A point lost in all this 2-stroke/4-stroke efficiency discussion is that twice in the recent history (last 20 years) of F3A, 2-strokes got dumped very quickly as soon as a decent 4-stroke alternative came along because they turn bigger props at lower RPM and have bottom end grunt. Characteristics I might add forced upon us by the design of the schedules, because a snaproll at the bottom of a loop or on an up-line is a far better way to determine the skill of a pilot than a snaproll in a down-line or at the top of a loop.

Electrics turn even bigger props at even lower RPM with even more bottom end grunt, but like everything there's a happy medium and you just can't keep halving the rpm and doubling the torque without introducing new problems.

Two-strokes in a heli are a whole different ball game because they're always up on the pipe, running in their sweet spot waiting for the command to dump their available torque into the gearbox. A geared, constant rpm 2-stroke in an F3A ship with a variable pitch prop would probably work well but it would be a whole new sound for the judges to get used to ???