When's the last time you caught a ride in an autogiro?
- By T.A. Heppenheimer
- Air & Space magazine, March 2003
(Page 2 of 5)
Pick any airliner flying today: It’s probably powered by turbofan engines. With their gaping intakes and enormous, multi-blade fans, turbofans revolutionized aviation during the late l960s, propelling aircraft of undreamed-of size: the Lockheed C-5A and Boeing 747. They were able to do so because the great size of the fans—some are as large as 10 feet in diameter—enabled them to move huge quantities of air, increasing the engines’ thrust.
The turbofan also offers superb fuel economy. Most of the air pushed rearward by the fan bypasses the engine’s combustion chamber and flows at relatively slow speed, increasing the engine’s effective thrust without burning additional fuel. But after l980, when the price of oil had climbed higher than the aircraft it fueled, General Electric sought to develop engines that would push the fanjet’s efficiency still further.
Existing versions enclosed the fan within a cylindrical housing, or cowl. The cowl slowed the airflow to prevent the tips of the fan blades from rotating faster than the speed of sound, a condition that would have made the engine unacceptably noisy. Just as supersonic aircraft create sonic booms, fan blades spinning supersonically would cause their own loud disturbances in the air.
The cowl also held thrust reversers, clamshell-like structures that reversed the flow of the jet exhaust, directing it forward so it could be used for braking during landings. Making the fans larger would have also required larger cowls, which would have been heavy and difficult to install and would have needed even heavier thrust-reversers.
GE thus put its hope in an unducted fan—a fan without a cowl. Its blades were swept back to reduce the drag from supersonic rotation, and, like reversiblepitch propellers of earlier days, the fan blades swiveled to direct thrust forward during landing. The GE 36 UDF engine produced 15,000 pounds of thrust and weighed only three tons.
In NASA-sponsored ground tests, the engine consumed 20 percent less fuel than the standard turbofan, but flight tests showed that the unducted fan was unacceptably noisy, even in versions scaled down in power for the modest-size airliners of the McDonnell Douglas MD-80 series. For larger aircraft, the noise problem would have been far worse. What really killed the unducted fan, however, was the fall in oil prices in the mid-1980s.
Biplanes flew for decades with great success, but fell by the wayside when they failed to match the speed of monoplanes. The autogiro, a rotary-wing aircraft, suffered a similar fate. A Spanish inventor, Juan de la Cierva, flew the first of them in 1923 and introduced an entirely new mode of flight.
The autogiro was based on the conventional monoplane: It sported stubby fixed wings and a front-mounted engine, but it had a large, unpowered rotor, which turned in the craft’s slipstream and yielded additional lift. An autogiro could fly with a short takeoff run and land nearly vertically, its rotor whirling like a pinwheel as it descended.