What´s the logic behind the Power Curve?

Question

Aircaft thrust is what counteracts drag, but in flight school they tought us that propeller aircraft are "power producers" and not thrust producers as jets. (That´s because the propeller needs the power of the engine to rotate and generate thrust, while a jet engine generate it by itself). In both cases we can look at the thrust curve to get some performance information, but for props, we need to get a power curve also (get it multiplying thrust times airspeed values of the thrust curve) to get other performance features as Vy. (found where greatest excess power is).

I can´t understand the logic behind that, I mean, why does power affect airplane´s aerodynamic? Why is it important how much power does the engine has if what really makes the aicraft move forward or climb faster is thrust?

Answer 1

why does power affect airplane´s aerodynamics?

It doesn't. The aerodynamics will determine how much excess power is left for acceleration or climb. But not the other way around.

Why is it important how much power […] the engine has?

Because the power of a piston engine is constant for the same air density, regardless of speed. It is more helpful to give a power rating because that figure is valid over the whole speed range, unlike thrust. Thrust is inversely proportional to speed and so a thrust figure is only valid for exactly one operating point.

Pure turbojets have approximately constant thrust at subsonic speed, so here the thrust rating is more useful. The power a turbojet can produce increases with flight speed.

Answer 2

Definition of Power = Force × Distance/Time = Thrust × Velocity

Another definition Fuel burn/Time.

By the second definition both jet and piston produce "Power", and certainly both jet and piston produce thrust.

We describe prop "power" vs drag because the thrust a fixed prop generates is much more dependent on its forward speed, there for we compare these values to drag at given speeds.

Interestingly, in the real world, we must increase rpm to achieve higher airspeed. A cleverly designed aircraft would have 2x the rpm at 2x the airspeed, helping maintain the same prop AOA throughout the speed envelope. This would make a horrid propeller thrust curve much flatter, but there are so many other airspeed/engine combinations to consider.

The jet, spinning at much higher rpm, is much less influenced by forward velocity, so the term is dropped (but could be left in).

The important thing to remember is not to confuse fuel burn power with the "power state" of the aircraft thrust (as Thrust x Velocity).