Subject: RE: [baidarka] Superiority of Native paddles
From: Peter A. Chopelas (pac@premier1.net)
Date: Wed Jun 20 2001 - 22:30:07 EDT
On Wednesday, June 20, 2001 5:37 PM, Michael Daly
[SMTP:michaeldaly@home.com] wrote:
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!
so what, you still get lift off of it. Besides the low AR blades (like
3:1) are almost impossible to stall anyway. You also get a lot of vortex
lift from most Euro blades. If you do not know anything about vortex lift
than you will likely not understand this very well, but vortex lift even
generates huge amounts of lift at 90 degrees angle of attack. It is very
costly in terms of drag but works great to prevent stalls in fighters in
"pop-up" and high-G moves.
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!.
I did not write this, it was the person I was responding to who made this
claim. Again, what I am interested in is the useful forward thrust vs. the
effort in at the handle. Clearly if you turn the blade sideways you get
little drag, but no thrust either, efficiency goes to almost zero. Please
do not put attribute claims to me based on your own assumptions, and then
say I am wrong.
> 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.
> .... Your use of AR is wrong. How can I be more explicit?
What you wrote here is exactly what I wrote in my original posting, please
go re-read it. AR is the span squared divided by area, it reduces to
span divided by cord only in a rectangular platform. Where is my use
wrong? You wrote exactly what I did, just differently (not to be overly
picky but technically YOU are wrong, for most shapes other than a
rectangular one, you will not get true AR with your definition).
>
> You've assumed the velocities cancel out. I don't see why.
You do not sound like you read the original posting because I gave the
reason there.
> There is a change in the angle of the useful thrust, not the flow.
Not relevant, I am only interested in forward thrust, not any thrust or
component of thrust that is in any other direction.
The water
> is nominally stationary relative to the speed of the blade ...
Not really true but irrelevant anyway: the relative velocity of the fluid
over the blade (whether the blade is moving or the fluid is moving) is what
generates the thrust. How is your statement relevant to what I want to
measure? Again I want to compare USEFUL forward thrust vs. Total power-in.
Your statement appears to get caught up in unnecessary complications and
details, and then you do not even appear to consider whether it is relevant
to what I am comparing: USEFUL forward thrust/total power in.
>
> > > 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?
I do not recall ever make such a statement, if I did it was in error.
Velocity of the fluid relative to the blade determine both the thrust, and
the drag, since this is the same it cancels. Please go back and carefully
read the earlier posts.
>
> > 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!
>
It has nothing to do with beliefs, it was first a suspicion, than a simple
observation and finally I pulled the text books out to see how strong the
effect is. I would welcome carefully controlled tests, and will likely do
this eventually. But you have not shown me where my derivation is wrong,
and my definition of AR is the same as yours, so if it is wrong, so are
you. (But it is not).
You appear to me to want to argue rather than carefully consider what I
have written and respond. By your answers it looks to me like you did not
even read what I had posted, or if you did you clearly did not understand
it. If you want clarification I will happily provide it, but please do not
argue with me about things I did not write.
Peter
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