Subject: Re: [baidarka] Lift and Drag
From: craig o'donnell (dadadata@friend.ly.net)
Date: Thu Oct 07 2004 - 08:37:13 EDT
>> On a wing the curved upper surface causes the air to moving across it
>> to travel farther than it has to on the flatter under side. Again
>> according to Bernoulli, the pressure of the air above the wing becomes
>> less than that under the wing and the wing is lifted.
>
>As a physics prof once declared to me - that satisfies force
>equilibrium but what about momentum? While that description is a
>suitable introduction to the idea, once you get past that, you have
>to consider the flow off of the wing and the momentum being carried
>in the air that is accellerated over the wing. I hesitate to try to
>describe it as I'm sure Peter can do a better job.
"Bernouilli" clearly has applications in some places, it's a pressure
phenomenon, but one of those places is not wings or sails.
"Lift" on a wing is due to equal and opposite reactions; the wing pushes a
lot of air down with a lot of force and the plane remains airborne.
This, of course, is a vast, vast oversimplification.
A typical paddle is generating power by drag and by viscosity (related to
drag). It is possible to slice a blade, even a flat blade, downwards into
the water at a slight angle and move the boat using lift. Your boat won't
move as fast but it's a lot less work. If water were chocolate syrup, you'd
swing that blade back and nothing would happen (or you'd tear some
muscles). Too viscous. If it were more like air, you'd go nowhere, not
enough viscosity for you to shove the fluid backward and move the boat
forward.
Part of the issue for oars and paddles is that the available power (you) is
limited but is quite large in terms of the drag it has to overcome (boat
moving through water) so this power by drag (tossing water backwards,
basically) works well.
--
Craig O'Donnell
Sinepuxent Ancestors & Boats
<http://freepages.genealogy.rootsweb.com/~fassitt/>
The Proa FAQ <http://boat-links.com/proafaq.html>
The Cheap Pages <http://www.friend.ly.net/~dadadata/>
Sailing Canoes, Polytarp Sails, Bamboo, Chinese Junks,
American Proas, the Bolger Boat Honor Roll,
Plywood Boats, Bamboo Rafts, &c.
_________________________________
-- Professor of Boatology -- Junkomologist
-- Macintosh kinda guy
Friend of Wanda the Wonder Cat, 1991-1997.
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