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The flight operations handbook for the Douglas X-3 Stiletto (an experimental aircraft from the early 1950s) says that the instrument panel has instruments that indicate the thrust of the two Westinghouse J34 turbojet engines. The actual aircraft in the USAF museum has instruments as in the picture. Despite being labeled "RPM", is it so that the outer scale actually shows the estimated thrust of the engine? Presumably calculated using some simple fixed mechanical linkage directly from the RPM.

The J34 model used is indeed said to have produced around 3000 lbf thrust, so that matches. The smaller inner scale shows the RPM, which for this engine goes up to 12,500 rpm, so that also matches.

Aircraft with more modern turbojets or turbofans rarely or never have instruments that directly would show the thrust, right? Is that because such an instrument is highly unreliable anyway and the actual relationship between RPM and thrust depends on factors that a simple electromechanical instrument can't know? (A glass cockpit could calculate it more accurately and display it, but I assume there is little interest in that information?)

RPM instrument of the Douglas X-3 in the USAF museum
Cropped from nationalmuseum.af.mil

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  • $\begingroup$ It doesn't seem logical that you would label something RPM that is actually intended to represent something else. Could it be for 2 different spools, and if so could there be that much difference between low and high pressure sections? $\endgroup$
    – Michael Hall
    May 10, 2022 at 18:05
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    $\begingroup$ @MichaelHall This is an early single-spool turbojet. $\endgroup$
    – tml
    May 10, 2022 at 18:08
  • $\begingroup$ @ymb1So that for instance when the rotational speed is 9000 rpm, the outer needle would point to 1, and the inner needle would point a quarter of the way between 8 and 12? Weird. Possible, sure. $\endgroup$
    – tml
    May 10, 2022 at 20:16
  • $\begingroup$ @tml I found a similar model by the same manufacturer and posted an answer with a quotation. $\endgroup$
    – user14897
    May 10, 2022 at 20:46

1 Answer 1

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It's a sensitive tachometer made by GE, a variation of another GE model used on the F4U Corsair (piston engine). Side-by-side (X-3's on the right):

enter image description here

[Following text is for the F4U's tachometer on the left] As it passes the 12 o'clock position, the smaller pointer would indicate the RPM in 1000's. This is a 'sensitive' tachometer in that it provides more precise resolution of the engine speed vs those with a single pointer for the full RPM range. (aeroantique.com)

Among other similarities, both have the General Electric logo right below the 0 of the outer scale:

enter image description here

The F4U one goes up to 3,000 rpm, but the single-spool Westinghouse J34 went up to 12,500 rpm (nationalmuseum.af.mil), so the X-3's tachometer allows for better precision on the outer scale.

Community wiki answer since @Bianfable noticed the multiples of 4,000 being used.

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  • $\begingroup$ I stil don't understand why the outer scale goes from 0 to 4 (and not 0 to 10) if the X-3 one is supposed to work and be read like the F4U one. But sure, they look almost identical otherwise. $\endgroup$
    – tml
    May 10, 2022 at 22:02
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    $\begingroup$ The outer scale gives you the remainder of the rpm divided by 4000. So the inside scale shows you whether you are between 0 and 4000, or 4000 and 8000 and so on, and then you add the reading from the outer scale to the lower end of the range of the inside scale. Example: inside scale on the 5 o'clock position (between "4" and "8"), outside scale 2 ticks past the "1", would give you 5200 rpm. $\endgroup$
    – Raketenolli
    May 10, 2022 at 22:12
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    $\begingroup$ @tml: what Raketenolli wrote, plus the small needle of the F4U needs to point at only 3 numbers, but on the X-3, imagine the small needle pointing to 12 lines(!). $\endgroup$
    – user14897
    May 10, 2022 at 22:23
  • $\begingroup$ @Raketenolli Ah! Now I get it. $\endgroup$
    – tml
    May 10, 2022 at 23:44

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