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Aviation History
1928
1928 - 0064.PDF
SUPPLEMENT TO FLIGHT 10 JANUARY 26, 1928 THE AIRCRAFT ENGINEER The design of correctly balanced brakes resolves into the location of the fulcrum pins, with reference to the disposition of the arc of shoe contact. In order that the relation between these points can be readily determined, Fig. 14 has been prepared. If the positions given on this figure are adhered to, then complete balance can be assured. It should be understood that the fulcrum pin may be placed at any distance from the drum centre, provid- ing the fulcrum is located on the angle stated. Taking an example from Fig. 14. When X = 120° and Y = 80° then cos Z = cos 0-85 = 31° approx. The included arc of contact for one shoe is equal to X-Z and in this case it is approximately 89°. protection for the brake apparatus as is possible with the disc type of wheel, where, the disc sides form their own fairings. The practice of attaching brake drums to the spokes of wire wheels does not appear to be entirely satisfactory, because it is difficult to imagine that adequate rigidity can be secured in this manner. The only aircraft wheels, complete with brakes, that have so far been manufactured in quantities, are the Bendix and Sauzedde wheels. From the published weights, it appears that the disc wheel has still a further recommendation, for it is appreciably lighter than the wire type wheel. A table of these wheel weights is appended. Tyre Size. 30 X 5 32 x 6 36 x 8 44 x 10 Weight Wheels (less tyres) with Brakes. Disc Wheel. Bendix T3Te. lbs. 22 30 31 60 Spoke Wheel. Sauzedde Type. lbs. 23-4 34-7 41-6 97-5 Weight of Brake only. Bendix. lbs. 9-0 Sauzedde. lbs. 9-r> 12-0 12-0 110 ! 12-5 24-0 21-5 If the frontal area and aerodynamic resistance are not increased bv the addition of brakes, then the total Fig. 13. Operating Cams. Cams are generally designed to give progressive brake action, and their form is determined by the travel and the rate of action required. Due to excessive wear on the faces, cams should be mounted with a view to ease of renewal. Disc v. Wire Wheels. It would appear that the disc type wheel possesses con- siderable advantages over wheels of wire spoke construction. Disc wheels provide a large internal space in which the brake drums can be accommodated, and there is no doubt that advan- tage will ultimately be taken of this space, for housing the undercarriage springing. It is usual practice to enclose spoke wheels with canvas or aluminium fairings, but these cannot provide such good loss in performance will be due to the added weights of the brakes and operating gear. Control. Brake control is usually carried out by connecting cables from the wheels to tilting type rudder pedals. This system makes it possible to apply the brakes either simultaneously or separately, while rudder control can be maintained at the same time. In the case of undercarriages fitted with a shock absorbing leg of varying length, it is necessary to carry the operating cable down the fixed strut or radius rod, in order that brake action can be controlled without interference from the shcok absorbing member. Various operating systems have been proposed using hand, foot and tail skid control. This latter method suffers from the inability to provide independent braking on each wheel. In the case of hand or foot operated control, the effort of braking comes through the pilot's muscles, and the maxi- mum effort is therefore limited. It is not considered possible to obtain a greater effort than 70 1b. through the hand or 45 lb. through the foot, and it is not usually possible to obtain a greater velocity ratio (between foot movement and travel of brake shoes) than about 50-1. In view of the strictly limited effort available, it appears that it must be augmented on machines above 3,000—4.000 lbs. gross weight. This external help will come from some form of servo mechanism, but it is not necessary to elaborate on any of these various systems at the present time. There are many possible combinations of control, and these present a wide scope for the exercise of ingenuity. 54; (For Fig. 14 of Mr. Dowty's article, see next page.)
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