Counter-Rotating Propellers
Authors: McCoy H. M. 
Published: August 2016

This paper is concerned mainly with that application of counter-rotating propellers referred to as “ dual-rotation ” propellers. This term is here defined to apply to a pair of oppositely-rotating propellers operating in close tandem, mounted on concentric shafts. Only the application to aeroplanes is discussed herein, for the application of these principles to helicopter design is considered another subject.

This paper presents a summary of the development and background of dual-rotation propellers and the benefits that can be derived from their use. Complete references are furnished for designers, no attempt being made to reproduce the somewhat involved theory and lengthy reference data.

A short history of the three applications of counter-rotating propellers is presented. The applications include multi-motored aeroplanes with oppositely-rotating propellers, aeroplanes with engines mounted in tandem (back to back) so that the propellers revolve in opposite directions, but primarily, aeroplanes with dual-rotation propeller installations.

Most present-day aeroplanes with maximum velocities of 350 m.p.h. and over have single-rotation propellers. At this velocity the single propeller is operating past the peak of its efficiency. The loss in efficiency increases rapidly with increasing forward velocity. Dual-rotation propellers offer an excellent means of maintaining peak propeller efficiencies up to aeroplane velocities of about 450 m.p.h. by absorbing the rotational energy losses in the propeller slipstream. At velocities above 450 m.p.h. up to the limiting velocity there will be only a small reduction from the peak in dual-rotation propeller efficiencies.

Sustained level flight aeroplane velocities appear to be limited to about 550 m.p.h. when propellers are used as the means of propulsion. The efficiency of a conventional single-rotation propeller will be so low, however, that this limiting velocity can be approached with such a propeller only at a prohibitive cost in engine horse-power. Any given velocity above 300 m.p.h. can be obtained with less horse-power if dual-rotation propellers are used to replace a single-rotation propeller with the same total number of blades. The percentage gain in performance will increase with increasing forward velocity when dual-rotation propellers are used.

It is possible that single-rotation propellers with more than four blades will never be commonly used. Instead, dual-rotation propellers with a total of four, six or perhaps even eight blades, will be used in the next decade on many high performance aeroplanes, especially those operating at high altitudes. The types of aeroplanes will include small single-seater racers and fighters and high performance, long range bombers and transports, particularly those with supercharged cabins.

The discussion brings forth the fact that there are few experimental data available in this country to allow a quantitative engineering evaluation to be made of the worth of dual-rotation propellers. Sufficient substantiated theory exists, however, to permit the general statements made above. An outline of the experimental tests that are required to check the theory is presented.

Following the discussion of tests made by the U.S. Air Corps on a pursuit type aeroplane equipped with a dual-rotation propeller system, comments are made on the relative advantages and disadvantages of dual and single-rotation propellers as they affect aeroplane design. Hub design problems associated with dual-rotation propellers are discussed.

Because of the increasing importance of dual-rotation propellers, additional pertinent references other than those used in this paper are presented in the Appendix.