[3] While at MIT, Caldwell and fellow student Hans Frank Lehmann designed a contest winning glider.

[4] At that time, MIT did not offer courses in aeronautics, yet working with Hans Lehmann, Caldwell titled his graduate thesis "Investigation of Air Propellers.

[3] In Ohio, Caldwell designed the whirl test by mounting the subject propeller on a fixed stand to measure thrust, endurance, speed, efficiency, and structural strength.

The controllable-pitch or variable-pitch propeller tied together the major aeronautical advances of aerodynamic drag reduction and increased engine power.

[3][8] In 1933, the most modern aircraft in the world, Boeing's Model 247 operated by United Air Lines struggled to reach 6000 feet.

After adding Hamilton Standard's hydraulic two position controllable-pitch propeller developed by Caldwell, the Model 247 entered transcontinental service over the Rocky Mountains.

[3][11] Almost all United States Army Air Forces aircraft in World War II used hydromatic constant-speed propellers.

[2] He later married Majorie Snodgrass (1897–1976), their son Frank Walker Allen Caldwell (1934–1962) was born in West Hartford, Connecticut.