All aircraft must be shown to be free from flutter for flight safety and per civil and military airworthiness requirements. TLG can perform the required certification flutter analyses for all required aircraft configurations and conditions. We can also plan and participate in the required ground and flight testing and subsequent analysis.
TLG Aircraft Flutter Services:
- Flutter DER, Robert Lind
- Feasibility Studies
- Product Development
- Vibration Analysis and Test
- Flutter Evaluation and Design for Aeroservoelastic Stability Margin
- Simulation of Failure Modes including FCS Failure, Structural Failure and Engine Blade-out Conditions
- Full MSC.Nastran/Patran Aeroservoelastic Calculation Capabilities
- Flutter Flight Test Plan and Support
What is Flutter?
Flutter is an aeroservoelastic phenomenon in which unsteady aerodynamic forces combine with structural vibrations to produce a self-feeding oscillation which usually leads to airplane damage! Flutter analysis is done to ensure that the aircraft is safe and free from flutter at all points in the flight envelope.
What are the Regulatory Requirements for Flutter?
The FAA as well as European Aviation Safety Agency (EASA) and other agencies require that the aircraft is shown to be free from flutter for all nominal flight conditions and for critical combinations of failed systems and structures. All possible combinations of fuel, payload and operating conditions (such as holding ice accumulation) must be considered. Flight flutter tests must be performed for some of the conditions to demonstrate the accuracy of the analysis and to prove the airplane is safe to operate. Flutter is critically dependent upon airspeed because the dynamic pressure in the airstream provides the energy for the self-feeding oscillations. Therefore the analyses must be performed to higher airspeeds than the airplane is otherwise designed for. This provides a margin of safety in speed and is required by civil airworthiness regulations.
How are Flutter Calculations Made?
The airplane interaction between structural vibrations and unsteady aerodynamics are modeled using an engineering tool such as MSC.Nastran. These models show whether an airplane will respond in a stable or unstable fashion to an atmospheric disturbance. Stable responses will damp out over time and are not a problem. Unstable responses will grow, generally within seconds, and usually with serious results.
How is Freedom from Flutter Designed into an Airplane?
A flutter analysis shows which airplane types, or modes, of vibration result in flutter and at what airspeeds the flutter occurs. Flutter outside the analysis envelope is not a problem but flutter inside the envelope is. Variations in the analysis model can show which design changes can be used to improve flutter behavior. Typical design features that contribute to good flutter characteristics are stiff, lightweight structure, mass-balanced or hydraulically stiffened control surfaces and structural mass concentrated toward the wing or tail leading edges.
What Tests and Analyses are Performed?
Airplane structural vibration modes are usually calculated with a Finite Element Model (FEM). Validation of these models is from a Ground Vibration Test (GVT) of the actual as-built airplane or component. The aerodynamic data are taken from Computational Fluid Dynamics (CFD) and wind tunnel tests and validated during airplane certification flight testing. Flight flutter tests are also made during the certification flight tests. In these tests the airplane is taken to critical flight conditions (this usually requires a shallow dive) and control pulses or other means are used to excite structural vibrations. These vibrations are shown to have stable damping. A speed buildup approach and other flight test procedures are used to ensure flight safety during these tests.