Transonic Model

Supersonic wind tunnels are rather simple (though very expensive) to build. When their air moves well above the speed of sound, it passes through the experimental chamber in a smooth, even stream. "Transonic" tunnels (close to the speed of sound) are much more difficult. At this critical speed, very important in the study of aerodynamics, the tunnel tends to "choke." Shock waves form inside it, making accurate experiments difficult.

To get around this hurdle, the National Advisory Committee for Aeronautics has been developing "gravity propelled" models to be dropped from airplanes high above the earth. They are heavy, bomb-shaped objects which carry experimental wings, tails and control surfaces. During a fall from 40,000 ft., they usually attain speeds above the speed of sound.

The early models could be used only once; they smashed themselves to bits when they smacked the earth. So the models had to be equipped with telemetering devices to send the readings of their instruments by radio to recording receivers on the ground--an expensive and not always efficient process.

Last week the NACA told about a recoverable model that can be used over & over again. In its tail is a strongly made dive-brake which opens gradually as the model approaches the ground (see cut). When the speed has been reduced sufficiently, a parachute pops out and lowers the model to the earth. The model's instruments are self-recording. After the records have been taken out and the parachute repacked, the model can be sent aloft for another drop.

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