Re: [Paddlewise] lift and efficiency: D

Aerobatic airplanes also have symmetrical profiles on their wings.
That is what allows them to fly upside down.

When the wing is at a positive angle with respect with the flow,
the air has a longer path around the round edge to travel to the top
of the wing, while the flow that goes under the wing has a shorter
turn.

Hence, *the key is in the round shape of the front edge*. That is why
symmetrical profiles have the center of lift closer to the front than
asymmetrical profiles.

I know this is very hard to explain without a drawing, but if you go
to a hobby shop, any book on model airplanes should have good drawings
about it.

On Greenland paddles, for the same reason explained above, it is very
important to have the edges properly rounded. A sharp edge would make the
paddle stall at even the smallest angles (tested, been there).

The same principle applies to rudders, and skegs. If the leading edge
is too sharp they just stall and only act as drag.

- Julio

> 
> However, lift is created when there is a difference in pressure between
> opposite sides of an object regardless of the means used to create that
> pressure difference. (John, correct me if I am wrong about this.) Despite   
> the word, the direction of lift does not have to be up. You can also
> create lift by moving an inclined object through a fluid. A prime example   
> is holding your hand outside a moving vehicle and tilting it so the wind
> moves it up. Other examples are when you scull your kayak paddle either
> on or under the surface of the water, or support yourself with a sweep
> stroke, or roll using a sweep or screw roll. I think this is the kind of
> lift most people are referring to with regard to Greenland and other
> paddles.
> 
> If I understand them correctly, wing paddles are an exception in that
> they *are* shaped like airplane wings in section, and generate forward
> lift in the same way. However, they are specialized paddles used mostly
> for racing, and since they do not perform as well as standard paddles
> for bracing and rolling, they are rarely stocked by recreational paddling   
> shops. I think I have seen only one ad for them in Sea Kayaker.
> 
> Chuck Holst  
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