Return of the Prop

Designers of airplane propellers used to weep into their blueprints every time a jet plane roared over the plant. They feared that the fierce little jets, which have no propellers, were the wave of the future. These days the prop designers are more cheerful. The performance of Allison's XT-4O turboprop engine (5,500 h.p.) has made many airplane companies think once more in terms of propellers. The prop men are sure that propellers, drastically redesigned, can keep up even with the fastest airplanes.

Last week Hamilton Standard Propellers Co. told about its long, desperate, and, it thinks, successful efforts to bring propellers up to date.

Supersonic Tips. Even before the jet era, propellers were a headache. When an airplane is in flight, the tips of its propeller blades, describing a spiral through the air, reach supersonic speeds while the airplane itself is still lumbering along far below the speed of sound.

What made the problem serious was that anything moving at or near the speed of sound sets up shock waves (powerful sound waves) that interfere with the smooth flow of air. Beset by shock waves, the efficiency of a propeller falls off dismally. Extremely large propellers, geared to turn slowly, dodged the shock waves for a while, but as the speed of airplanes increased, even big props ran into trouble. When the turbojet engine came along, designers of speedy aircraft joyfully abandoned the propeller.

The turbojet engine, however, also has its troubles. A jet plane lacks power while taking off, when power is needed most. It is inefficient at low speeds, and therefore cannot save fuel by slowing down, as when "stacked up" over a fogbound airport. These faults were not so bad in the case of fast fighters, but when designers tackled jet airliners or long-range bombers, they missed the good old propeller, which gets a firm grip on the air at even slow take-off speeds.

Wasp Waists. Searching for a prop that could be used efficiently above 450 m.p.h., Hamilton's engineers, led by Chief Aero-dynamicist George Rosen, tried all sorts of shapes. One design, intended to sidestep shock waves, had curved blades, quite like the swept-back wings of a fast modern fighter. Another had a blade with a pinched-in "waist." Some blades were short and broad so that they could spin rapidly without nearing sonic speed. All these designs proved unsatisfactory.

Finally, instead of trying to keep the blades from tangling with shock waves,

Hamilton's prop men designed a thin, knife-edged blade of conventional, square-tipped shape that would move fast enough all along its length to leave shock waves behind. This did the trick. Tested in a wind tunnel, a scale model of the new propeller proved to be 80% efficient at 600 m.p.h. No shock waves roiled the air-flow over its smooth surfaces. Shock waves are not quick enough.

Hamilton's designers are sure that its new-style propellers will work efficiently even above the speed of sound. But their first jobs will probably be in subsonic turboprop fighters for use on carriers, where quick take-off is all-important. Other uses: long-range turboprop bombers and transports in the 4OO-500-mile speed range.

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