Silent airplanes have buzzed Soldiers Field for the past week in an effort to determine "how quiet is quiet."
The planes make less noise than an automobile. Under the terms of a federal contract, they are being flown to find how silent an airplane must be before the community won't object to it.
The project is is the hands of the Aeronautical Research Foundation, an independent group of University and MIT professors. Professor Lynn L. Bollinger of the Business School is executive director.
By varying the noise that the planes make, Bollinger can compare how many complaints are received from citizens. He also confers with hospitals and other groups on the effect of the sounds.
Two Ways to Keep Quiet
Bollinger uses two devices to tone his planes down. Most airplane noise comes from air currents around fast-moving propeller tips. The engines are therefore geared down to make the blades move slower, and four-bladed props are used instead.
His other method is mufflers on the engines.