Thread: Discussion Gyrocopter Aerodynamics
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Old Jan 09, 2007, 06:27 PM
mnowell129 is offline
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Mickey from Orlando. Really.
Joined Nov 2004
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Originally Posted by iter
Mick, thanks for the explanation. The way I had it explained to me was that Cierva came up with lead-lag hinges (which had shafts offset from vertical) to cyclically change blade pitch to offset asymmetric lift, and that even with a vertical shaft, the LLH helped offset AL because a lagging blade saw a reduced airspeed and consequently reduced lift. I thought it was probably the wrong explanation, now I know. Thank you!

It's interesting to me how asymmetric lift is a common explanation for so many things in rotor aerodynamics, and how many of them turn out to be stress-related, like delta-3 and lead-lag hinges. Thanks for giving us a coherent explanation.
My belief is there are a lot of "explanations" that come from pilots and mechanics who were told something by the engineers they didn't quite follow or the explanation was abbreviated to avoid a lengthy mathematical explanation or who made a good educated guess at the way something worked and people believed them because obviously they could fly the thing.....
As an example the seemingly most hallowed autogiro book there is, "Autogiro, The story of the Windmill Plane" by George Townson. The author is/was a pilot, not an engineer. Here are some gems from the theory section of his book:
"Cierva also put a vertical hinge on the blade to permit it to move fore and aft to relieve the bending as the drag increased on the "advancing" side and decreased on the "retreating" side."
(mickey: this completely misses the conservation of angular momentum discussion of blade in-plane lead-lag.)
"As the blades advance, the increased air speed causes the blade to climb or "flap.".. This action effectively equalizes the lift on each side of the rotor disc and permits the autogiro to fly level in forward flight instead of rolling because of the "unbalance of lift" across the rotor disc."
(mickey: this ignores the basic physics of flapping that have been covered here.)
"On the autorgiros produced by Pitcairn and Kellet, the rotating mast of the rotor was inclined to the retreating that way "encouraging" the blades to flap..."
(mickey: this is of course fully explained as cyclic to the retreating side to compensate for coning induced roll. It's also silly to think that the blades need "encouragement" to flap.)
I can't tell you how many times I've seen this used as reference. The rest of the book is a very nice coverage of the history and characteristics of various autogiros, but the theory section is just plain wrong in several places and it gets referred to over and over.
Originally Posted by iter
I've been looking at different coaxial helicopters lately, and I noticed that Blade CX and a few similar designs have no LL hinges, but others, like the Lama, do. Do you think that's related to the original Lama's light and weak foam blades? Lama's weaker, more flexible foam blades can't take the bending loads, but BCX's nylon blades can? (I think Lama's blades were designed to shatter on impact)

My educated guess is that the lead lag hinges on the lama are strictly to reduce blade failures when you hit something and make them easy to replace, which apparently you have to do fairly often. The lead/lag forces don't happen in a hover either, which is where lama's/BCX's spend most of their time. Keep in mind that coning is not flapping so a rotor in hover, even with coning has all the blades running at the same radius, therefore without varying lead/lag. It's when you start forward movement that the rotor flaps up in response and the lead/lag variations happen.

Originally Posted by iter
Yes, blade balancing! There is something that works for me and anyone who flies a Slow-G or a Penni, blades that have no tip weights, but they seem to be important, and I'd like to have a better understanding of why it works even though it shouldn't. Many smaller helicopters, especially the coaxial ones, seem to have no tip weights, but the larger ones all do. Is that related to higher torsional rigidity in smaller blades?

The short answer is yes. It's related to torsional rigidity. When I visit blade balancing I'll elaborate.

Thanks for the questions!
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