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RAT are common in airliners today to provide emergency electrical and hydraulic power. Yet, firsts airliners (prior to WWII) may not require the same amount of power than today's airliners (which are bigger and require electrical and hydraulic power to keep all electric-assist requirements, computers, hydraulic actuators,...).

The historic section of the wikipedia article speak about windmilling on the FW-189's engine as a precursor to RAT, but I fail to see anything related to first device whose only role is provide power.

My question is: What was the first aircraft equipped with a RAT? Was it military or civilian (it may not be a airliner)?

EDIT: given the comment by Peter Kämpf, I edited the question to include all aircrafts. Thus having an answer mentioning both airplanes and other aircrafts.

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    $\begingroup$ This being about electrics and first use, my entry would be a Zeppelin. But you are asking about airplanes, so lighter than air is not included. In the question text you say aircraft, so maybe a Zeppelin would qualify? Please clarify! $\endgroup$ Aug 27 '19 at 18:12
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    $\begingroup$ @PeterKämpf edited accordingly $\endgroup$
    – Manu H
    Aug 27 '19 at 18:51
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So far, the oldest I have found is LZ-127 Graf Zeppelin, first flight in 1928. It used a propeller-driven generator to power its radios and a second one (with buffer battery) to keep the ship illuminated. In the drawing below (source) they are indicated as "Wind-Dynamo", being on both sides of the main gondola.

I am rather sure that the wartime Zeppelins used those generators as well, but have yet to find proof for it.

Drawing of hull and gondola of the LZ-127

LZ-127 in Los Angeles

LZ-127 on August 26, 1929, in Los Angeles (source). The right generator is circled in red.

EDIT: Now I have found a source that attributes a RAT to the Zeppelin Staaken Riesenflugzeuge of WW I. The source is Tom Morrison's book "Quest for All-Weather Flight" and it says on page 28:

In November 1917 the Drexler bank indicator was introduced instead [of the Anschütz gyro]. Much simpler than the Anschütz instrument, it weighed only 5 kg (11 pounds). It was powered by electricity from a generator driven by a small propeller in the slipstream and took less than a minute to run up to speed.

So this dates the first RAT back to November 1917. Sorry, no online sources.

ANOTHER EDIT: I could have thought of that earlier. Really! Igor Sikorsky illuminated the cabin of his Ilya Muromets four-engined giant airplane using light bulbs, powered by a wind-driven generator, too. Wikipedia cites a Sikorsky source from 1938 as:

Lighting was provided by a wind-driven generator and heating was supplied by two long engine exhaust pipes which passed through the corners of the cabin.

Since the earlier S-21 Russky Vityaz had already a similar cabin, it is likely that the first RAT was used as early as 1913. But the first source I found so far firmly establishes it in 1914 if not earlier, in a civilian passenger airplane later used as a bomber. Not an airliner, though, because those came later.

Russky Vityaz

Russky Vityaz. Clearly visible is the electric searchlight in the open fuselage nose stand and less visible (circled in red) the wind-driven generator used to power it. At least that is what I think it is.

Wikipedia says about the legal status of this picture: This work is in the public domain in the United States because it was published (or registered with the U.S. Copyright Office) before January 1, 1926

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Even the very first aircraft that were retrofitted with electric instruments had air-driven generators (and Venturi tubes for gyroscopic instruments). Engine-driven generators and vacuum pumps came later.

Here's an account of the first non-stop transatlantic flight in June 1919, which suggests air-driven generators were pretty much standard by then. From Wikipedia:

It was not an easy flight. The overloaded aircraft had difficulty taking off the rough field and only barely missed the tops of the trees. At 17:20 the wind-driven electrical generator failed, depriving them of radio contact, their intercom and heating.

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