RE: [Paddlewise] sailing canoe troubles etc

Dr. Sutherland asked me a technical question: those very few who happen to
know both of us will see great humor in that, since I am in no way a technical
expert on much of anything.  That said, I'll come up with an impressive and
potentially accurate response.  Or not so much.

Chuck says:

"Here is a question for Joq. We once participated in a demonstration
helicopter rescue of a paddler in distress in the water. We had half a dozen
of us in a group with one swimmer to be picked up in a basket lowered from the
standard red and white rescue Huey. One of our fellows took the "victim's"
boat. The victim fired off a little red smoke canister-looked like a tiny
little pink fart that instantly was gone in the breeze. The rescue chopper
whooshed in and lowered the basket and the victim climbed in. The rest of us
stood off about 50 yards. We could see the down draft race toward us as a wall
of spray. We each had to get our heads down on the decks of our boats. The
spray felt like sand pellets. My 9 ft paddle kept me firmly upright. I was not
blown off across the bay. Clearly I am just not understanding how this paddle
is supposed to be used to cause endless disaster. I should have had one of
this little short busted paddles created in storms."

"My question for Joq:  What is the likely wind speed created by the downdraft
of the rescue helicopters? That breeze was a hell of a lot stronger than the
35 mph + breeze I paddled in on Long Island Sound one September afternoon in
the tail end of an east coast hurricane."

Joq's response

I'd much rather discuss the pink fart generated by the smoke canister -- it
had to have been expired or soaked or or a dud or something, because those
things put out great clouds of orange -- sometimes looks a little pink --
smoke.  Much more of a major flatulencial event than that described.  But
that's not what is being asked, unfortunately.  (Where is Dr. Peregrine
Inverbon when he is really needed; his explanation of so many pithy questions
by virtue of human intake of Caribou paunch would add significant clarity to
this issue and dispell pink clouds summarily.)

Okay, rotor wash velocities.  Gonna depend.  Gonna depend on the size and
weight of the helo being used, the density of the air and possibly the phase
of the moon.  Or not.  But lets use a medium sized commercial helicopter that
I know a little bit about -- the Sikorsky S-76.  (It was going to be the S-74,
but stuff happens, and it became the S-76, which was very patriotic -- while
also the year of its introduction.  Kinda.  (Of absolutely no value is the
fact that the S-76 was the first helicopter ever designed for commercial --
not military -- applications; first flight in 1977 -- which we thought was
pretty close to 1976, so we didn't rename it again.  People forget this stuff,
anyway.)

The S-76 has a rotor diameter of 44', and at a mission weight of about 11,000
pounds -- heavy but not untypical -- a rotor loading of about 7.5 pounds per
square foot of rotor disk is generated.  Trust me on this -- my math notes
don't appear in the margins.  (Contrary to popular belief, a rotor system is
not pushing the air below it down: it's using its rotating wings to pull the
helicopter up through the airmass -- although it still generates some pretty
awesome downdrafts as a side effect.  Maybe a distraction -- sorry.)
Initially, the helo downdraft is about 25 knots, but for reasons of physics
and stuff, it accelerates as it descends.  Typically, the speed of the
downwash doubles as it gets to the water surface when the helo is about as
high as its rotor diameter -- with a nominal 40 foot hover.  So, at the water
surface below the helicopter,  you may have an air velocity of about 50 knots
at the edge of the rotor footprint.  The air spreads out near the surface at
about the height of a kayaker's head, unfortunately, and at 75  feet from the
helo footprint, the airspeed disipates to about 20 knots.  At Dr. Sutherland's
150 foot standoff, the downdraft would have been considerably less than 20
knots.  But the real effect of downwash isn't the moving air: it's the stuff
in the air -- sand, pebbles, etc -- or droplets of water in Dr. Sutherland's
case that causes the problem, and makes people dive for cover.

So, sorry Chuck.  Your Long Island Sound hurricane's 35 knots trumps any
typical rescue helicopter's downwash at the standoff you report.  It was the
water droplets and stuff in the air that created the helo/human difficulty,
not the speed of the air itself.  And from my brief experience in paddling
Long Island Sound waters -- where I grew up -- I hope your exercise was held
at a different body of water.  I wouldn't want droplets of Long Island Sound
water hitting me in the face!  It took two weeks to get the grime ring off the
Pintail the last time I paddled the Sound -- and it wound up taking Brillo to
get it clean!

How's that for an English major's analysis of rotorwash?

Joq
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