Word: shocks
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...twelve-year-olds had never really seen their home town before. For weeks, they had peered and poked about Houston like so many junior sociologists in bobby-sox and sneakers. The things they reported were enough to shock some Houstonians. But Social Studies Teacher Marian Hiller of the Stonewall Jackson Junior High School was delighted...
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...
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...