"Michael McNulty" wrote in message ...
"Stephen Austin" wrote in message
...

snip


Yes, but that downwash is not able to continue unabated. Immediately on
leaving the disc it is compressed against surrounding air which slows it
considerably.

Wrong. In hover the downwash actually doubles in speed as gets leaves the
plane of the disc, due to wake contraction. This is a basic fact of
helicopter aerodynamics.


Your assertion was that for a large helicopter to get into SWP it would
take
"considerably higher" speed than a brisk walk down the stairs. Yet, in
all my
training, I have never heard any other reference used. Please define
"considerably higher".

You mention a simple formula for calculating downwash velocity yet you
didn't
post it. I'd be interested in knowing this too.

Here's a simplified example for an AS-350b2:
Main Rotor Radius: r = 17.5 (ft)
Gross Weight: gwt = 4961 (lbs)
Disc Loading: DL = gwt/pi*r^2 = 4961/3.1415*17.5*17.5 = 5.16 (lbs/sqft)
Air Density: rho = 0.002377 (standard day sea level)
Downwash Velocity: v = (DL*2/rho)^1/2 = (5.16*2/0.002377)^1/2 = 65.87 (fps)
v = v*3600/5280 = 44.9 (mph)
Which is considerably faster than a speed I can attain walking.