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It seems that the tips of the fan of a turbofan engine can and do break the sound barrier:

The GE-90 has a fan diameter of 3124 mm and a rotational speed of 3475 RPM. Their circumferential velocity is d·π·57.917 = 568 m/s or Mach 1.67 at sea level and ISO atmosphere.
Peter Kämpf's answer

While, on a propeller driven aircraft, it seems that having the prop tips exceed the speed of sound is a bad thing - Can turboprop blades break the sound barrier?

Why is it that exceeding the speed of sound is acceptable in one situation but not the other?

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FreeMan
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    The Mach 1.67 estimate is too high, because it ignores the fact that the airflow through the fan is constrained by the fan case. The speed difference between the blade tips and the air outside the fan case is irrelevant. The flow through the fan is indeed transonic in some situations, but the highest Mach number is much closer to 1.0 than to 1.67. – alephzero Oct 23 '15 at 22:30
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    @alephzero The blade tips are traveling in a plane that is (mostly) perpendicular to the flow through the engine. It's true that the flow through the engine will usually (probably always in a non-scramjet) be subsonic, but that doesn't mean the blade tips are subsonic relative to the air in which they're traveling, since they're moving perpendicular to said flow. In order for the airspeed of the blade tips to be less than their rotational speed, the intake air would have to be rotating around the turbine axis before encountering the intake fan. – reirab Oct 24 '15 at 00:06

4 Answers4

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Short answer:

  • Turbofans need supersonic speed at the fan blade tips to create their high thrust.
  • Turbofans can tolerate supersonic speeds because the intake creates constant flow conditions irrespective of flight speed.
  • Efficiency for propellers and fan blades is highest at subsonic flow conditions.
  • Propellers can turn at supersonic speeds, but since flow conditions are less controlled, the penalty for doing so is much higher than the penalty for a fan.

Explanation

It is a bad thing to have supersonic fan blade tips, just like supersonic propeller tips are best avoided. But in turbofans it is a price worth paying, because the faster tip velocity means higher dynamic pressure, and the pressure difference between both sides of the fan blade grows with the square of their velocity. This makes the high thrust levels of modern turbofans possible.

Propeller efficiency over speed

Propeller efficiency over speed (picture source). The plot for fan blades would look not much different. The very thin, uncambered airfoil of a supersonic propeller and the added wave drag lower the maximum efficiency, but hold efficiency up into supersonic air speeds.

Note that the propeller on the XF-84H Thunderscreech did move at supersonic speed. There is nothing inherent in propellers which prevents their tips from moving faster than the speed of sound. On the other hand, the big diameter of a prop requires proportionally more torque to keep the prop rotating against the drag from the supersonic tips. Thus, a fan engine requires less torque per blade to reach supersonic tip speeds on the fan blades.

Also, the shroud of a turbofan engine helps a lot to make the noise from supersonic tips manageable. The XF-84H's noise made people literally sick. But there is more to it: @FreeMan encouraged me with his comment to dive a little deeper.

A supersonic propeller will work well when the direction of flow at every station along the propeller blade is about equal to the local airfoil chord. Since the blade is uncambered, this means that the change in local flow direction at the leading edge can be minimized to the amount which is required to create the desired thrust. But to be able to fulfill this condition you need to match your propeller speed to the flight speed and twist distribution. Also, the angle of attack must be compensated for by sviveling this propeller axis into the direction of flight. It will have no p-factor, but can run at only one speed for a given flight speed.

Contrast this with a turbofan: The intake makes sure that the flow speed and direction at the face of the fan is the same no matter what the flight speed is. This is done by the pressure field in and around the intake which will spill excess air overboard at high speed or suck extra air in from the sides at low speed. In the fan you can indeed match the local angle of incidence to the airspeed so the fan will work well over its design range.

