U.S. soldiers in Vietnam heard rumors of ghosts; the Viet Cong chalked it up to bad luck.
- By Roger Warner
- Air & Space magazine, May 2004
(Page 2 of 5)
For the airframe, Hall chose a well-known commercial sailplane, the Schweizer 2-32. His team took an ordinary 100-horsepower Continental O-200 engine and mounted it behind and slightly above the cockpit, so it made a bulge in the top of the airframe, like a camel’s hump. The propeller shaft ran above the canopy, outside the airplane, to a vertical pylon attached to the nose. They tested several propellers and chose an eight-foot-diameter model with four wooden blades. To quiet the engine further, the Sunnyvale team lined the inside of the cowling with fiberglass batting and ran the exhaust through a muffler from a 1958 Buick. Instead of using noisy gears, they connected the engine to the propeller shaft with V-belts, similar to fan belts. Les Horn recalls that it was the “only aircraft flying that was powered by rubber bands.” But the engineering and workmanship were first-rate.
The prototype aircraft were designated QT-2: “2” for two-seater, and “QT” for “Quiet Thruster,” officially, though everybody knew it also stood for “on the Q.T.” (on the sly). The first flight was set for August 15, 1967, at an isolated municipal airport in Tracy, California, about 50 miles from San Jose.
Being modified sailplanes, the QTs had a single main wheel mounted in the center of the belly, two tiny wheels under each wingtip to keep the tips from dragging, and a small nosewheel. When the test pilot, Quint Burden, started the engine, he taxied down the runway listing to port until, at around 15 knots (about 17 mph), he had enough speed to level the wings.
After he took off, he circled the field, the big wooden propeller turning at a leisurely 800 rpm, about a third the speed of a normal prop for an engine of that size. “This was a really quiet airplane, I tell you,” recalls Hall, who was there for the test flights “We could fly it at 250 feet and barely hear it at all. At 800 feet it was completely silent” to ground observers.
There had been a few studies of techniques for quieting airplanes, but for the most part the Lockheed team had to figure out acoustic stealth for itself. There was ground-level masking noise, to start with—crickets and frogs in the countryside, or the background sounds of a small town late at night, which Lockheed pegged at 50 decibels. Lockheed found the QT’s overall sound level was 70 decibels at 1,000 feet.
Then there was the QT’s acoustic signature, which was different from other aircraft. And it was so close to the threshhold of hearing that it was perceived in very different ways. Hall thought it was “the gentle rushing sound of the ocean surf” while Burden, the test pilot, described it as an almost subliminal thub…thub…thub. Others were reminded of tires on a distant highway, the whirring of an electric fan, or a flock of birds overhead. The heart of acoustic stealth, the Lockheed guys discovered, is a widely observed but imperfectly understood relationship between detecting noise and perceiving and identifying its source. If you didn’t suspect an airplane was above you or notice that a few stars were being blocked and then reappearing, you might not be aware of anything at all—even if a QT-2 were only a couple hundred feet overhead.
Further tests revealed the QT was best flown cautiously, straight and level. A yaw, or turn on the vertical axis, could develop into a larger yaw than expected because the area around the nose pylon was so large it counteracted the stabilizing effect of the vertical tail. A banked turn could lead to a phenomenon called yaw-roll coupling; in a slow roll, which nobody ever tried, once upside down the wings would probably fall off. “It was a very tender aircraft,” says Les Horn, who notes that the original Schweizer has an 8-G rating, while the QT-2, weighed down by an engine and other gear, had a rating of barely 2.4 G.
They needed a long runway for takeoff, then the airplanes could slowly climb to 5,000 feet and cruise at 110 knots. For minimum noise, though, the best speed was down around 70 knots, which was just one knot over the stall speed. In this so-called quiet mode, the craft required only 17 horsepower to stay aloft, according to the tests.