Drop and Radio-Controlled Model Testing

A major disadvantage of free-spinning wind-tunnel model testing is that the Frisbee-throw method of launching the model into the tunnel prevents study of real-life stalls and spin entries. Also, model size and Reynolds number are limited by the wind – tunnel size. Both of these problems with spin tunnels are partially eliminated for drop or radio-controlled models. For spin research, NASA drops models from helicopters and air­planes and operates radio-controlled dynamically scaled models that resemble superficially those flown by hobbyists.

For example, a 1 /12-scale drop model of a typical fighter airplane would weigh about 300 pounds. The NASA Langley helicopter drop model work is led by Charles E. Libbey. Both radio-controlled and helicopter drop model testing go on at the NASA Plum Tree test site, near Poquoson, Virginia.

9.2 Remotely Piloted Spin Model Testing

Perhaps the closest that one can get to testing a new airplane configuration for spin entry and recovery conditions in advance of the real thing is the remotely piloted drop model technique originated by Euclid C. Holleman and other NASA staff people at the Dryden Flight Research Center. A test pilot controls the drop model from a ground-based simulator­like isolated cockpit that has instrument displays run by telemetered data. The airplane’s flight control system is simulated on a ground computer and commands are up-linked to the model.

Considering that the setup resembles a space vehicle control center, design and operating expenses must be high compared with alternative spin model testing methods. However, the method efficiently produces good flight test data in a short time. Unplanned angles of attack and sideslip reached in remotely piloted tests of a 3/8-scale model of the McDonnell Douglas F-15 fighter might have resulted in loss of control for a less forgiving airplane.

On the negative side, pilots report that the absence of motion cues is a serious drawback for spin testing and might lead to overly conservative results. Holleman notes that before the actual drop tests pilots practice the flight plan on fixed-base ground simulators. Speeding up the pace for the fixed-base training sessions by a factor of 1.4 prepares the pilot for the actual flight.

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