Subject: [baidarka] Aspect ratio of apples and oranges
From: Joe Street (jstreet@venus.uwaterloo.ca)
Date: Fri Jun 22 2001 - 10:12:03 EDT
In all these discussions about aeroplane wings (and I am also an aviator
so I know a bit about this) we have lost sight of a simple fact. On the
water we do not sweep our paddles through the water in the same way the
wing of a plane cuts through the air! Airplane wings fly virtually
parallel to the flow. There is some angle to the relative airflow which
is generally only around 10-15 degrees or so (if not much less). Even
the old rogallo hang gliders didn't fly with more than 30 degrees angle
to the airflow. This is analogous to trying to paddle with our paddle
blades parallel to the hulls of our kayaks. This is why all these
comparisons between paddles and wings may IMHO be a bit like comparing
apples and oranges. Paddles are generally used at a very high angle,
almost 90 degrees to the flow, in other words in a deep stall as someone
put it. This is analogous to considering the effect of the aspect ratio
of a sailplane wing when the sailplane has got its nose pointed at the
sun and the tow plane is dragging it horizontally through the air, if
that were possible (don't try this at home kids). In that 'flight'
regime the sailplane would be better considered as a drag producing
device than a lift producing device, if you don't believe me, just ask
the tow plane pilot! I'm really not convinced that aspect ratio is a
good explanation for why greenland style paddles are superior for long
haul travel.
With the flow going 90 degrees to the face of the blade, the cord for
use in any calculation of aspect ratio would have to be considered as
the thickness of the blade not the width of the blade face, which
doesn't make any sense and is not appropriate to what those formulas
were developed to deal with, as someone pointed out earlier.
The more I think about it the more I am starting to believe that it is
just a matter of the reduced blade area of the native paddle which
allows a higher cadence that is more favorable to the human body for
long duration power output.
This is where another airplane analogy may be used and I am talking
about the variable pitch propellor. When maximum power is needed the
pitch of the propellor is reduced which allows the engine to run at a
higher speed where it is at the peak of its power curve while the
airspeed may be fairly low, whereas at altitude and when cruising the
prop is dialed out giving it more bite and allowing the engine to run
slower at the peak of its fuel efficiency curve while still providing
cruising power. This analogy is not meant to compare the propellor to
the paddle, again propellors fly at low angles of attack, but rather to
illustrate that the powerplant can be utilized in different ways
according to the need by the use of different propellor pitches. Each
is more efficient for the circumstance in which it is intended to be
used. Similarly a racing paddle is not appropriate as a cruising paddle
and vice versa.
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