Generally, a fully subsonic fan would be more efficient. But then the diameter would need to be as large as that of big turboprop engines, and the shroud would become impossibly heavy and produce too much drag. The high dynamic pressure on the fan blades is needed to produce the thrust with the relatively small diameter of a turbofan.

Peter Kämpf
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    If I read your text and that graph correctly, a very thin, uncambered, supersonic propeller would actually be preferred if one could build a practical shroud/nacelle around it, and that, essentially, is what the "fan" portion of a turbofan engine is. – FreeMan Oct 26 '15 at 12:38
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    @FreeMan: There is more to it. This propeller would only work well if the direction of flow at the leading edge is parallel to the airfoil chord, so the local incidence must be right. This requires to match twist, speed and airspeed. For every Mach number you would have only one prop speed where the twist would be right. In subsonic flow the round leading edge gives you much more leeway to be off with your local incidence. The intake of a turbofan gives you that: Uniform and equal airspeed no matter what flight speed. A propeller needs to be more flexible. – Peter Kämpf Oct 26 '15 at 12:48
  • @FreeMan: Yes, the fan is really a propeller with easier flow conditions and a very high activity factor. – Peter Kämpf Oct 26 '15 at 13:12
  • (1) In your first bullet point you state "Turbofans need supersonic speed to create their high thrust". Should it not be " ... subsonic ... " instead? (2) Also, in the third bullet point "Efficiency for both ...", you do mean that the efficiency for both the intake and the fan is highest at subsonic conditions? If so, maybe you could add "intake and fan" to the sentence for clarification. –  Dec 13 '15 at 09:46
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    @Thesis: The question is about the blade tips, so I refer to the speed at the blade tips here. The intake flow speed and the flight speed are indeed subsonic, but the circumferential speed of the blades must be added. I will edit to answer for clarification. – Peter Kämpf Dec 13 '15 at 09:59
  • Thanks, Peter. Two more questions: (1) "Thus, a fan engine requires less torque to reach supersonic tip speeds on the fan blades." - The reason being a smaller diameter D of the fan? (2) "Generally, a fully subsonic fan would be more efficient. But then the diameter would need to be as large as that of big turboprop engines, ..." - I cannot follow that. The circumferential velocity is proportional to D, so for a subsonic fan (tip), D should be small? On the other hand, the engine power is proportional to D^5 and revolution speed n^3, so for D=const, n could theoretically also be increased? –  Dec 13 '15 at 13:41
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    @Thesis: (1) Yes, but strictly true only for the single fan blade. Since the solidity of a fan is higher than that of a propeller, the torque to drive the whole fan is immense. But the torque/thrust ratio of a fan is lower. (2) A subsonic fan would run much slower and have less dynamic pressure along the blade span. Not D but RPM needs to be small. To produce thrust, the subsonic fan would need more air, which it gets by increasing D. Thus, it becomes a propeller. – Peter Kämpf Dec 13 '15 at 14:05
  • Does this mean that it actually is practical to make a supersonic prop aircraft, so long as you have a shroud, the engine can be gimbaled, and the propeller blades have variable pitch AND twist? Also, I don't understand why, if turbofans are shrouded and I think meet your conditions, they are basically never used for supersonic flight. Thanks a lot. – Gus Jul 05 '16 at 02:39
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    @Gus: Supersonic flight needs high exit speeds of the exhaust gasses, so a turbofan is at a disadvantage. The Tu-144 used turbofans first and switched over to turbojets (which the Concorde used from the start) with considerable performance improvements. See here for more. The shrouded supersonic prop is indeed feasible, but much easier to achieve when coupled to a jet engine. Most supersonic aircraft today use turbofans with a low bypass ratio. – Peter Kämpf Jul 05 '16 at 09:06
  • @PeterKämpf Thanks. I'm wondering now if the shrouded supersonic prop would be practical for a supersonic electric aircraft? The issue I see is the mass of the motor and duct needing to be gimbaled. The Concorde's engine, Olympus 593, has a power to mass ratio of 36 kW/kg, while high performance electric motors only have 9 kW/kg, which means the assembly would be heavier than that with a turbo instead (but maybe it's smaller in volume). However, relative to other options for supersonic electric aircraft (compressor+nozzle combo) perhaps it's optimal? Also, any reading suggestions for this? – Gus Jul 06 '16 at 01:00
  • Maybe some sort of contra-rotating scimitar blade could be used, as well. – Gus Jul 06 '16 at 01:07
  • @Gus: Forget electric propulsion for supersonic flight. This is utter nonsense with current (and projected) energy storage and realistic range. Also, the 593 power is calculated at Mach 2 and includes intake thrust; you will be surprised how much this will affect the calculation. Look at static performance instead for a fair comparison. – Peter Kämpf Jul 06 '16 at 06:15
  • I was thinking an advantage propellers have is relatively constant efficiency with altitude. So you could go really high (85,000 ft) and have about 1/3rd of the drag of a Concorde assuming drag is ~proportional to rho (not sure w/ wave drag though). And if it's possible to design a supersonic prop like the one you described, near the efficiency of low speed props, then perhaps the engine can be 1.5-2x as efficient as the 30-40% of jets. And so then the cruise power requirement would be cut to ~25% of the Concorde, then maybe it makes sense if batteries double in J/kg from their current state? – Gus Jul 06 '16 at 22:12
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    @Gus: Electric propulsion has the advantage of high efficiency over a high motor speed range, so the high altitude propeller must be very large (to compensate for the low $\rho$ at altitude) and slowly spinning at low altitude. But with an intake-like shroud for supersonic efficiency the propeller needs to become much smaller and weighs a lot (and still efficiency will be lousy compared to subsonic efficiency). Look here for subsonic electric flight. – Peter Kämpf Jul 07 '16 at 12:54
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Propeller tips can and sometimes do go supersonic (like XF-84H or Tu-95). However, while it is tolerated in case of turbofan (some measures like swept blades and low speed fans are used in turbofans to counter this), it is not so in case of propellers for a few reasons:

