I was under the impression that helicopter rotor blades couldn't be allowed
to reach Mach 1? Approaching Mach 1 also includes a massive increase in
drag as the airfoil approaches that barrier. Doing that around 600 times a
minute (I'm assuming a rotor rpm of around 300 x 2 rotor blades) would
introduce one hell of a vibration problem, not to mention the stresses that
would be imparted to the blades and control systems as they repeatedly pass
in and out of the sound barrier. I thought it was one of the factors that
limit, or can limit, a rotorcrafts Vne speeds.

Take a light, flat object, like a boat oar, and swing it as hard and as fast
as you can with the flat side of the oar against the direction of swing. If
you can do it hard enough and fast enough, you'll get a pretty good "whoosh"
sound. Now visualize that same phenomenon on a helicopters rotor blade,
like a Huey at max take-off weight of 9500 lbs. The tips on the advancing
blade are running somewhere around 514 mph (rotational speed, assuming that
300 rpm with a 48 ft rotor diameter) + the forward speed of the aircraft
which can be what, around 110 to 115 mph? That means the blade tip speed on
the advancing side would be 624 to 629 mph? Every time the blade comes
around, it's going to make that whopping sound. Heavy, two bladed systems,
like the Huey, do it the best. The Model 47 does it too but the machine is
lighter and not quite as loud, most of the time. Bell 206's will also do
this. It's even there on an R22 but the effect isn't as pronounced because
the bird is a lot lighter and they carry a higher rotor rpm so the frequency
is higher and as such, not as dramatic.

Birds like the 412 (4-bladed Huey for all intents and purposes?) don't
display this effect, I think, for two reasons. First, the blade loading
(for a given weight) is lighter and, second, because there are four blades
instead of two, the frequencies are higher and the effect is just not as
obvious because of that.

That's how I look at it. I could certainly be off on any of this and I'm
sure I'll be corrected somewhere along the way if I am! ;-) I got the
rotor diameter (48 ft), and the max take-off weight (9500 lbs) from a web
search. Different models might be a little different but you get the idea.
Couldn't find anything on rotor rpm so the 300 is just a guess but it works
as an example.

FWIW!

Fly Safe,
Steve R.


"Guy Alcala" wrote in message
. ..
Over on the rec.aviation.military forum a discussion got
around to the doppler effect and/or helo rotor blades, and
the following exchange took place:

------------------------------------------------------

if the air reaches M 1.0 going over the
thickest part of the airfoil, bang!. Whop, whop, whop . .

Is that what gives the Huey its distinctive (not to say
spooky) sound?
I always assumed it had something to do with the fact that
it was
turbine powered, the blade downwash acting against the jet
blast. No?

No.

If it's just a matter of blade speed, why does no other
helo sound
like a Huey?

I've read an explanation somewhere; IIRR it's an artifact of
the two-blade teetering rotor design, but I'm going to ask
over on r.a.r. The four-blade Hueys (Model 412) now
available don't make that sound.
-------------------------------------------------

Can anyone confirm (ordeny) my memory as to what causes the
Huey's sound? And if I'm correct, does this only happen on
Bell's teetering rotor design for the Huey, or is it also a
feature of the Model 47 as well as the Hiller UH-12,
Robinson etc.(or would be if they had large enough diameter
blades to get the tips up near supersonic)?

TIA,

Guy