[ARC5] APR-4

[Robert Eleazer]

"OK, a dumb question:  Why would some models of the APR-4(*) be configured to work with 60 cps power?  Mine does, works great.  But why/where??"


The older WWII models used bigger and heavier transformers that would work on 60 HZ and therefore would also work on 400 HZ.  I would suspect that they wanted to be able to use the gear shipboard as well.

The postwar models, the APR-4Y, 1950 is the date on mine, reduced the weight by going to 400 HZ transformers.  By that time, 400 HZ was getting pretty common on at least larger aircraft.  By the late 50's 400 HZ was common on even single seat fighter jets.  This was because the Constant Speed Drives used with jet engines were built to accommodate 115V 400 HZ alternators as well as 28 VDC generators.

On the U-2, to save weight they dispensed with the CSD and drove the 400 HZ alternator directly off the engine.  This meant that the alternator only produced the correct voltage and frequency at the designed engine speed, at long range high altitude cruise, but that was not a problem, since the avionics that needed it really only had to work during that portion of the flight, anyway.

Wayne