  • As the supersonic speeds are approached (or exceeded locally), shock waves form over sections of the propeller blades- This significantly reduces the propeller efficiency while at the same time causes increased loads on the blade. This causes a problem- For supersonic speeds, the blades have to be extremely thin, while the loads require the blades to be thicker.

  • The drag due to the (tip) shock waves increases the required engine power tremendously. For example, the Tu-95's engines had to be uprated from 12000 shp in the prototypes to 15000 in production units to reach the required speed.

  • Another main reason is sound- XF-45 was so loud that it caused seizures. In case of turbofans, the bypass fan is covered, which mitigates the noise issues somewhat.

aeroalias
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The most important difference is that the fan blades are running inside a casing, and propellers operate in free air. Most of the noise and energy loss comes from the vortices shed from the tips of the prop blades. The fan case prevents those vortices from forming, except for the small amount of air leakage between the blade tips and the casing.

There is inevitably a small clearance gap between blades and casing, for example because the external aerodynamic forces on the casing may deform it into non-circular shape in some flight conditions, but the maximum gap around a 3000mm diameter fan would typically be less than 5mm.

Ducted propellers are used in small sizes, but the weight penalty of a large duct that is strong enough to survive conditions like birdstrike (and the collateral damage from bent or broken prop blades) would be prohibitive for large props.

alephzero
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    "The fan case prevents those vortices from forming": This seems to be the best and most direct answer to the question. – mins Dec 13 '15 at 20:37
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Why is it that exceeding the speed of sound is acceptable in one situation but not the other?

Because lift (or thrust of the propeller which is a wing that rotates) at supersonic speeds decreases while compression is not influenced. The air that strikes the blades of a compressor has no other way to go but to pass through it and feed the engine.

In the case of a propeller at supersonic speeds the air instead of flowing around the blades forms V waves, a phenomenon that increases drag and reduces lift (thrust).

Robert Werner
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