At a high level, this is simple. Hot gas goes out the back, and Newton's third law says the engine is pushed forwards. But when you look inside the engine, it's less clear which part experiences the thrust. Compressor? Turbine? Combustion chambers? Nozzle? How about if there's an afterburner?

For other kinds of aircraft engine, this is comparatively easy to figure out:

  • For a piston engine, the thrust is felt on the propeller blades, since they're doing the work on the air.
  • For a turboprop, most of the thrust is on the propeller blades, and the residual jet thrust presumably works like a turbojet.
  • For a turbofan, most of the thrust comes from the fan blades, and, again, the thrust from the hot core presumably works like a turbojet.
  • For a rocket, some of it comes from pressure on the head of the combustion chamber, and the rest from the divergent part of the nozzle.

Reading about turbojet history, largely in Anthony Kay's books, has shown me that compressors sometimes pull forward and turbines backwards, but hasn't shown me any rules about this. How does it work?


1 Answer 1


This is a great question with no simple answers.

When we draw the control volume around the outside of the engine, we greatly simplify things. The thrust is the result of the change in momentum $F_n=\dot{m}(V_j-V_i)$. Here, we've drawn the control surface such that the pressure is the freestream pressure everywhere, so there aren't pressure contributions.

As we shrink the control volume down, things get more complicated. If you go all the way down such that the control volume is the metal-air interface everywhere, then all the forces must resolve to pressure on the metal (except for the momentum from the mass flow of the fuel injectors, which crosses the boundary).

If you split a piece of metal -- say a compressor blade or a turbine bucket -- it will have a 'pressure side' and a 'suction side' -- but like a wing, the pressure on both surfaces may well be lower (or higher) than ambient. I.e. one 'side' of a component may produce positive thrust -- while the other 'side' may produce negative thrust.

This becomes particularly apparent when you talk about installing an engine on an airframe. Thrust/drag accounting -- resolving the forces to either the engine or the airframe -- is a complex matter that blurs physics with business. We end up with concepts like cowl lip thrust and spillage drag.

When you get to these details, you have to be meticulous to make sure you are consistent -- and you communicate all of your assumptions to anyone who may use your results. Otherwise, it is easy to mess things up.

This gets us back to the reason we like drawing the control volume where it makes the problem easy.


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