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Answer by Robert DiGiovanni for Why does an aircraft require less power when it is flying slower than the most efficient speed?

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all aircraft are moved with thrust

THRUST ACCELERATES AIRCRAFT UNTIL DRAG EQUALS THRUST

"Potential energy" of fuel is turned into thrust, no matter if it's a propeller, a rotor, a jet, or a rocket.

Minimum "power" is the lowest aircraft fuel consumption rate, in liters per second, the keep the aircraft in level flight.

Minimum drag is the airspeed at which the lowest amount of drag is produced. This is expressed as best L/D ratio

Why are not minimum power and minimum drag speeds the same?

Because the efficiency, $\eta$, of propellers is affected by the freestream velocity or airspeed and the propeller angle of attack.

For a given RPM, propellers can produce more thrust at a lower airspeed because force, or thrust, is derived as air m(V2 - V1). The slower the freestream, V1, the greater the thrust per unit of fuel burned.

Even though aerodynamic drag is a bit more at V min power, for the lowest amount of fuel burned, thrust = drag at V min power.

So, if you have to glide, do it at best L/D, because $\eta$ of gravity does not change with speed!!


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