Development of the aircraft powerplant has been the greatest single contributing factor to the progress of aviation. A logical field for future development seems to be the improvement of its altitude performance, and the best of the several proposed methods for doing this. Consequently, the Sub-Committee on Powerplants for Aircraft of the National Advisory Committee for Aeronautics recommended that the Roots-type compressor be investigated, and a complete supercharger was built and sent to the Langley Field laboratory for test.
The design, principles of operation and characteristics of the Roots-type compressor are described, and its slip-speed due to air leakage past the rotors, its pulsating discharge, type efficiency, and variation in torque are discussed. The Roots-type supercharger built for the National Advisory Committee for Aeronautics, as designed for use with the Liberty-12 engine, is described and illustrated. The set-up for laboratory tests is illustrated and described and the results of the tests are presented in charts.
Attempts to synchronize the pulsations of the supercharger with the engine induction-periods did not measurably increase the engine-power, due perhaps to the uneven induction-timing of the Liberty-12 engine.
For flight tests the supercharger was installed with a Liberty-12 engine in a modified DeHaviland-4 airplane, and an air-scoop on the side of the fuselage was connected with the supercharger intake. It was also mounted with a Wright J-4 radial air-cooled engine in a UO-1 airplane and was geared-down to give full supercharging to 18,000 ft.
Twelve reasons are given for the conclusion that the Roots-type compressor seems to be well adapted for use as an aircraft-engine supercharger for many service requirements. In opposition to the advantages enumerated, its bulk and weight are cited as disadvantages, although the supercharger described in the paper is not to be taken as an example of the possible ultimate reduction in size and weight.