Birthing the Testing Techniques

The development and use of free-flying model techniques within the NACA originated in the 1920s at the Langley Memorial Aeronautical Laboratory at Hampton, VA. The early efforts had been stimulated by concerns over a critical lack of understanding and design criteria for methods to improve aircraft spin behavior.[441] Although early aviation pioneers had been frequently using flying models to demonstrate con­cepts for flying machines, many of the applications had not adhered to the proper scaling procedures required for realistic simulation of full – scale aircraft motions. The NACA researchers were very aware that cer­tain model features other than geometrical shape required application of scaling factors to ensure that the flight motions of the model would replicate those of the aircraft during flight. In particular, the require­ments to scale the mass and the distribution of mass within the model were very specific.[442] The fundamental theories and derivation of scaling factors for free-flight models are based on the science known as dimen­sional analysis. Briefly, dynamic free-flight models are constructed so that the linear and angular motions and rates of the model can be readily scaled to full-scale values. For example, a dynamically scaled 1/9-scale model will have a wingspan 1/9 that of the airplane and it will have a weight of 1/729 that of the airplane. Of more importance is the fact that the scaled model will exhibit angular velocities that are three times faster than those of the airplane, creating a potential challenge for a remotely located human pilot to control its rapid motions.

Initial NACA testing of dynamically scaled models consisted of spin tests of biplane models that were hand-launched by a researcher or cat­apulted from a platform about 100 feet above the ground in an airship hangar at Langley Field.[443] As the unpowered model spun toward the ground, its path was tracked and followed by a pair of researchers hold­ing a retrieval net similar to those used in fire rescues. To an observer,

the testing technique contained all the elements of an old silent movie, including the dash for the falling object. The information provided by this free-spin test technique was valuable and provided confidence (or lack thereof) in the ability of the model to predict full-scale behavior, but the briefness of the test and the inevitable delays caused by dam­age to the model left much to be desired.

The free-flight model testing at Langley was accompanied by other forms of analysis, including a 5-foot vertical wind tunnel in which the aerodynamic characteristics of the models could be measured during simulated spinning motions while attached to a motor-driven spinning apparatus. The aerodynamic data gathered in the Langley 5-Foot Vertical Tunnel were used for analyses of spin modes, the effects of various air­plane components in spins, and the impact of configuration changes. The airstream in the tunnel was directed downward, therefore free – spinning tests could not be conducted.[444]

Meanwhile, in England, the Royal Aircraft Establishment (RAE) was aware of the NACA’s airship hangar free-spinning technique and had been inspired to explore the use of similar catapulted model spin tests in a large building. The RAE experience led to the same unsatisfac­tory conclusions and redirected its interest to experiments with a novel 2-foot-diameter vertical free-spinning tunnel. The positive results of tests of very small models (wingspans of a few inches) in the apparatus led the British to construct a 12-foot vertical spin tunnel that became operational in 1932.[445] Tests in the facility were conducted with the model launched into a vertically rising airstream, with the model’s weight being supported by its aerodynamic drag in the rising airstream. The mod­el’s vertical position in the test section could be reasonably maintained within the view of an observer by precise and rapid control of the tun­nel speed, and the resulting test time could be much longer than that obtained with catapulted models. The advantages of this technique were very apparent to the international research community, and the facility features of the RAE tunnel have influenced the design of all other ver­tical spin tunnels to this day.

turning vanes

test section

documentation

camera

data

acquisition cameras (2 of 8)

honeycomb

This cross-sectional view of the Langley 20-Foot Vertical Spin Tunnel shows the closed-return tun­nel configuration, the location of the drive fan at the top of the facility, and the locations of safety nets above and below the test section to restrain and retrieve models. NASA.

When the NACA learned of the new British tunnel, Charles H. Zimmerman of the Langley staff led the design of a similar tunnel known as the Langley 15-Foot Free-Spinning Wind Tunnel, which became opera­tional in 1935.[446] The use of clockwork delayed-action mechanisms to move the control surfaces of the model during the spin enabled the researchers

to evaluate the effectiveness of various combinations of spin recovery tech­niques. The tunnel was immediately used to accumulate design data for satisfactory spin characteristics, and its workload increased dramatically.

Langley replaced its 15-Foot Free-Spinning Wind Tunnel in 1941 with a 20-foot spin tunnel that produced higher test speeds to support scaled models of the heavier aircraft emerging at the time. Control inputs for spin recovery were actuated at the command of a researcher rather than the preset clockwork mechanisms of the previous tunnel. Copper coils placed around the periphery of the tunnel set up a magnetic field in the tunnel when energized, and the magnetic field actuated a magnetic device in the model to operate the model’s aerodynamic control surfaces.[447]

The Langley 20-Foot Vertical Spin Tunnel has since continued to serve the Nation as the most active facility for spinning experiments and other studies requiring a vertical airstream. Data acquisition is based on a model space positioning system that uses retro-reflective targets attached on the model for determining model position, and results include spin rate, model attitudes, and control positions.[448] The Spin Tunnel has sup­ported the development of nearly all U. S. military fighter and attack aircraft, trainers, and bombers during its 68-year history, with nearly 600 projects conducted for different aerospace configurations to date.