On Fri, 17 Mar 2006, Olin Lathrop wrote:

> Herbert Graf wrote:
>> Only problem with them as I understand it is they can't start on their
>> own, they have to be spun up to a certain speed before they are good
>> to go.
>
> Yes, but again what produces the net torque?  Let's say the blade is set up
> so that when it's at the front it pushes to the right.  When that same blade
> gets around to the back it has now pivoted 180deg.  If it's just a flat
> blade, it will now be pushing again to the right, counter acting the torque

It is a regular aerofoil optimized for the calculated airspeed. If you 
look at the lift/drag vs. AoA curve of an aerofoil you will see a lot of 
lift and relatively little drag. If the foil is used 'backwards' (the 
case for the receding blade in a 2-blade vertical Darreius) the lift is 
nowhere near that which appears in 'forward' 'flight' because the 
profile is all wrong. Thus the blade that generates lift wins. The ratio 
between lift and drag can be 5:1 for part of the rotation.

A vertical Darreius can be made easily by gluing two aerofoils (foam, 
cardboard, whatever) onto the edge of a turntable style base, at a 
certain AoA. In fact if the platter has enough inertia one wing half is 
enough. Thin, short chord aerofoils are preferred. A wire should be 
attached from the blade tip to an anchor on the platter to prevent 
centrifugal force from tearing the blade out.

Peter
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist