This story begins in 1929, a full 12 years before the United States was plunged into the maelstrom of World War II. That year, the air just north of the tidewater town of Hampton, Virginia, was filled with the sound of saws, loud hammering, and heavy construction equipment. In 1929, the National Advisory Committee for Aeronautics (NACA) authorized the construction of a massive wind tunnel to be located at the NACA Langley Memorial Aeronautical Laboratory in Hampton. Becoming operational in May 1931, this wind tunnel had a test section 60 feet in width and 30 feet high, large enough to mount not just a model of an airplane but rather the whole airplane itself. For this reason, the tunnel was called the NACA Full Scale Wind Tunnel (FST). At the time, the FST was the largest wind tunnel in the world – a stunning facility for aerodynamic testing.
But why a wind tunnel of this huge size? Wind tunnels had existed since 1870, when the English inventor Francis Wenham designed and built the first wind tunnel in history at Greenwich. By 1929, scores of wind tunnels had been built world-wide for aerodynamic testing, including one by the Wright brothers. However, all these tunnels were small and were limited to testing relatively small models. The NACA’s Full Scale Tunnel was the first facility large enough to test a whole airplane.
But why was the testing of a whole airplane in a wind tunnel so important as opposed to testing a smaller model? There are scale effects in applying aerodynamic data obtained on small models that compromise the data when applied to the “real thing.” This is especially true with the data for aerodynamic drag, an important datapoint to measure. Everything else being equal, the maximum velocity of a given airplane in level flight depends strongly on drag – the lower the drag, the faster the maximum speed of the airplane. In particular, for new airplanes coming off the drawing boards at the beginning of World War II, the maximum speed of a combat fighter aircraft became a major design criterion. Therefore, the accurate prediction of aerodynamic drag was critical to the success of these new designs.
However, as any design engineer knows, many compromises go into the design of a given airplane. In terms of drag, there are necessary appendages, accessory equipment, protuberances, holes, etc., on the surface of an operational aircraft that tend to destroy the sleekness of an otherwise low-drag streamlined configuration. The identification of how much extra drag was created by these “blemishes” became important, with an eye towards decreasing this drag. The question became just how to carry out this identification.
The answer surfaced rather abruptly in April 1938, when the Navy was having a major problem with a new fighter design, the Brewster XF2A Buffalo. The 250 mile-per-hour speed of this fighter was disappointingly low, and yet it was already starting to come off the production line. The Navy needed help immediately, and it came in the form of the NACA Full Scale Tunnel and its engineering staff. The Navy Bureau of Aeronautics quickly arranged to have the Buffalo tested in the Full Scale Tunnel at Langley. Even before the ink was dry on the paperwork, the Navy flew an XF2A-1 to Langley, and within days the NACA engineers mounted the airplane on the stilt-like balance in the FST.
Recognizing that the deficit in top speed for the Buffalo was most likely due to a higher than expected aerodynamic drag, the FST team conceived a brilliant and perfectly logical test procedure to identify the sources of the extra drag. They removed protuberances like the radio antenna, smoothed irregular surfaces like the cockpit canopy, used putty or tape to cover holes, and smoothed the bays for storing the retracted landing gear. Attention was paid to the exhaust stacks, machine gun installation, and gun sights that projected into the airflow outside the airplane.
When the airplane was perfectly smooth and aerodynamically clean, the drag of this faired and sealed airplane was then measured in the FST. This was the baseline drag measurement. Next, one of the protuberances was added back to the smooth configuration, and the drag was measured again, thus identifying the increase in drag due just to this addition. The tests continued by adding one-by-one each of the drag-causing items, measuring the airplane drag after each addition, until the airplane configuration was fully restored to its original condition. In this fashion, a detailed test base was acquired, identifying which items increased drag, and by how much. The design of airplanes was not the primary role of the NACA, so these test results were then reported to the manufacturer, giving Brewster the necessary information to modify the design of the airplane in order to reduce its drag. Amazingly, the top speed of the modified Buffalo was increased from 250 to 281 miles per hour, a meaningful 10 percent increase.
The process of smoothing out the canopy of the XF2A-1 Buffalo is shown in the slideshow below.
What a wake-up call to the airplane industry! The Army and Navy suddenly stood in line for such drag testing in the Full Scale Tunnel on other new aircraft designs. The pioneering test procedure developed by the NACA engineers was labeled drag cleanup. Over the next two-and-a-half years, 18 new prototype military airplanes were thoroughly run through this drag cleanup test procedure, each design benefiting to a greater or lesser extent by the tests.
One of the more telling stories that came out of the drag cleanup program was its effect on the P-39, a beautifully sleek but somewhat unconventional airplane.
The P-39 Airacobra had its engine mounted behind the cockpit at the airplane’s center of gravity (note the air scoop at the back of the cockpit windshield), and a cannon in the nose. Designed by Bell Aircraft, the Army had expectations for the P-39 to be its first fighter to exceed 400 miles per hour in level flight. Indeed, the prototype XP-39 had reached 390 miles per hour during early tests at the Army’s Wright Field near Dayton, Ohio. Production airplanes, however, were expected to weigh about 2000 pounds more than the 5550-pounds prototype, a weight increase that was predicted to bring its top speed to barely 340 miles per hour. This did not make General Henry H. “Hap” Arnold, the iconic commander of the U.S. Army Air Corps, happy, and certainly did not satisfy the Bell aircraft designers. So the P-39 was quickly inserted into the NACA Full Scale Tunnel Drag Cleanup program and testing began on June 8, 1939.
The tests showed that the completely faired P-39 had a drag that was about 50% that of the original value, a dramatic decrease. In addition to these drag cleanup tests, the P-39 also went through a series of flight tests at Langley. Ultimately, the heavier production models of the P-39 never achieved the desired 400 mile-per-hour maximum speed, and the Army seemed to accept this situation. The Airacobra went on to become a successful ground attack aircraft, with over 700 sent to Great Britain and the Soviet Union under the Lend-Lease agreement. The ultimate success of the P-39 would not have been possible without the benefit of the Langley FST Drag Cleanup tests.
The FST continued its contributions to America’s aerospace industry for the next half-century, testing such modern jet fighters as the F-22, until it was finally decommissioned in 2010. The drag cleanup studies from World War II, however, remain one of its finest hours.
For additional reading on this subject, check out the following books: Engineer in Charge by James R. Hansen (NASA SP-4305, 1987) and The Airplane: A History of its Technology by John D. Anderson Jr. (American Institute of Aeronautics and Astronautics, 2002).
John D. Anderson is the curator of aerodynamics at the National Air and Space Museum and professor emeritus of aerospace engineering at the University of Maryland.