Ornithopters

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Ornithopters

Post by Mike Everman » Thu Dec 04, 2008 5:34 am

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larry cottrill
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Re: Ornithopters

Post by larry cottrill » Thu Dec 04, 2008 11:58 am

Yes, that is an amazingly good one.

But, on the other hand, I've seen ornithopters in films, magazines, etc. over almost a lifetime, and the one thing I notice that is a basic difference in the success of various designs is this: Successful ornithopters are always small. Now, there's something to be said for ultramodern materials (I mean the really high strength/weight ratio stuff like carbon fibre resin materials and so on) so that you could theoretically get one a lot bigger than that, but there still seems to be something basically wrong once you get up to a certain size. It seems to me to have something to do with exactly what you mean by a term like "flapping wings".

I have a wonderful old(now) book called Borne on the Wind, The Extraordinary World of Insects In Flight by Stephen Dalton, arguably the first man to accurately photograph insects in perfectly natural flight (using high speed strobes and laser triggering, etc.). He goes to careful lengths to point out the vast differences between insect flight (usually simple hinge action on flat, rigid surfaces) and bird flight (very complex muscular action on complex flexible structures with well-developed airfoils). There was a very nice PBS special years ago that I think was called C'mon, Geese! where a guy raised a small group of wild geese, imprinting them to fly alongside him as he flew an ultralight. Doing this, he was able to obtain incredible video closeups of the adult birds in flight -- you could actually see the muscle contractions in the back of a bird as it was flying along just in front of his wing, for example. Doesn't sound like much, but when you see it you realize how incredible it is. The point is, it makes you realize how difficult it would be to design a wing-propelled flight system for an object of reasonably high loading (weight / surface area). It's that kind of complexity that would need to be perfected for man-size ornithopters, I think. (Note that some insects, such as moths, use their wings in a more complex, wavelike motion, but that would present mechanical difficulties at a large scale, too.)

It just seems very difficult to build a powered, highly flexible airfoil surface that can be intricately controlled in a way that will both lift and propel a large payload that imposes a reasonable loading. As I said, with modern materials it might be within the realm of possibility, especially if microcontrollers could be employed to distribute and synchronize multiple power inputs near the hinge points. Might take five or six high power servoes per wing or something, perhaps a system on the order of complexity of some robot centipede.

L Cottrill

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Re: Ornithopters

Post by larry cottrill » Thu Dec 04, 2008 2:25 pm

Thinking on these lines a little more, it occurs to me that one solution to the basic problem is actually revealed in helicopter design. In the helicopter in forward flight, the "advancing" rotor blade moves forward at a low-drag, low angle of attack, typical of high speed flight. The "receding" blade moves rearward (I am assuming that the aircraft's flight speed is always lower than the relative speed of most of the blade!) at a higher-drag high angle of attack, typical of low speed flight. So what we need in an ornithopter is simply an oscillating (rather than rotating) blade which makes this transition in a way that is synchronized with the changes in forward motion of the blade.

Probably, we would have each wing move forward in a slightly rising angle with the airfoil allowed to partially "streamline" itself, but restricted to still provide a low but positive angle of attack. Then when the wing moves rearward and slightly downward, it would pivot a few degrees to present a very high drag angle of attack. This would need to be adjusted significantly as forward speed is attained, but that could be done with a tail plane pitching the aircraft in a more-or-less conventional manner. The wing design could be an airfoiled plane pivoting (between stops) around a strong tubular spar.

It would be interesting, and not outlandishly expensive, to make a model like this in around a two-metre span, with conventional model airplane construction. Of course, such a model would still have a pretty light wing loading compared to man-carrying craft, but it would be enough to show you whether the idea is going anywhere, so to speak.

L Cottrill

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