Most aircraft in the world have vertical tails or rudders to prevent Dutch roll instabilities, a combination of yaw and sideways movements with roll that somewhat resembles the movements of an ice skater. Unfortunately, a vertical tail adds weight and drag, reducing fuel efficiency in passenger aircraft. It also increases the radar signature, something you want to keep as low as possible in a military aircraft.
In the B-2 stealth bomber, one of the few rudderless aircraft, Dutch roll instabilities are addressed using drag flaps placed at the tips of the wings that can split and open, causing one wing to generate more drag than the other. others and thus stabilize the machine laterally. “But it is not really an efficient way to solve this problem,” says David Lentink, aerospace engineer and biologist at the University of Groningen. “The efficient way is to solve this by generating lift instead of drag. This is something birds do.”
Lentink led the research to better understand the rudderless flight mechanics of birds.
Automatic airplanes
When birds fly, there is almost constant turbulence: “When they fly around buildings, near trees, near rocks, near cliffs,” says Lentink. The leading hypothesis as to how they go about this in a seemingly graceful, effortless manner was put forward by a German scientist named Franz Groebbels. He argued that the ability of birds depended on their reflexes. When he held a bird in his hands, he noticed that its tail flapped down when the bird was moved up and down, and when the bird was moved left and right, its wings also responded to movement by moving asymmetrically to the left and right to stretch out. “Another reason to think that reflexes matter is to compare this to our own human locomotion: when we stumble, it is a reflex that saves us from falling,” Lentink claims.
Groebbels' intuition that bird reflexes are responsible for flight stabilization was later supported by neuroscience. The movements of birds' wings and muscles were recorded and found to be proportional to the extent to which the bird was thrown or rolled. However, the hypothesis was extremely difficult to test with a flying bird; all experiments aimed at confirming this hypothesis were performed on birds held in place. Another challenge was determining whether those wing and tail movements were reflexive or voluntary.
