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.
Cierva formed a partnership with U.S. airplane builder Harold Pitcairn, who also ran a lucrative airmail route (which he sold to a business that developed it into Eastern Airlines.) Amelia Earhart flew one of the Pitcairn-Cierva craft and praised its safety and ease of control.
But another aircraft was under development in the 1930s that would outshine the autogiro, and by the end of the decade, Igor Sikorsky had mastered its design. A helicopter, which had a powered rotor, could hover, take off and land vertically. Even though the autogiro was faster, the helicopter’s eventual success in military operations proved to be so sweeping that the autogiro vanished entirely.
Custer Channel Wing
There’s something valiant, if clown-like, about the Custer Channel Wing. The semi-circular troughs on either side of its fuselage are suggestive of the oversized ears on Disney’s cartoon elephant, Dumbo. But these channels, through which air was sucked at high speeds by propellers, also created enough lift to bestow on the aircraft Dumbo’s amazing capability for short takeoff and landing (STOL). The aircraft could take off in as little as 150 feet.
The Channel Wing was the lifelong obsession of Willard Custer, a distant relative of the general who died fighting the Sioux at Little Big Horn. Custer flew his first version in 1942 and submitted it for military evaluation, but the Army saw no reason to pursue it. Undaunted, Custer sought backers for a civilian aircraft. A 1953 version flew successfully at speeds as low as 22 mph, and actually hovered in an 11-mph wind, but Custer proved a more talented inventor than financial manager. Although his aircraft never went into production, Custer does have a place in aviation history. His first Channel Wing is in the collection of the National Air and Space Museum, and his third can be seen at the Mid Atlantic Air Museum in Reading, Pennsylvania.