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Aviation History
1940
1940 - 0917.PDF
MARCH 28, 1940 DYNAMIC SUSPENSION Isolating Engine Vibration : Three Degrees of Freedom : Attachment by Cylinder Head CTEST models of American aircraft with radial enginesare now being fitted with what has come to becalled the " dynamic engine mounting." The vibration problem is calling for more and more attention from designers. As far as the engine is concerned, the vibration problem is dealt with in two ways, first, to design an engine as free as possible from vibration, and secondly, to prevent as far as possible, the transmission of that vibration to the airframe, on which it may have dangerous as well as uncomfortable effects. The engine mounting has always been a part of the structure on which it was desirable to be liberal in fixing the sizes of structural members. The size of member shown to be necessary by stress analysis has usually only been taken as a guide and the designer's final choice has been influenced considerably by his past experience of what has been used on former designs. Such liberality in design has been found neces- sary to eliminate the chance of unexpected failures from fatigue due to the transmitted engine vibration. So a design of engine mounting which would reduce the transmission of engine vibration to the mounting and the whole airframe, where it could have such undesirable effects as loosening rivets and making passengers not merely un- comfortable but also fatigued, is welcomed by designers. The old idea of mounting engines was, for stationary types, to attach them rigidly to large blocks of concrete, apparently on the principle that the greater the weight they had tied to them, the less they could move. But the fact that movement was prevented did not in any way eliminate the forces at work. Modern ideas in regard to automotive engines are entirely different. Having reduced 3-0 ^2-5 82-0!„ e! i-o •5 1 r r-—' i TOTAL VERTICAL AMPLITUDE OF REAR COCKPIT STRUCTURE. MGEIvV ...—< TIAL , 1 MOU "V / —1—j—^ ^—,, DYNAMIC SUSPENSION 1200 14OO 160O I8OO 20O0 22OO CRANKSHAFT R.P.M. 1400 g!200 geoo I 400 •3 200 ) i r STRESS VARIATION IN LONGITUDINAL - MOTOR MOUNT TUBE 1ANGENTIAI \ t . MO -- JNT ^- mmeSJ / , DYN/ y IN SUSF .,--•' •ENSI \ ON- 1200 KOO 1600 1800 2OOO CRANKSHAFT R.P.M. F«g. 3- Reduction of vibration and stress. Fig. I. (Right) Firstlink-type dynamic suspension made in1937 for a double- row Wright Cyclone. Fig. 2. (Below)Link-type dynamic mounting. 2200 the unbalanced forces, and therefore the vibration, in the engine to a minimum, the designer seeks to isolate the vibration from the rest of the structure. But unfortu- nately the source of power must be connected to the vehicle which carries the passengers—the horse must be tied to the cart. And—no doubt by chance—the horse is connected to the cart in a most scientific manner. By shafts, traces and swingletrees the rhythmic motions of the horse's body are isolated as much as possible from the cart while still allow- ing the "thrust" to be transmitted. The horse was the first example of " floating power." The dynamic engine mounting is the latest example, but the problem here is a little more difficult. For not only must engine and air- frame be tied together to transmit the airscrew thrust but the airframe must support the weight of the engine, while the horse, of course, supports itself. The Fundamental Principle The new mounting gives the engine three degrees of freedom and works on the principle that vibration can be isolated by allowing the unbalanced forces acting on the power plant to create their own counterbalances by acceler- ation 0/ the total mass, so leaving the supporting structure virtually undisturbed. Angular freedom is provided by attaching the engine to the ring of the mounting by means of several tubular rubber adhesion " springs." These fittings are familiar air- craft accessories these days and mounted with their axes tangential to the ring, give a slight degree of freedom in angular movement. Such movement, of course, generates a restraining force by putting the rubber in shear. This was the first development away from the older type of mounting in which the engine was simply bolted rigidly to the structure. The latest development, which gives the new mount- ing its name, is to insert a small link, of something like 3 in. in length, between the tubular rubber "spring " and the ring of the mounting. This link is parallel to the longi- tudinal axis of the aeroplane, so that small movements of it about the pin holding it to the mounting give a side-
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