Dr. Lewis' Limit

The world's airplane speed record has risen from 30 m.p.h. in 1903 to 440 m.p.h. in 1936. Last week for the first time man knew the point at which this progression must stop. In Manhattan Dr. George William Lewis, research director of the National Advisory Committee for Aeronautics, demonstrated to the Institute of Aeronautical Sciences that the highest speed airplanes can attain with present wing design is 575 m.p.h.

Presenting motion pictures taken in the NACA's new "superspeed" wind tunnel at Langley Field, Va., where airflow up to 750 m.p.h.* is possible, Dr. Lewis proved that at 575 m.p.h. the smooth flow of air over the wing top suddenly breaks away in a feathery "shock wave" which completely nullifies lifting power. Cause is an area of excessively compressed air, built up by the airfoil's passage. Same phenomenon occurs at the tips of propellers. Only chance for speed greater than 575 m.p.h. is a mechanism for changing wing and propeller shape when the plane reaches Dr. Lewis' limit.

* Approximate speed of sound.

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