# How can a curve in Gabrielli-Von Karman graphs curve down?

Gabrielli-Von Karman graph plots the specific power, that is, power required to move certain weight, against speed. It gives an idea how much power is required to move certain mass at a certain speed using a certain vehicle.

There are some vehicles whose curves go down at some intervals, such as helicopter, racecar and jet fighter. How does this happen? Curving down implies that we need less power at higher speeds. Why would this be true, especially for aircrafts as we know air resistance is highly proporsional to speed?

• Perhaps the shape has an effect over certain speed ranges... Commented May 3, 2020 at 15:29

There is a good reason for airplanes and I guess it can be extended to some race cars with spoilers which function similar to flaps and air-brakes.

An airplane's thrust demand graph (power needed to keep it going) is divided into two parts of the drag curve as per the attached graph.

At slow speeds such as take-off or landing the angle of attack is higher and some flaps are deployed adding to the drag to the advantage of more lift, so there is a lot of "Incidental" drag ( as if when you hold you hands out of a moving car window in a tilt), as the speed increases the angle of attack is reduced and the "Parasite" drag which is basically the friction of the stream of air in which the plane is flying increases. That is why we get the total drag ( power demand ) sloping down in midrange speeds and then go back up.

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