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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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  • $\begingroup$ 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. $\endgroup$ – alephzero Oct 23 '15 at 22:30
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    $\begingroup$ @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. $\endgroup$ – reirab Oct 24 '15 at 0:06
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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.

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  • $\begingroup$ 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. $\endgroup$ – FreeMan Oct 26 '15 at 12:38
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    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Dec 13 '15 at 9:59
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    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Dec 13 '15 at 14:05
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    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Jul 5 '16 at 9:06
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    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Jul 7 '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.

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  • $\begingroup$ I thought the Tu-95 question established that the blade tips do not in fact reach Mach 1.0? $\endgroup$ – egid Oct 23 '15 at 16:05
  • $\begingroup$ @egid I think that question establishes that the propeller tips went supersonic after the engine power was increased. See the edit in that question. $\endgroup$ – aeroalias Oct 23 '15 at 16:19
  • $\begingroup$ Also, Peter just edited his Tu-95 answer to correct the prop speed. $\endgroup$ – FreeMan Oct 23 '15 at 16:31
  • $\begingroup$ @FreeMan ah, yes, indeed. Hadn't seen that - it was M0.87 or something before $\endgroup$ – egid Oct 23 '15 at 16:34
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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).

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  • $\begingroup$ Do you have some link where one could find more about the V waves (searches tend to return the medical version, which is clearly irrelevant here)? $\endgroup$ – Jan Hudec Oct 23 '15 at 17:14
  • $\begingroup$ Instead of "V waves" search instead for "Shock waves". $\endgroup$ – Robert Werner Oct 23 '15 at 22:34
  • $\begingroup$ Oh, shock waves are obvious. I hoped that I'll learn something new, but apparently not. $\endgroup$ – Jan Hudec Oct 24 '15 at 9:39
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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.

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    $\begingroup$ "The fan case prevents those vortices from forming": This seems to be the best and most direct answer to the question. $\endgroup$ – mins Dec 13 '15 at 20:37

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