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Is this only due to the various flight conditions encoutered during operation, such as side winds and turbulence, or are there other reasons? Also, is interference between fuselage and power plant involved?

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No, these losses are well known and are called installation losses.

Their reasons are:

  • Intake losses. In a ground test the engine will be fitted with a screen to avoid foreign object damage, but none of the long intake tubes which are common especially in supersonic aircraft.
  • Generator loads: The engine drives generators to supply the aircraft with electricity. This can be simulated in a ground run, but often the generator load is higher in flight.
  • Bleed air: Some of the compressed air inside the engine is piped to a heat exchanger and then into the cabin for pressurization and heating, or used for leading edge de-icing. Again, this can be simulated in the static test but sometimes bleed air losses are higher in flight.

Interference between fuselage and power plant is only to be expected with fuselage-mounted engines. This can be both beneficial (as in the precompression at high angle of attack and speed on the Rafale or the F-16), or it can distort the flow when side-mounted intakes are operated at a high sideslip angle.

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  • $\begingroup$ Also keep in mind that such losses don't help "sell" the engine to customers, and also prevent apples-to-apples comparisons, so manufacturers have a vested interest in having the highest test thrust. It's up to the aircraft manufacturer to determine what other loads they may be applying and how that will affect the actual thrust. $\endgroup$ – Adam Davis Oct 23 '17 at 14:49
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    $\begingroup$ @Harper That's not really true, I don't think. You can't design the aircraft around not expecting those loads or your APU essentially becomes a PU and you'll need a backup for it. If the APU dies, you're on one engine, and the other can't keep you flying and provide bleed air and generator power then you're in rather serious trouble. I may be wrong, but I don't think you'll find many commercial aircraft designed that way. $\endgroup$ – J... Oct 23 '17 at 16:54
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    $\begingroup$ @J... Are you not familiar with a bleeds-off takeoff? It is a normal procedure for most if not all large transport aircraft today, and fairly routine. If the APU is deferred, you can depart unpressurized, and start pressurizing shortly after takeoff. $\endgroup$ – Ralph J Oct 23 '17 at 22:05
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    $\begingroup$ @RalphJ I'm not, and I've learned something - thanks! I'd imagine, though, that the aircraft could still safely take off in those circumstances with bleeds on. Most of the time you're not taking off at maximum thrust anyway, generally, simply to reduce fuel consumption and engine wear. I guess I was taking particular exception to the notion of "if you ever need maximum thrust" - it's not a question of need, this is just a measure for economy, I think. I was taking need to mean a case where the aircraft cannot perform unless bleed and generators are off. $\endgroup$ – J... Oct 23 '17 at 22:32
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    $\begingroup$ @RalphJ In any case, I'm happy to accept the statement that if you ever want that performance or want that economy, then the option is there, but for the aircraft to routinely need that performance would be an unacceptably risky design. $\endgroup$ – J... Oct 23 '17 at 22:34
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Static thrust is quite more than in-flight thrust, because the efficiency of any thrust generating engine or propeller is directly related to the incoming flow.

If there's no inflow, the accelerated air will generate more thrust. If there's inflow on the other hand, the thrust will be lower. Non installed engines are doing a simpler work.

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