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Aviation History
1931
1931 - 0390.PDF
SUPPLEMENT TOFLIGHT THE AIRCRAFT ENGINEER APRIL 24, 1931 possible, for the same reason. Where this is not feasible they should be thoroughly sealed, even at the expense of a little extra weight. In such cases it is a good idea to weld the joint if possible or even to go to the extent of machining out of the solid and thus avoiding a joint altogether, the latter method ensuring both absolute tightness and accuracy. Another point which might conceivably be overlooked, is the fact that in certain arrangements the piston rod decreases the effective volume of the oil cylinder and if the oil has to pass from one side of the piston to the other it may be neces- sary to extend the rod beyond the piston so that the volume on either side is unaffected by the movement of the rod. The following example indi- cates the lines on which to proceed in the design of an oleo-pneumatic leg; and as th< theory involved is similar, up to a point, to that of the rubber and oil combination, the one example will suffice for both. For the sake of / simplicity, it is proposed to / / consider the design of a leg in which the oil is forced through a needle valve directly into the air reservoir without any piston or other separating medium between the oil and the air (see Pig. 1). Inci- dentally, it has been considered as an objection to this type that the oil is liable to froth on entering the air chamber. This, however, does not seem to be the case in practice—• ~_ -_^ FIG. probably on account of the high pressure. In other respects, this type of leg has '/TO V?//>M \7 • AJR CHAMBER OIL LEVEL ORIFICE - MOVING JOINT - NEEDLE OIL CHAMBER distinct advantages, in simplicity and lightness, over other arrangements. Considering the normal type of aircraft with two com- pression legs, and taking the weight to be 10,000 lb. when fully loaded, the load per wheel at the moment of impact under conditions of level landing with tail up and acceleration of once " g," is equal to half the weight of the machine, that is, 5,000 1b. This is based on the perfectly tenable assump- tion that aerodynamic forces resist rotational movement for a fraction of a second. Taking the maximum permissible load on the structure to be 3 X " g," the maximum load per wheel = 15,000 1b. If the vertical velocity of descent is taken as being 10 ft. per second (the usual figure) and the oleo absorbs 100 per cent, of the kinetic energy (i.e.., no allowance for tyre resilience) then K. E., to be absorbed = £ x 5,000 X V1 = 7,770 ft.-lb. To allow for one wheel landing, take 1J times this value (since the time elapsing before the other wheel touches will be very short). ThenK.E. = 7,770 X 1£ = 10,350 ft.-lb. Since the maximum allowable load per leg may not be inoreased, the extra K.E. will necessitate increased travel of the wheel. Wheel travel = K.E. 10,350 = 0-69 ft. = Therefore maximum load on oleo 15,000 X 8-288 15,5001b Bursting of Cylinder Walls. The maximum pressure at which it is permissible to work is a somewhat moot point. In the opinion of the writer, 2,000 lb. p.s.i. is not too high, though 1,500 is more general. For the purpose of this example, however, 2,000 lb. p.s.i. will be taken. Using the following notation .— pa = air pressure (lb. per sq. in.). pu = oil pressure (lb. per sq. in.). P = maximum load (lb.). A,j = gross area of piston (sq. in.). A/, = orifice hole area (sq. in.). A? = nett piston area = A(J — A/,, at the end of the stroke, Ay = = — and Po — P — pa Pa P;— ptl Ah Po Pa Since at the end of the stroke (i.e., when fully compressed) Po = pa- A/j should be kept as small as~possible—consistent with a sufficiently rigid needle—in order, that the orifice may be wide. If the orifice is very narrow it necessitates excessively fine machining limits on both the orifice hole diameter and the needle diameter, and makes 'absolute concentricity of the needle with the hole imperative. As the simplest method of fixing the needle is by screwing it into the base of the cylinder, this concentricity is not easy to obtain with a very great degree of accuracy. -I 1U FIG. 2, DIAGRAMMATIC REPRESENTATIONOF AIR PRESSURES load 15,000 8 • 28 in. Usually, it will be found that, owing to the geometry of the undercarriage, the travel of the oleo will not be the same as that of the wheel. Also, if there is any great differ- ence, it will be necessary to plot the travels one against the other, to ascertain whether or not the ratio is constant throughout. If it varies greatly, the resistance figure of the oleo will have to vary accordingly, in order to ensure constant deceleration of the aircraft. For this example, however, the ratio will be taken as such that for a wheel travel of 8-28 in. the oleo travel is 8 in. and will be assumed to be constant, so that the resistance figure for constant deceleration will be constant. 362d If the orifice hole diameter be arbitrarily fixed at J in. (this being a standard reaming size), Ah = 0 • 441 sq. in. _ . 15,500 - 2,000 x 0-441 . ThenA, - — — = 7-309 sq. in. Hence Ag = A« 4- Ah — 7-309 + 0-441 = 7-750 sq. in., and cylinder diameter = 3-14 in. Therefore, maximum hoop stress developed in the cylinder PaVwalls = taken as 60,000 and t = which, if the material is mild steel, may be 2,000 X 3-14 2 X 60,000 in. This gives a factor of safety of nearly = 0-0524 in. Say,
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