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dragon6172
Posts: 1140
Joined: Sat Jul 14, 2007 9:56 am

RE: Starting Helicopter In Free Fall

 Quoting Klaus (Reply 34):And if the wikepedia explanation is correct, autorotation-enabled rotors already have a spanwise-variable pitch built in which even becomes negative along the inner span in the lowest collective range.

And that is why you do not quote Wikipedia. The aerodynamic twist of a rotor blade is the opposite of what you quoted, the blade TIPS are always at a lower pitch angle than the inner span.

This makes sense for a couple reasons. First, the tip of the blade has air flowing over it much more quickly than the inner part. This means the blade tip needs a lower pitch to create the same amount of lift as the inner portion. This also helps keep the blade tip from stalling at high pitch settings and high forward airspeeds.
During an autorotation, having the the blade tips at negative pitch ( only a few degrees at best ) provides rotational force to the rotor system in the desired direction, and it does this in the most efficient way possible by applying the force at the end of a long moment arm.

dragon6172
Posts: 1140
Joined: Sat Jul 14, 2007 9:56 am

RE: Starting Helicopter In Free Fall

After further review of the wiki page on autorotation, it seems to me that they have the "driven" region and the "driving" region backwards. The driven region is the vast majority of the rotor blade at or near zero pitch. The driving region is the region at negative pitch. The stalled region is the portion at positive pitch. So, you put the portion that will create drag near the center of the rotor disk (small moment arm) and the portion that will create rotational force near the outer portion of the disk (large moment arm) and make everything else neutral.

If this were reversed... the rotor disk would never accelerate (rotationally) in the desired direction because the larger drag moment arm would always win.

Also, the reason the "driven" region is the largest, is because it is essentially the "energy storage area" for stopping the descent.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Starting Helicopter In Free Fall

 Quoting dragon6172 (Reply 51):seems to me that they have the "driven" region and the "driving" region backwards

I had thought the same thing but hadn't yet taken the time to read the article. I went away from my computer wondering about their definitions of "driving" and "driven" because it didn't match...

 Quoting dragon6172 (Reply 50):The aerodynamic twist of a rotor blade is the opposite of what you quoted, the blade TIPS are always at a lower pitch angle than the inner span.

Correct. Therefore the blade region with the lowest possible angle would have to be out near the tips. Any chance of having negative angle (from plane of rotation) would be there.
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