Chapter 3: Figure and Ground
52 - Ride the Wind (Part Two)
His colleague T. K. Mueller picks up the conversation as he demonstrates an experiment set up at another wind tunnel. Here the matter at hand is stall. One of the most dangerous times for aircraft of every kind is the point at which a plane is landing and loses speed. It’s then that the balance between the high- and the low-pressure zones above and below its wings can become unstable and cause a stall. If that occurs the plane can drop abruptly from the air—something that has happened in a number of commercial air disasters.
Showing film of a bird landing, Mueller points out how, as the bird slows, a separate layer of feathers on the top surface of its wings begins to lift. The layer forms a gentle, rippling pattern. By turning on the wind machine and gradually angling a wing in front of it, he produces the same effect.
What happens in a stall, he says, is that “if the airspeed gets too low or the angle of attack too steep,” the lift that comes from air moving across the top of the wing weakens as that air fails to reach all the way back to the wing’s trailing edge. When this happens, an eddy begins to creep up from that edge, advancing forward along the top of the wing. The eddy works its way beneath the air that’s rushing back, separating that airflow from the wing. “When that flow is lifted off the surface,” Mueller points out, “it does not produce lift anymore.” As a result, a feedback loop forms in which the eddy propagates faster and faster and the lift gets weaker and weaker until all lift fails. “Most airplane accidents,” he says, “happen because of a stall.
“Now we look at birds,” he continues, “and we find that they have a solution.” Pointing to the rising layer of feathers along the tops of bird wings, he calls them “smart” devices. “They sense when the eddy is spreading forward,” he says, “and as that happens, they lift up, to separate it from the air coming back from the front edge. So there is a wedge effect going on.” A slender, rippling wave of feathers rules out a failure of lift.
The Rechenberg lab has worked with the German glider company Stemme, which tested a similar system of flaps on its plane wings. It worked, says Mueller. “We have tried out the flaps and the pilot says that he could not get into a stall that easily. He went up to one thousand meters and tried to stall or tried to tumble, and he felt a real difference.”
As Mueller talks, far above his head a clear plastic tunnel that is about three feet in diameter and filled with water can be seen extending across the lab. Another scientist in the group, Rudolf Bannasch, propels life-size models of penguins through it while filming them. Says Bannasch, “We have studied the energetics of penguins swimming in the Antarctic. These animals spend most of their lives in the water of the cold ice sea. They had to learn how to use their energy in an ergonomic way—how to spend as little as possible for swimming.”
Bannasch has been looking at how the penguin’s body contours affect the water in which it swims. And he has found something quite similar to how the stall effect occurs on wings, although in this case it works to the animal’s advantage. Rather than being one smooth contour from head to tail, a penguin’s body has small undulations—subtle ridges around its circumference—which raise the speed of the water passing over them. Behind those undulations the passing water slows and curls in on itself. This creates a roller effect that separates the water flowing past it from the animal’s body, dramatically reducing friction.
Penguins feed on krill, a small crustacean, Bannasch says. “We analyzed their energy consumption and found that they need only one kilogram of krill to travel about a hundred and thirty kilometers in the sea.” That’s the equivalent, he translates, of using a liter of gasoline to travel about fifteen hundred kilometers in that water.
Back at his desk, Bannasch displays with some pride a photo of an athlete dressed in brightly colored shorts and T-shirt, standing next to an unusual-looking bicycle. It’s enclosed in a streamlined white shell that looks oddly like a penguin. The bike is a racer, and the light fiberglass shell was formed in a Mercedes-Benz lab. They have raced it, he says, and with impressive results.
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