The question How do airplanes regulate the output frequency of AC generators? here on SE already answers why we need an IDG with a constant speed drive on airliners.

[IDG's] have been used in aircraft electrical power generating systems since the 1950's.—Wikipedia

Before the 1950's, in the early jet airliners, turboprops, and big piston planes, how did they manage the same task of regulating the AC frequency?

Or did they rely purely on DC via alternators and a bank of batteries? I'd think old big planes had plenty of pumps, etc., to require AC.


1 Answer 1


Inverters, it would appear, were not uncommon. I get the feeling that the quality of the (unregulated) AC from alternators was insufficient for instrument applications, and was primarily used for less sensitive systems. For more stable AC, it appears that the inverters were used.


AC System

Alternating current for the autosyn instruments, drift meter, radio compass, and warning signals transformer is furnished by either of 2 inverters, one of which is a standby for the other. One inverter is under the pilot's seat and the other under the copilot's seat. A single-pole, double-throw switch on the pilot's control panel controls the DC power to the inverters and selects the inverter to be used. In the "NORMAL" position the left-hand inverter is on and in the "ALTERNATE" position the right-hand inverter is on. Source

Anecdotes on PPRUNE also seem to offer support for this.

B-47 Stratojet

Finally, a decent source. Two (unregualted) AC alternators for 'windscreen deicing and some radar equipment', with 2+1 spare single-phase inverters for other AC users.

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KC-97 Stratostanker

Has 6 generator dials, and 4 inverters on the flight engineer's station, looking at a panel diagram and the bottom right of this photo. That being said, there are two switches labelled alternators on the AC section of the panel. They only seem to offer "NESA Bus" (B-47 manual seems to imply this is for window deicing) or Bus No. 2, depending on how the right switch is set. The left switch is for the voltmeter/ammeter dials.



Credits to brewbooks on Flickr

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    $\begingroup$ In this era the inverters would have been electromechanical in the form of a motor-generator set. They probably produced a reasonably good sine wave, much better than the stepped approximation generated by early solid state ones. $\endgroup$
    – user9394
    Commented Aug 4, 2017 at 19:56
  • $\begingroup$ I believe electro-mechanical inverters/vibrators would be more likely. At least in rail and building contexts, motor-generator sets were expressly known as such, or as M/G sets. $\endgroup$ Commented Mar 23 at 7:46
  • $\begingroup$ Early versions of the B-52 used pneumaticly powered generators. $\endgroup$ Commented Mar 23 at 7:49

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