Many books use the Bernoulli principle to explain airplane flight. These explanations show diagrams of airplane wings that are flat on the underside, but convex on the upper side. The diagrams typically show that air moving across the top of the wing has a further distance to travel than air moving across the bottom of the wing, and conclude that because air moving across the top of the wing must travel faster, the air pressure on top of the wing is less than the air pressure on the bottom of the wing.

There are several problems with this idea:

1. Real airplane wings are rarely shaped as the diagrams show. Most airplane wings are curved on both sides. In fact, in the case of delta-wing planes, the wings are essentially flat.
2. If the Bernoulli effect were all that held a plane up in the air, then what would happen if a plane flew upside-down, as stunt planes sometimes do? The Bernoulli model predicts that lower pressure on the surface of the wing that is normally the upper surface would now be drawing the plane earthward, and it would fall out of the sky. According to this model, upside-down flight is impossible!
3. In order to create enough lift to hold the plane in the air, very large planes would have to have wings with highly convex surfaces. Yet we see that even wings of jumbo jets are only slightly curved instead of resembling hills.
4. Each time the plane accelerated, the Bernoulli effect should cause it to rise because of faster airflow over the wing. Yet this doesn't happen.

While it's true that air does move over the wing and does create an airfoil effect, this isn't enough of a force by itself to cause lift.
What contributes more substantially to lift is the force of the air diverted downward by the wing. According to "How Airplanes Fly: A Physical Description of Lift" the total lift of the wing is directly proportional to the amount of the air diverted downward times the velocity of that air. If you think of a helicopter blade as a long, thin wing, you know what happens when you stand under it: you feel a massive rush of air downward! The equal and opposite reaction (Newton's third law) pushes the helicopter up in the air. A wing "catches" air in a way similar to how a sail catches air. The actual forces around an airplane wing are complex, but the net effect is a downward rush of air. Pilots change their elevation (go up or down) by changing the shape or angle of the wing, which changes the angle of attack and increases or decreases the downward rush of air.

Objectives:

1. Students will be able to describe Bernoulli's Principle.
2. Students will be able to give examples of Bernoulli's Principle in toys and technology.

Vocabulary:

Bernoulli's Principle: For a fluid undergoing steady flow, the pressure is lower where the fluid is flowing faster.

Materials needed for all activities:

• Bernoulli Blower (or hand-held hair dryer)
• various balls
• 10 ping pong or small styrofoam balls (one for each group)
• 5 funnels (one for each group)
• alcohol wipes (to clean the funnel)
• 10 ring stands or other supports (2 for each group)
• 5 index cards (one for each group)
• 5 large wooden spools (one for each group)
• 5 straight pins (one for each group)
• thread
• string
• tape