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.



So are you telling me that you believe that it will take a 44.9 mph descent
before an A-Star will get into SWP? I'm sorry, no offense, but I find that
hard to believe. I've got a little A-Star time, but I never entered SWP with
it. I do remember during my checkout though that the IP was adamant that
standard precautions against it (SWP) be maintained. Heck, if what you're
saying is true it would be pretty much impossible to get into SWP in any
aircraft.

The 3rd rule of thumb in the 1-2-3 SWP is a greater than 300 feet per minute
descent. I've always heard 4 miles per hour is a pretty average walking speed.

4 mph = 21,120 ft/hr / 60 = 352 feet per minute. Which is a speed that is not
considerably faster than I can attain walking.


Stephen Austin
Austin Ag Aviation
Charleston, Missouri