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Aviation History
1949
1949 - 0093.PDF
JANUARY 2OTH, 1949 FLIGHT A SHOCKING PROBLEM Details of the Unique Undercarriage Legs Devised by Lockheed for > the Cierva Air Horse TO eyes used to the slim beauty of modern aircraft:designs, the ungainly box-and-brace structure of theCierva Air Horse presents an almost Heath Robin- son picture. Yet it is doubtful if any aircraft built has ever been more purely functional in design than this three- legged unwinged Pegasus. Mr. Shapiro, of the Cierva Auto- giro Company, in his paper to the R.Ae.S. and Helicopter Association (Flight, December 2nd, 1948) gave an admir- able exposition of the technical aspects of the Air Horse design, and interest was added by the successful first free flight of the aircraft at Eastleigh on December 8th, under the gentle hands of Mr. H. A. Marsh. It will, we understand, be some little time before we are able to present a detailed analysis of the Air Horse, but one of the more important features of the aircraft—the unique landing gear—we are able to deal with immediately. The normal oleo-pneumatic undercarriage is, of course, a very well-known piece of airframe equipment, the essen- tial features of which have not undergone much change for many years, although constant development has naturally resulted both in refinement of design and better perform- ance. But in the main, it can broadly be stated that the orthodox oleo-pneumatic undercarriage leg of today is but a rather more thoroughbred version of its forebears of a few years ago. , r This generality, however, does not apply to the legs j designed for the Air Horse. The quite extraordinary • performance demanded influenced the physical aspects, and the result is a leg which, in all respects other than t fundamental principle, is unique. But before we proceed \ to an examination of the leg, it might be well worth \ while to review the factors which resulted in the singular performance demands. These are stated in detail by Mr. Shapiro in the lecture already mentioned, but we may here summarise,them as being (i) the need to cater for loss of power, which most often occurs just after take-off, when there is little time for the pilot to take effective corrective action; and (ii) the fact that, by building safe emergency landing into the undercarriage instead of the rotor, saving in weight is achieved and the need to impair normal rotor efficiency is avoided. Designed. Dissipation There is also the aspect that, as one of the primary operational roles of the aircraft will be crop-spraying (for which it has been given the delightful title of Spray- ing Mantis) and this involves normal flight at heights of between 10 and 40 feet, the siting of safe emergent landing in the undercarriage would seem to offer better chance of surviving a sudden power failure than would investing the emergency function in almost any other medium. For these reasons it was estimated that a vertical rate of descent of 41 ft/sec should be taken as a datum, and the undercarriage was, therefore, designed to cater for this. When, however, it is stated that the normal military aircraft undercarriage is designed to cope with 12 ft/sec, and naval deck-landng types with 14 ft/sec, whilst only the most ambitious of conventional designs get into the 16 ft/sec class, the magnitude of the task con- fronting Automotive Products, Ltd., in designing the job, may be appreciated. To put it into familiar scale, it means that in a shock landing at 41 ft/sec each of the three legs has to be capable of dissipating energy at the maximum rate of about 70,000 horsepower. On the basis of past experience of absorption efficiencies which can reliably be achieved, Lockheeds found that a 41 ft/sec rate of descent could not well be accommodated with a leg compression stroke of much below 6oin. Neces- sarily, a stroke of 5ft means that there must be an exten- sion of like amount below the cylinder tube and, as an overlap bearing length between inner and outer tubes when fully extended is required in order to give strength in bend- ing, it meant that the compressed leg length from head to axle centre-line came out at 11ft 5in, whilst the fully extended length came to 16ft 5m. • \;'Z'-' iii- :\> 'Structural Factors -*. * ;S* Having ascertained the length, diameters were decided by pressure requirements and seal loadings, together with the physical weight considerations which, of course, state that a large-diameter, thin-walled tube will be lighter than a smaller-diameter, thick-walled tube for the same strength. These considerations were, however, coloured by the fact that the cylinder tube is required to act as a structural member of the rotor-support trussing, taking tension loads in flight and compression loads whilst static. Again, the normal helicopter requirement for ordin- ary landings is that the shock strut shall be capable of accommodating a combined load of 0.3 vertical reac- tion applied as a drag load, and 0.25 vertical reaction applied as side load. With a free castering wheel, it* is necessary to cater for 0.4 vr in any direction in which the wheel can swivel. The slender length of the Air Horse leg can tolerate the 0.4 requirement at all values above 34111 of closure. Leonhard Euler, from his seat in Elysium, must surely gaze down with particular benevolence at Leamington Spa. Extensive ground manoeuvring is not normally re- quired of helicopters, and the small wheels and tyres therefore fitted meant that Lockheeds were unable to obtain any assistance from the tyres in meeting the shock case. The tyres, incidentally, are Palmer 7.25- inflated to 50 lb/sq in, and each tyre bot- toms at a lojVd of 7,700 1b, that is 15,400 1b per leg. The m'»rt,the energy absorption side was to jfi~ft/sec rate on a stroke of 5ft. This ion meant that the maximum ground re- 850 Jt) (£>e^tag) had to be accommodated the whole shock absorbei The designed entropy can be seen e accompanying family of curves, the of 3^,850 lb being the maximum with le was designed to cope. <©)quirement involved the provisionLof variation in damping orifice arft^ overCa-Avide range, and a somewhat automatic damping valve had be Evolved to satisfy the conditions, as no easy task, for the total energy to be absorbed in the 41 ft/sec case is 145,300 ft. lb. per leg, and it is from this factor, against a closure time of 0.226 seconds, that the dissipation value of about 70,000 h.p. is derived. As shown by the curves, the variation in damping orifice area follows closely the retardation of descent velocity, whilst the ground reaction remains virtually constant over the whole closure range. Friction and air reaction remains low up to about hali closure, then increases rapidly up to the peak at full closure, the disparity between When fully extended, the head of the struct is over 17ft above the ground. B i
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