Science: Hybrid Aircraft

The helicopter used to be the airplane’s eccentric poor relation. It could do a few odd jobs (sea rescues, short-range shuttling), but its high cost and its lack of range and speed weighed heavily against its advantages. The Korean war turned Cinderella into a fairy princess. The helicopter’s ability to take off from anywhere and to land almost anywhere made it just the thing for evacuating the wounded, supplying isolated positions, carting specialists and brass around. Most recent and spectacular helicopter mission: landing a full battalion of marines with their weapons on a mountainous front-line sector (TIME, Oct. 22).

Last week all the services, especially the Army and Marines, were demanding more helicopters. But they still remembered the helicopter’s handicaps: lack of range and speed. Many experts believe that the ideal aircraft for assault, supply and evacuation will be a “convertiplane”: a hybrid that takes off as a helicopter, flies like an airplane, then drifts down to land like a helicopter again. Last April, the Department of Defense announced development contracts for convertiplanes to Bell, Sikorsky and McDonnell Aircraft Corporations. A fourth manufacturer, Gyrodyne Co. of America, which has no contract, claims that its convertiplane is the only model now actually flying.

Helicopter’s Handicaps. The chief trouble with a helicopter is the rotor. It enables the helicopter to rise vertically and to hover. But it wastes power (cutting the helicopter’s range to a third or fourth of a comparable airplane’s), and limits the helicopter to a top speed of about 140 m.p.h. There is no such limit to a convertiplane’s speed—if there is some sort of propeller for times when the craft is flying like an airplane.

The most obvious method, favored by Gyrodyne, is to put a small wing and one or more propellers on a conventional helicopter. After the craft is in the air, the rotor is disconnected from the engine and the propellers take over. The rotor continues to spin, driven by the air rushing past it, like the rotor of an autogiro.* This “windmilling” supplies some lift; the wing provides the rest.

Bell’s convertiplane will resemble a conventional airplane with helicopter rotors spinning above each wingtip. After it is in the air, the rotors will be tilted 90° forward, thus turning into propellers to fly the craft like an airplane. When the time comes to land, the rotors will return to the helicopter position. Bell believes that the changeover can be accomplished safely and in only a few seconds.

Jet Rotor. McDonnell will not tell what its convertiplane will be like. Sky-side gossip believes that it will have a rotor driven by some sort of jet. One possibility is small ram-jets on each blade tip to push the rotor around. Another is a central turbojet engine blowing hot gases through hollow rotor blades. The gas will escape as jets from one side of each blade tip, making the rotor spin. When the aircraft has gained enough altitude, the central engine will be used to propel it forward, supported partly by the windmilling rotor, partly by small wings.

Sikorsky, like McDonnell, will not tell what design it is working on. Igor Sikorsky points out that convertiplanes have many serious mechanical and aerodynamic problems that have not yet been solved. He believes that only moderate increases in speed and range are likely while the hybrid aircraft still has a whirling rotor to get in the way of the airstream. For many years, Sikorsky thinks, conventional helicopters will hold the ground that they have recently won. Eventually, perhaps, a convertiplane will be perfected that can retract its rotors completely while flying as an airplane. Such a craft, free of the rotor’s drag, might have very great speed and range. Presumably Sikorsky is working on this design.

*Predecessor of the practical helicopter, in vented in 1933 by Juan de la Cierva. It had a conventional propeller and a non-powered rotor that supported it by windmilling.