Wednesday, July 20, 2016

Airplane Aerodynamics don’t Apply to Insects

What is surprising is that drag is barely relevant with small flapping wings.  Thus we have a linear response to application which is great.  This also explains why flapping has been conserved in insects particularly for so long.

 Applying this to  bird dynamics suggests that our dismissal of very large birds who flap their wings  may be a real mistake.

In fact the absurd idea of humans flying with flapping wings may not be absurd at all.  Accomplishing that mechanically is obviously a challenge but may well be possible today.  The birds have shown us how and an effective power assist is to particular trick.  Add in titanium feathers and it may be doable.

Airplane aerodynamics don’t apply to insects

Posted by James Devitt-NYU July 11th, 2016

Flying insects don’t obey the same laws of aerodynamics as planes do, new research finds. This could explain why even the best pilots can’t match the insects’ maneuvers.

“We’ve known for quite a while that the aerodynamic theory for airplanes doesn’t work so well in predicting the force of lift for flapping wings,” says Leif Ristroph, an assistant professor at New York University’s Courant Institute of Mathematical Sciences who directed the study. “We found that the drag or wind resistance also behaves very differently, and we put together a new law that could help explain how insects move through the air.”

“To double its flight speed, an airplane must increase its thrust four-fold to counter the stronger wind resistance,” Ristroph explains. “In contrast, we found that flapping wings have a drag that is in direct proportion to its flight speed—to go twice as fast, an insect simply needs to double its thrust.”

The significance of aerodynamic drag and its strong increase with speed has been known since before the Wright brothers took flight. This fact is summarized by a mathematical law that posits wind resistance increases as the square of speed; hence, moving twice as fast requires four times the thrust to overcome the higher drag.

Previous studies of flying insects, which beat their wings hundreds of times a second, suggested that these creatures do not obey this same relationship.

To make this sense of this discrepancy, the researchers in the Applied Math Lab built a robotic wing apparatus for measuring the motions, flows, and forces. The apparatus allowed the team to compare steady motions of a wing, as would occur for airplane flight, to the maneuvers of insects, in which their wings flap as they move through air.

The team’s results show that the back-and-forth motions cause the drag to resist the movement in some instances; however, at other times the drag is actually directed forward, more like a thrust. The net force that results depends on the flight speed as well as the flapping motions, all of which the authors include in a new drag law.

This law may not be news to insects, which have been flying with flapping wings for hundreds of millions of years. But the authors think that their findings could help guide the design of tiny flying robots that mimic the wing motions of insects.

The study appears in the journal Physical Review Fluids.

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