Model Forum / Radio Controlled / Helicopters / March 2007

If I understand your question properly, one answer that comes to mind is
that many of us are running higher rotor speeds than were common in the
past.  That would increase the frequency of what you're talking about and
make it less noticeable.

Having said that, the "thump, thump, thump" sounds I get in certain flight
modes are still there, just as they've always been but it is only in certain
flight modes.  I don't remember hearing a model helicopter that didn't sound
pretty much like an airplane while in a hover.  There are some helicopter
noises that are unique to the rotor but they've never been as obnoxious as
I'd like them to be!  :-)

Fly Safe,
Steve R.

The thumping sound comes from "blade-vortex interaction", also called
vortex slapping.

The tips of rotor blades leave trailing vortices just like the tip
vortices of airplanes -- except that since the rotor tips travel in a
circle, the vortex trails are circular too.

In forward flight each tip describes a series of loops, and its tip
vortex is strung out along that path. The vortices move slowly down,
so they don't normally contact the blades -- except in a forward
descent. In that condition, each blade sometimes descends into the
vortex left by the blade ahead of it (or its own vortex from the
previous revolution) and literally slaps it. That makes a thumping
sound -- along with a bump felt in the cyclic control and in your
butt.

Student airplane pilots get something similar to this when they
practice steep 360s. If you let your altitude slip a bit during the
turn -- which you usually will at first -- you'll get a bump from the
vortices you left the previous time round.

BVI is much less noticeable in model helis because of the high rotor
speeds.

rj

The thumpers are usually two bladed heads with a huge rotor diameter and
f.cking great heavy blades which rotate relatively slowly. All of which make
them thump.

More blades, less diameter, less weight per blade and higher rotor speeds.
All of which make them quieter. Add to that the tail rotor design which are
supposed to make them quieter too and you've got a heli that sounds like a
plane.

This is true of both full size and models btw (apart from the quiet tail
rotors that is).