Contradictory Screws

Like one of the fastest airplanes that ever flew,/- the two-man Japanese submarines which attacked Pearl Harbor had contrarotating propellers --two tandem screws placed close together, turning in opposite directions. Blades on the second screw are pitched counter to those of the other screw so that both thrust in the same direction. Such devices for U.S. warplanes were announced last week to be "in the developmental stage" by Curtiss-Wright, United Aircraft, other plane builders.

Purpose of this mechanism in planes (and submarines) is to overcome torque or sideways twist created when 1) the air's resistance to the rotating screw makes the engine tend to rotate the plane itself, 2) the whirling air stream behind the propeller hits the lifting surfaces at a skew angle. Torque must be counteracted by ailerons and rudder, especially in small planes whose bodies--like those of small submarines--do not in themselves provide enough stable ruddering.

Even when thus counteracted, torque is bothersome because displaced ailerons and rudders are not as efficient or powerful as controls in normal position. Contrarotating propellers develop two self-canceling torques and, in pursuit planes, increase the maneuverability vital in dogfighting.

Forward thrust, engineers formerly thought, would also be increased, since the second screw would bite into an already moving air stream. But wind-tunnel research at Stanford has shown only a disappointing 2 1/2% increase in thrust efficiency at low speeds, an actual loss of efficiency at higher speeds.

Chief disadvantage of contrarotating propellers is maintenance: warplanes, landing on small unlighted fields by night, snub and flatten their noses all too often, and the propellers' intricate gearing mechanisms are hard to repair. But as motors become ever more powerful, torque--and the need to overcome it--becomes an increasing problem.

Because he believes propeller speeds are nearing the upper limit of possible efficiency, an Italian engineer, Secondo Campini, has invented the first successful plane in aviation history to be propelled by a jet of compressed air. So reports The Aeroplane, a British journal which reached the U.S. last week.

Campini's plane sucks air into its cavernous nose, compresses and heats it in the fuselage, ejects it through the tail--a rocketlike principle, though it is not a real rocket plane, since the propelling jet is not a rush of gases supplied by combustion of the fuel. Campini's plane weighs 11,000 lb., recently flew from Milan to Rome (300 miles) in 2 1/4 hours. Average speed was only 130 m.p.h., but the plane is a pioneer venture.

/- An Italian single-motored (3,000 h.p.) sea plane which attained 440.67 m.p.h. in 1934. This record stood until 1939, when a German plane hit 469.22 m.p.h.

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