Can someone explain why turboprops are less effective in terms of thrust than turbofans? But at the same time, why turboprops burn less fuel? For example, the Bombardier Q400/De Havilland Dash 8 has a fuel burn of 2.79L/seat/100km, whilst the 737 MAX 8 with the same number of engines burns 2.93L/seat/100km, but the Bombardier has a max. cruising speed of 360 knots and the Boeing plane can go up to 526-527 knots, even though it's heavier.
$\begingroup$
$\endgroup$
3
asked Apr 21, 2020 at 15:15
-
1$\begingroup$ Most, if not all, turboprops can put the propellers in beta and generate reverse thrust. $\endgroup$– Ron BeyerApr 21, 2020 at 15:16
-
2$\begingroup$ For your second question: How does propellers' pitch change during the use of reverse thrust in turboprop/propfan propelled planes? -- and others -- Note: it's advised to search the site before asking. $\endgroup$– user14897Apr 21, 2020 at 16:44
-
$\begingroup$ Because they are attached to faster airplanes? $\endgroup$– quiet flyerApr 21, 2020 at 23:59
Add a comment
|
3 Answers
$\begingroup$
$\endgroup$
To generate a given thrust, a turbofan accelerates less air but faster than a turboprop. This means that turbofans can fly faster. However thermodynamically it is more efficient to accelerate a greater mass of air slower, so turboprops are more efficient and that translates into lower fuel burn.
$\begingroup$
$\endgroup$
It’s all dependent upon what operating speeds your aircraft will be in. Propellers can move a great volume of air at slow speeds but accelerate the gas to a lower exit speed than does a turbojet or a turbofan. This is the principal reason that a helicopter is the most efficient means of VTOL flight out there for a heavier than air aircraft. But as speeds increase, the the specific impulse drops and the propeller efficiency drops off with it. At around 400-500 mph, the specific impulse of a high bypass turbofan becomes superior to the turboprop, as, while it moves less air than a turboprop, it can accelerate the smaller air mass to a much higher exit velocity. This trend continues through low bypass turbofan engines, turbojets and then ramjets.
And as pointed out turboprops have a beta and reverse pitch range which can produce reverse thrust.
answered Apr 21, 2020 at 16:19
$\begingroup$
$\endgroup$
Turboprops are less fast because of the mechanical constrainsts at the blade tips,efficiency and noise near the speed of sound.
For reverse thrust:yes
related : How does a turboprop engine keep running during thrust reversal?
