This film is nicely done with some incredible inflight view points on flying birds. Click it to see what I mean. You have camera angles that look like they could only have been shot from another bird, except for one thing. The camera is perfectly steady.
Here is part one of it on Youtube: (follow the links for the other segments)
This serves to highlight the aspect of bird flight that concerns me a lot when contemplating a man sized flapping winged machine. What you notice when viewing a goose in flight from what appears to be a few feet away, is the way the body of the bird moves up and down rhythmically with the beat of the wings. The head, which in most of these birds is on the end of a long flexible neck, is isolated from this bobbing and is compensated by the neck so it is relatively steady. This bobbing of the birds body is what makes it unlikely that the video is from a bird mounted camera.
Most of the birds in the video are geese or storks or related bird types that share the characteristic that they migrate over very long distances. There are very few raptors or soaring birds featured. This means that the birds in the video spend most of their flight time actively flapping their wings.
Noticing this constant flapping, I tried to imagine what the ride would be like if one were to be shrunk down to a size where you could ride one of these birds. And assuming that you could not ride on the steady head of the bird, then you would be either carried underneath or sitting on the shoulders with legs astride the neck. How would that ride feel?
The only thing I come up with is riding horseback. Its been many years since I rode a horse and the reason it has been that long is that it was not fun for me. Damned uncomfortable actually. Now I know that there are many people who love horses and riding. But there is a reason we invented wheeled vehicles. Two words, saddle sores.
Even though the shoulders of a huge bird would presumably be softer than a leather saddle there is still the ride. Riding a horse that is just walking is hard enough on your butt, legs and spine, but the faster he goes, the rougher the ride. These migratory birds cover thousands of miles at relatively slow speeds. They fly, flapping constantly, sometimes around the clock.
This is the aspect of ornithopter design that I believe most of the people who are designing their own ornithopters are ignoring. If you are designing a machine that flaps, how are you going to deal with the bobbing, bouncing, bumping up and down of the ride?
Here is an idea. Imagine a swimmer weighted down so that he is negatively buoyant. He must swim in order to stay above water. If he stops paddling with his arms or legs, he will sink and drown. Now, there is one thing he can do so that he can rest. He can catch a wave and ride it. Disclaimer, I surf. In surfing of course, you have a board that is minimally buoyant. But to go any where, you have to paddle or ride a wave.
Non soaring birds must flap or they sink. Nature has given them an incredible ability to power their muscles for hours on end. The only down side is that bouncing ride. On the other hand, the soaring birds can ride the lifting currents of ridge and thermal lift to gain altitude and then glide for miles all without beating a wing.
So this is the question: If I were to offer you your choice of a machine to spend a whole day riding, one is modeled on a goose and the other was modeled on a condor in terms of flying styles, which would you choose?
Instead of the constant bobbing up and down, I imagine the ride of the condor machine to be perfectly smooth with the exception of the need to flap occasionally and for short duration as needed to gain initial altitude and extend glides between lift sources.
The limitation is that straight line flight from point A to point B would be more difficult to manage and less predictable. But given a machine that is capable of extended flapping flight when required, and a willingness to put up with the rough ride occasionally, this shouldn't be a problem.