Engine horsepower vs thrust

[Dreamlinerwannabe]

I understand that engine thrust is about climb angle, engine horsepower is about climb rate.


since engines thrust and power are for different purposes, isn't it should be quoted with 2 things? why when people talk about jet engine they talk about thrust while piston aircraft they talk about horsepower?

Thank you very much!

[ATCast]

The main engine parameter of piston aircraft is horse power because the engine delivers approximately the same power anywhere in the normal speed envelope of the aircraft.

The faster a piston aircraft flies, the less thrust it gets from the engine. The power is approximately constant, so the thrust force is decreasing with speed:
Thrust force = power / speed

A (pure) jet engine delivers approximately the same amount of thrust anywhere in the normal speed envelope of the aircraft, and therefore thrust is the main engine parameter for jet engines.

The faster a jet aircraft flies, the more power the engine is generating. The thrust is approximately constant, so the power is increasing with speed:
Power = thrust * speed

The above are very coarse approximations, as always it gets much complexer the closer you look at it.

[Slatye]

My understanding is that it's because different engine technologies behave differently. The key equation here is that Work (energy) = Force * Distance. Dividing by time, Power = Thrust * Velocity. Therefore if you know power and you know airspeed, you know thrust. If you know thrust and you know airspeed, you know power.


For a propeller-based engine (piston or turboprop), power is (more or less) constant. The result of this is that thrust varies inversely with airspeed (ie if you double the airspeed, you halve the thrust). Therefore specifying thrust for a piston engine would be a bit pointless - you'd have to say "10kN thrust at 100kts" because without the airspeed being included the number is meaningless.

For a turbine, the opposite is true: thrust is roughly constant so you get more power at higher airspeeds. Specifying a power rating would only be relevant if you also specified the airspeed (ie "10MW at 100kts").

The practical outcome is that for low-speed work, you want a propeller (a low airspeed means you generate the same thrust at less power). For high-speed work you want a turbine, because to generate useful thrust at high speed with a propeller you'd have to put in an exhorbitant amount of power.

[Dont Hang Up]

The faster the aircraft is travelling the greater the volume of air that can pass through the jet engine, therefore the faster that fuel can be burned and the greater the power developed.

Therefore, for a jet engine, a simple measure of maximum horsepower is meaningless because it depends how fast you are going. Maximum thrust, on the other hand, is approximately constant (at any given altitude).

[flyboyike]

It's all just PFM, no sense sweating it.

[HazelNuts39]

It's basically the output of the engine that can be measured on a test stand: shaft horse power for a shaft engine, thrust for a jet engine.

[Dreamlinerwannabe]

Thank you very much for all the constructive responses! It makes my head much more clear now! :):)

[Dreamlinerwannabe]

Why piston engine power is roughly constant?
Physics theorem: Power = thrust * speed
When airspeed increases, drag increases, making “excess thrust” less, so thrust decreases, that’s why power is constant.

For jet engine, thrust is roughly constant, when airspeed increases, engine can suck in more air, making power increase. Doesn’t it mean thrust increases then according to this physic equation?

[barit1]

For jet engine, thrust is roughly constant, when airspeed increases, engine can suck in more air, making power increase. Doesn’t it mean thrust increases then according to this physic equation?

Yes, thrust does increase, but not so great as to invalidate the principle. And there are other factors such as spill drag which muddy the waters; so at this level of discussion, a reasonable approximation is that jet thrust is constant.