I have a testing windmill with a set of 3 plywood blades. Each blade is approx 1 meter long and 18mm thick. Front is flat, back has a basic aerofoil shape. The blades were angled at approx 15 degrees.
These blades have been in use for over 12 months and survived a tower fall 9 months ago.
I tilted the windmill down to try a new lighter steel propeller hub that I lasercut a few days ago. Recently I had read an article about blade twist and the effects of stalling blade tips at high speed, so I decided to fire up the power plane and reshape the blades to provide 5 degrees of twist at the tips while keeping the original 15 degrees at the centers.
A quick coat of paint and the windmill was back up in the air. Straight away I noticed a dramatic increase in speed, and less noise. Giving the blades a bit of twist has made a big difference in performance.
My next set of blades will definately have the correct twist angles, the gains are worth the extra work involved.
I've taken some photos of the new propeller hub, will put on my website in the next few days.
http://www.thebackshed.com
Glenn
