# Why is the available power in propeller engine is almost constant for every velocity?

I surfed on the internet for an answer but I didn't find any. I'm taking Aircraft Performance course. I saw that available power for propeller engine is almost constant for every velocity. But I don't know why...

• I added a graph showing it – Volkan Aydıngül May 25 '17 at 13:14
• Why do you not expect it to be more or less constant? It will help people to answer. – Simon May 25 '17 at 17:16

Because the propellor blades have variable incidence and operate at constant rotational speed. As the plane picks up speed, the propellor blade angle is adjusted so that the local angle of attack stays the same.

• This is only true for variable pitch propellers which are not found on all prop planes. – Dave May 25 '17 at 14:00
• @Dave is correct, but ALSO incidence will change with the aircraft angle of attack, and that will affect P factor, and reduce efficiency. However, incidence is a tertiary factor for aircraft power. – mongo Jun 1 '17 at 21:59
• @ Mongo Yes Dave is correct, variable pitch propellors are only found on aircraft with variable pitch propellors. In the bit of airspeed that the OP shows, power curves are pretty flat. Helicopter manufacturers would disagree with incidence being a tertiary factor for power: the collective is what applies power via incidence change, and the fuel governor keeps the rpm constant. – Koyovis Jun 1 '17 at 23:40
• @Koyovis the OP asked within the context of what appears to be an airplane. I agree that in rotorcraft pitch is a primary factor in power availability, and that incidence is of greater consequence than in a conventional airplane. However, the curve provided by the OP is consistent with airplanes, and in that context incidence is tertiary. As a point of clarification, in airplanes, prop incidence is a function of mounting of the engine, and the AOA. In rotorcraft it has slightly different usage, and rotor incidence often refers to pitch of the rotor relative to the hub. – mongo Jun 2 '17 at 13:51

It is a gross simplification, assuming a constant speed prop. If a fixed pitch prop, it is non-linear. Even with a constant speed prop it is not exactly linear.

Lesser factors for power available are the angle of attack/angle of incidence, which will change with airspeed, assuming level flight. Ram air is also a minor factor on some engines.

The diagram you show is a simplification meant to show you that the power is the same, regardless of airspeed, but in reality it varies. How much? I can't say for certain in general, but on one aircraft I collected data on, I recall it varying about 6% in the 20 knot window we were looking at. And to be clear, that was power converted to thrust, not simply power plant power.

available power for propeller engine is almost constant for every velocity

This is because full power is always, in theory, available to you no matter the velocity of the aircraft. In other words, you can always slam the throttle full forward.

The required power is the power that you would need to move at a given velocity (through the air). When the required power equals the available power you would hit your maximum velocity. Above the available power limit when the power required to go a given velocity exceeds the power available from the engine you would not be able to go that velocity (with the exception perhaps of in a dive).

• While these statements may generally be true, they do not address why the available power in the chart is almost constant. But to a finer point, the engine prop/engine transfer efficiency is less in a propeller when the plane is stationary. As the plane moves, the prop efficiency increases. Therefore, the power produced also increases. – mongo Jun 1 '17 at 22:11