# What power have I obtained?

I am currently working on a question in aeronautical engineering. The question has asked me to provide the available power. I have calculated the drag, and am given the constant velocity. With that, I have calculated that into thrust. Upon using P=F*V, I have obtained a value.

Is this Brake or Available power for the engine?

• If your airplane has a propeller or is a jet with installation losses, you need to consider propeller efficiency rsp. installation losses before arriving at the required engine power. Jul 11 '21 at 10:05

The drag that you have calculated for your plane is probably the drag at cruise velocity, straight and level flight. When you multiply it by the airspeed, the figure that you get is the net power required under those flight conditions. It would be exactly the same as the output engine power only if the propeller had an efficiency of 100%, converting all the engine output power into the 'wind power' that will produce the thrust reaction needed to push your plane. That's not the case in the real world, where props have efficiencies around 60-80%...

Neither, what you have calculated is the power state of the aircraft as Drag x Velocity at a given velocity. This is power required to maintain steady state flight. Units are kg x m$$^2$$/s$$^3$$ or kg x v$$^2$$/s.

Power output of an engine is literally fuel burn/s x the efficiency factor of the prop to produce thrust at a given airspeed.

We can see, at a given airspeed, thrust from engine = drag from aircraft.

One can reference power available performance graphs for a given aircraft to see at what airspeeds available power from engine exceeds the power state of the aircraft. Maximum excess power is usually found at the airspeed corresponding to Vy.

Interestingly, with props, there is greatest excess thrust at lower airspeeds, but less power because V is smaller. The aircraft can climb at a steeper angle, but at a lower airspeed.

"Brake horsepower" is the actual torque output of the engine at a given rpm. It will equal the angular drag of the prop (units Force x distance/second).

"Available horsepower" is considered the maximum torque output at a given airspeed. This value can be described as a (maximum) "brake horsepower" rating.

It will be the brake or available power for the engine only if the engine is running flat out at that speed. What you’ve calculated is the net power required to fly at that particular V.

Power available will be the max. power at that altitude, minus the power required you’ve calculated.