The Bernoulli Effect and Flying Slowly

We have marveled at the ability of birds to reach incredible speeds and make quick turns. However, the greatest challenge is flying slowly. The wing shape allows flight, as air traveling along the top surface moves faster than air on the wing bottom. This is an application of a principle of physics known as the Bernoulli effect.

The Bernoulli effect says that when air moves rapidly, it exerts minimum pressure at right angles to the direction of the motion. You can demonstrate that with a simple experiment. Place two identical books on a table with a small space between them. Lay a piece of paper over the top of the two books and blow through the channel between them. The paper will collapse because the pressure of the moving air is lower than the air pressure above the paper. Blow hard, and the paper will collapse rapidly. Blow slowly, and the paper will bend down a little.

Airplane wings use this principle to get lift. As the plane moves faster, the lift increases, and the plane can rise. When the plane slows down, there is less lift, and the plane decreases its altitude. Birds do the same thing but have a unique design feature that allows slow flight. This special structure called a bastard wing or alula enables them to stay aloft even when flying slowly.

The alula consists of several feathers attached to the first digit of the wing bones. By moving that digit, the bird can separate the feathers of the alula from the rest of the wing, creating a slot that helps channel air over the wing. This enhances lift and allows the bird to stay aloft when flying slowly, such as when landing. When a bird seems to hover, the Bernoulli effect created by the alula design gives them that ability. Humans have not found a way to duplicate all the designs of the bird wing, so hovering is still a challenge for modern aircraft.

Much of what engineers know has come from studying the design God built into His creatures. Birds continue to teach us a lot about flying in various applications, including slowly flying. (See Romans 1:20.)

— John N. Clayton © 2024