Tell Me Why Airplanes Fly
The Kid's Answer
Akshay (left), age 5:
Because an airplane has wings like a bird. The wings don't flap, so the engine has to push.
The Scientist's Answer
Turns out Akshay's on the right track. Sort of. "This is very difficult to explain to people who are not aerodynamicists," says John Anderson, curator of aerodynamics at the National Air and Space Museum in Washington, DC. "Airflow over the wings creates a force on the wings called lift. That force lifts the plane up. The engine keeps the airflow going."
So how does airflow cause lift? The physics of drag and thrust do come into play, but here's the simplified answer: As the engine pushes the plane forward, it creates wind moving over and under the wings. The airflowing over the top is being pushed up against the atmosphere, and the atmosphere resists, effectively "pinching" the air between it and the wing and causing it to move more quickly than the air below the wings. (It's just like a garden hose: Ordinarily, the water moves through a hose at a steady rate; squeeze it, though, and the water is forced out faster.) The faster the air moves, the more the pressure on top of the wings drops.
Meanwhile, below the wings the air is moving slower and the pressure is increasing. As the plane speeds down the runway, that difference in pressure increases until presto — you have lift.
How to Explain It to Kids
The concept of pressure differentials may be beyond the grasp of an average 5-year-old, but he's felt the force of the wind. Remind him that an airplane uses engines to make its own wind, which streams over and under its wings, lifts the plane into the sky, and keeps it there.
To feel the principle of lift in action, try going outside on a blustery day. Have your children face into the wind and hold their arms out, palms down, like wings. When a good gust comes along, their arms will be lifted up a little (just as when you stick your hand out the window of a moving car — not that you should do that, of course). Unless you're Michael Jordan, this is about as close as you can get to unassisted takeoff.