Speeds for max range and endurance
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Join Date: Jan 2004
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Speeds for max range and endurance
I'm dusting off my performance notes in preparation for an interview with a turboprop operator. It's been a while since i've looked at the books so i've got a few questions:
(1) Do the speeds for pistons apply to turboprops as well?
(2) I understand that the speed for best range in a piston is the tangent to the power required curve. My notes also say that this speed coincides with the minimum drag speed, Vmd. So where on the drag curve is minimum power speed, Vmp, and why do you need less power at this speed even though drag is higher?
Thanks,
Al
(1) Do the speeds for pistons apply to turboprops as well?
(2) I understand that the speed for best range in a piston is the tangent to the power required curve. My notes also say that this speed coincides with the minimum drag speed, Vmd. So where on the drag curve is minimum power speed, Vmp, and why do you need less power at this speed even though drag is higher?
Thanks,
Al
(1) As I tried to point out in the last post of the thread that Intruder cited, the jet and prop cases are extremes on a spectrum that has real aircraft in the middle. With all other things equal, a real prop-driven aircraft's best endurance will probably be higher than Vmp, and a turbo-fan's best endurance will be lower than Vmd. I believe turboprops, like piston-engine driven props, have approximately constant BSFC, so the prop case is applicable.
(2) With usual assumptions about the shape of the drag curve, Vmp is usually at about 70% of Vmd. You need less power because energy is force times distance moved and thus power is drag times speed. At Vmp you have a little more drag but much less speed, so the product, power required, is less.
(2) With usual assumptions about the shape of the drag curve, Vmp is usually at about 70% of Vmd. You need less power because energy is force times distance moved and thus power is drag times speed. At Vmp you have a little more drag but much less speed, so the product, power required, is less.
