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Whenever, I come across turboprop, the author always mentions that they have higher bypass ratio than turbofan but no one explains the reason for this.

So, is the bypass ratio higher due to higher disc radius of turboprop or is it due to less air flow through the core or the combination of both?

If it is only due to less air flow inside the core, then they can still have a higher bypass ratio in turbofan right? (Since advanced turbofan produce less noise due to the nacelle)

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    $\begingroup$ When you say "propellor" do you mean a piston engine, or a turboprop or both? $\endgroup$ – Jay Carr Oct 5 '16 at 20:11
  • $\begingroup$ I've never heard anyone refer to a bypass ratio in reference to a prop. The ratio is the amount of intake air that does not enter the core vs the air that does. Without a ducted system I don't know if you can really come up with an amount of air though the prop $\endgroup$ – TomMcW Oct 5 '16 at 23:39
  • $\begingroup$ Related: The fan/propeller speed aspect is developed in the answer of Is it possible to drive a turboprop directly from a jet engine without a gearbox? $\endgroup$ – mins Oct 6 '16 at 10:12
  • $\begingroup$ I seem to recall the big difference between the two is that the prop is generally bigger than the fan, but that the prop also has an effect where it's not just the air going through the prop that adds to the thrust, but the air immediately surrounding the prop too. I believe this additional air is included in the BPR calculation but I can't remember the name of the effect. $\endgroup$ – Notts90 Oct 6 '16 at 14:54
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Bypass ratio is the ratio of the overall flow to the core flow (which goes eventually through the combustion chamber). Overall flow is either the flow captured by the intake or going through the propeller disc. Even for piston aircraft it is possible to define a bypass ratio if you interpret the air which flows through the cylinders as the core flow.

A turbofan essentially uses a shrouded propeller of high solidity (or activity ratio) which helps to accelerate the flow through it as much as possible. It needs to produce the most thrust from the limited air volume captured by the shroud's intake. The limitation is needed to limit the tip speed of the fan and the overall size and drag of the shroud.

Propellers, on the other hand, accelerate a bigger flow by less and need to cover less of the propeller disc for doing this. To keep thrust constant, a bigger mass flow needs to be involved into the acceleration.

Plot of thrust specific fuel consumption over bypass ratio

Plot of the thrust specific fuel consumption in lb of fuel per lb of thrust per hour of different engines over the logarithm of their bypass ratio (picture source).

A comparison is only sensible if we look at the same core component: If a turbofan and a turboprop share the same high pressure components. One example would be the Rolls-Royce/Allison 2100 and the Rolls-Royce/Allison 3007. Wikipedia gives the mass flow of the 3007 as 109 - 127 kg/s but gives no information for the 2100. Using the maximum flight speed of the Alenia C-27J of 167 m/s and its propeller diameter of 4.15 m, we can find a lower bound for the mass flow through one of its propellers if we make the heroic assumption that the maximum speed is reached in 25000 ft (7620 m) where air density is 0.55 kg/m³.

Mass flow through the propeller of one 2100 is at least 1242.4 kg/s or about ten times of what it is for the 3007. The mass flow though the core is the same because both engines share the same hot section. The static thrust of the 2100 is higher than that of the 3007 but drops more quickly with speed, so at high speed the 3007 produces more thrust.

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  • $\begingroup$ I have a doubt. If mass flow through turbofan is limited due to shroud and this is the reason for lower bypass ratio (than propeller), why don't we remove this conventional shroud and fit a shroud which only starts from the fan disc (more like a ducted fan but the duct extending till the rear nozzle)? $\endgroup$ – Selva Oct 8 '16 at 15:16
  • $\begingroup$ @Selva: No, the shroud helps to make the flow uniform. Without it the fan could not be optimised to the extent possible, and thrust per frontal area would need to be lower. The part ahead of the fan face is the most important. Maybe this answer helps. Or this answer. $\endgroup$ – Peter Kämpf Oct 8 '16 at 21:39
  • $\begingroup$ Thx for the reply and I apologise for asking too many questions. Wikipedia says RR2100 has 28.7in diameter and RR3007 as 38.5in. So, clearly 2100 has lower diameter (right?). Then, What is the reason for higher mass flow in 2100 (is it due to solidity?). Moreover, after reading your other answers I have a new doubt. Why fan in turbofan has more thrust than prop? Isn't it should be other way around? Because the turbofan also generates thrust from jet. $\endgroup$ – Selva Oct 9 '16 at 12:08
  • $\begingroup$ @Selva: The 5:1 bypass ratio of the 3007 requires a fan, while the 2100 has an added compressor stage in its place which is only sized for the core flow. The fan is like the propeller, but then the 2100 would have 4.15 m diameter. Thrust varies with speed, and more so for a propeller than a jet. If you look at static thrust (at zero speed), the 2100 has more. When I say in the other answer that the fan has more thrust than a prop, I mean this per area. $\endgroup$ – Peter Kämpf Oct 9 '16 at 12:14
  • $\begingroup$ Oh. Got it now. Again, thanks a lot for your reply :) $\endgroup$ – Selva Oct 9 '16 at 14:18
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There is a limitation when designing a fan, the speed at the tip to not reach sonic conditions. Essentially, that means that is basically rotational speed times diameter of the blade. So... at same conditions the factor of rotational speed and diameter of the blade will be a design condition.

That is coming up with 2 different design solutions:

  • We design a fan that we connect directly the fan to the turbine. So higher rotational speed of the fan but smaller balde diameter.
  • We install a reduction gear, so we reduce the rotational speed of the fan and we can increase the diameter.

It is a simplification, but essentially the first is call turbofan and the second turboprop.

By design, the turboprop will have higher diameter and so higher bypass ratio but will carry a heavy reduction gear. So... we have a tradeof.

That is why some low subsonic airplanes prefer turboprop in comparison to turbojet.

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    $\begingroup$ Fan tips quite regularly go supersonic $\endgroup$ – TomMcW Oct 5 '16 at 23:44

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