Growing Up Boeing by Rebecca Wallick

Author:Rebecca Wallick
Language: eng
Format: epub
Tags: aviation, boeing, 747, aviation history, aeronautics, 767, 737, 757, test pilots, jet age, 727, flight test, dash 80, lew wallick
Publisher: Rebecca Wallick

Flutter

Next, in terms of adding data points into the box, come tests of the high speed/high altitude handling characteristics of the airplane—the upper portion of the box. The flight test crews need to determine the handling characteristics and structural capabilities of the airplane under extreme speeds and weights, and that requires a fair amount of flying, taking the airplane to the very edge of its envelope. The crews call it high-speed flutter clearance—in other words, they are proving that the airplane doesn’t have aerodynamic flutter, or vibration, at high speeds and/or high altitudes, and that it will maintain its structural integrity under those extreme conditions.

Flutter. It sounds so innocuous, doesn’t it? One thinks of a butterfly, a falling leaf, an eyelash tickling a cheek; something gentle and soft. But in the world of test flying, flutter means vibration, and uncontrolled vibration is one of the most dangerous events that can happen during flight. Flutter is anything but innocuous.

Galloping Gertie (the first Tacoma Narrows Bridge) in Washington State is a well-known and dramatic example of resonance (or vibration) and its consequences. Completed in 1940, the bridge earned its nickname when workers building it noticed a bounce, or ripple effect, that had them chewing on lemons to counteract motion sickness. On November 7, 1940, just four months after opening to traffic, wind gusts of up to 42 mph funneled through the Narrows, hitting the bridge. It was designed to withstand winds of up to 120 mph, but on this fateful day, the wind hit it just so. The bridge began to oscillate dangerously up and down—to resonate—until ultimately the oscillations increased so dramatically that the bridge broke apart and fell in huge pieces into the waterway far below, all of it caught on film.

When pilots talk about flutter, they’re referring to the vibration induced on an airplane’s wings, tail, or control surfaces by the force of airflow during flight. The amount of vibration depends upon the airplane’s structural characteristics (its flexibility and stiffness) as well as its altitude, airspeed, and Mach number.44

If flutter is present, it usually increases with increased airspeed. In well-designed airplanes, any flutter will damp out on its own within a reasonable time. If damping doesn’t occur or is insufficient, however, resonance occurs, dramatically increasing the vibration and potentially leading to catastrophic structural failure—a wing, or the tail, could break and the airplane could crash. During a six-month span in late 1959–early 1960, two Lockheed L-188 Electras disintegrated in midair, killing all crew and passengers onboard.45 Beyond ascertaining that something originating in the left wing caused the first accident, investigators were stymied. After the second, nearly identical incident, the Civil Aeronautics Board determined that harmonic coupling between the wing and the rotating propeller caused of both disasters.

In new aircraft, where the aerodynamics and mechanical properties can’t be fully known until it flies, stall characteristics and flutter—and flutter clearance—can only be determined through flight testing. These early stall and flutter clearance tests are what Boeing refers to as “minimum crew” flights because of their danger.

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