Rotor Slap.

Rotor Slap.

Post by Guillaume Hasto » Sat, 28 Aug 1999 04:00:00



I love that sound too and i use it on a 3D like manouver :

it's a low altitude circle , sideways - inverted flying + tail down ( heli
tilted to 50deg or so ) . The manouver is impressive in itself but that
flapping sound you're talking about really adds something great to it :-)

bye,

Guillaume

http://www.hastoy.com - R/C helicopters
downloadable videos
MH-Design online shopping


Quote:
>What causes rotor slap and which manoeuvres will best reproduce
>that great sound?

>Regards to all...........Allen.

 
 
 

Rotor Slap.

Post by Peter Seddo » Sat, 28 Aug 1999 04:00:00


According to my full size helicopter manual blade slap occurs during partial
power descents when one rotor blade intersects its own vortex system or that
of another blade. When this happens the blade experiences high velocities
and rapid angle of attack changes. This can drive one section of the rotor
blade into compressibility and possible shock stall both of which produce
aerodyanmic loading variations.

Charts show that slap can occur over a narrow band centered on a rate of
descent of 500fpm at an airspeed of 80mph

So now you know !!!

Regards Peter

Quote:

>What causes rotor slap and which manoeuvres will best reproduce
>that great sound?

>Regards to all...........Allen.


 
 
 

Rotor Slap.

Post by Brian and Aleci » Sun, 29 Aug 1999 04:00:00


I fly rc helicopters and work on HH-60 Pavehawks in the US Air Force.  And
from what I understand that cool slapping noise is called Blade Stall.  In a
real helicopter if you continued to perform a blade stall maneuver you would
eventually trash the main rotor head.  I am unaware whether or not it will
do the same to RC.  I like the sound myself!

--
Brian and Alecia Bowers
Psc 79 Box 20441
Apo Ap 96364


Quote:
> I love that sound too and i use it on a 3D like manouver :

> it's a low altitude circle , sideways - inverted flying + tail down ( heli
> tilted to 50deg or so ) . The manouver is impressive in itself but that
> flapping sound you're talking about really adds something great to it :-)

> bye,

> Guillaume

> http://www.hastoy.com - R/C helicopters
> downloadable videos
> MH-Design online shopping

> Allen Wilkinson a crit dans le message


- Show quoted text -

Quote:
> >What causes rotor slap and which manoeuvres will best reproduce
> >that great sound?

> >Regards to all...........Allen.

 
 
 

Rotor Slap.

Post by 76.. » Sun, 29 Aug 1999 04:00:00


Allen -

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

Quote:

> What causes rotor slap and which manoeuvres will best reproduce
> that great sound?

> Regards to all...........Allen.

 
 
 

Rotor Slap.

Post by Steve Simpso » Sun, 29 Aug 1999 04:00:00


Quote:
> 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.

Model heli rotor blade tip speed rarely exceed the 500 feet per second
range.

You may have been thinking of propellers on real planes where the situation
that you describe does indeed exist.

As correctly pointed out elsewhere in this thread, the sound is caused by
one blade slamming into the turbulence of the other (stalled) blade.