🚀 go-pugleaf

    It is not at all impossible to produce a constant frequency by
    and alternator directly driven by an aircraft engine. I used to fly
    in the US Navy P-3 Orion, a Turboprop aircraft. The engines ran at
    constant speed and were in fact PHASE LOCKED at the A/C power
    distribution phase. They ran alternators that directly powered the
    equipment.

The original message from Larry referred to WWII aircraft, which used
piston engines.  It isn't easy to get a constant-speed drive from one
of these.  The Orion (military Electra) has turboprops.  I'm not sure
how their aux power takeoff works, but if it's like a turbojet, the
alternator runs off a separate constant-speed turbine fed by the
compressed air from the engine.  This is easy on a turbine engine
(turboprop, turbojet, fanjet, etc).  Unfortunately, these engines
were not available during WWII, mostly because of materials technology.
So the alternator ran directly off the piston drive shaft, and the
power frequency varied with engine speed.