# What does RPM mean in a propellor aircraft, compared to a jet aircraft? [closed]

What is the practical significance of speed RPM in a propeller aircraft for a pilot? I am not able to correlate it with RPM in jet engines?

• I don't really understand your question. By "propellor aircraft" do you mean "piston engine"? Or turboprop? How and why are you trying to compare RPM values? Can you give a specific example to help clarify your question? Mar 30, 2018 at 14:25
• @Adder. If you are trying to equate power in a prop aircraft, with thrust in a jet powered aircraft, just knowing the RPM of the prop, and the RPM of the gas turbine, is not sufficient information. So, the answer is, there is no correlation between just these two parameters. Mar 30, 2018 at 23:07

in propeller aircraft, the practical significance of propeller RPM is it is one component of the equation that determines propulsive power, the other being shaft torque.

If the propeller has a fixed (nonadjustable) pitch, then the RPM of the propeller is a simple indication of the power output of the engine, and the pilot manages engine power by adjusting tachometer RPM with the throttle.

Controlling the power output of a turbine engine is a more complicated process, and depending on the type of engine and its application, the pilot may use pressure measurements in the different stages of the engine or ratios of those pressures or shaft RPM's expressed as percentages of full power to manage the power output of the engine.

RPM in a propeller aircraft is typically under 2400, as the prop spins at the engine RPM speed. Above 2700 the tips of the propeller begin going supersonic and make more noise than anything.

Jets engine spin much faster, as discussed here Which are typical rpm values for aeronautical turbines?

• Just two nitpicks: 1) the prop spins at a multiple of the engine speed, as a lot of them have a gearbox in between. 2) The blade tips will go supersonic at lower rpm if they are longer, so giving just an rpm number as a limit is misleading. Mar 30, 2018 at 13:18
• What @AEhere said (and given the answer's focus, I'm not sure I'd call that just a nitpick). The specific value will depend on gearing and propeller size. As one example, for the plane I'm flying, the magneto check is done with the engine at 4000 rpm, warmup is 2000 rpm, and idle is somewhere around 1800 rpm. (I don't recall off hand what the cruise rpm is.) And of course, while it'll be a multiple of the engine rpm, it won't necessarily be an integer multiple (and technically it doesn't need to be a multiple >1 either, but I'm not sure what would be the value of a sub-1 multiplier).
– user
Mar 30, 2018 at 20:07

There are three primary cases for use of RPM in aircraft that differ by engine type: piston, turboprop and turbofan. In all three cases RPM refers mainly to two speeds, propulsor and engine. They may be indicated in percent of maximum (usual for turbine) rather than as a specific speed (usual for piston).

• Piston engine- the propulsor is the prop and the engine speed measured is the crankshaft. Usually there is no gearing and both are the same, so there will only be one gauge in an aircraft. Notable exceptions are warbirds with high power and correspondingly large props.

• Turboprop - the propulsor is again the prop and the engine speed being measured is the turbine output shaft, fixed to the low pressure compressor. This is input to a gearbox since turbine speeds are too high to be useful in directly driving a prop. There can be two speed gauges in the aircraft.

• Turbofan - the propulsor is now the front fan and the engine speed being measured is usually the core, the high pressure compressor. The front fan historically runs on the low pressure compressor shaft. PW inserts a gearbox between the front fan and the output shaft, making the drivetrain resemble the turboprop configuration. More complex engines may have three shafts, all running at different speeds.