At-Home STEM: Bernoulli vs. Coanda

This week we’re looking at different aspects of flight and ways that we can explore those at home. We’ll start with lift and how air flow helps things like planes get off the ground.  

This black-crowned night heron uses both flapping and gliding flight. Photo by J Van Cise Photos

You may have watched a bird flying across an open area and noticed that sometimes the bird flaps its wings and other times it glides with wings stretched out. The flapping helps the bird move forward faster, while the gliding tends to involve more motion up and down in the air column. Airplanes use the same method for flying- engines move the plane forward while the wings influence movement up and down. So, how does a plane get off the ground? You may have heard that it’s connected to Bernoulli’s Principle and air pressure, and part of that is correct, but not all of it. 

In the 1700s Swiss mathematician Daniel Bernoulli found that changing the speed of moving water and air changed the pressure as well- the faster air or water moved, the lower the pressure it had. Want to see how this works at home? You can demonstrate changing air pressure with some items that are probably easy to find.  

1. Hold a piece of paper between your hands in front of your mouth and blow above the paper (not into the paper- this is important)- you will see the paper start to lift up as the faster air above drops in pressure and the air below pushes up.  

2. Take two inflated balloons (if you put a little water in each one, it will help steady the balloons) and tie an equal length of string to each (You could also tape string to ping pong balls, whiffle golf balls, etc.)- hang them side by side with about an inch between them, and then blow into the space between the balloons. You will see them move toward each other as the faster air in between drops in pressure and the air to the sides pushes in. Now blow on the other side of a balloon and notice how it moves away from the other balloon.  

Earlier hypotheses about flight thought this was how an object got off the ground- you got the air on one side to move faster and the rest of the air pushed the item in that direction. When the area of low pressure was above an object and the rest of the air pushed that object up, lift would be created. 

That sounds good, but it’s not quite how things work- rather than speed creating a difference in air pressure, a difference in air pressure changes air speed. And then the wing itself changes the direction of movement. An airplane’s wing is designed to have a curved upper surface, while a bird can change the shape of its wing in response to flight conditions. That curved upper surface is a longer distance to travel than the underside of the wing, meaning that the air above can spread out over a larger volume, which decreases air pressure. This, in turn, makes the air travel faster above than below the wing. Because of the Coanda Effect (named after Romanian inventor Henri Coanda), a stream of gas or water will follow along a curved surface (like a wing) rather than flowing out straight behind it- this means that the air running along the top of the wing continues down from the edge of the wing and the airplane gets pushed up by lift. Engineers can increase or decrease lift by changing the curve of the wing. The plane is still flying because of a combination of air pressure and speed, but it’s a different set up than you might have heard. 

Diagram of different airfoils from NASA

If you want to get a sense of how engineers work to create wings for different kinds of aircraft and different styles of flying, NASA has free software for simulating how different wing and flight qualities impact lift.