# What´s the logic behind the Power Curve?

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?

• Like I said in a comment to the similar question aviation.stackexchange.com/q/81809/34686 --" For a given setting of the throttle or power lever, piston engines with propellers tend to produce constant POWER regardless of airspeed, and jet engines tend to produce constant THRUST regardless of airspeed. ... Of course that is a gross oversimplification. I strongly suspect that this has been covered on ASE before ... search on ASE for other related content Commented Oct 23, 2020 at 22:29
• … of course piston engine only produces constant power if it is attached to a good constant-speed propeller so it can be operated at the optimal RPM independent of speed. With fixed propeller at low speed the engine power is limited by not being able to spin up to the optimal RPM and at high speed it has to be throttled down to avoid overreving. Commented Oct 24, 2020 at 20:49

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.

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).

• We only have to increase RPM to achieve higher true airspeed with fixed propeller. And because engine power increases with RPM up to some limit (and then declines quickly and the engine can't handle more anyway), it wouldn't work for large speed range, because then the engine wouldn't be able to produce enough power for take-off. That's why all faster propeller aircraft have variable pitch (constant speed) propellers. And spin them slower at higher airspeed to keep the parasite drag low. Commented Oct 25, 2020 at 11:17
• Turbines, i.e. ducted fans, are not influenced by forward velocity because the flow speed through them is almost independent of forward velocity, the inlet converting the speed to pressure (which increases the effective pressure ratio) instead. Commented Oct 25, 2020 at 11:20
• @Jan Hudec. "Engine power increases with rpm up to some limit, it won't work for large speed range": electric motors have a much greater rpm range. A real opportunity for the application. Regarding converting speed to pressure, and applying energy to that pressurized air, I am seeing a similarity to the turbojet and fan jet portions (and also to the prop flow stream): if we increase rho, we can get more thrust (more mass being accelerated). Commented Oct 25, 2020 at 14:38
• Electric motor would indeed work with fixed propeller over longer speed range, but ultimately the increasing parasite drag of the faster spinning propeller would become a problem anyway and a variable pitch propeller would be needed for further increase in speed. Commented Oct 25, 2020 at 22:23