Select all that apply. If, after extensive research, the expected observations predicted by a theory fail to materialize, then the theory .
1 is probably false
2 should never be retracted
3 should be abandoned or modified
4 is confirmed

Engines for propeller-driven aircraft are limited in their maximum rotational speed by the fact that the tip speed of the propeller must not approach the speed of sound in air (mach i). taking 6 ft as a typical diameter for a propeller of a light airplane and 1100 fils as the speed of sound, find the upper limit on the rpm (revolutions per minute) of the propeller shaft.