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Aviation History
1953
1953 - 0202.PDF
200 FLIGHT MODERN BOGIES . . . of adoption. In any case, modern bogies would stand to gain little from such a practice. The arrangement of the wheels on each leg is governed by the runway-loading index. The spacing of the wheels is of greater importance than the tyre pressure, and it is advantageous to spread the load over the greatest possible area of runway, both to prevent distortion of the runway at the point of contact of each tyre, and also to relieve runway bending-moments. The size of tyre adopted is determined by the maximum pressure which can be used and the loading on the tyre. Shortly after the second world war, some attention was given to the "Compacta" tyre, of small diameter and flat profile, excessive tread-width being used to provide considerable ground-contact area. It will be appreciated that the tyre was unsuited to high pressures—which, ideally, demand a circular-section tyre—and "Compacta" development has now been all but abandoned. Although high-capacity expanding-tube brakes are still being made, this country has turned its attention chiefly to the manu facture of plate brakes. Braking conditions have today reached a somewhat critical stage, where the energy to be dissipated cannot be carried away by slipstream during the landing run. This in turn has demanded the provision of a considerable mass of metal within the wheel, sufficient to absorb the kinetic energy of the aircraft as heat energy—at over 1,000 B.Th.U./sec—without causing undue rise in temperature. Modern plate or disc brakes operate at up to 1,500 deg F, the materials used being either copper for high thermal conductivity, or steel for high specific heat. The wheel is so designed as to maintain tyre temperature within permissible limits. Regarding brake actuation, it is now considered that a pneu matic system cannot readily be applied to large aircraft; as it is incapable of ensuring instantaneous brake application. Hydraulic systems can approximate to this ideal, and it is interesting to note that the Fokker S.14 Mach-Trainer, which has no hydraulic system, is to go into production with a pneumatic/hydraulic transfer unit for hydraulic brake application. It is of importance so to design the bogie that an equal load is carried by each wheel during most of the landing run, thus offering the maximum braking power. The brake plate capacity is a function of rim diameter and this is in direct conflict with most modern stowage requirements. Again, civil aircraft require brakes with even greater energy-capacity than those fitted to military types,, and it is expected that there will be some re arrangement of the undercarriage on the civil developments of both the Vulcan and the Victor. As a final note upon brake systems, it may be said that, although several types of anti-skid device have been developed to ensure safe application of maximum brake torque, it is essential on a bogie-type undercarriage to fit one of these units to each wheel. If the device were fined to one ivheel only the slightest trace of bogie porpoising could cause all the wheels to lock solid. The requirements of modern aircraft tyres are, if anything, even more difficult to fulfil than are those for the remainder of the undercarriage. Particular difficulty has resulted from the enforced adoption of high pressures and, of course, high rotational speeds. In 1945, the average bomber had a tyre pressure of some 70 lb/sq in and could, therefore, be operated from dry grass. Today, tyres are operating efficiently at over 200 lb/sq in and developments are in hand that will increase this figure to above 250 lb/sq in. High pressures not only make great demands upon the runway surface, but they also imply the provision of new tyre construc tional materials. Nylon is being increasingly used in aircraft tyres and, among many other attributes, it is 60 per cent stronger than rayon and has much greater resistance to concussion loading. Terylene shows even greater promise. In all cases, it is the designer's aim to obtain the highest possible strength-to-weight ratio. Incidentally, it may be noted that, although it was British practice during the war to use smooth-treaded tyres, the majority of tyres now in use feature a tread formed from circumferential grooves. An important factor in the design of wheels is the prevention of the spread of heat from the brakes to the tyres. The type of wheel currently used in the new British bombers has a flat rim with a detachable flange in three segments which lock into a circum ferential slot in the wheel. The wheel itself is generally of cast magnesium alloy. This is a convenient point at which to introduce the name of Dunlop. This company is responsible for a vast amount of undercarriage progress, and supplies the complete tyres, wheels and brakes for the Victor and Vulcan. For any bogie-equipped aircraft, power steering is considered mandatory and a hydraulic system is usually provided for this purpose. For retraction, the nose-leg must be made self-centering; on most modern bombers the limits are of the order of plus or minus 50 deg. On the Vulcan this is accomplished by a specially designed jack which can be extended or shortened during castoring or steering, but which is pressure-loaded to a central position by hydraulic pressure before retraction of the nose undercarriage takes place. The desired centering force for the Vulcan is obtained with a very small jack, weighing only i\ lb, which is some 23 lb lighter than a steel spring of comparable performance. Nosewheel centering on the Victor is effected by applying hydraulic pressure to both sides of both the steering jacks, which are brought to a positive centre by virtue of their special design. Ground manoeuvring problems can be acute in bogie-equipped aircraft. If an unduly sharp turn is attempted, the main-wheel tyres will experience severe scrubbing in their sideways move ment across the ground, so causing damage to the tyres and a very high torque on the undercarriage. To reduce this effect, a minimum turning-radius is specified for each aircraft type, and is generally enforced by placing a stop on the castoring angle of the nosewheel steering jack. Additionally, a special towing arm may be required for ground manoeuvring, used in conjunction with warning lights on the tractor. Bogie shimmy is rare; even after very severe landings, no measurable play has been discovered either on the Victor or Vulcan. This is partially due to the selection of large pin-diameters with correspondingly reduced bearing pressures. Bogie axles are invariably mounted rigidly in the bogie beam, with each wheel free to rotate independently. On the Vulcan and Victor, each main wheel carries two tyres, making a total of 16 in all. Most large bogie units are built up from large light alloy castings or forgings. The biggest part of die Dowry bogie for the Avro Vulcan is a massive casting produced by Sterling Metals, Ltd., in Z5Z magnesium alloy. Use of this material has kept the weight of the component down to 360 lb; an aluminium alloy casting of the same strength would weigh 540 lb. The Electro- Hydraulics undercarriage of the Victor has two large struts attached, Y-fashion, to the yoke arm; it also has a large girder- type radius-rod with top and bottom extrusions and plate webs. The Victor unit employs a bogie beam formed from two light- alloy forgings dowelled and bolted together to form a box section. The Vulcan bogie beam is a one-piece casting. It is worth noting that both units are housed in the wing with the bogie inverted. In the Dowty undercarriage this condition is achieved by hauling up the front axle by means of a small trimming-jack in the main-leg; the Electro-Hydraulics under carriage retracts with the bogie hanging at its acute trail angle, in which condition the front of the bogie strikes a cam situated on the radius rod, thus rotating the bogie beam to the inverted position. Enough bogie experience has now been gained for it to be saici that such undercarriages can be made lighter than those employing fewer, but larger, wheels. The leg itself is usually heavier in
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