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Aviation History
1931
1931 - 0389.PDF
tl APRIL 24, 1931 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT U1 (i.e., fiti) in y2. Drop a perpendicular y^y1 on/1/ to cut Wx in X^ OXj, OX2, and OXS are the three diameters for the circular arcs to be drawn in sub-sectors 1, 2 and 3 respectively. Draw these cutting arcs and then draw the loading arcs kjcj, k2kz, and £s&s as described in Case I. The radial intercept between the two arcs gives the bending moment for any angular position of the radius. It will be seen from the above construction that the first step is to draw the locus lines and their rotated positions, and so locate the X for the last sub-sector. Having found the position of X8 on ZZa(2) we can only obtain the position of Xs on II2 by first using the rotated position of the latter. If Ut had not been rotated, U3(2) would have been parallel to it, and X2 would have been found directly by drawing a line through X, parallel to g3gr of Fig. 6. The same thing happens when proceeding from X2to find Xx; it is first necessary to find the auxiliary point y2 on the rotated position of II ^ before the perpendicular «/2X! can be dropped on the boundary line J1/ to give Xx. The above description may appear a little involved, but once the procedure is thoroughly mastered it is surprising how quickly a particular problem can be solved. It is important to use as large a scale as can conveniently be handled and to make all measurements with care. Note.—In each of the above cases a figure has first been drawn with the object of obtaining the necessary data for the two equations and the positions of the locus lines, etc. The actual bending moment diagram has then, for the sake of clearness, been drawn separately. It is now pointed out that it may often be more convenient not to draw a separate dia- gram, but to make use of the first figure for the purpose of drawing in the actual bending moment diagram. SHOCK ABSORBERS FOR AIRCRAFT LANDING GEAR. By W. S. HOLLYHOCK. THE most generally used shock absorbing media for aircraft landing gear are oil, air, rubber and springs. These are employed in a variety of ways, the two most common of which are combinations of oil-and-air, and oil-and-rubber. The all-rubber shock-absorbing unit is practically obsolete, and for various reasons, springs are n.ot viewed with any great favour hi this country, so it is not proposed to discuss either of these latter methods in detail in this article. In both the oleo-pneumatic and oleo-rubber combinations, the oil serves the dual purpose of absorbing the shock of impact on landing, and of dissipating the energy so absorbed. The air and rubber on the other hand, give the necessary " cushioning " effect in taxying, and in certain cases also serve as factors of safety in the event of unduly harsh landings. Oil being incompressible, cannot store energ 7, and therefore cannot be used alone. Conversely, air and rubber being incapable of dissipating energy, need to be used in conjunction with oil to prevent bouncing of the aircraft when taxyiog, and to keep down the size and weight of the shock-absorbing unit. Incidentally, it is chiefly in this respect that springs are not considered satisfactory, because a damping medium must be used in conjunction with them, and if oil is used the weight becomes prohibitive. The alternative is some sort of Mo- tional device with its concomitant evils of wear and unrelia- bility. The oleo-pneumatic combination is undoubtedly the best arrangement in every way. It is the lightest, offers the least aerodynamic resistance, and is more durable than others. The big bogey of the pneumatic leg is the high air pressure necessitated. At least, it is considered an objection, though it is difficult to understand just why. Certainly accidents have happened, but aeroplanes have been known to break up in the air—and the aircraft industry has survived the shock. Actually, there is no reason why pneumatic cylinders should be any more prone to bursting than engine cylinders, and as regards accidents on the ground, these can be elimi- nated by rendering the unit foolproof—not a very difficult proposition. There is one other disadvantage about this arrangement, namely, the filling of the leg with air. This operation, if carried out by hand (as is usually the case) takes something like half an hour to perform. Nevertheless, to allow such a trivial matter to bias design would show a distinct lack of sense of proportion. As a matter of fact, if the unit is properly designed, the leakage will not be great so that frequent replenishing will not be necessary. The principle on which all oleo combination units are based is the converse of the hydraulic ram acting either in advance of, or simultaneously with, an energy storage device. The unit consists of a cylinder containing oil and a piston which on impact forces the oil through a valve or small orifice. In the case of the oleo rubber arrangement the piston com- presses the rubber after exhausting the cylinder of oil. In the pneumatic leg, the compression of the air may take place concurrently with the evacuation of the oil cylinder, or follow it, according to the type of leg concerned. It follows that such energy as is absorbed by the oil is actually dissipated ; while that absorbed by the rubber or air is stored and given out again in the form of a rebound action. In this connection, it should be noted that the rebound action should be controlled by suitable stops and/or throttling of the oil passage to prevent the aircraft bouncing unduly when taxying. To this end, the rubber is initially compressed from one third to one half " g." With air, the higher figure should be taken (owing to its greater elasticity) if the two actions are consecu- tive as in the case of the rubber leg. Where the oil and air act concurrently, the air must, of course, be compressed initially to one " g " to ensure correct functioning of the oil valve. In all cases the return velocity of the oil must be controlled by throttling, though the amount of retardation is not of great importance except where the oil and air act concurrently—in which case, great care is necessary on account of the high air pressure acting on the piston. With regard to the design of the oil valve—which, by the way, is usually of the needle variety—the cushioning effect of the tyres should not be taken into account. The amount of energy absorbed by the tyres, even when fully inflated, is not great and when they are deflated—a not unknown, occurrence—obviously does not exist. So that if the leg is designed to allow for tyre effect, the aircraft will sustain a serious shock when landing on flat tyres. WThereas the effect of landing on fully inflated tyres with oleos not designed to take account of tyre resilience will be to ease, somewhat, the first shock of impact, though there must, of course, be a slight harshness when the tyres cease to deflect. With regard to the actual design of compression rubbers for use in oleo-rubber units, it is not proposed to go into details in this article, as the matter has already been discussed in these columns. So long aB the proportions of the individual rubbers are not freakish there is not much to choose between different shapes. The one really important requirement, of course, is a first quality material. Even so, they need to be carefully watched in service as they rapidly deteriorate- particularly under conditions of extreme temperatures. With regard to oleo-pneumatic combinations, there are some important facts which should be borne hi mind when designing such units. Firstly, it is absolutely imperative that the leg be so arranged as to be quite foolproof as far as the releasing of the air pressure is concerned. In other words, it must be so protected by locking devices, that it will not be possible for a mechanic to release any part in such a manner that it can become detached from the leg while there is any appreciable air pressure behind it—whether the leg is actually in place on an aircraft or not. The possible consequence of such an accident need not be elaborated if it is remembered that the air pressure may be of the order of 1,000 lb. per sq. in. Secondly, owing to the high pressures attained, joints and glands need careful consideration in order to eliminate loss of pressure as far as possible. To this end, glands exposed to high air pressures should be so arranged that oil is present to keep them tight. In fact, it is really desirable to avoid glands exposed to air at high pressure, altogether, and it is quite possible to do so if the right arrangement is chosen. Joints which are exposed to high air pressure should be balanced by high oil pressure on the reverse side, where 362 c
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