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In my question about helicopter noise, I made the assumption that one reason they are so darn noisy is that the main rotor tips go faster than the speed of sound. @FreeMan questioned whether that was the case.

I've tried to find information about this but most pages end up discussing VNE -- the forward speed at which the advancing blades go supersonic while the retreating blades lose lift and stall. This is not about that situation.

This is about regular flight -- is the extremely loud noise from helicopters because their rotor tips are supersonic, or is from other sources?

p.s. There is some mention that the "WOP-WOP" of descending helicopters is caused by the rotor tips going supersonic. Is this the case - and if so, is it an edge case (the only time the tips are supersonic) or just one example of when they do.

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The "wop wop", usually known as blade slap, is heard when the tip of the blade passes through the vortex created by the previous one.

It can be avoided. The most common flight regime when this happens is a shallow descent but still with quite a lot of power - e.g. fast and shallow. The vortex starts to move down as soon as it leaves the tip of the blade so in level flight, the following blade passes over it. In a shallow descent with high pitch angle, the following blade can literally "slap" into the previous vortex. The vortices of the two blades now interact and can cause local, transient supersonic flow. To avoid it, simply lower the pitch to establish a more positive descent or pull back on the cyclic to increase disc loading and flatten the attitude.

The blade tips do not go supersonic. In fact, in nearly all helicopter designs, the rotor rotates within a very narrow range of speeds, typically between 90% and 110% of the normal speed. In most flight regimes, the rotor is rotating at 100%, +/- a few percent, whether you are climbing, descending or cruising. Only during auto-rotation and aggressive manoeuvring does the range vary by 10% or more. It depends on the type of helicopter but absolute limits would be something like 85% (panic time, risk of complete stall) and 115% (smaller panic, risk of damage to the machine, especially in the tail rotor drive shaft).

In normal operations, and design aims to achieve this, the rotor tips do not go supersonic since when they do, there is a sudden and large decrease in performance with more power required, higher blade loads, vibration and noise.

Think about a helicopter flying forwards. The advancing blade at its most perpendicular position experiences a relative airflow which is equal (ignoring all kinds of minor side effects) to the forward speed plus the speed of the blade. The retreating blade is experiencing a relative airflow equal to the speed of the blade minus the speed of the helicopter.

If the blades rotate so fast that the tips are supersonic, then the main lift generating part of the retreating blade, the outer two thirds of the span, would experience such a low airspeed, for some of the span it will even be negative, that the blades will stall causing a catastrophic roll into that side. It is this phenomenon which ultimately limits the rotational speed of the blades and the maximum speed of the helicopter.

Let's look at the R22 as an example. The following figures are approximate.

The rotor tip speed is about 670 fps (feet per second). The speed of sound at ground level on a standard day is about 1100 fps. The R22 is flying at close to VNE, let's say 100kts which is about 170 fps.

The tip on the advancing side, at it's fastest, is therefore flying relative to the airflow at 840 fps and on the retreating side, at it's slowest, at 500 fps.

The blade length is about 11 feet, so the middle portion of the blade on the retreating side is only flying at 190 fps (half of 670 minus the airspeed). When you get to about 4 feet out from the blade root, it's now only 50 fps and not much further in from that, becomes zero, then negative.

Remember that lift is proportional to the square of the speed. You can now see the huge discrepancy between lift on either side as airspeed increases.

To answer your question directly, the R22 would need to fly at 530 fps to approach supersonic tip speed which equates to about 330 kts which it can't get anywhere near achieving.

PS. The R22 POH speaks in Imperial measures. When I get some time, I'll redo the figures in metric which I, and most of the world, prefer.

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  • $\begingroup$ The first sentence of your last paragraph seems to contradict itself: the disk rotates so fast the tips go supersonic, therefore the outer 2/3 of the span would experience slow airspeed. Could you please clarify how the tip (part of the outer 2/3 of the blade) can be supersonic, yet that same 2/3 of the blade can be moving too slowly? I'm not telling you you're wrong, I'm just really confused. $\endgroup$ – FreeMan Jul 17 '15 at 12:52
  • $\begingroup$ I will re-word it slightly. I am talking about the retreating blade. Thanks for pointing it out. $\endgroup$ – Simon Jul 17 '15 at 13:57
  • $\begingroup$ Is not the case that the forward moving rotor hitting the speed of sound is a limiting facter in the speed of the helicopter itself? I have a vague recollection of a Lynx pilot telling me that when I was a kid. $\endgroup$ – chriscowley Jul 20 '15 at 9:40
  • $\begingroup$ @chriscowley It is, but you will hit VNE (velocity never exceed) first. Read about dissymmetry of lift and how it relates to VNE $\endgroup$ – Simon Jul 20 '15 at 9:45
  • $\begingroup$ @FreeMan I found the answer confusing, too. I think it's saying this. The rotors spin fairly slowly so, when the helicopter is stationary, the tips are far from the speed of sound. The only way to get the tips to break the sound barrier would be to move the helicopter forwards very quickly. So quickly, in fact, that the retreating blade would actually still be moving forwards, relative to the ground. That blade would would generate no lift because the airflow over it would be from what's supposed to be its trailing edge to the leading edge. $\endgroup$ – David Richerby Nov 17 '16 at 10:18
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The characteristic beat of a helicopter rotor is caused by the interaction between the rotor blade vortices, in particular between the main rotor and the tail rotor vortices. As the shockwaves of these impulses coincide, they create powerful (loud) harmonics. This effect can occur at rotor speeds well below supersonic.

The vortex interaction can be reduced by surrounding (a smaller, multi-bladed) tail rotor - more like a fan - with a shroud. Such an installation is called a fenestron ("windowed", and actually a trademark belonging to Eurocopter), a ducted fan or fan-in-fin. This development was originally designed for improved safety and performance.

Modifications to the main rotor to reduce the impulse from the vortex typically trade away power or economy.

On the topic of supersonic speed, helicopters have a theoretical top speed of 417 kph in conventional flight mode because of the problem of the advancing blade reaching supersonic speed over too great an area and the retreating blade losing lift abruptly.

Someone asked how only part of the blade could be supersonic while the majority of its length was subsonic. This is because the movement is angular. A point on the outer portion moves through the air much quicker than a point on the inner region, to cover the same angle in the same time. This supersonic condition is reached sooner in flight than in hover. When the blade is moving "forward", the airspeed is added to the rotational speed of the forward moving blade and subtracted from the backward moving blade. A common solution to accommodate the difference in lift of opposing blades is to hinge them at the root to allow the blade with a higher airspeed to flap upwards to a limited extent. Some "rigid" designs replace the hinge with a flexible section.

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  • $\begingroup$ Nice answer. Welcome to aviation.se! $\endgroup$ – Ralph J Feb 28 '17 at 17:16

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