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Aviation History
1950
1950 - 1046.PDF
666 FLIGHT, i June 1950 TESTING AIRCRAFT BRAKES . _ figures that an allowance of this magnitude is essential, at least until further experience is gained and the necessary data obtained. Test No. 2 is to determine the static torque developed backwards—which should be sufficient to hold the aircraft on a slope of one in ten. It can be omitted if the design of the brake renders it unnecessary, and, as an expander- tube brake functions equally well in either direction, this test was not carried out in this instance. It is usually only necessary in the case of brakes incorporating a servo action. In the Reduced-speed Stopping Test, two stops were made to determine the action of the brake when applied fully at speeds equal to 10, 30 and 50 per cent of 1.1 times the flaps-down stalling speed of the aircraft concerned. Under certain conditions the torque developed could con- ceivably be in excess of the allowable tolerances of +50 per cent and — 25 per cent of the normal torque. The General Performance and Wear Test entailed making 25 "full K.E." stops and, as the title suggests, was made to determine the general characteristics and reliability of the brake. During each stop a series of observations were -Biade, including the time to bring the flywheels to rest, the maximum temperatures reached by the-brake dram a»d brake expander-tube, and the wear that had occurred in friction lining. This information and the general performance in the test are useful in compiling the relevant maintenance and servicing instructions and in determining provisional periods between inspections and lining replacements. The brake drum at the end of each run was glowing red and the tem- perature reached was estimated to be 1,200 deg F. At the end of this test the drum was blackened and scored but was still considered serviceable. Guidance must be given in the Pilot's Notes to avoid inadvertent use of the brake beyond the kinetic energy absorption-capacity. Normal everyday use should not exceed more than three quarters of tie maximum capacity. The Elapsed Time for Repeat Stop Test was then carried out. A run was made and the temperatures of the brake drum and brake expander-tube and wheel rim at the end of the stop were recorded. The temperatures were then measured at minute intervals. This is necessary since there is heat radiation from the drum (which obviously cools steadily) to the brake and wheel, the temperatures of which rise until a peak is reached. The time taken for lie brake assembly to cool sufficiently to allow a repeat stop to be safely made is noted and the information given in the Pilot's Notes. Re-application of the brakes for parking an air- craft immediately after they have been used to arrest a landing-run may result in the rapid overheating and conse- quent damaging of the lining through its contact with the very hot drum. From the above test it is possible to establish a minimum cooling period which must be allowed before parking-braking, and this figure is given in the Pilot's Notes. Test No. 6, described in the B.C.A.R. Manual as the Overload (or "Accelerate-Stop") Test, was next carried out, and for this the inertia machine was set to give 1.5 times the normal kinetic energy absorbed by the brake. The purpose of this is to prove the brakes capable of func- tioning should they be applied with the aircraft speed higher than normal. It covers the case of brakes being applied excessively early during the landing run, and also the occasional instance in which brakes have been used and, in the event of emergency during and immediately follow- ing take-off, have to be used again. The test is undoubtedly the most severe of the whole series. During the run, thick smoke was emitted from the brake and the exceptionally high temperature of well over 2,000 <leg F was estimated to have been reached by the drum. It is a requirement that such overloading shall not cause total failure of the brake, nor should the torque exceed twice the certified maximum. That the Palmer brake, although exceedingly sooty and blackened, was still serviceable after this test, was due in considerable measure to the special heat-resistant expander- tube developed in this design. For the final or Over-use Test, two stops were made within five minutes of each other with no cooling between them, and the torque recorded. The requirements are that the condition of the brake throughout and after the test must not be such as might cause damage to the tyxe, wheel, or any part of the airframe, although damage to the brake itself is permissible provided both stops are accomplished, while the torque should not exceed three times_the certified maximum continued braking torque. In this instance these conditions were fulfilled. The purpose of this particular test is to reproduce the effect of continuous use of the brake —for example, heavy application during a landing run with immediate and extensive use during taxying. During the test seen by the author, nearly 1,000 separate readings of temperatures, stopping time, drum distortion and lining wear were taken by the Palmer test engineer and recorded on an impressive set of forms prepared for this purpose. More readings were taken than the test con- ditions required; they will be studied in the development of fulmg brakes. Rarely are the rewards for such rigorous testing so readily apparent as in this case. During the recent 40,000-mile Australasian tour by the first production Marathon, GA-ALUB, 250 landings were made; between these, it is stated, no servicing, adjustments or replacements had to be made to the brakes, which at all times gave full perform- ance. Since the return of the aircraft to this country they have been inspected and are still serviceable. It is interesting to compare the weight of this brake, which complete with the drum is "under 15 lb, and the kinetic absorption capacity of 1.175 x ioeft/lb, *or which it is now fully approved. Dividing one into the other gives the figure of 13 lb weight for an absorption of one million ft lb of kinetic energy, and on this basis it is probably the lightest brake of its type yet developed. It appears to bear favourable comparison with some of the latest types of disc brake where figures of 18 lb weight per million ft lb of kinetic energy absorption are obtained. Whilst there are many factors affecting the applicability of a certain type of brake for use on a particular aircraft, these figures appear to indicate that, upon a weight basis, the drum brake is still suitable in many cases for the most modern aircraft. Left, the test brake assembled in its drum In readiness for the Vrun ; right,it, tne assembly is seen withdrawn for inspection.
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