“Flutter” may sound like a benign word when associated with a flag in a breeze, a butterfly, or seaweed in an ocean current. When used in the context of aerodynamics, however, it describes a highly dangerous, potentially deadly condition.
Consider the case of the Lockheed L-188 Electra Turboprop, an airliner that first took to the skies in 1957. Two years later, an Electra plummeted to the ground en route from Houston to Dallas. Within another year, a second Electra crashed. In both cases, all crew and passengers died.
Lockheed engineers were at a loss as to why the planes’ wings were tearing off in midair. For an answer, the company turned to NASA’s Transonic Dynamics Tunnel (TDT) at Langley Research Center. At the time, the newly renovated wind tunnel offered engineers the capability of testing aeroelastic qualities in aircraft flying at transonic speeds—near or just below the speed of sound. (Aeroelasticity is the interaction between aerodynamic forces and the structural dynamics of an aircraft or other structure.) Through round-the-clock testing in the TDT, NASA and industry researchers discovered the cause: flutter.
Flutter occurs when aerodynamic forces acting on a wing cause it to vibrate. As the aircraft moves faster, certain conditions can cause that vibration to multiply and feed off itself, building to greater amplitudes until the flutter causes severe damage or even the destruction of the aircraft. Flutter can impact other structures as well. Famous film footage of the Tacoma Narrows Bridge in Washington in 1940 shows the main span of the bridge collapsing after strong winds generated powerful flutter forces. In the Electra’s case, faulty engine mounts allowed a type of flutter known as whirl flutter, generated by the spinning propellers, to transfer to the wings, causing them to vibrate violently enough to tear off.
Thanks to the NASA testing, Lockheed was able to correct the Electra’s design flaws that led to the flutter conditions and return the aircraft to safe flight. Today, all aircraft must have a flutter boundary 15 percent beyond the aircraft’s expected maximum speed to ensure that flutter conditions are not encountered in flight. NASA continues to support research in new aircraft designs to improve knowledge of aeroelasticity and flutter. Through platforms such as Dryden Flight Research Center’s Active Aeroelastic Wing (AAW) research aircraft, the Agency researches methods for in-flight validation of predictions and for controlling and taking advantage of aeroelastic conditions to enhance aircraft performance.
“Flutter clearance is a big part of the cost to approve a new aircraft for flight,” says Marty Brenner, aerospace engineer at Dryden. “What we’ve been supporting is how to estimate what this boundary is based on flight test data.” To do this, Dryden partnered with ZONA Technology of Scottsdale, Arizona, through the Small Business Innovation Research (SBIR) program.
An industry leader in aeroelastic modeling software, ZONA engaged in multiple SBIR projects with Dryden for predicting flutter boundaries, developing adaptive controls to help suppress impending flutter, and innovating new ways of conducting flutter testing without wind tunnels. Through these partnerships, ZONA has developed unique technology that help aircraft designers ensure the performance and safety of their vehicles in efficient, cost-effective ways.
The ZONA Online Flutter Estimator (ZOFE), one of the outcomes of the company’s collaboration with Dryden, is a software tool that not only helps manufacturers design safe, flutter-free aircraft, but also helps maintain the safety of the flight tests of these designs.
“During the flight test, you don’t want the aircraft running into flutter,” says PC Chen, ZONA’s president. “At the same time, you do want to know where the flutter boundary is. This software allows you to fly the aircraft in preflutter conditions, then calculate or predict the flutter boundary at the higher speed.”