Rotor slap is a "compressibility effect", in other words, under certain
conditions the air near the rotor blade tips (where the relative wind
speed is greatest, and also where most of the lift occurs) exceeds the
speed of sound (approx. 1100 feet per second) and a slap is heard above
the normal rotor blade sound level. Certain conditions like a fairly
steep approach/descent with power applied and sharp turns will cause
slap. The condition is caused not only by the rotor blade tip speed but
the actual "angle of attack" of the "relative wind" that strikes the
rotor blade. You can easily calculate the rotor blade tip speed of your
model, and then factor in the "relative wind" resulting from the blade's
angle of attack and the ship's actual speed in the air. These are
conditions that full scale helicopter pilots have to be aware of when
making approaches in densely populated areas, to avoid complaints from
the public, and to "fly neighborly" as the FAA pamphlet on this subject
puts it. Smooth control inputs and smooth application of power will
help to minimize "slap". Abrupt control inputs, like sharp turns, etc.
will give you slap.
The shape of the main rotor blade tip will also affect "slap". A blunt
square tip blade like a Huey will give more of a slap than a swept tip
blade like an Apache, since the blunt edge causes more air disturbance
and turbulence. Any basic text on Helicopter Flight (the $8 volume
available at any flight school is a bargain) will explain these
principles with drawings better than I can verbally.
I fly both full scale and RC model helicopters, and personally I enjoy
trying to make my RC models fly as realistically as possible. I find
"3D" flying rather disturbing to watch since it is so "unrealistic".
John, Nashville, TN
> What causes rotor slap and which manoeuvres will best reproduce
> that great sound?
> Regards to all...........Allen.