Frank W. Caldwell (1889-1974)
Courtesy of the National Air and Space Museum Smithsonian Institution
As America's leading propeller engineer and designer during the aeronautical design revolution of the 1920s and 1930s, Frank Walker Caldwell (1889-1974) was a significant contributor to the development of propulsion technology. Caldwell oversaw the invention, development, and innovation of the metal, ground-adjustable pitch propeller and the hydraulically-actuated two-position controllable-pitch and constant-speed propellers during his tenure as the United States government's chief propeller engineer (1917-1928) and his work in industry (1929-1938). In the process, he pioneered the fundamental propeller testing facilities and techniques needed for successful engineering development.
A native of Lookout Mountain near Chattanooga, Tennessee, Caldwell received his mechanical engineering degree from the Massachusetts Institute of Technology in 1912. When the United States entered World War I, Caldwell joined the Propeller Department of the Airplane Engineering Division of the United States Army Air Service located at McCook Field in Dayton, Ohio, as chief engineer. He was responsible for the research, design, and testing of all aircraft propellers used by the army and navy during the war.
The war accelerated the development of aeronautical technology. As engine horsepower increased, the American military required heavier, larger, and more complicated propellers. It became clear that the wooden, fixed-pitch propeller, which was only efficient in one flight regime, would not be adequate. What was needed was a variable-pitch propeller, a propeller able to vary the angle, or pitch, of its blades to meet different performance regimes. Caldwell effectively set the tone for developing a new propeller that consisted of separate detachable blades joined to a central hub that would allow pitch adjustment on the ground. This ground-adjustable pitch propeller would be a crucial technology to the success of milestone flights such as Charles A. Lindbergh's solo transatlantic flight in May 1927 even though it was still essentially a fixed-pitch propeller.
Recognizing that the key to performance was the ability to change pitch while in flight, Caldwell joined the Hamilton Standard Propeller Corporation in 1929 to develop a controllable-pitch propeller. His hydraulic, two-position design provided efficiency at both takeoff and cruise, the two main operating regimes for the airplane. Performance tests revealed that Caldwell's invention maximized the performance of revolutionary aircraft such as the Boeing Model 247 and the Douglas DC-2. The National Aeronautics Association recognized Caldwell and Hamilton Standard for their achievement by awarding them the 1933 Collier Trophy.
Caldwell and Hamilton Standard went on to begin development of a propeller that changed blade angle automatically according to engine speed, the Hydromatic constant-speed propeller. A major feature of this new propeller was its ability to "feather," which positioned the blades to prevent propeller windmilling after engine failure. Virtually the entire air force frontline inventory during World War II, from multi-engine bombers to fighter and transport aircraft, employed Hydromatic propellers. The feathering feature alone was crucial to the safety of Allied bomber crews over Germany and Japan.
To ensure that these new propeller designs were efficient, structurally sound, and ready for production, Caldwell developed the propeller whirl test. The test involved mounting a propeller to a stationary test stand where instruments measured the effective thrust of the propeller while it underwent long endurance runs at high speeds. He designed the whirl-testing facilities for the United States government at McCook Field (1918-1927) and Wright Field, Dayton, Ohio (1926-2000). Caldwell's pioneering testing procedures were indispensable to the development of all modern high-performance propellers during the twentieth century.
The Institute of Aeronautical Sciences, the present day AIAA, awarded Caldwell the Sylvanus A. Reed Award in 1935 and an honorary fellowship in 1946. He also served as the institute's president in 1941. During World War II and the early Cold War, Caldwell was the corporate director of the United Aircraft Corporation Research Division until retiring in 1955. On December 23, 1974, Caldwell passed away at his home in West Hartford, Connecticut, at the age of 85.
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