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Trying to understand the drag and power required/available graphs for a piston + propeller airplane. Both graphs are from current book/website and while they are showing mostly the same thing, I noticed the shape of the power available curves are different.

If I have to guess I think the second graph make more sense, since power available from a piston engine should be almost the same throughout the entire speed range.

*If the above is true, can you please also explain why it's not a perfect horizontal line across? Ie: if the engine is rated for 150 BHP, then the line should be at 150 BHP, other things equal (density altitude)

I feel like I'm missing something fundamental here but can't seem to work this out. Thanks

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    $\begingroup$ As far as I can see the graphs are the same and could represent the exact same aircraft, the only difference being that the ratio of vertical units to horizontal units has been changed, "stretching" out the upper graph. $\endgroup$
    – quiet flyer
    Jan 19, 2021 at 13:14

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Whether or not the graphs are identical certainly would be up to the artists eye, but working through the units and definitions may be helpful.

Most of us know "power" as engine output in "horsepower" but it can also define the energy state of an object that is propelling itself as Power = Force x Velocity with units of kg m/s$^2$ x m/s. The propelling force is Thrust.

Horsepower carries the same units as: Torque Force x distance/time as engine "power" output, which is proportional to fuel burn/time and equal to prop drag x distance/time.

Which "Power Available" graph is more correct?

They both are if we understand them. The first one is based on Thrust x Velocity of the aircraft. Factor out Velocity and you have a perfectly sensible graph with L/D max right at the bottom of the curve.

The second one shows "horsepower available" based on engine RPM, which improves slightly for fixed pitch props that reach optimal AOA and less blade interference at higher airspeeds. Essentially, the prop allows the engine to run at a higher RPM because it is creating less drag.

"Horsepower required" is how much engine torque is required to turn the prop at a sufficient velocity to create sufficient thrust to match the aircraft drag.

Why they don't just plot thrust vs drag is beyond me, but working through the units and nomenclature seems to help.

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Power is work getting done over a period of time. Work is force multiplied by distance. So when you are sitting at the beginning of the runway, at full throttle and brakes on, while thrust available is very high, power available is 0, because no “work” is getting done.

Edit- basically you are applying a lot of force, or using a lot of thrust, but since distance traveled is 0, no work was actually done. Which is why the power available is indeed 0 at a velocity of 0, and increasing rapidly.

Note- the second graph is exactly the same as the first, only it is showing the curve beginning at some point after the stall speed.

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