View Full Version : Could we rotate rotors and fly like bees?
11-28-2005, 12:45 PM
11-28-2005, 02:14 PM
However, a tiny "aircraft" such as an insect is far more affected by issues of boundary layer than a big one. The difference is so great that it's almost as if there's a different aerodynamics for tiny fliers. We see the same effect, but less dramatically, in the differences between model helicopters/gyros and the real thing.
The air molecules right next to a wing cling to the wing's skin. The air molecules a little farther away from the wing skin move past the skin as expected. In between, there's a layer of molecules that are moving at an in-between speed. They may also swirl and tumble.
This boundary layer is pretty thin on fast wings, and also on large-chord wings. The layer is huge (relatively) on teeny wings like those on a bee. Therefore, boundary layer takes up the whole neighborhood from a bee's perspective. You can see how the little swirls and eddies that are incidental on human-scale wings could amount to a whole different lift-making mechanism for a bee.
Scaling up a bee doesn't look too promising to me, just as scaling up birds didn't pan out. Even most large birds don't flap much, but rather are really self-launching sailplanes.
11-28-2005, 03:51 PM
It's true that scale is important in terms of the aerodynamics of a wing. The reynolds number is one way to quantify these effects. It is the ratio of inertial to viscous properties of air.
On the other hand, some effects , such as Von Karman vortices can be seen across all scales. The two pics below show von karman vortices around a spoon in a bowl of Jello and in clouds as they swirl by mountains.
The vortices are formed as pressure differences, created by an obstacle, force some of the air (or fluid) to move more quickly than the rest as it flows around the obstacle.
Recent studies show that insects borrow energy from the vortices that form around their wings during flight. Normally vortices just create drag. But insects can recapture some of their energy and use it to improve speed and maneuverability. Insects learn to rotate their wings before starting the return stroke, and the wings are lifted by the eddies of air created on the downstroke.
It may bee that vortex manipulation can soon be used by larger aircraft to improve lifting efficiency.
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