Subject: Re: [baidarka] baidarka speeds
From: Peter Chopelas (pac@premier1.net)
Date: Wed Oct 06 2004 - 12:47:40 EDT
> I seem to be missing something here. My old "airplanes for dummies" book
> explained that 'lift' is the force generated by a foil perpendicular to
the
> direction of flow, and 'drag' the force parallel to the direction of flow,
> but opposing it. OK - If this is wrong, stop me here and can I please
have
> more correct definitions?
this is approximately true for a conventional airplane wing in normal
flight, but as in all things relating to fluid mechanics, it is an over
simplification that applies only in this circumstance. While drag is
actually quite complex as well (there are at least three components and more
depending how you analyze it), but it is handy to define it as acting in the
streamwize direction. Lift can act in any direction on the surface, but it
is only caused by the curvature of the fluid. On a surface pulled through
the water the LOCAL flow on the surface will generate lift perpendicular to
the local flow (which may be different from global flow direction). On an
Aleut blade the large vortexes that form on the backside cause local
cordwise flow, creating a lift vector on the forward facing side of the
paddle, pointing forward, the same direction as the drag vector.
Also consider that conversation like this can be confusing if you only think
of "lift" as up, but the lift vector can be directed in any direction. The
lift vector on a propeller for example is forward (a prop is a foil going
around in a circle), as it is on a paddle. The biggest confusion I think
comes from people who can not visualize "life" going in any other direction
other than "up".
>
> Given that the above is a close approximation to the truth, then the lift
> component of a paddle blade being pulled through the water will tend to
roll
> the kayak one way or the other (as in a bracing stroke) depending on
whether
> the blade is canted forward (a la Maligiaq) or backward (a la Europeans).
this is only true if you think that lift is only in the "up" direction. You
must stop thinking this way if you want to understand this properly. The
horizontal tail on a conventional airplane configuration has its lift vector
pointing DOWN. This is how stability is maintained. Props and paddles have
their lift vector pointed forward. a rudder when deflected points its lift
vector sideways to turn the hull (or aircraft). The "lift" of a fluid pump
vane is what pumps the fluid through the pump. It is what drives a sail
boat forward.
> Either w
ay, lift is a very useful property that all experienced kayakers use
> to help keep the scratched side down.
Lift is used for all kinds of things, including rolling, bracing AND going
forward.
> I am puzzled to find that Michael Daly seems to be citing Winters' work as
a
> confirmation of the hypothesis that lift is more important than drag as a
> driving force in paddles, when Peter's quote (above) tends to suggest that
> Winters concludes the opposite.
I read Mr. Winters work as claiming that paddle thrust is primarily drag.
This would only be true if he defines drag as any force that is in the
opposite direction of paddle movement. The industry convention is that lift
and drag are studied separately, no matter what direction they act. If they
are not examined separately (even I they act in the same direction) you can
not develop optimum shapes since lift and drag are caused by different
phenomenon.
Again, I think because this is not obvious to the average person, it is not
an easy subject to explain accurately without confusion or arguments based
on lay peoples misconceptions from their experiance.
Peter
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