hey guys ! a new type of windmill (french site)

[Earl]

Hi,

All airfoils whether flat or curved have a characteristic called L/D or Lift / Drag.
The L/D ratio and how it varies with the attack angle is the most important thing.
Stalling performance is another.

A flat plate has very bad L/D and should never be used.  Sorry.  You can not
argue against 100 years of aerodynamic research.

The air foil must not be flat.

There have been lots of boats built with vertical wing sails; google Walker wingsail or drivesail, for example.  The Walker WingSail had a movable front part to change the shape of the airfoil from symmetrical, to unsymmetrical on either side.  This achieved a very high L/D over wide range of attack angles without stalling.  It was beautiful and worked well, but had higher weight.  There are other designs, you will find lots of interesting info on people's search for the ultimate sail.

As in all branches of engineering, there are always compromises necessary.  That's life.  Good engineering is to find the best compromises for a certain goal.

Earl

A plate blade works as well !

Hi titof,
Surely with flat blades its working as well, I don't argue that.

[Earl]

My latest design is a pure lift based design.

Impossible to have a pure lift design.  There is always drag.
See my above post.

See also http://en.wikipedia.org/wiki/Airfoil
and
http://www.aerospaceweb.org/question/airfoils/q0035.shtml

Earl

[minesapint]

The foils are not flat.
A flat foil was in fact used by the wright bros with an angle of attack to produce life but flat foils are inefficient.
I use a symetrical foil with a angle of attack of about 20 degrees - if you go above this you stall, less than this reduces lift.
You are right every foil has a drag / lift coefficient. The best foils are non symetrical - interesting effects when the pilot tries to fly his plane upside down!!!!
You have to keep the drag low and the lift high for a chosen angle of attack.
A drag based machine will only go as fast as the wind whereas a lift based one can exceed wind speed that is why sailing boats can zip along.
Like I said I have done a number of sims - there are some quite good foil design apps on the net that you can use to get the design right.
The real clever bit in the software - how to control all the variables (such as blade rotation at any given point in the sweep and the angle of attack) for a given wind speed and rotor loading You have to stop the thing from stalling whilst keeping maximum torque.
.

[Earl]

I agree with everything you say, except about the Wright brothers.  They used airfoil wings on their plane, in fact they built a wind tunnel and studied their designs.  Maybe the first wind tunnel ever built?  These two bicycle builders were not dumb.

Earl

The foils are not flat.
A flat foil was in fact used by the wright bros with an angle of attack to produce life but flat foils are inefficient.
[snip]

[minesapint]

Wright brothers - I stand corrected.
I am sure I read it somewhere....