The very complete laboratory tests of airplane engines at ground level were of little aid in predicting performance with the reduced air pressures and temperatures met in flight. On the other hand, it was well-nigh impossible in a flight test to carry sufficient apparatus to measure the engine performance with anything like the desired completeness. The need clearly was to bring altitude conditions to the laboratory where adequate experimental apparatus was available and, to make this possible, the altitude chamber of the dynamometer laboratory at the Bureau of Standards was constructed. The two general classes of engine testing are to determine how good an engine is and how it can be improved, the latter including research and development work.
Recent standard comparative tests between an Hispano-Suiza and a Liberty engine were made to provide a reasonably satisfactory basis for predicting an engine's value for a given service and such runs were selected as seemed most properly to belong to a program of this kind. Four general classes of these runs are specified and the test results are given in a series of curve charts which are explained. Altitude and propeller-load results are presented in charts and discussed and the omission of temperature runs is justified to a certain degree. The decrease in engine power at great altitudes is considered and, in conclusion, the value of carrying out such a program of tests upon a large number of engines is emphasized. Knowing that one engine is better than some other, the difference between them furnishes a suggestion at least as to where to search for points of superiority.