Flutter Analysis: For Safer Air Travel

Recent aviation incidents, including a commercial plane crash attributed to a loss of climb performance shortly after takeoff, have sparked worries about flight safety. These events have brought attention to problems related to malfunctioning engines and insufficient maintenance protocols. As a result, it has become increasingly critical for both commercial and private aircraft to conduct thorough pre-flight inspections. A look back at trends in aircraft accidents reveals that some incidents have been attributed to a phenomenon known as flutter, which can severely compromise an aircraft's structural integrity. A notable example is the crash of the F-117 Nighthawk, which highlights the serious risks associated with flutter.

For better safety, flutter testing has become a vital part of the aircraft development and certification process. With the industry's current focus towards a “shift-left” approach in design, conducting comprehensive flutter analyses is essential. This analysis should involve a multidisciplinary simulation workflow that includes the interaction between aerodynamics and structural dynamics, which is known as fluid-structure interaction (FSI), to yield more accurate real-world scenario results. FSI simulations, as an important part of comprehensive simulation and analysis, enhance aircraft reliability but also further the overarching responsibility of air carriers to ensure passenger safety in an increasingly crowded airspace.

This blog aims to provide an in-depth understanding of flutter analysis, along with insights into Apoorva Rangathagiri's presentation titled “Flow Physics of Forced Pitching Wing Simulations at Low to Moderate Reynolds Numbers” at CadenceLIVE Silicon Valley 2025.

What is Flutter and How Does It Affect Aircraft Performance?

Flutter is a complex aeroelastic phenomenon characterized by rapidly growing oscillations within an aircraft structure. This instability arises from the interplay of aerodynamic forces, structural elasticity, and inertia. When the wings of an aircraft begin to flutter, it leads to these oscillations increasing swiftly. If the oscillations persist, they can result in significant structural damage.

Feedback Loop Interaction Between Aerodynamic Load and Structure

There exists a positive and a negative feedback loop based on the interaction between the aerodynamic load and the structure. A positive feedback loop occurs when structural deformation leads to an increase in aerodynamic load. This increase further causes more deformation, which in turn elevates the aerodynamic load even more. This cycle continues until the aircraft experiences uncontrolled vibrations or oscillations. In some cases, vortex shedding can also produce unsteady loads that contribute to the positive feedback loop, leading to flutter.

Conversely, a negative feedback loop occurs when structural deformation results in a decrease in aerodynamic load. This reduction then decreases the deformation and, subsequently, the aerodynamic load, creating a stabilizing effect. This process continues until the aircraft stabilizes and regains control.

Aeroelastic flutter arises from the positive feedback loop, which causes the aircraft to enter a cycle of self-excited vibrations. As the amplitude of these vibrations increases with each iteration, so does the likelihood of structural failure. This risk is significantly heightened when the vibration amplitude surpasses the structural limits of the aircraft, which can lead to loss of structural integrity and catastrophic failure. Flutter can manifest at various speeds, depending on the aircraft's design and the conditions of the airflow, making it a significant safety concern. Therefore, conducting aeroelastic flutter analysis during the design phase is essential in predicting the loads exerted and ensuring the structural integrity needed to prevent flutter issues.

Insights from CadenceLIVE Silicon Valley 2025 Presentation

The presentation explores the mechanisms that govern flutter instability and the aerodynamic behavior of a pitching wing under various conditions. The research focuses on understanding the interplay of critical parameters such as Reynolds number, reduced velocity, amplitude, and angle of attack, and how these factors contribute to flow dynamics, turbulence, and flutter development. By utilizing Fidelity CharLES high-fidelity CFD simulations and advanced computational techniques, the study aims to explain the relationships between aerodynamic forces, structural dynamics, and oscillatory instabilities, ultimately enhancing the modeling and prediction of flutter phenomena.

Impact of Reynolds Number, Velocity, and Amplitude on Wing Stability

The presentation reveals that when a wing undergoes flutter, oscillations increase significantly, potentially leading to structural failure if unchecked. For instance, as the Reynolds number increases from 1,000 to 50,000, the flow transitions from laminar to turbulent, with the boundary layer forming a laminar separation bubble followed by vortex breakdown. These changes correspond to increasing non-linearity and chaotic behavior in flow dynamics.

The reduced velocity, representing the ratio of free-stream velocity to the wing's pitching frequency, significantly impacts oscillatory behavior. Higher reduced velocities exhibit structured and stable flow fields, while lower reduced velocities intensify flutter instability. Similarly, larger amplitudes exacerbate turbulence and enhance the formation of coherent flow structures, such as hairpin and tip vortices, demonstrating the sensitivity of aerodynamic forces to the wing's oscillatory motion. Phase-averaged analysis further highlights the conditions under which lock-in and negative damping occur, confirming the onset of flutter.

These findings provide critical insights into the dynamic complexities of flutter, offering a foundation for developing more efficient flutter mitigation strategies in aerospace systems.

Watch the on-demand video of Apoorva’s presentation "Flow Physics of Forced Pitching Wing Simulations at Low to Moderate Reynolds Numbers" in the Multiphysics from Chips to Systems track under the CadenceLIVE Silicon Valley 2025 event.­