Who doesn’t want to fly like a bird in open air, with nothing but wings and a jet on your back? A few—like Swiss pilot Yves Rossy (see video below), whose self-designed rigid wing comes complete with jet engines—have succeeded, at least after a plane gets them up to the proper altitude. Now German researchers at ESG Elektroniksystem and Drager Aerospace are working on the jet-pack fantasy with the military in mind. They are developing a high-tech rigid wing with jet pack, the Gryphon, for the ultimate sneak attack: paratroopers with the maneuverability of a fighter plane.
Wearing the jet fuel–filled, 110-pound Gryphon, paratroopers would hop off a plane at 33,000 feet and head for their destination. Air rushing across the 5-foot 9-inch wing would create lift, while drag slows and gravity tugs downward. After a few moments the engine would kick in. At that point, the fight between maneuverability and stability—a perennial challenge of aerospace design—comes into play. Too much stability makes turning difficult. Too much maneuverability makes the device hard to control, imperiling the flier. To solve this, Gryphon designers are planning to add a computerized steering system that adds stability while allowing the pilot enough control to roll and turn, much like systems already used in fighter planes.
Of course, the ultimate jet-pack fantasy doesn’t include jumping from a plane. But no winged jet pack yet exists that offers true ground-to-air flight. A plane’s rapid movement down a runway creates the aerodynamic boost that lifts it into the air. To lift off from a standstill, a birdman would need enough upward thrust to overcome his own weight. “They would have to strap themselves to a rocket,” says Tom Benson, a NASA engineer who has studied the history of flight. “That, or a very, very powerful jet engine.”
Although the Gryphon is conceived as an improvement over the military parachute, its designers also have an eye on the extreme sports market—and a computerized steering control system might be just what would-be jet packers need. For now, though, if you want to fly like a bird—complete with climbing, swooping, and diving—you’d better be either a German paratrooper or a daredevil inventor with skills to spare.
Should Newton Get Credit For the Jet Pack? While the exactitudes of flying are complicated, the basic description of flight is pretty simple: Gravity pulls you down, lift pushes you up, thrust moves you forward, and drag holds you back. The key element is lift, the only force that can free us from the surly bonds of gravity. Typically, lift was explained by Bernoulli’s principle, which says that because air moves faster over the top of the wing than the bottom (the bottom of the wing is straight while top of the wing is curved, so air going over the wing goes futher in the same amount of time), the pressure of the air over the wing is lower. The low pressure above the wing "sucks" the wing (and, hopefully, the plane) up, the same way that low pressure in your mouth pulls soda up a straw.
But many physicists believe that this traditional textbook explanation is incomplete. Looking at flight from the perspective of Newton's laws of motion, the main function of the wing is to push air down. Wings are tilted backwards, so as air pushes into the surface of a wing in flight (the wing actually pushes into the air, but it's the same effect), the air is redirected somewhat downward. Newton's third law ("equal and opposite reaction") says the air getting pushed downward pushes the plane upward. That—in addition to the Bernoulli effect—gives a plane lift.
The Newtonian explanation is why we need a jet in jet pack. All wings can do is slow a fall. But with a forward boost from a set of rocket engines, the wings bump into more air, pushing more of that air down, which pushes the jet pack (and, hopefully, the person wearing it) up. That’s why Yves Rossy’s wing includes four rocket engines, and why the Gryphon will eventually be rocket-powered.
