The main purpose of the Propeller Research Tunnel was researching the aerodynamic efficiency of propellers on radial engine aircraft. In 1917, William F. Durand published NACA Technical Report 17 on his work with isolated propellers in Stanford University's wind tunnel, however these results did not match with the data NACA had collected for propellers connected to fuselages. Additionally, little was known about the limitations of propellers. Propellers had efficiency issues caused by loss of compression at the tips at high speeds. In 1923, Langley engineer Fred Weick, suggested NACA build a wind tunnel with a 20 foot diameter throat, capable of speeds up to 100 miles per hour in order to perform full-scale propeller tests. According to Weick, British engineers were running tests on scale propellers at the time, but were unable to obtain accurate results due to a scaling issue related to the Reynolds number in the smaller wind tunnels. NACA had been using the Variable Density Tunnel in order to increase the density of air to keep the Reynolds number in testing similar to the Reynolds number experienced by full-scale aircraft. However, the Variable Density Tunnel was not able to provide consistent data for propellers, so NACA built the Propeller Research Tunnel.
History
NACA began work on the Propeller Research Tunnel under direction of Director of Aeronautical Research George W. Lewis in 1925 and completed its construction in 1927. Built using two 1000-horsepower diesel submarine engines, and an 8-blade, 27-foot diameter fan, the Propeller Research Tunnel was capable of pushing air in a 20-foot stream at 110 miles per hour. The PRT remained operational until it was demolished in 1950 when NACA needed a place to build their 8-foot Transonic Pressure Tunnel.
Use in Research
Propeller Research
The Propeller Research Tunnel was used in the development of more efficient propellers that did not lose compression at the blade tips at high speeds. NACA was also able to test full scale propellers to find a blade shape that maximized efficiency and performance where previous designs had failed.
NACA cowling
The PRT was also used to develop a way to reduce the drag produced by the exposed pistons of radial turbine engines. By testing various cowlings on full-scale models in the PRT, NACA was able to produce the NACA cowl, which won the Collier Trophy in 1929 for its impact on aeronautics. It was predicted that the cowling, by reducing drag and increasing engine cooling, would save the American aircraft industry upwards of $5 million, and the cowling and its variants were quickly adopted by plane manufacturers.
Other
Similarly, the PRT found that engine placement and the fixed landing gear contributed greatly to drag. NACA engineers worked to create a retractable landing gear and found that multi-engine planes benefited from having their engines in-line with the leading edge of the wing. Both of these discoveries were also quickly adopted by airplane manufacturers. Data collected in the PRT was used heavily in the design of many World War II planes including the Boeing B-17 Flying Fortress, Boeing B-24 Liberator, and the Douglas DC-3.
