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Aviation History
1952
1952 - 0787.PDF
28 March 1952 359 service load. Of course, as the aircraft weight goes up so does the catapult end-speed come down, but this latter must not fall below a minimum for safe take-off speed. The overload conditions, which include the carriage of various types of external stores, will follow as a natural sequence at this juncture, and provide a simultaneous check on the suitability of these stores (e.g., bombs, long-range fuel tanks, rocket projectiles, etc.), and their carriage attachments to withstand the accelerations imposed by the catapult. Failures are not so common during catapulting as they are in arresting, and usually occur at the overload condition, when bombs may fall off or a torpedo may have its air-tail cut off by the thresh ing wire launching-strop as it frees itself from the aircraft hook at the end of a launching run. From time to time peculiarities are met, such as the directional instability of the Sea Fury prototype which left the catapult in a violent swerve; but remedial action is usually simple. In this particular case the fitting of a tailwheel lock was the answer. We next come to the R.A.T.O.G. tests. Two units are fitted, one on each side, usually located near the trailing edge of the main- plane and arranged so as to give a thrust as nearly as possible through the centre of gravity and along the power-thrust line of the aircraft when the rockets are fired, thus giving the minimum change of longitudinal trim. Each unit consists of a carrier for two, three, or, indeed, any requisite number, of five-inch flashless- cordite rockets, which each produce about. 1,200 lb thrust for a period of four seconds. The tests are, firstly, a functional check on the electrical circuit which fires the rockets, and a check on the suitability of the pilot's firing-button position. Then actual static firing is done so that the aircraft merely moves forward under the impulse of the rockets with the engine idling. Thus, the rigidity of the units on their mountings and the flash effect on the runway and the tail end of the aircraft are noted. IN the design of the "Vardel" pump, Dowty Equipment, Ltd., have employed the two-stage principle; the first stage is a gear-pump of light construction, the driver gear being on the same shaft as the second stage, which is a radial seven-plunger unit. The gear-pump output exceeds the capacity of the second stage, the excess being recirculated through a relief valve. The bulk of the first-stage delivery passes through a control valve to the second stage; from the inlet port, fluid enters each cylinder in turn from an annular gallery, and is pumped via non-return valves into the H.P. supply-line. The sequence of operations is seen below. The gear pump draws from the reservoir at a pressure which might be as low as 1 lb/sq in absolute (at 60,000ft) through inlet B. Delivery is made at 80 lb/sq in to the annulus D, the excess recirculating through relief valve F. A limited quantity is returned to the reservoir through the bleed G, in order to maintain fluid viscosity and temperature at a uniform level. High-pressure delivery from the second stage is made through annulus H to the outlet K. Oilway R transmits delivery pressure to piston L acting against spring load on valve M. At maximum pressure, piston L causes valve M to seal off the first-stage delivery except for that which passes bleed N and reaches the second stage via a by-pass annulus and passage C. The driving gear of the first pump-stage is actually on the same shaft as the eccentric driving the second stage, although this diagram shows the unit "opened-out" for clarity. Troubles are most frequent at this stage, due mainly to weak mountings and damage to tailplanes and elevators from pieces of stone, etc., blasted up from the runway by the powerful efflux of the rockets. I have seen a tailwheel-well so full of tarmac that the wheel could not possibly retract into it. In the good old days of fabric-covered elevators, as on the Swordfish and Walrus it was a common occurrence to set the whole rear end alight, but stressed skin construction has now removed that hazard. From the static tests we pass on to the full flying tests, which consist of checks on any change of longitudinal trim, measured take-offs at various loads, asymmetric take-off characteristics and jettisoning of the carriers after firing. From the measured take-off tests are produced the information required by the Service for operating R.A.T.O.G. This includes handling notes for the pilot, and firing-distance and unstick-distance charts for various loads and wind-speed conditions under which the particular type of air craft is operated. The asymmetric tests are designed to assess the effect on directional trim and unstick performance of the misfiring of any number of rockets, up to the maximum carried, on one side of the aircraft. Finally, jettison tests are made to ascertain the safe speed-range within which the empty rocket cases and carriers may be jettisoned without danger of hitting the tail or otherwise damaging the air craft. In most cases it has been found that normal slipstream is not sufficient to carry them clear, and elastic bands have to be fitted to them, under tension from the trailing edge of the wing, so that extra downward and outward pull is obtained to give adequate clearance when they are released. These tests described by no means embrace the full scope of the test-flying programme carried out on naval aircraft at Farnborough, but they are the core of the test syllabus and constitute the essential checks in which failure is totally unacceptable. The second-stage plunger-pump is housed in the large-diameter section on the left. The pump is available with American or British mounting flange and drive-shaft. This flow pressurizes the H.P. circuit until piston L uncovers the escape ports from channel R at full pressure. The fluid then returns to the reservoir via the spring housing and passage P. When the pump is idling, the depression on the inlet side of the second stage—at D—moves the piston so as to enlarge the opening of the relief valve port; this reduces the gear-pump recirculating pressure. The delivery, at 3,000 r.p.m., is 5 gal/min; this is maintained up to 60,000ft with only a very slight loss in pressure. General characteristics of the pump are shown above. Dry weight, at 13 A lb, compares favourably with earlier designs dependent upon automatic cut-out valves and accumulators for pressure regulation. Dowty Equipment state that the factors governing the choice of a two-stage design of this nature were, principally, reduction of plunger-pump output when idling, maintenance of fluid circulation at high altitude and provision of a pressurized supply to the plunger pump. Though the component is chiefly designed for aircraft use, the company point out numerous other applications (such as plastics moulding) in which the pump is finding favour with hydraulic engineers in other industries. NEW HYDRAULIC PUMP The Dowiy "VardeV : Delivery at 4,000 Ibjsq in
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