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The XB-70 has a "digital" gauge showing the total fuel quantity using six rotating digit wheels (as in mechanical counters, odometers etc), and another showing the fuel quantity in the tank selected (out of 11 tanks) by a knob.

enter image description here

I wonder, surely these indicators did not work like mechanical odometer-style counters so that when going from displaying quantity, say 56432, to another, say 45123, the units wheel would be spinning like crazy, the tens wheel would be spinning at one tenth the speed of that, the hundreds wheel one tenth of that, etc?

But instead, I assume each digit wheel was individually electromechanical driven, and the they all just moved simultaneously to the new positions showing the relevant quantity.

Or was the total amount displayed perhaps rounded to the closest thousands anyway so that the four rightmost digits were actually coupled, and the selected tank quantity rounded to hundreds, so that the three rightmost digits were coupled? The position of the digit wheels in the picture do seem to suggest that. But then one wonders what the point in having such a "digital" display was, if it is rounded so much that some round gauge with a rotating pointer would have given the same accuracy anyway.

Anybody have any definite knowledge, or an educated guess? Hmm, I probably should search on patents.google.com, things like these were surely patented.

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    $\begingroup$ It looks to me as though there are four wheels: three for the leftmost three digits, and one wheel that can display ten different things: "000", "100", "200", ... , "900". So the two rightmost digits displayed are always "00", and the display has four digits of precision. That is more accurate than a round gauge would be. $\endgroup$
    – rclocher3
    Commented Dec 8, 2021 at 20:24
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    $\begingroup$ Thanks @manassehkatz-Moving2Codidact, corrected it. $\endgroup$
    – tml
    Commented Dec 8, 2021 at 20:50
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    $\begingroup$ If I understand correctly, this question is really about how the lower "LBS SEL TANK" gauge changes if one were to go from the selected "6L" and quickly rotate the knob to, for example, "4" (number made up, don't know if it's "4" or "4L" and "4R"). $\endgroup$
    – FreeMan
    Commented Dec 9, 2021 at 17:12
  • $\begingroup$ @FreeMan Yes, that is correct. (Because the "TOTAL LBS" gauge hardly changes very rapidly.) (And yes, there is a tank "4". Thew wing tanks are 6 to 8, L and R.) $\endgroup$
    – tml
    Commented Dec 9, 2021 at 23:18

1 Answer 1

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It depended on the indicator and what it was trying to measure. The encoding altimeters on some earlier Boeing jets had a similar style readout that had the two rightmost digits linked and displayed in increments of 20 ft but the rest of the digits were independent. Meanwhile the EAS-Knots, Digit Altitude and Digit Mach Number mechanical readouts on the SR-71 were single digit precision readouts.

But then one wonders what the point in having such a "digital" display was, if it is rounded so much that some round gauge with a rotating pointer would have given the same accuracy anyway.

If the range of measurement is sufficient, even with rounding the digit style display can provide more information than a round gauge style readout, in some cases they also provide more direct information that prevents the pilot from needing to interpret multiple arms to derive a single measurement (e.g. digital clocks vs an old 3 hand clock).

when going from displaying quantity, say 56432, to another, say 45123, the units wheel would be spinning like crazy, the tens wheel would be spinning at one tenth the speed of that, the hundreds wheel one tenth of that, etc?

Many aircraft instruments of the "steam gauge" era do not provide instant results. The common vertical speed indicator has a quoted 6 to 9 seconds of lag before the measurement should be considered accurate so depending on how these gauges are fed they will change readings as fast as they can and that was largely considered acceptable. Again, if they are individually driven that would be dependent on design and implementation.

A bit of a side note but important to the question, fuel burn schedules in aircraft are typically prescribed before hand, very specific and very well planned in reality fuel gauges are used for lose reference inflight and flow meters + timing + good engine information is whats used to calculate fuel burn, these gauges would have been used to loosely confirm numbers inflight and not necessarily constantly flopped around.

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