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Aviation History
1949
1949 - 0451.PDF
MARCH IOTH, 1949 For Happier Landings FLIGHT 287 easy to show by calculation and model tests that, whilst thewheel is spinning-up on landing, the nose-strut will castor through a considerable angle and then, as the wheel becomesfully accelerated, will return almost to its normal position. It is, however, not so easy to deter-mine the speed at which the strut will deflect, which, if hydraulic shimmy-damping is employed, is of paramount importance in that means must be pro-vided for the fluid to escape without building up high pressures. Research work and the employmentof an energy predictor have yielded data enabling a suitable relief valveto be designed to meet the imposed conditions. One particular case maybe cited as exemplifying the magnitude of the problem: the aircraft was alarge tricycle-undercarriage type, and, with one nose tyre flat, the strut de-flected through 57 deg, the maximum angular velocity being 32 radians/sec,that is, 1833.5 deg/sec. This ultra- rapid "flick " caused the damping fluidto be expelled at the rate of 200 cu in/ sec, which, at a relief pressure of 3,700lb/sq in, meant that the relief valve had to deal with something of the orderof 150 h.p. In order to cater for high ground-reaction values, the company has developed a duplex shock-absorber foruse in nose-wheel struts, in which a second air chamber is introduced intoan otherwise normal unit in such a manner that the air curve can be trans-ferred from one compression ratio to another at any predetermined pointduring a stroke. The advantages of the Duplex shock-absorber are brieflyas follows: (a) the maximum reaction during braked taxying is considerablyreduced—in fact the dynamic effect of pitch during braking is reduced to half that experienced withthe normal type of shock absorber not fitted with low-velocity orifices or twin air chambers; (b) the total travel of the noseundercarriage can be reduced. Electro-Hydraulics have excellent drop-testing facilities intheir research department. A small, beam-type drop-test machine accommodates undercarriages suitable for aircraft upto approximately io,ooolb all-up weight, as well as tailwheels, whilst a large, vertical-guide machine is capable of handlingundercarriages for aircraft up to approximately 100 tons a.u.w. The vertical-guide machine, which was manufactured by pr y The Mk VIII fork-lift truck shown here is the petrol-enginelgearbox-transmission type, and can lift 10 cwt to a height of 9ft. Complementing the large vertical rig is this smaller beam-type machine for drop-tests at weights up to the equivalent of 3,000 \b. Rubery Owen, Darlaston, to the designs of Electro-Hydraulics,Ltd., is 35ft high and is stressed for a maximum vertical reaction of 150 tons, with side loads of 0.3 and drag loadsof 0.4 vertical reaction. Research naturally forms a large part of the company'sactivity, and although that concerned with hydraulic systems is, of course, confined chiefly to specific lines of inquiry applic-able to given aircraft installations, development research on such things aspumps and seals does, however, go on constantly. It is generally felt that the optimumpressure has not yet been reached and may well lie between 4,000 and 5,000lb/sq in, having regard to the strength of materials now available, the pipesizes required to transmit the necessary fluid horse-power and the many otherfactors involved. A system operating at a pressure of 3,000 lb/sq in, with arelief pressure of 3,700 lb/sq in, is peration, and the time isys"rant when a system operat- ing at 4,000 Ib/sq in will, in the viewol Electro-Hydraulics, be an accom- plished fact. Towards this end, thecompany are proceeding with work on the design of an engine-driven pumpemploying a nov;el principle which will be suitable for these higher pressures. Electro-Hydraulic Valves Considerable attention is also being devoted to the development both of pneumatic and electrical equipment, and perhaps the broadest field of acti- vity in the last-mentioned direction is in the designing of electrically oper- ated hydraulic and pneumatic valves. Apart from the electro-hydraulic valves designed for incorporation in aircraft systems, the company has produced a self-contained valve for use in control- ling fuel-flow supply to the torch igni- tion on the Armstrong Siddeley Python and Mamba gas turbines. A wide range of valves designed especially for torch ignition is avail- able, such types being fitted to all Rolls-Royce gas-turbine units. They incorporate a self-contained atomizer and filter and operate at a terminal pressure-drop of 6 v, although they are temperature-rated at 29 v. One of the more interesting lines of exploration at present under way is that of powered controls, several schemes, rang- ing from the conventional type of power system to large electrically-controlled units being now in the development stage. The conventional assister unit consists of a hydraulic jack and selector coupled into the normal, manual-control linkage in such a way that mechanical follow-up and cut-off is obtained. The unit incorporates a device enabling the pilot to revert to manual control in the event of hydraulic failure. Units of this type have been built in small quantities largely utilizing poppet-valve selectors, special valve forms having been evolved which eliminate "judder." When the unit is at rest, all the selector valves are closed and a high degree of irre- versibility is maintained by the trapped fluid in the jack. In the pure powered-control system, the mech- anical linkage between the pilot and the control surface is replaced by an electrical signalling system which transmits signals dependent on jvr. The displacement error between the two signalling devices is used to control the position, direction and rate of movement of the control surface. Another activity of Electro-Hydraulics is the production of fork-lift trucks, models being avail- able in a range of sizes from 10 cwt up to 2 tons capacity, and in petrol-engine/gearbox, petrol- engine/electric, and all-electric versions. One of the refinements of these trucks is that, 3- conjunc- tion with hoists varying between 9 and 0.2ft, the fork can be tilted laterally in order to coincide with the ground angle of tailwheel aircraft, and so facilitate loading and unloading with a sloping floor.
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