Illustration: jason lee

When is a flying car not a flying car? When it’s a Transition and when its maker, Terrafugia, insists on calling it a “roadable aircraft.”

Think of it as an airplane prototype suited for limited operation on land, the Cambridge, Mass., company says. In other words, do your best to imagine an eccentric flying machine that only occasionally moonlights as a car.

Like platform shoes and plastic flamingos, the convertible car–airplane concept just won’t go away. During the past century, dozens of models have emerged, including one capable of vertical takeoff and one with bolted-on wings that pilots detached and carted along in a trailer. None have risen above the status of historical footnote.

Here’s the problem: it has proven to be virtually impossible to craft a light, agile plane that also handles well on the road. Basically, a car is safer and more stable when it’s heavier, while a plane flies better when it’s lightweight.

So far, Terrafugia has completed a detailed design and is now trying to raise funds to build a prototype by 2008. To hide the fact that the Transition is a flimsy car and a feeble plane, Terrafugia designed the vehicle to weigh just under 600 kilograms, the cutoff for the U.S. Federal Aviation Administration’s new light-sport aircraft class. Such planes are to be flown only in good weather at low altitudes and relatively slow speeds, and Terrafugia hopes that in this class, the Transition’s limitations won’t stand out.

In Terrafugia’s simulations, the Transition looks like a stubby little plane affixed to a four-wheel platform. At the push of a button, the wings fold and tuck in alongside the body, sort of like the wings of a perched eagle. With bumpers on both ends, the car-shaped body would cause the Transition to experience more drag and fly at slightly lower speeds than other light-sport aircraft. Unlike those on normal planes, its wings would be flat on the bottom so that they could fold completely—which reduces the Transition’s aerodynamic performance.

As a car, however, the Transition fares far worse. Although 600 kg isn’t unreasonable for a light prop plane, it is decidedly wispy for a car. For comparison, a Mini Cooper, one of the smallest four-seat cars that is widely available in North America, weighs almost twice that much. A strong gust of wind could cause the Transition to fishtail. To make things worse, its folded wings would create mammoth blind spots on a vehicle the length of a large pickup truck and would be easy targets for fender benders.

Terrafugia says that in car mode, the Transition would typically be used only to drive between home and airport. Still, that might be enough to exasperate its owners. “We have become very demanding when it comes to safety issues in cars,” says C.P. (“Case”) van Dam, a professor of mechanical and aeronautical engineering at the University of California at Davis. “We expect comfort and high performance.” The same expectations have shaped pilots’ preferences in small aircraft, van Dam notes, adding, “If you end up compromising in both areas, do you really satisfy a large enough group of people?”