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If two fixed-wing aircraft almost identical except for the engines, one turbofan and one turboprop, which one would need less runway to take off?

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    $\begingroup$ Takeoff distance is much more a factor of wing loading and other factors rather than engine choice. $\endgroup$ – GdD May 17 '17 at 20:05
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    $\begingroup$ GdD is correct. Lots of factors go into takeoff performance; engine type being just one. That said, if all things are equal and somehow you had the engines on the exact same aircraft, I would guess that you'd also have to reduce weight on the turbofan to keep it equivalent with the turboprop, thus reducing engine size and thrust capability. At slow speed, a turboprop will generate thrust more efficiently, so my guess would be that the prop would be able to get the aircraft to takeoff speed faster and use less runway. But it would quickly get smoked as speed increased. Pure guess though. $\endgroup$ – Shawn May 17 '17 at 20:30
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    $\begingroup$ I'm not sure that I agree with @GdD. Jets produce thrust more efficiently at higher airspeeds, while turboprops produce thrust more efficiently at lower airspeeds. All other things being equal, a turboprop should accelerate more quickly, and therefore need less runway, than a jet. $\endgroup$ – J Walters May 18 '17 at 3:32
  • $\begingroup$ Why "requires"? I guess you wanted to ask "which engine type allows the shortest take-off run". Please clarify. $\endgroup$ – Peter Kämpf Dec 22 '18 at 20:36
  • $\begingroup$ Requires because takeoff runway is a limited precious thing. $\endgroup$ – vasin1987 Dec 24 '18 at 17:27
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Even with similar weight, turboprop and turbofan aircraft will have different design characteristics. A turboprop will fly at lower altitudes and airspeeds, resulting in different aerodynamic designs. A turboprop also burns less fuel per hour, so it will need less fuel capacity.

That being said, we can look at some aircraft to get an idea of what the trends might be. The best example is probably the Dornier 328. It's a turboprop aircraft that seats about 30 passengers. There also happens to be a version of the plane with jet engines. This is probably about as similar as you can get in both weight and aerodynamics. According to Skybrary, the figures are as follows:

Dornier 328
MTOW 13990 kg
Distance 1000 m

Dornier 328JET
MTOW 15200 kg
Distance 1382 m

You can see the the jet derivative is a bit heavier but requires a much longer takeoff distance.

The trend is similar between other aircraft.

Dash 8-300 Turboprop
MTOW 18600 kg
Distance 1085 m

ERJ-135 Jet
MTOW 20000 kg
Distance 1580 m

CRJ-200 Jet
MTOW 21523 kg
Distance 1527 m

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Everything else being equal, and for the same engine power, a turbofan will need more distance, since turboprops give more thrust than turbofans at small airspeeds. In general, and for the same power, thrust at low airspeed is higher the lower the 'disk loading' is. That's why helicopters have big rotors...

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A shorter take-off run equals a greater acceleration in order to reach take-off speed earlier, which means applying engine's power output (converted into thrust) to the greatest possible surface of air, from 0m/s to take-off speed.

Imagine a 10 meter race starting at 0m/s, helicopter (R44) accelerates faster vertically, than an airplane (Cap 10b) does horizontally when releasing the brakes. Yet Robinson R44 and Cap 10b have the same power to weight ratio.

Turboprops move a greater frontal surface of air at a slower speed, turbofans move a smaller frontal surface of air at a higher speed. Most force application surface you have, most efficent thrust you get.

A 1kg steel sphere sinks faster in water, than a 1kg steel disk displacing a lot more water.

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For two identical airframes, one turbofan powered and the other powered by prop jets, it's going to be a matter of the total available thrust from each as we as the thrust to weight ratio of each engine installation. The lighter the engine / propeller and the greater the thrust, the shorter the takeoff roll will be by Newton's Laws

F = m*a

Here, the total force propelling the aircraft forward is the engine thrust force Fe subtracted from the sum of the Aerodynamic drag forces and wheel friction.

F = Fe - Da - Df

So a becomes

a = (Fe - Da - Df)/m

And the time of takeoff roll from standing start using basic kinematics.

Vr = a*t

And the total ground roll as

L = 0.5*a*t^2

In the real world, jets are designed for different cruise and mission profiles than turbopropeller aircraft, which requires the use of different airfoils and wing planforms. Typically swept wing aircraft have higher stall speeds and consequently higher Vr than do rectangular or tapered planforms usually found on turbopropeller aircraft.

The comm nets made about the section from Aerodynamics for Naval Aviators are interesting. This effect would be most noticeable on a fixed pitch propeller but minimized with a proper constant speed propeller which can keep propeller efficiency around the theoretical maximum of 85%-88%.

One advantage which a turboprop will have - at least for a tractor configuration - is that the prop wash over the wings creates additional lift, which can reduce Vr. Propwas over the tailplane can provide additional elevator authority as well, reducing the takeoff roll here.

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    $\begingroup$ I did not downvote, but the quote "Everything should be made as simple as possible, but no simpler." seems applicable. $\endgroup$ – Sanchises May 18 '17 at 13:51
  • $\begingroup$ This doesn't take into account the dynamic variation of thrust throughout the takeoff roll for propeller aircraft as opposed to the more constant thrust of a turbojet aircraft throughout the takeoff roll. See Aerodynamics of Naval Aviators, p 185. $\endgroup$ – J Walters Jun 7 '17 at 3:59

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