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When aircraft engines are evaluated in a test flight, how is the thrust of the engine measured?

You would need accurate thrust numbers to calculate the real-world specific fuel consumption (i.e. not in a controlled environment like an engine on a test bed).

I'd assume it is nearly impossible to measure thrust indirectly with any accuracy since so many factors are involved (wing shape, wind, air density, lift, ...).

So I guess there should be a way to measure thrust directly, probably in the engine pylon.

So how is engine thrust measured in flight?

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    $\begingroup$ Specific fuel consumption is property of the engine (and propeller if applicable) assembly only. Those can be just put in a wind tunnel or on a test rig! $\endgroup$ – Jan Hudec Oct 15 '14 at 13:30
  • $\begingroup$ My question concerned the specific circumstances of a real test flight, including wind, temperature, air pressure, moisture, ... $\endgroup$ – florisla Oct 15 '14 at 18:11
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    $\begingroup$ I don't think it's actually interesting value. You need to know consumption at given speed under given conditions, which can all be measured directly and the thrust is mostly irrelevant intermediate value at that point. It is really only useful for comparing engine performances when choosing engine for an aircraft and for that purpose measurement on a test bench is plenty good. $\endgroup$ – Jan Hudec Oct 15 '14 at 18:37
  • $\begingroup$ You would still need to measure thrust to detect anomalies. If your engine uses the right amount of fuel but produces less thrust than expected, that's a very dangerous situation. $\endgroup$ – florisla Oct 17 '14 at 6:31
  • $\begingroup$ It is unclear if your question only address engine based on turbine (trubojet, turbofan, ...) or also include other engines (piston, electric, rocket, ...) $\endgroup$ – Manu H Oct 19 '14 at 8:11
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Engine thrust is measured in flight by EPR - Engine Pressure Ratio.

EPR is the ratio of the turbine exhaust pressure divided by the pressure measured at the fan or inlet. Indeed this is the measure used for a number of engines for setting thrust.

More detailed airborne testing of engines is done during development, most manufacturers have airborne testbeds. The parameters recorded here probably measure in the hundreds or even thousands...

Rolls Royce B747 Testbed - image from Flight Global

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  • $\begingroup$ Not all airplanes have EPR. For instance, my jet engines only have N1%, N2% and ITT (well, and fuel flow, oil pressure/temperature, vibration meters, etc. too).... $\endgroup$ – Lnafziger Oct 21 '14 at 3:20
  • $\begingroup$ Sure, but EPR is the closest thing to an airborne measure of thrust, whereas N1 is more a measure of power set (percentage fan speed). $\endgroup$ – vectorVictor Oct 21 '14 at 15:30
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    $\begingroup$ Absolutely, but your answer does not address how thrust is measured in an aircraft without EPR gauges.... $\endgroup$ – Lnafziger Oct 23 '14 at 3:33
  • $\begingroup$ I suppose it isn't directly. Its the same as Peter's answer on piston engines. Whilst in piston aircraft you generally set RPM or Manifold Pressure. In Jet aircraft setting a percentage N1, is setting a percentage of the available thrust, albeit predetermined and not measured "live". $\endgroup$ – vectorVictor Oct 23 '14 at 6:17
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VectorVictor gives the right answer for jets.

However, for propeller aircraft this is really hard, and in the end only the difference between drag and thrust can be measured. By measuring the brake power of the static engine and the drag of the aircraft in a windtunnel, some data points can be acquired which help in calculating what the real thrust in flight could have been.

You are right, in the end it is impossible to produce a precise measurement of thrust. The most important part of the measurement is actually a precise definition of what thrust is: How do you account for the increased friction drag in the propeller slipstream? Is cooling drag part of the airframe drag, or should it reduce thrust? Measuring thrust is foremost an exercise in precise, meticulous bookkeeping.

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For simple platforms, load-cells can be attached in between the propulsion system and the airframe. This allows the propulsion force to be measured.

This force data excludes aerodynamic integration effects, such as:

  • propeller stream effects for nose-propped aircraft, and fuselage blockage for rear-propped aircraft
  • engine cooling drag (which is an aerodynamic property of the particular engine/propulsion integration)
  • accelerated air over the airframe (in-body jet engines, or propeller aircraft)

This is not an absolute requirement to assess the performance of an engine. Especially, if the aircraft can fly with another engine or if the drag characteristics of the aircraft is known by some valid method, instead of a direct measurement, indirect calculations can be done. For example,

  • for constant speed and level flight, thrust = drag
  • for accelerated level flight F_net=Mass * acceleration, etc.
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A new or overhauled engine's thrust on a manufacturer's test bed is measured against a load cell or thrust meter at given RPM, TGT (Turbine Gas Temperature),fuel flow and EPR (Engine Pressure Ratio).

When the certified engine is installed in the airframe and ground run, after taking into account local atmospheric conditions,intake and jet pipe losses, the maximum EPR, or thrust, and TGT and fuel flows can be cross checked.

In a test flight, given the correction factors for altitude and speed etc, against the known parameters of EPR, TGT, RPM and fuel flow, the engine can be seen to be giving required performance. If the the engine parameters are correct but aircraft performance (i.e. speed and rate of climb) are low then suspect the airplane's weight and/or drag (flap, doors, panels etc poor fit) as possible culprits. Its not fair to expect an engine to push a dirty heavy airplane up that big hill on a hot day is it?

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