Dynamically Scaled Free-Flight Models

Joseph R. Chambers

The earliest flying machines were small models and concept demonstra­tors, and they dramatically influenced the invention of flight. Since the invention of the airplane, free-flight atmospheric model testing—and tests of "flying" models in wind tunnel and ground research facilities — has been a means of undertaking flight research critical to ensuring that designs meet mission objectives. Much of this testing has helped identify problems and solutions while reducing risk.

N A HOT, MUGGY DAY IN SUMMER 1 959, Joe Walker, the crusty old head of the wind tunnel technicians at the legend­ary NASA Langley Full-Scale Tunnel, couldn’t believe what he saw in the test section of his beloved wind tunnel. Just a few decades earlier, Walker had led his technician staff during wind tunnel test oper­ations of some of the most famous U. S. aircraft of World War II in its gigantic 30- by 60-foot test section. With names like Buffalo, Airacobra, Warhawk, Lightning, Mustang, Wildcat, Hellcat, Avenger, Thunderbolt, Helldiver, and Corsair, the test subjects were big, powerful fighters that carried the day for the United States and its allies during the war. Early versions of these aircraft had been flown to Langley Field and installed in the tunnel for exhaustive studies of how to improve their aerodynamic performance, engine cooling, and stability and control characteristics.

On this day, however, Walker was witnessing a type of test that would markedly change the research agenda at the Full-Scale Tunnel for many years to come. With the creation of the new National Aeronautics and Space Administration (NASA) in 1958 and its focus on human space flight, massive transfers of the old tunnel’s National Advisory Committee for Aeronautics (NACA) personnel to new space flight priorities such as Project Mercury at other facilities had resulted in significant reductions in the tunnel’s staff, test schedule, and workload. The situation had not, however, gone unnoticed by a group of brilliant engineers that had pio­neered the use of remotely controlled free-flying model airplanes for

predictions of the flying behavior of full-scale aircraft using a unique testing technique that had been developed and applied in a much smaller tunnel known as the Langley 12-Foot Free Flight Tunnel. The engineers’ activities would benefit tremendously by use of the gigantic test section of the Full-Scale Tunnel, which would provide a tremendous increase in flying space and allow for a significant increase in the size of models used in their experiments. In view of the operational changes occurring at the tunnel, they began a strong advocacy to move their free-flight stud­ies to the larger facility. The decision to transfer the free-flight model testing to the Full-Scale Tunnel was made in 1959 by Langley’s manage­ment, and the model flight-testing was underway.

Joe Walker was observing a critical NASA free-flight model test that had been requested under joint sponsorship between NASA, industry, and the Department of Defense (DOD) to determine the flying charac­teristics of a 7-foot-long model of the North American X-15 research aircraft. As Walker watched the model maneuvering across the test sec­tion, he lamented the radical change of test subjects in the tunnel with several profanities and a proclamation that the testing had "gone from big-iron hardware to a bunch of damn butterflies.”[440] What Walker didn’t appreciate was that the revolutionary efforts of the NACA and NASA to develop tools, facilities, and testing techniques based on the use of sub­scale flying models were rapidly maturing and being sought by military and civil aircraft designers—not only in the Full-Scale Tunnel, but in several other unique NASA testing facilities.

For over 80 years, thousands of flight tests of "butterflies” in NACA and NASA wind tunnel facilities and outdoor test ranges have contrib­uted valuable predictions, data, and risk reduction for the Nation’s high-priority aircraft programs, space flight vehicles, and instrumented planetary probes. Free-flight models have been used in a myriad of studies as far ranging as aerodynamic drag reduction, loads caused by atmospheric gusts and landing impacts, ditching, aeroelasticity and flut­ter, and dynamic stability and control. The models used in the studies have been flown at conditions ranging from hovering flight to hyper­sonic speeds. Even a brief description of the wide variety of free-flight model applications is far beyond the intent of this essay; therefore, the following discussion is limited to activities in flight dynamics, which

includes dynamic stability and control, flight at high angles of attack, spin entry, and spinning.