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Aviation History
1949
1949 - 0094.PDF
64 A Shocking Problem FLIGHT this curve and the ground reaction curve being a measure both of the efficiency and of the work done by the damping valve. The figures relating to oil displacement in the strut are not without interest: the volume of oil passed through the valve is 500 cu in, and, as this takes place in 0.226 seconds, Starboard main leg showing how upper part of strut forms a stressed member of the rotor outrigger. the rate of flow is equivalent to 7.9 gall /sec, or 478 gall/min. These are, of course, mean values; the maximum flow velocity through the valve occurs on the initial landing impact, the flow rate at this time being no less than 860 gall/min—this through an orifice area totalling 0.5 sq in. The pressure generation complementing this peak flow is no less than 3,55olb/sq in. In common with the provision for emergencies which one finds in all aircraft today, this accommoda- AIR VALVE Cut-away drawing of leg showing relative positions of units at 20in of com- pression. The action of the ram in forcing 01/ through the damping valve into the oil chamber, so further compressing ths air below the separator piston, may readily be fol- lowed, together with the physical structure of the leg. Detail of threaded sleeve joints in cylinder tube (top), and plunger tube (bottom). tion in the Air Horse under- carriage for a landing im- pact at 41ft/ sec is scarcely likely to be exercised, for it is equivalent to the free vertical drop from a height of 26ft. It is, perhaps, rather more likely although, we trust, still remote, that emergency landings would be made in autorotative pitch, and in legislating for this condition, it has been assumed that the landing path of the aircraft will approximate to 45 deg, and the equivalent vertical velocity will be 29ft/sec. Catering for this case does not, of course, require the full per- formance range of the damping valve, and so the unit was designed as a valve within a valve, one half alone being employed for the 29ft/sec case and the whole being used for the 41ft/sec case. There are permanent orifices in the valve which provide for the very slight damping required in performance of the normal '' perfect'' landings, whilst for progressively worse landings, the first half of the valve will accommodate any bumps up to the 29ft/sec rate. Although the provision of the extra- ordinary shock absorption characteristics was difficult enough, the design and manufacture of the legs them- selves propounded problem? of virtually equal difficult^7 in their own particular way. The cylinder tube and plunger tube are 8ft 4|in and 8ft 11.84m long, respectively, and of 6.542m and 5.6i6in dia- meter, whilst the wall thick- nesses are 0.104m and o.i88in. Obviously enough, the machining of these tubes to tolerances of +o.oo2in for the bores, + o.oo2in for the outer diameter of the cylinder tube and ±o.ooiin for the outer diameter of the plunger tube, presented no small problem. The machining was carried out by J. L. Jameson, Ltd., of Ewell, Surrey, and the surface finishes are as follows: the upper half of the cylinder tube has an ordinary machined finish externally and a polished bore, whilst the lower half has an ordinary machined finish both externally and internally. The upper half of the plunger tube has a ground and chromium-plated external finish and an ordinary machine-fiinished bore, but the lower half is ground and chromed externally, whilst the lower f-length of the bore is honed. As the supply position of British steel tubes of the required type meant a delay in delivery of 16 months, the Cierva Company obtained the necessary stock from America, the specification being equivalent to our own D.T.D. 254, i.e., a 75-ton h.t. steel. In addition, the small quantities involved made it necessary to machine them from the hollow billet tather than finish-machine from drawn stock. It was also necessary, in view of the B 6
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