THE rapidly increasing use of aircraft engines fitted with superchargers for improving the power output at high altitudes has focused attention on means for predicting their performance in advance of actual flight tests in an airplane. A considerable amount of engine testing has been performed in several well-equipped laboratories in the past. These results have been carefully compared to determine the degree of similarity of the performance of these engines, and to form conclusions from which the performance of other engines may be predicted. Since the gear-driven centrifugal supercharger has demonstrated its superiority for use at moderate altitudes over other types on the grounds of simplicity, capacity, weight and space requirements, the data considered are almost entirely concerned with this type. It is shown, however, that naturally aspirated engines have quite similar characteristics.
The test results examined indicate that if the horsepower output is determined at any one altitude, the horsepower output for the same engine speed at other altitudes may quite accurately be computed by a simple formula or by use of graphic methods.
Many engines are fitted with superchargers capable of producing such high manifold pressures that the engines cannot be run at full throttle at sea level. For such engines, means are developed for making tests at sea level with a moderate outlay for equipment, from which the altitude performance may be predicted with substantial accuracy.
The methods described in this paper are believed to be simple and accurate enough for general engineering use, and may aid in checking results during the progress of research work.