Aeroelastic flutter is a phenomenon when the aerodynamic load acting on the aircraft causes it to vibrate or oscillate.
The positive feedback loop between the aerodynamic load and structural deformation causes flutter in an aircraft.
CFD simulation of the fluid-structure interaction establishes the relation between aerodynamic load and structural deformation to help identify potential flutter risk.
Aircraft structure and airflow conditions have a significant impact on aeroelastic flutter analysis
An aircraft structure is subjected to various aerodynamic forces and moments as it interacts with the surrounding airflow during flight. The aircraft is designed to withstand these dynamic loads to maintain flight stability. However, under certain conditions, these aerodynamic loads can cause the aircraft to vibrate or oscillate, causing a phenomenon called flutter.
Aeroelastic flutter is a significant concern in aircraft design given its propensity to lead to structural failure. One mitigation strategy is aeroelastic flutter analysis, which can provide greater detail about the flutter behavior and identify the design changes required to ensure the aircraft’s safety.
In this article, let’s look further into the concept of flutter and the importance of aeroelastic flutter analysis in aircraft performance.
What Is Aeroelastic Flutter?
Aeroelastic flutter is the high-frequency vibration that occurs in the aircraft due to the interaction between the aerodynamic load and the structure. As the aircraft moves through the air, the aerodynamic forces may cause the aircraft structure to deform. In turn, the aerodynamic load applied to the structure experiences change as well. There can be two types of changes:
Feedback loop interaction between aerodynamic load and structure
Positive feedback loop
Negative feedback loop
Aeroelastic flutter occurs due to the positive feedback loop, which causes the aircraft to fall into the cycle of self-excited vibration. As the amplitude of the vibration increases with each loop, the risk of structural failure increases. This is due to the risk of the vibration amplitude exceeding the structural limit.
Depending on the aircraft geometry and airflow conditions, the flutter can occur at different speeds, making it a major safety concern. Therefore, aeroelastic flutter analysis is crucial during the design phase to predict the load generated and structural integrity required to avoid the issue of flutter.
Aeroelastic Flutter Analysis: Identifying Influencing Factors
Aeroelastic flutter analysis focuses on predicting and analyzing the flutter behavior in the aircraft and its impact on aerodynamic performance. The impact includes issues like increased aerodynamic load, increased risk of losing aircraft control, and reduced aerodynamic efficiency. Various analytical, computational, and experimental methods or their combination can be used for performing aeroelastic flutter analysis during the design phase to get accurate and reliable results.
The flutter analysis requires the identification of some key components.
The flutter speed is the measure of airspeed when the natural frequency of vibration and frequency of the aerodynamic load is equal. These frequencies can be identified at different speeds for flutter analysis for different aerodynamic models. The analysis can help identify the ideal aircraft design susceptible to flutter. If otherwise, the models and simulation can be used to identify optimization strategies to prevent flutter and enhance aircraft safety.
The different flutter modes or vibration patterns influence how the aircraft experiences flutter. The flutter modes commonly include:
- Wing bending torsion flutter
- Wing leading edge flutter
- Tailplane flutter
- Control surface flutter
- Propeller whirl flutter
The prediction and analysis of these components for aircraft design can be made using methods like the finite element method (FEM) or computational fluid dynamics (CFD).
CFD-based aeroelastic flutter analysis includes modeling the fluid-structure interaction to study the response of the aircraft when subjected to the aerodynamic load from the surrounding airflow.
Computational Analysis of Aircraft Performance
Simulating aircraft for aeroelastic flutter analysis
The process of aeroelastic flutter analysis using CFD includes merging fluid flow and structural models to calculate the aerodynamic load and associated structural stresses and deformation. The fluid model uses the Navier-Stokes equation to simulate the flow field under the defined flow conditions and calculate the acting forces around the structure. Similarly, the structural model uses the equation of motion to solve for the deformation of the structure.
With fluid-structure simulation and analysis, it is possible to understand if the aircraft is experiencing a positive or negative feedback loop. The CFD tool also makes it possible to analyze the behavior of aircraft structures such as wings or propellers to identify any unstable flutter modes under different operating conditions. The identification of the flutter-inducing factors and analysis of the looping effect between aerodynamic load and structural deformation can be conducted repeatedly until an ideal solution is obtained, i.e., the one optimal design with reduced flutter and improved performance.