved and the elimina tion of the pilot's fear of over-braking has resulted in a marked reduction in landing distances. The summary to a recent report on a series of tests carried out on aircraft with and without automatic braking states quite simply, "From the evidence avail able to date it would appear that the fitting of Maxaret brake units results in up to 30 per cent reduction in braking distances." The Maxaret, a small unit weighing 4.7 lb, is introduced into the hydraulic brake circuit. No major modification is necessary as it can be mounted on the brake torque plate or any convenient structural member. Each unit comprises a sensitive valve mechanism controlled by a small flywheel assembly housed in a rubber-tyred shell, and provision must be made for this shell to be rotated by direct contact with the rim of the landing wheel. The Maxaret unit is coupled into the normal brake feed- line, the only additional piping necessary being from an exhaust connection to the hydraulic return line. Briefly, the unit functions as foEows. The shell is keyed to a spring clutch which provides a positive drive for a flywheel drum in one direction only, the drum in turn driving the flywheel. The driving segments of the drum permit 60 deg of relative move ment between drum and flywheel. With the wheel spinning normally these components all attain the same rotational speed, but, should conditions cause the landing wheel and consequently the shell to decelerate, the clutch immediately disengages the drive between the shell and the drum and the flywheel advances 60 deg to drive the drum. This relative movement operates a valve mechanism to release the pressure in the brake until the wheel regains speed and the driving sequence is restored. Immediately after touch-down the pilot can apply full braking pressure in the knowledge that any tendency to wheel locking due to lack of weight on the wheels will be counteracted auto matically. Thus full pressure may be used from the very beginning of the landing run, and the braking effort applied at the wheels will automatically increase as the load on the wheels increases. A major factor hitherto prohibiting the use of full pressure immediately after landing is the low value of the coefficient of friction between tyres and runway at high speed. For dry concrete it is estimated at betwen 0.75 to 1.00 at 30 m.ph., 0.5 to 0.7 at 80 m.p.h., and only 0.3 to 0.5 at 120 m.p.h. To avoid locking the wheels under these conditions it has been common practice to apply intermittent or progressive braking This prevents serious damage to tyres but, as the pilot of neces sity has to play for safety, it does not result in the shortest run. After the initial touch-down, the wheels, or an individual wheel, may be thrown clear of the ground as a result of surface irregu larities. When this occurs the Maxaret immediately relieves the brake pressure in the wheel or wheels concerned, enabling them to continue to spin and ensuring against their renewing contact in the locked condition—an inevitable result if the brake pressure continued to be applied in the normal way. In the event of the aircraft being bounced clear of the ground, the flywheel will con tinue to rotate for four seconds, holding the brakes in the off position, so that, on touch-down, the wheels will be spun up freely (to a speed consistent with the forward speed of the air craft) before the brakes are again applied. Where the danger of overshooting exists in conditions of bad (Right) The compactness of the Maxaret unit is emphasized in this typical fighter mainwheel instal lation. (Below) Assembly of the unit. MOUNTING BRACKET BACKPLATF FLANGED SE HOUSING BALL BEARINGS VALVE BLOCK FLYWHEEL AND SHELL ASSEMBLY visibility, or when an emergency arises on the rare occasion when a runway becomes obstructed, good braking is of vital importance. By the use of automatic brake control, one of Britain's fastest transport aircraft, under test, was brought to rest in less than 500 yards from the point of brake application. Should wheels become locked during a strong-cross-wind landing on a wet or icy runway, an aircraft may quite easily be blown sideways off the runway. Automatic braking enables the pilot to maintain directional control by the effective differential use of his brakes under these conditions. In the unlikely event of a Maxaret unit becoming unservice able in use, it is designed to "fail safe" and will continue to trans mit maximum fluid pressure to the brakes in the normal way. In practice, as the unit functions, the brake rarely has time to come" fully off. Immediately it operates, braking pressure is reduced by a very small spill of fluid to return. As full operating pressure remains available at the supply to the Maxaret unit a quick reaction is ensured as the wheel speeds up again. Following exhaustive laboratory testing, hundreds of landings were made with Maxarets on a D.H. Devon (converted to hydraulic braking) and a Handley Page Hermes V. Some very con siderable and satisfactory experience with the units has sub- sequendy been obtained on a number of the latest types of service aircraft, including the English Electric Canberra, the Handley Page Victor, Avro Vulcan and Avro Canada C.F. 100, and the Short and Harland S.A.4. After successful trials made under the severest airline operating conditions, Maxarets are now being specified for Comets and Viscounts. In an article on the Vickers Viscount, Mr. G. R. Edwards said of the automatic brake control: "More recently, the application of Maxaret brakes has demonstrated the possibility of achieving extremely short landing distances. The tests on icy runways in Canada have been most encouraging." A Swedish report comments upon the ease with which take-
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