Understanding Flutter: The Feedback Loop Behind Aerodynamic Stability in Aircraft
Flutter is the engineering study of potentially unstable interactions between the aerodynamic forces on an air vehicle and the deformation due to those forces. The formal name for these studies acknowledges interaction in ‘aeroelastic stability’ by combining the terms ‘aerodynamic’ and ‘elasticity’.
All flight vehicles experience forces due to traveling through the air. This is a requirement for aircraft because it generates the lift that keeps the airplane in the sky (as well as the drag that must be overcome by engine thrust).
But there’s more going on outside that airliner cabin window. The airplane deforms under those loads like bending and twisting a stick or a straw. Thinner wings generally deform more than thicker ones, and fuselages usually deform the least, but they all deform under load. That change in shape affects the aerodynamic forces. But those forces were causing the deformation in the first place, so now the deformed shape is different. And the aerodynamic forces are different again, and so on. We call this a ‘feedback loop’. Flutter is the study of whether this feedback loop is stable or unstable. A stable response converges to a final static state. An unstable response, once started, will grow on its own until the structure breaks.
Every airliner trip you have taken has been on a flutter-free airplane. Have you looked out the window and seen the wing shaking in turbulence? This is the airplane’s dynamic response to the unsteady gusts of air. When the turbulence stops, the wing stops shaking. That is a stable response to the gust input.
Here are two examples. In the first clip, this NASA experimental airplane is intentionally flown close to a flutter boundary speed. You can see the tail starting to deform. In the second clip, you can see the wings of a glider bending and twisting at high speed.
Lighter-weight structures may flutter more easily, but lightweight is not a requirement. The most famous flutter example is the Tacoma Narrows Bridge across Puget Sound. The bridge shape allowed for unstable coupling with the wind, and the bridge tore itself apart not even a year after it was finished:
Flutter engineers analyze the airplane using mathematical models of the structure and the aerodynamics. Airliners must be free from flutter throughout the ‘flight envelope’ – all speeds and altitudes at which the airplane is designed to operate – including a ‘dive’ margin for accidental overspeeds, and a 15% additional margin on top of that. All possible loads of passengers/cargo/fuel must be considered in addition to a laundry list of ‘non-normal’ conditions, including equipment or structural failures. Ground and flight test campaigns validate the analysis data and provide the final demonstration of safety.
Next time you ride through turbulence, you can watch the wing flex and know that motion has been thoroughly evaluated by flutter engineers.
TLG Aerospace specializes in aircraft flutter analysis and certification. We help ensure aircraft are free from flutter across all configurations and conditions for flight safety and compliance with airworthiness standards.
Our services include ground and flight testing, as well as advanced modeling techniques like Finite Element Models (FEM) and Computational Fluid Dynamics (CFD) to validate structural and aerodynamic data. TLG also conducts Ground Vibration Tests (GVT) to assess structural vibration modes and simulate failure scenarios.
Our series on Flutter will be expanded in the coming weeks with discussions on various types of Flutter.
Connect with TLG’s Director of Engineering, FAA Flight Analyst DER, and FAA Flutter DER, Robert Lind.
