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Let's say an F-104 Starfighter is flying at sea level at 600 KIAS with throttle at 100% RPM.

Then, let's say the F-104 is still flying at sea level, but the pilot reduces throttle and airspeed falls to 250 KIAS. The pilot then pushes the throttle to 100% RPM and starts accelerating.

My question is, at 100% RPM, will fuel consumption be greater at 250 KIAS and less at 600 KIAS?

Does airspeed impact fuel consumption? The higher the airspeed, the less fuel consumption, and the lower the airspeed the greater the fuel consumption?

note: I added this chart after I made this post: enter image description here

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  • $\begingroup$ Perhaps you should frame it in terms of ram air effect on specific fuel consumption (lbs fuel per lb thrust per hr). Thrust declines with speed initially, until ram effect with airspeed starts to provide a "compression boost" and net thrust goes up. To the extent that net thrust goes up, FF should also go up so the answer is probably airspeed increases fuel consumption to the extent that it is increasing net thrust in the speed region you're talking about. Unless SFC is going down due to efficiency increases from ram effect, so maybe FF doesn't go up. I have no idea if that is the case. $\endgroup$
    – John K
    Jan 10, 2021 at 15:49
  • $\begingroup$ John, Let's say a fighter produces 10,000 lbs of thrust at sea level. When the fighter is flying at, say, 250 KIAS, it might be only producing, say, 8,000 lbs of thrust at that airspeed? And then when speed increases to, say, 500 KIAS, it would be producing 10,000 lbs of thrust? $\endgroup$
    – delta2014
    Jan 10, 2021 at 16:18
  • $\begingroup$ John, See the chart I added to the post. It shows thrust variation with airspeed. $\endgroup$
    – delta2014
    Jan 10, 2021 at 16:34
  • $\begingroup$ You need to check the method of fuel injection in the Starfighter. If it only has to do with throttle setting and nothing else, airspeed is not a factor, it will dump in a given amount of fuel for a given throttle setting. But I would imagine, even with a plane that old, a primitive computer compensates for air flow/ram pressure. The linear increase of "lb m/hr/lb fuel" graph of the turbo jet seems to show "my drag squares but my speed doubles". $\endgroup$ Feb 10, 2021 at 19:07

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I don’t believe that airspeed directly affects fuel flow, but it may impact other variables that can have an influence.

I don’t have a flight manual for an F-104, but for the EA-6B the manual says the following about the fuel control unit on the J52P-408A engine: (straight turbo jet, non-after burning, 1970s technology)

“The engine fuel control is divided into a computing and metering section. The computing section monitors throttle position, burner pressure, engine speed, and compressor inlet temperature. These data are translated into a fuel demand that is relayed to the metering section, which measure the correct amount of fuel from the engine fuel pump the fuel pressurizing and dump valve.”

Of those four inputs the primary would be throttle position, with metering occurring to dampen out pilot inputs and ensure smooth operation. The other three may be influenced by environmental conditions such as airspeed, altitude, etc. but airspeed itself is not an input.

Modern FADEC, (Full Authority Digital Engine Control) engines might be different.

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  • $\begingroup$ So it sounds like that engine used compressor rpm instead of EPR as an indication of power? Was there an EPR indicator? $\endgroup$
    – John K
    Jan 10, 2021 at 22:09
  • $\begingroup$ @John K, correct, no EPR. But I really didn’t use RPM as an indicator of power. RPM was for when I introduced fuel on start, checking at idle, then again at max blast to make sure the limit wasn’t exceeded. All other times I’d just set power for a target airspeed or fuel flow. RPM was largely supplementary info in flight. $\endgroup$ Jan 10, 2021 at 22:21
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It would appear to be yes. See Page 22 of this paper by the Hamburg University of Applied Science. In the chart (below) on Page 22 of representative engines, the turbojet's Thrust Specific Fuel Consumption increases from .8 lb/hr/lbs thrust at 0 airspeed to about 1.0 at Mach. So the engine is most efficient from a SFC perspective while stationary and efficiency declines with speed. Then, when you allow for the potential for the higher speed to increase the net thrust available, the fuel burn goes up just from the increase in energy conversion, on top of the reduced efficiency in doing that conversion.

enter image description here

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  • $\begingroup$ So John, if a pilot is flying a fighter at sea level at 100% MIL power, and he is flying at 300 KIAS, he will see his PPH gauge increase and increase as his airspeed keeps increasing? $\endgroup$
    – delta2014
    Jan 11, 2021 at 22:39
  • $\begingroup$ I would expect so as net thrust would be going up at some point so more fuel required to make it, plus efficiency would be declining as TSFC goes up with speed which would require more fuel even if you kept the net thrust value constant. $\endgroup$
    – John K
    Jan 12, 2021 at 1:15

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