Helicopter whump-whump

We’ve all heard them. The sounds of a helicopter moving overhead, its rotors describing a whump-whump of varying speed. But why? Why not the constant whirrrr of a plane propellor? Both used pitched blades. Both make their signature sound regardless of horizontal or vertical alignment while doing so. Why, then, this difference? Does the helicopter rotor pause for a millisecond after each rotation or what?

That’s the downward “wake” of each main rotor blade passing over the tail boom. There’s also a higher-pitched sound from each tail rotor blade, on helicopters arranged that way.

See also http://boards.straightdope.com/sdmb/showthread.php?t=452889
and
http://boards.straightdope.com/sdmb/showthread.php?t=256849

There seem to be several different explanations in those linked threads. I’m not sure we’re all on the same page (so to speak) in exactly what question we’re trying to answer; what particular characteristic of the sound each person is trying to explain.

I wrote my doctoral dissertation in aerospace engineering on this. What you are talking about is called Blade-Vortex Interaction, or BVI. It use to be known as Blade Slap.

In order to understand it you first need to understand what happens…

So at this point I read the attached links and the first thread is chuck-full of wrong information. The second thread is too… up until I got to my post from five years ago saying just what I was about to say here. Weird. I haven’t checked out GQ in months.
Anyway, I suggest you check out the second thread, particularlythis post to save me from retyping everything.

Oh, note particularly that the wake hitting the tailboom makes no noise that you would ever be able to identify over the rest of the helicopter noise.

I’m still not sure you’re answering the question that the OP is trying to ask.

Blade-Vortex Interaction may be the reason that a rotor blade mades its characteristic “whump” noise, but I think the OP is trying to find out why we perceive the sound of a helicopter as a series of discrete events (“whump-whump-whump”) compared to an airplane propeller (which sounds to me more like a steady drone).

Is that a fair restatement, dobieman?

Speed of rotation. Helicopter rotor speed is much less than aircraft propeller. Diameter of the blades is significantly shorter on aircraft. This is related to tip velocity which needs to be kept below the speed of sound (I believe the tips can get quite close but getting into SOS territory introduces lots of instability and vibration possibilities). Slower speed gives a much lower sound frequency where the individual rotations (pulses) are distinguishable rather than mushed together with the high rpm aircraft propeller. You can distinctly hear the sound frequency rise when a small plane fixed pitch prop revs up. The amount of air moved also relates to the sound. As a variable pitch prop or rotor is re-angled to provide more thrust; the sound deepens as well. The prop/rotor will slow slightly and begin to move a greater volume of air.

Asked and answered. Blade slap is the whap whap whap noise that comes through on top of the normal whirring noise. It’s a series of discrete events because the wake of the tail rotor only hits a blade hard enough to slap it when both blades are in the right position, which isn’t every pass of the tail rotor or main rotor.

Robot Arm…that sums it up nicely. I suspect I am asking two questions without realizing it. The question you address…why do we perceive it as a punctuated sound…being the first question and then the corollary, is it indeed punctuated or is it in reality a constant sound such as that of a plane propellor and we just hear it differently?
Thanks for the restatement!

Amongst the answer provided by many of you I found what I was looking for, thanky kindly. The combination of interactions between the air movement of the main rotor and that of the tail rotor accounts for the periodicity of the sound as opposed to the constancy one hears in a plane propellor which does not interact with another propellor the same way.

Actually that is not it. The tail rotor (in most rotorcraft) makes most of the noise you hear in a helicopter very far away and much of the loudest noise, but it is not responsible for the whop whop noise. The main rotor wake can and does interact with the tail rotor, and this can cause aerodynamic inefficiencies, but it also is not responsible for the whop whop noise.

That noise is caused when the vortices created by the main rotor blow back through the rotor and hit another blade (or even the same blade on its next pass around). The old term “blade slap” is helpful here as it is the blade slapping the vortices. Think of the vortices as smoke trails off the tips of the blades. When another vortex hits one of them it makes a “whop” sound.

