How birds turn (and free wings)
How birds turn
(and free wings)
...Airplanes and gliders have vertical tails, birds do not. So how do they turn?
...Some years ago already an American scientist, a paleontologist, had built a flying model of a pterodactyl, in order to try to assess the animal's ability to stay in updrafts. It is known that the head of these critters was extended by a flat bony crest.
Our man had deduced that this must serve it as a vertical tail and he made it steerable, to ensure, with the help of a remote control, the piloting of his model. But in doing so he forgot that bats are very agile in flight and perfectly capable of turning very tightly, whereas they have no bony crest, nothing but two membranous wings. So where was the trick?
...When men took flight hanging from the first Rogallo wings, piloting was done entirely by shifting the center of gravity. By pulling or pushing the control bar of the trapeze, pitch control was ensured. There, no problem. The first Mantas had "fixed crossbars". To turn, the center of gravity had to be shifted. Turns were always performed in a sideslip, with a strong increase in the sink rate (from 2.5 meters per second in stabilized horizontal flight to 3.5 m/s in a turn, even a shallow one).
The first "Mantas". Flying in the upright position.
**...**A pioneer, whose name I can't recall, had the idea of detaching the crossbar from the keel. A link rod (see drawing) allowed a certain lateral movement.
...The result: an extraordinary gain in maneuverability, was immediate. When the pilot shifted his weight to the side, dragging the keel, which was attached to the trapeze, along with him, he deformed the wing. The drawings show this effect in a very exaggerated way. But one can easily imagine the result. By flattening one of the "lobes" by a centimeter, while the other hollowed out by as much, everything happened as if both wings had been fitted with ailerons running along the entire trailing edge. The flying comfort changed completely and safety increased.
...Let's return to the bat, which also has flexible wings, with a free trailing edge, which are simply attached to the arm and to one of the animal's hypertrophied fingers. What does it do when it wants to turn? It simply extends one of its wings while slightly folding the other. Same effects on the trailing edges. No need for a vertical tail.
...Birds turn in a similar way. Those whose wings have high aspect ratios, like albatrosses, use "reverse effects". The feathers are connected to muscles. Thus the bird's entire wing surface is variable-geometry and constitutes a very complex system. These are "multi-slotted" wings that allow air to flow from the lower surface to the upper surface, which causes "boundary layer blowing" and allows flight at high angles.
...When a glider pilot makes his landing approach, he deploys what he calls his "airbrakes", which are also "lift destroyers". Birds partially fold their wings. You have doubtless seen a bird of prey on approach, its two wings half folded.
**...**In short, we haven't invented much. Hang gliders already existed in the time of the dinosaurs. They were called pterodactyls.
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