The story of the prototype 707.
- By R.G. Thompson
- Air & Space magazine, May 1987
(Page 4 of 5)
Flutter is a vibration in the airframe that is induced at high speed in response to aerodynamic forces. It usually arises on an extremity, and, if left unchecked, it can intensify until it breaks up the strongest airframe. Dutch roll—so called because of its resemblance to the rolling side-to-side gait of ice-skating Dutchmen—occurs in all airplanes, but is harder to check in those with swept wings. As the airplane yaws from side to side, one wing advances and develops additional lift, causing the airplane to roll to the opposite side, which results in a combined rolling and yawing motion. If this motion continues, it creates a cycle of alternating, increasing yaw angles that can result in uncontrollable roll.
Dash 80’s original tail fin was short, compared with the fin of the B-52, and not much of a yaw inhibitor. Its size, coupled with the lack of a power boost for the rudder, may have contributed to its tendency to flutter. “Flutter…it was a black science then,” Dix Loesch says. “When the flutter guys started talking to their bosses, everybody else just sort of looked at the ceiling.”
Johnston hunted for flutter in Dash 80 early on, and he found it near maximum speed, where it can be expected. Even though the flight engineer’s instrument panel was shaking so hard the mounting bolts broke, Johnston coolly reported, “We’re experiencing an appreciable vibration up here.” Later, however, Loesch encountered flutter during normal climb.
“I did the normal things to fight it—leveled out, throttled back. They didn’t work. I thought the airplane was going to shake itself to pieces. All of a sudden the rudder froze, and the flutter stopped.” A minor structural failure saved the day—a balance weight had broken loose and jammed the rudder.
Whatever it was that caused Dash 80 to shake, rattle, and roll, Boeing discovered that changing the fin’s internal balance weights, increasing its size, and adding an electronic yaw damper and a hydraulically boosted rudder control ended the problems with flutter, yaw, and Dutch roll.
Late in 1955, orders started trickling in, then flowing. Douglas’ DC-8 orders were not far behind. By the end of 1959, 100 707s had rolled off the production line at Renton, and the first of several hundred KC-135s had been delivered to the Strategic Air Command. Dash 80’s career as a prototype and dealer demo was finished, but Boeing was not yet ready to put it out to pasture.
In the early 1960s, Dash 80 was used to test some modification that would later show up in the 727. These tests led in turn to a long stint with NASA and Boeing testing wings that can generate enough lift for the airplane to remain airborne at extremely low speeds. Dash 80 had averaged 612 mph during a transcontinental speed record flight in 1957. Now it was creeping around Seattle skies at 80 mph and landing at 92 mph. Dismayed commuter airline pilots had to S-turn their Douglas DC-3s on final approach to Boeing Field to avoid overrunning what appeared to be a 707. To preserve control at such ridiculously low speeds, Dash 80 sprouted a lush profusion of leading- and trailing-edge devices on its wings. Their appearance alone was enough to make test pilots blanch.
Gannett continued to fly Dash 80 throughout the low-speed tests, but Johnston and Loesch had moved on and were replaced by S. Lewis Wallick, recently retired from Boeing, and Thomas Edmonds, who is still a test pilot there. In test of leading-edge slats for the wing, engineers experimented with the curve of the slats by applying the file and fiberglass to the devices between flights. Leading-edge symmetry is critical—without it, an airplane tends to roll uncontrollably in a stall. This imprecise shaping of the wing made for occasional imbalance and some very sporty flying. Edmonds recalls a day when one flight was enough: “We stalled, rolled over to about 90 degrees to the horizon, did a split-S, and ended up headed in the opposite direction. We looked around, kind of startled, and decided there was no point in doing any more stalls that day.”