This is a picture of a rotor wake seen from above at one point in time. The solid lines are blades and the curved lines are vortices coming off of the blades. Where a solid line hits a curved line there is an interaction (if they are close enough together in the up/down direction). When they are parallel when that happens you get a “whop”. This happens several times per blade rotation. Specifically it happens a*N times per full rotation where N is the number of blades and a is an interger between 0 and 3 (depend on the flight conditions).

The reason a plane propeller does not make this noise is not because it does not have a tail rotor, but because it is tilted 90 degrees relative to a helicopter rotor like you see here. In the propeller case, the vortices fly away from the propeller instead of through it (the same thing happens to a helicopter when it is hovering, the wake just goes down so there is no whop whop noise).

The helicopter moving forward makes a lot of noise that isn’t punctuated as well as the punctuated noise, while the propeller only makes the non-punctuated noise (because there is no blade slap).

Blade slap is a main-rotor-only phenomenon, the tail rotor does not come into it, and even helicopters with no tail rotor (like the NOTAR system or coaxial helicopters) have it. Also, it is periodic and if it happens at all it will happen at least as many times per rotor revolution as you have blades.

That affects the general sound of the helicopter, but it is not responsible for the whop whop noise the OP is asking about.

I think this is answered earlier in this post, but to specifically address this point, the noise sounds like a series of discrete events because the BVI is a series of discrete events. Every time the blade hits a vortex at the right angle/strength/miss-distance/etc it makes a whump. No vortex strikes in a propeller, so no whumps.

Gotcha. From your earlier post, I thought you were saying the vortices from the tail rotor hitting the main rotor caused the noise.

The diagram is helpful.

The problem with trying to explain this is that you really need to see good pictures to really get it and finding those on the internet isn’t terribly easy. So, a quick recap with what I have been able to find.

  1. All lifting bodies (like wings) make tip vortices. Under certain atmospheric conditions you can see them like here:

  1. Propellers and helicopter rotors are just wings spinning, so they make them too:

Here

  1. When a rotor is flying, the wind carries the wake away behind it do it doesn’t interact with any blades:

Explore James Nicolaus

  1. The same thing happens with a helicopter in hover (movie, opens easier in Internet Explorer):

here

  1. When the helicopter is moving forward though, those vortices hit the blades and make whop whop noises:

Movie (open in Internet Explorer)

Very good explaination. Thanks!

When the corporate Dash-8-200 (Q200) drops people off at Bombardier’s Dorval plant, before turning around and flying back to Toronto, it usuallyloads and unloads people and mail right behind the office building, in that semi-closed off area just south of the building that kind of looks like it has wings, to the left of that line of bizjets.

The pressure waves from it’s propellers bounces off the hangars and comes back to it, and it sounds pretty much like a helicopter; whomp-whomp and definitely not quiet as in Q!

I kind of intuitively knew how this phenomenon worked, but I’m glad to have read this thread anyways!

You might already know this but the Q series Dash 8s have active noise reduction technology in the cabin. The “Q” refers to it being relatively quiet for passengers compared to other turboprops, it is not supposed to be quiet on the outside.

Yes, I do know that! But I always found it a little funny to be covering my ears as I walked by the airplane with “Q for Quiet!” written on the side! And as the plane comes in off the runway and to the hangars, the sound just becomes louder and louder, and more whomp-whomp like as the prop blades cut trough the pressure waves returning to them.

That corner really does amplify sounds a lot - it makes the Wichita-bound Learjet 45 XR (IIRC) shuttle sound like a much, much larger plane!

Can someone please explain the Italian joke from the original thread Flight provided?

‘Wop’ is a pejorative term for Italians.

The most common joke is:

[spoiler]How can you tell an Italian helicopter?

Da little blades Dago Guinea-Guinea-Guinea, and da big blades Dago Wop-Wop-Wop.[/spoiler]

Sorry.

As an interesting aside to all of this a “huey” has a very distinctive “whop whop” because its rotor has only two blades. It’s very distinctive.