I believe I understand everything exept this one statement which others have said in the past also. But the blades are FIXED at ONE ANGLE (say 4 or 5 degrees at tip) to the hub, so how can the 'angle of attack' ever change!? The blades don't twist. I always figured it was related to SPEED of the blades moving thru the air in rotation the way the air either 'falls off' the back side of the blades or 'wraps' around the back side of the blades??.
Of course, with a variable pitch blade system (which almost nobody has here) , this would make perfect sense. It also makes perfect sense with airplane wings in flight. But with fixed angle blades on windmills, this 'changing angle of attack' statement confuses me as it always has. But I understand everything else.
A couple RPMs before cutin, the blades are running faster than the design TSR. A little bit into stall, the blades are running slower than design TSR.
The say 5 degrees at the tip is fixed, but the angle of attack is related to the RPM of the blade.
It sounded better in my head, G-[ Parent ]
The blades don't see the wind as you see it, they see a vector of the real wind and the component due to the blades rotating typically 6 times wind speed at the tip.
That is why the things have extreme difficulty starting from standstill. The angle of attack from the wind alone is over 80 deg and stall starts about 12 deg.
As the blades pick up speed the apparent wind moves round to far nearer the blade flying direction and at the maximum power point the angle of attack will be about 4 deg. As you slow the prop the apparent wind comes more into the direction of the true wind and at about 12deg angle of attack you start to stall.
Try Hugh's site, http://www.scoraigwind.com/ He has diagrams showing the real wind, the apparent wind, the pitch angle and the angle of attack.
Flux[ Parent ]
