Subject: RE: [baidarka] Superiority of Native paddles
From: Peter A. Chopelas (pac@premier1.net)
Date: Wed Jun 20 2001 - 21:38:16 EDT
On Wednesday, June 20, 2001 1:56 PM, Hendrik Maroske
[SMTP:HMaroske@exmail.de] wrote:
> Well, there are different opinions on how these equations can be used for
> calculating an airplane. I hnow of some guys who really are earnestly
> convinced
> that all this does not apply for their type of plane, so if there are
> differing
> opinions about how to use this for aircraft, how can we try to use this
for
> something else?
This is not a totally accurate statement: there are a large number of
theoretical models out there, all work to one degree or another, there is
just argument under which conditions is each method more accurate. On
unusual configurations such as canard, tandem wing, or all wing designs you
have to carefully review your assumptions (a good idea no matter what you
design), and sometimes when you are at the edge of the method's limits, you
get large errors.
There is no argument about the validity of the methods, the argument is
where the limits and assumptions are valid. That is not really an issue
here since we are not talking about a complex structure but a single blade
in a fluid.
Do not get confused with the "lift" convention used in aircraft: what we
are interested in is thrust available for forward motion, it is derived
from low pressure on the forward face of the blade, and high pressure on
the aft (or "power" face) of the blade. You have a whole bunch of other
"lift" or thrust forces on the blade, but in this case we only want to
measure the part of it that is used for forward motion only. So I have
defined my "lift" or thrust as only that portion that is in the desired
direction.
For example in a kayak that is not directionally stable (i.e. does not
track well) you spend a lot of lost effort keeping the kayak pointed in a
strait line. Necessary effort but by my definition it is "lost" effort, it
contributes nothing to forward motion.
Like wise I have defined my drag as the TOTAL effort you feel at the shaft
or handle. On an aircraft the total drag is defined as the direction of
the streamwise flow, and lift normal to it. But realize that there are
some components of lift that are pure drag under this definition. The
horizontal tail on a conventional configured airplane has it's lift vector
pointed down, this forces the main wing of generate more lift to counter
it, increasing induced drag, and the tail must generate induced drag too.
This downward lift from the tail is necessary for stability and control
but manifests itself as drag only on the airframe. So keep your
assumptions strait, and please do not use irrelevant examples.
So do not attempt to redefine my terms and tell me the analysis is wrong
because drag on an airplane is defined differently than the way I defined
it.
A propeller on a boat or an aircraft produces "lift" or thrust in the
forward direction, this is not lift as defined by the wing, but it is still
lift.
Also be careful when imagining the paddle as a pure drag device, it is just
not so. Try this, cover one paddle blade with a very draggy surface, say
wrap it with some artificial plastic grass ( the stiff type) and go
paddling, you will quickly see you get way more thrust for your effort by
the smooth blade for your effort. Even a parachute is not a drag device
(contrary to popular opinion) the lift vector is strait up to slow the
decent, if it was a pure drag device that a canopy of turbulence generators
would work better than a smooth canopy, but it does not (and no one has
ever made one work). Also ask any sport jumper what happens when they
stall the parachute, if there was no lift from it than it could not stall,
and the rate of decent would not rapidly increase.
If you design a paddle (or a parachute or spinnaker for that matter) as a
drag device, you get very poor performance. Alternatively if you design
and use them as a "lifting" device, you get very good performance.
Also keep in mind that even a fully "stalled" surface, such as a paddle
pulled strait back in the water (which is actually seldom done), you still
get lift from it, it is just a lot less than if it is not stalled and the
drag goes up. Also you can get vortex lift that is quite powerful, but
also at the expense of high drag.
"Lift" vectored forward, or thrust, is always good, drag is always bad.
Drag costs energy, heats the water, causes turbulence, and loss of useful
power. I doubt there is any component of the drag that is useful to
generating thrust as I have defined it, if you could make the drag go to
zero, then you would have a perpetual motion machine, and infinite
efficiency. Not possible of course, but you want to minimize the drag as
much as possible, and maximize the thrust (or forward directed "lift").
Peter
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