Subject: Re: [baidarka] Superiority of Native paddles
From: Hendrik Maroske (HMaroske@exmail.de)
Date: Thu Jun 21 2001 - 15:28:24 EDT
Okay,
having had a night's sleep about this, let me make a different approach
towards that aspect ratio thing...
Let me start with a flying wing whose purpose it is to keep some heavy
mass from falling out of the sky. The basic trick here is, the wing is
moved fast enough through the air; while passing through air, it moves
air down to keep its own mass aloft. This air movement has been
photographed
in wind tunnels and calculated with computer software. There is a good
web site that demonstrates this:
http://www.aa.washington.edu/faculty/eberhardt/lift.htm
If we slow this wing down, it will cover less air in the same time, it
has to move that air quicker downward, like a plow, to stay aloft. For
this, some wings have flaps that are extended at slow flight; all others
will be rotated to point the nose a bit higher.
If the wing moves faster, it covers more air within a given time, so
we can afford to move that air down more slowly, in order to stay aloft.
We either retrace these flaps or rotate the wing to point the nose a bit
lower.
The effort of moving air down results in two large vortices at the
wing tips; the slower we fly, the faster we move the air down, the
more violent those vortices are.
Now, what happens if aspect ration increases? The wing can cover more
air within a given time, since it is wider now, thus we can afford
to move that air more slowly, thus we get smaller wing tip vortices.
Vortices eat energy; here, this energy is provided and calculated
as induced drag. If at the same time we have a high aspect ratio,
a very clean profile and move very fast, those vortices get really
small. That's the trick of high performance sailplanes.
Okay; for a paddle with a given area and a given thrust, if a larger
aspect ratio coincides with a smaller pair of vortices respectively
a smaller vortice rotational speed, I think it makes sense to argue
that it has a higher efficiency.
The mayor problem here is to define an appropriate test, with appropriate
conditions, to show this.
Please forgive the oversimplification above, but I have tried here
to give an image of what happens around the wing/paddle; maybe
someone else gets a good idea from this, to help define that
experiment.
Clearly, as long as that vortex is small enough so as not to
influence efficiency, I do not see any improvement. OTOH I
think the old aleut paddlers were able to make much more of
a push and thus larger vortexes than I, so it might have been
much more important for them.
And then there is that problem, when these vortices become
too violent, the paddle blade begins to flutter. Increasing
aspect ratio, thus decreasing vortex size/speed, may help
to shift this undesirable effect towards higher thrust force.
I personally prefer those sleek paddles since paddling upwind
with them seems a lot easier; just to be able to use that
"sliding stroke" helps a lot, and if it's just mentally :-)
Cheers
Hendrik
P.S. To heck with theory. Go Paddle :=)
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