In a search for a rational basis for a transport-economical comparison of jet and airscrew propulsion of aircraft, a method has been developed which is generally applicable for evaluating any kind of propulsion of aircraft, as well as the efficiency of any principle or type of aeroplane design. The method is based on the assumption that the ratio between the useful load and the gross weight (or empty weight)—when related to the speed —is representative for the overall efficiency of the aircraft, including the power plant, from a transport point of view. General equations have been deduced for this useful load ratio as functions of, in addition to the speed and altitude, all major design parameters, as wing loading, power loading (or thrust loading for jet aeroplanes), specific engine weight, specific fuel consumption, parasite drag coefficient, effective aspect ratio, and two structural weight coefficients. This has been possible by the deduction of a formula for the gross weight, which appears to be usable even for general analysis of other aircraft performances, whenever the useful load has to be considered.

The method presented has been applied in this paper particularly for a comparison between jet-propelled aeroplanes and piston engine-airscrew aeroplanes. This has revealed that pure jet propulsion seems to be superior to the conventional system at speeds as low as 200 to 300 m.p.h., depending upon the duration in hours of flight and the detailed assumptions made.

Lack of accurate data for the gas turbineairscrew combination has prevented the writer from making a closer investigation of this system, but a preliminary study of the same kind seems to indicate that the turbineairscrew aeroplane—although for most purposes definitely superior to the piston engine aeroplane—is inferior to the pure jet aeroplane for durations up to about two hours of flight. If, however, all the mechanical complications involved in retaining the airscrew are taken into consideration, it appears doubtful whether the turbine-airscrew system will be any more than an intermediate step in aeronautical development. At the best, it might remain only for extremely long ranges, or transport of cargo at comparatively low speeds.

The conclusions drawn are based on the present stage of development of jet engines, but naturally it will take at least some three to six years before the jet aeroplanes contemplated are built and put into service.