FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1949
1949 - 1246.PDF
FLIGHT JULY 7TH, 1*549 WHEEL BRAKES Their Development, Design and Use Part I: Construction of the Kxpnnding-diameter Types A successful pioneer design—the Palmer aircraft brake of 1927. ON very early aircrait, the low-powered engines thenavailable demanded extreme lightness in the air-frame and the need for brakes, in view of the low landing-speeds, was insufficient to warrant their additional weight. For running-up, engine chocks were invariably used. A few of the machines fitted with the first wheels and tyres specially developed for aircraft were fitted with a very crude form of cable-operated hand brake, but these were probably proprietary articles originally intended for other forms of transport. It was not long, however, before speeds increased, and the proportionate lengthening of landing-runs began to raise pioblems 'of airfield size. The need for an efficient method of braking By MAURICE was recognized. Early experiments were mainly directed towards the development of car-type shoe-brakes. None of these was completely successful, and the distinction of being the first manufacturer to produce a brake specially for use on aircraft goes to the Palmer Tyre, Ltd-, which firm placed the PalmeT Aero Wheel Brake on the market in 1927. It proved to be just what operators had been waiting for, its chief merit being its extreme lightness. By comparison with present-day- brakes it did not nave to do much work, but for its weight it gave an exceptionally powerful braking effort in com- parison with earlier designs. Efficient Early Design The brake consisted of an annular expanding tube to which was attached a complete ring of brake blocks. The tube lay in the bottom of a castellated channel, and when expanded either by air or fluid brought the blocks into frictional contact with the brake drum. The blocks were shaped to fit into the castellations and were thus pre- vented from rotating with the drum. There was nearly 100 per cent contact between the blocks and drum, and even a small pressure resulted in a much higher braking effort than hitherto had been attainable, This first brake is noteworthy, because the basis of the design has remained unaltered in the modern drum brakes of to-day, and only now are the larger sizes gradually being superseded by the disc brake. The then comparatively new British division of the Goodyear Tyre and Rubber Co. marketed their first brake one year later, in 1928. It was not altogether successful, and a disc type of brake was introduced the following year, incorporating bronze and steel discs controlled by hydraulic pressure. This was the first of its kind, and its success is proved by the fact that' modified versions are still fitted to many aircraft, although now, like the expanding tube types, it is being superseded by the latest types of single-disc brake. The first Dunlop brake—produced for a Westland air- craft—was marketed in 1929. The soundness of this and subsequent designs from Fort Dunlop needs no emphasis here. Brakes in general use to-day may be divided into three distinct classes: internal-expanding shoe; expanding tube or drum ; and disc-type. The shoe type is extensively used on light aircraft and, on helicopters, where demands are light and simplicity is important. It is interesting to recall that every operational German aircraft in the late war was equipped with such brakes. Most shoe-brakes are of the two-shoe type, but various methods are used for expanding the shoes and for making provision for adjustment to counter- act lining wear. Shoe brakes generally consist of a pair of rigid shoes to which are attached renewable linings. The shoes are hinged on one or two fulcrum pins and F. ALLWARD are operated by a cam located approxi- mately diametrically opposite the fulcrum-point. The cam has a symmetrical outline and is in contact with the shoe "tips," so the respective shoe- displacements are equal for any given angular rotation of the cam spindle. Each shoe will dissipate half the total energy dealt with by the brake and the wear of the linings will be equal, due to the mechanical action of the cam. ; The two' shoes in such a brake, however, have an un- equal tip-operating force from the cam, as one shoe is "leading" and the other "trailing." The leading shoe is the one in which the created friction force is resisted by the fulcrum pin, and the trailing shoe is the one the friction of which is resisted by the expander. To obtain an equal retarding force from each shoe, therefore, the tip pressure on the trailing shoe has to be greater than that on the leading shoe. If this is arranged, any form of located expander will then carry out-of-balance force, due to the unequal shoe-tip pressures. " Floating Expander '' Brakes Many modern twin-shoe brakes have a "floating" expander, which may be mechanical—in the form of a cam or wedge free to. float between the shoe tips—or alter- natively, an open hydraulic cylinder with a piston at each end to engage the shoe tips. With a symmetrical expander, or cylinder pistons of the same diameter, the shoe-tip pressures are then bound to be equal. This alters the distribution of work done by the two shoes : the whole of the pressure on the leading shoe is available for creating force between the lining and the drum, whereas the equal shoe-tip pressure on the trailing shoe has to resist the created friction force on that unit. It follows that in such brakes the rate of lining wear will be unequal. Comparing two brakes of similar size, how- ever, one with a located and the other with a floating expander, the latter will be slightly more powerful. A further development of the simple, located- expander, twin-shoe brake is the two-leading-shoe type, in which full advantage is taken of the inherent self- energizing factor of each shoe. On such brakes the fulcrum points of each shoe are placed diametrically opposite each other, and the operating mechanism is arranged to apply the same effort at each shoe tip. The reason why this
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
