Subject: Re: [baidarka] Superiority of Native paddles
From: Michael Daly (michaeldaly@home.com)
Date: Wed Jun 20 2001 - 20:36:55 EDT
From: "Peter A. Chopelas" <pac@premier1.net>
> > The chord is measured streamwise. Below you use the width of the paddle,
> > which in typical usage is normal to the flow. Why are you not using the thickness
> > of the blade?
>
> Typical usage is not normal to the flow, try it. You get flutter, vortex
> streets, turbulence, loss of thrust.
If it's not normal to the flow, I'm sculling. Not too useful for forward motion.
The predominant use of the paddle is normal to the flow creating drag -
not lift. That's certainly the way Euro paddles are used. If you claim that
native paddles are not, then you are comparing apples and oranges in
your model, since the behavior is different. Compare the same behavior!
Either both are used in drag, normal to the flow or both are wings.
>
> But more importantly, by definition the aspect ratio HAS to be a function
> of the area of the blade, you do not affect the area with different
> thickness. The thrust is not affected by thickness either, so how could it
> affect the efficiency?
>
Peter, you claim to be an expert in fluid mechanics and then you come up with
this. The aspect ratio is the ratio of the span to the mean chord. At least that's
what I learned 26 years ago in university and used when working on the Canadair
Challenger aircraft as an engineer in the late '70's.
The chord is measured roughly streamwise from the leading edge to the trailing
edge. When you use a paddle you are in a very deep stall! For all intents and
purposes, the thickness of the blade is the chord - not the width! What you are
trying to say is that the orientation of the blade is irrelevant in calculating drag.
I don't think that there are too many folks that will agree with this!.
> >
> > If you use the thickness, you'll find the difference in aspect ratios between Euro
> > and native paddles not so great because if the considerable mean thickness of
> > the native paddles.
>
> Thickness not related to AR, and not related to thrust you get from the
> paddle.
The AR is the length of the blade divided by the mean thickness. You've got the paddle
turned 90 degrees to the way it's used. That's how the equations you are using were
defined. Changing this arbitrarily, as you have, makes your results invalid.
> There is no other definition for the AR. If you do not want to take my
> word for it go look it up for yourself because I can not prove a negative.
I've got Fluid Mechanics with Engineering Applications, 6th edition by
Duagherty and Franzini right here. Your use of AR is wrong. How
can I be more explicit?
> > >
> > > efficency=P-out/P-in = TxV/DxV = T/D since the velocity cancels
> > >
> >
> > There are several problems with this assumption.
>
> This is NOT an assumption! It is my definition of efficiency, it is what I
> want to compare.
You've assumed the velocities cancel out. I don't see why.
> Besides, did not you just say it is typically used normal to flow? Now you
> say there is a change in angle?
There is a change in the angle of the useful thrust, not the flow. The water
is nominally stationary relative to the speed of the blade unless you're in
WW.
> > Three - Why is the "handle" velocity the same as the blade velocity? Given
> > the mechanics of paddling, I'm having a hard time mapping your "handle"
> > to a real paddle in such a way that I can see the equality.
>
> Has nothing to do with handle velocity.
You said above that the handle velocity is equal to the blade velocity. Why?
If they're not equal, why do they cancel out?
> My own observation is that the high aspect ratio paddle is more efficient,
> and the equations point in that direction too.
You are trying to get your theories to prove your beliefs and won't consider anything
else. The first step in your derivation is wrong. You are using the wrong AR!
Mike
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