The reliability of wind tunnel airfoil data, also, was established during high-speed flights performed by the NACA’s Curtiss Jennies – which now were accompanied by several other borrowed aircraft in the committee’s growing research fleet. In 1924, NACA purchased its first aircraft, a Boeing PW-9, built expressly for research. By the following year, Langley had 19 different aircraft dedicated to a variety of test operations.
One of the longest and most intensive research programs during the NACA’s first two decades was a series of evaluations of the pressure loads borne by airframes under stresses encountered during high-speed maneuvers. Undertaken after a series of Army Air Service crashes and fatalities, the NACA’s pressure-load research occupied some of Langley’s brightest minds for eight years.
Pressure-load test flights began at Langley in 1926, the year Henry J.E. Reid became the lab’s new engineer-in-charge, a post he would retain for more than three decades. First a Curtiss JNS-1 and then the PW-9 – a stocky, heavily-braced airplane – were subjected to almost every conceivable maneuver and condition of flight. During level flight, spins, loops, pull-ups, inversions, and dives, all the aircraft’s main surfaces underwent moment-to-moment stress analysis at multiple points on the wings, elevators, vertical tails, rudders, and horizontal stabilizers.
The wind tunnels, meanwhile, proved useful in addressing questions that test flights couldn’t answer, and gave NACA engineers useful data about the proper sizing of pressure orifices. Test flights and wind tunnel tests often raised more questions than they answered, opening new avenues of inquiry – and by the late 1920s, pressure-load data coming out of Langley was changing the way aircraft were being designed and manufactured. The laboratory’s work had also, in the meantime, earned the NACA an international reputation as one of the world’s preeminent aeronautical research institutions.
The technical and procedural experience gained by the NACA in its pressure-load investigations enabled the committee to undertake a number of other programs. Some of these research problems could be investigated with instrumentation and test flights, while others required a multi-pronged approach that included ground-based modeling. Within a period of five years, the NACA launched a wave of tunnel construction. The new facilities included:
- The Propeller Research Tunnel (completed in 1927). Built to correlate NACA data with tests conducted at Stanford University, the diesel-powered tunnel created a 20-foot-wide airstream that reached 110 miles per hour.
- The 11-inch Hypersonic Tunnel (1928) allowed researchers to investigate aerodynamic effects near the speed of sound.
- The 5-foot Vertical Wind Tunnel (1929) enabled researchers to analyze the spin recovery of models without risk to pilots or aircraft.
- The 7 by 10 foot Atmospheric Wind Tunnel (1930), a replacement for Tunnel No. 1, enabled studies of stability and control in the low-speed range.
- The Full-Scale (30 by 60 foot) Tunnel (FST), the largest wind tunnel in the world until 1945, allowed an approximation of free-flight conditions by bringing entire aircraft indoors. Declared a National Historic Landmark in 1985, the FST was in operation for 64 years before being retired in 1995.
By the beginning of the 1930s, the NACA’s collection of wind tunnels at Langley comprised the greatest aeronautical research capability in the world – and one of the era’s greatest advances would be developed in the Propeller Research Tunnel, beginning in 1927.
After World War I, most American planes used air-cooled radial engines, with cylinders arranged around the propeller drive shaft – and the exposed cylinders created considerable drag. But the drawbacks associated with liquid-cooled engines, such as the weight of their liquid cooling systems, were more considerable – especially for the Navy, whose planes were required to withstand jarring impacts with carrier decks and needed the superior reliability of radial engines for over-water flight. The Bureau of Aviation asked the NACA to look into the possibility of using a circular cowling to reduce the drag created by radial engines while still allowing adequate cooling.