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The general design problem is considered by reference to the principle of similarity. The principle is briefly reviewed and applied to lubrication, propeller drives, cooling and detonation. It is shown that while small engines should be capable of a larger output per square inch of piston area, in practice the reverse is the case. It is also shown that small engines should, in general, operate at a higher efficiency than large ones. A promising line of attack for improving the lubrication of small engines is indicated.

CRANKSHAFT torsional vibration has become a serious problem in aircraft engines. Thanks to much experimental work, we now have a good working knowledge of the two phases of the problem, the elastic and inertia characteristics of the crankshaft-propeller combination and the forces to which this system is subjected. Methods used in the past to reduce vibration have been to change the elastic characteristics of the crankshaft, or to incorporate direct damping or some form of vibration damper of which the Lanchester and the resonant damper are examples. All of these methods have serious limitations. An interesting device which is capable of eliminating vibration in constant speed machinery is the undamped absorber. For variable speed machinery this absorber is of no value. By arranging an undamped absorber so that the restoring force varies with speed, it is possible, theoretically, to eliminate vibration in certain variable speed machinery.