I Didn’t Know That . . .

Q: If the shape of an airplane’s wing provides lift, how does a plane fly upside-down?

A: The wing’s shape is important, but it is not the whole story, said aeronautical engineer Russ Cummings of Cal Poly San Luis Obispo. The wing’s angle of attack--its angle relative to level ground--is also important. For maximum lift, a positive angle of attack is required; that is, the front of the wing must be higher than the rear. By analogy, if you stick your hand out the window of a moving car and tilt it upward at an angle of 45 degrees, you can feel the lift. When a plane is upside-down, the wing can still provide lift if the angle of attack is greater than 4 degrees, according to Cummings, but the process is not very efficient.

Inverted flight can cause other problems. If the engine’s fuel intake is located at the bottom of the gas tank, for example, the engine can stop because it will be sucking air when the plane is upside-down. Planes that perform acrobatics require special fuel systems to keep the flow going.