by Victoria Robinson, Scholastic Science World, January 1997, vol. 53, No.8
Stephanie Malby's airplane plummets, spiraling to the ground. The faster it falls, the louder its engines roar. Has Malby lost control? Just when the plane seems certain to crash, it swooshes upward. Whew-- that was a close one! Or was it?
Like all stunt--or aerobatic--pilots, Malby flies for thrills. "When you see the world right-side up, then upside down, then right-side up again," she says, "it's exhilarating, no matter how many times you do it."
Air shows featuring stunt flyers are a huge hit these days. With 27.4 million people attending in 1995, air shows have soared into second place behind baseball as the country's most popular spectator sport.
It's certainly not the safest. In 1995 three U.S. daredevil pilots died in air-show stunts. Last year five pilots lost their lives.
But aerobatics pilots know the dangers and can minimize the risks with years of practice and an understanding of the physics of flight. When you understand the science behind their moves, you could be one step closer to piloting yourself!
The first "trick" for any pilot is getting the plane in the air by overcoming gravity, Earth's downward pull on all objects. To go up, the plane must go forward. Its engines provide thrust, the backward-pushing force that propels the plane down the runway. At takeoff speeds of 70 knots (80.5 mph), air rushes over and under the plane's wings.
As air sweeps across the wings' curved upper surface, it speeds up. The fast-moving air creates an area of relatively low pressure above the wings. The higher pressure beneath the wings exerts an upward force called lift, which nudges the entire aircraft into the air.
But to thrill an airshow audience, stunt pilots have to do far more than fly straight. The loops, dives, and rolls, after all, make spectators "ooh" and "ah." The secret to performing these tricks is to redirect the forces pushing on the plane by changing the way air flows over the wings.
The maneuver can be scary. "When you roll, you lose lift and the airplane will start to fall out of the sky," says aerobatic pilot Dennis James. To compensate, aerobatic pilots make allowances for these slight "falls."
At the beginning of a roll, for instance, the pilot pulls up the nose about 10 degrees by flipping up hinged panels called elevators on the plane's tail. Like ailerons, elevators change the flow of air to make the plane climb or descend. With the nose up entering a roll, "you can let the plane fall and end up at your original altitude," says James.
Another popular trick, the loop, is just an extreme climb--so extreme that the plane loops over backward. The pilot just keeps the elevators in the up position. "Once you're upside down," says Malby, "gravity pulls you around the rest of the way."
When a plane and pilot are standing still, Earth's gravity pulls on them with a force of "1G." But when either speed or direction changes dramatically--as in, say, a corkscrew turn--the plane and the pilot experience a stronger pull.
Some aerobatic pilots, like Cecilia Aragon, may experience a pull of 8 to 10Gs! Since weight is determined by the pull of gravity, during those few seconds, Aragon feels like she weights 8 to 10 times her body weight--or like nine people her size are standing on her shoulders. "It always feels to me like a giant hand comes down and--boom!--clamps your chest," she says. "It's as though you're slammed into your seat. It becomes hard even to lift your hand off your lap." So imagine what it's like to try to steer a plane!
All that "extra" gravity also pulls on the insides of the pilots' bodies. It makes blood "pool" in their legs. "Pulling" Gs is "like shaking a thermometer to get the mercury down into the bulb," says pilot James. "Picture doing that to a human being."
So how do pilots handle Gs? Fighter pilots, who experience high Gs when training to evade enemy planes wear "G-suits." A G-suit has air bladders, or pockets, that inflate to squeeze the legs and abdomen so blood doesn't pool. Some military stunt pilots even wear G-suits for shows.
But civilian aerobatic pilots don't. These pilots usually don't endure high Gs for more than a few seconds at a time. Instead, to fight the effects of Gs, they "grunt and groan and stress and strain," says James. In other words, they flex their leg and stomach muscles during high-G maneuvers to keep their blood from pooling. "You have to pretend you're a contender for the World Wrestling Federation and pump yourself up," James says.
For aerobatic pilots, the thrill of performing for cheering crowds makes the physical strain of stunt flying worth it. So next time you watch a stunt-flying show, give a cheer for the pilots--and the physics you see taking flight.