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Old Apr 13, 2012, 01:00 PM
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United States, UT, Salt Lake City
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Originally Posted by DPATE View Post
There is a lot we can learn from nature, but we have to be careful about our assumptions. I have never measured a bird, so I do not know if it has what we would call "longitudinal static stability" (which is what we are talking about when we talk about CG placement). However, do not assume a bird is statically stable because it could still undergo steady level flight using active control.

Take bipedal walking, for example. Using our definition of static stability, this motion is unstable, just like an inverted pendulum. So if I was trying to build a robot that could move around, would it be a good idea to use two legs? That depends.....
I only have one leg and I get around-
(also one very $$ prosthetic device)
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Old Apr 13, 2012, 01:02 PM
Grad student in aeronautics
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I think you're missing the point.
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Old Apr 13, 2012, 01:07 PM
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Originally Posted by BMatthews View Post
A great test for all this was done by someone here in the MS forum a few months back. He took a simple wing with a ClarkY airfoil and tried, unsuccesfully of course, to make it fly as a plank style flying wing. He then turned the negative pitching moment into a positive pitching moment by turning the wing upside down. It trimmed to a stable glide with no issue at all. Of course the upside down shape brought a lot of drag and limited the maximum attainable lift coefficient. But it DID glide in a stable manner... just not very far.
I never tried an upright Clark Y plank, since it is obvious a negative Cm airfoil will not work. I think you might be surprised at how well an inverted Clark Y plank glides - I just chucked it off my balcony, and it looks like over a 6:1 glide, which isn't that bad for a little low Re chuck glider.

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Originally Posted by Sparky Paul View Post
I'm trying to finger out how this guy works!
Birds routinely fly with negative static margins, even when they have tails. Watch them when they land, their tails go down, which can only work if there CG is behind their neutral point. And gulls and albatrosses routinely fly with their tails completely folded.

It doesn't take much of a computer to fly an unstable flying airplane, so I don't think birds have much trouble flying when statically unstable. We manage to walk upright, and that is statically unstable. Flying with an unstable CG position is an advantage for both performance and manoeuvrability, so birds would have evolved to take advantage of that pretty quickly. Even the Pterosaur model that McCready's team built was yaw unstable, and I'm pretty sure the real one would have been pitch unstable too.

Supposedly even a simple rate gyro will make an unstable CG glider flyable, so I'm sure birds would have no trouble with being unstable. I haven't tried this myself:

http://www.rcgroups.com/forums/showp...8&postcount=11

Kevin
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Old Apr 13, 2012, 01:18 PM
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Quote:
Originally Posted by DPATE View Post
There is a lot we can learn from nature, but we have to be careful about our assumptions. I have never measured a bird, so I do not know if it has what we would call "longitudinal static stability" (which is what we are talking about when we talk about CG placement). However, do not assume a bird is statically stable because it could still undergo steady level flight using active control.

Take bipedal walking, for example. Using our definition of static stability, this motion is unstable, just like an inverted pendulum. So if I was trying to build a robot that could move around, would it be a good idea to use two legs? That depends.....
.
Observing the standard issue raven flying, especially sloping off the wind going up the front face of a building, their tail is -always- working!
This one may never have had a tail!
I'm gonna watch a lot closer now when out walking, that I know it's a local and not a transient. Takeoffs and landings would be neat to see!
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Old Apr 13, 2012, 01:28 PM
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Quote:
Originally Posted by kcaldwel View Post
I never tried an upright Clark Y plank, since it is obvious a negative Cm airfoil will not work. I think you might be surprised at how well an inverted Clark Y plank glides - I just chucked it off my balcony, and it looks like over a 6:1 glide, which isn't that bad for a little low Re chuck glider.



Birds routinely fly with negative static margins, even when they have tails. Watch them when they land, their tails go down, which can only work if there CG is behind their neutral point. And gulls and albatrosses routinely fly with their tails completely folded.

It doesn't take much of a computer to fly an unstable flying airplane, so I don't think birds have much trouble flying when statically unstable. We manage to walk upright, and that is statically unstable. Flying with an unstable CG position is an advantage for both performance and manoeuvrability, so birds would have evolved to take advantage of that pretty quickly. Even the Pterosaur model that McCready's team built was yaw unstable, and I'm pretty sure the real one would have been pitch unstable too.

Supposedly even a simple rate gyro will make an unstable CG glider flyable, so I'm sure birds would have no trouble with being unstable. I haven't tried this myself:

http://www.rcgroups.com/forums/showp...8&postcount=11

Kevin
.
McCready's Pterosaur had the head extended, if memory serves.
The local herons and egrets retract the head to the shoulders, which would lessen the area ahead of the c.g.
But then, cranes and storks fly fully extended...
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Old Apr 13, 2012, 01:50 PM
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Not to mention canards... err, ducks. if you look at their shape while in a cruise they have no tail surfaces to speak of. They control pitch by shifting the wings forward and aft, which makes for much lower overall drag. Only when they need sudden pitch changes or increased drag they extend the tail feathers.
[edit] your tailless raven probably controls pitch the same way, but a raven does not need the maximum efficiency that migrating birds require, and the added manoeuvrability pays off when, for example, stealing food from other ravens.
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Last edited by Brandano; Apr 13, 2012 at 02:00 PM.
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