Flutter is a dynamic aeroelastic instability caused by interaction of unsteady aerodynamic forces with elastic vibration of the structure. At the so-called flutter critical velocity, the damping ratio of one system mode becomes zero. If the flow velocity is increased beyond this critical point, the damping ratio becomes negative and even smallest disturbances (e.g. turbulence) will lead to amplified oscillations and to mechanical failure of the airframe.
Whirl Flutter
Whirl flutter is an aeroelastic phenomenon that can occur on propeller-driven aircraft. In whirl flutter, the gyroscopic effects of the spinning propeller and its aerodynamic forces must be considered. Whirl flutter is also a point of concern in the design of tilt-rotors, which are vertical take-off and landing (VTOL) vehicles where rotors with gimbal and collective plus cyclic rotor blade pitch control are installed at wing tips.
To better understand and mitigate whirl flutter issues (for propellers and rotors), engineers conduct wind tunnel tests to validate and improve new designs, testing methods and simulation models. Online monitoring of modal parameters - such as eigenfrequencies and damping ratios - is important to support decision-making whether to safely continue or to abort a test.
ATTILA Project
The ATTILA (Advanced Testbed for TILtrotor Aeroelastics) project focused on developing, building, and testing a platform for aeroelastic wind tunnel experiments on tiltrotor aircraft. The centerpiece of the project was a specially designed testbed: an instrumented, aeroelastically scaled half-wing with a powered proprotor see Figure 1.
The multi-disciplinary nature of this project required a consortium of diverse expertise, ranging from model design and piloting systems to wind tunnel operations, simulations, model updating and stability monitoring. The project consortium consisted of the Royal Netherlands Aerospace Centre (NLR), the German-Dutch Wind Tunnels (DNW), the Polytechnic University of Milan (POLIMI), Leonardo Helicopters (LHD) and the German Aerospace Center (DLR). This article provides an overview of an online monitoring system for whirl flutter analysis within the ATTILA project, part of the Fast Rotorcraft IADP under the European Clean-Sky 2 program (https://www.attila-project.eu/).
Instrumentation and Ground Vibration Testing
In order to perform accurate online stability monitoring the vibration of the testbed needs to be measured precisely. A sensor optimization was therefore performed during the design stage of the project. The wind tunnel model was instrumented with 26 accelerometers in the wing and nacelle, a picture of the model and sensor locations can be seen in Figure 2. Load balances were also used at the wing root and wing-nacelle interface to monitor the forces. It is very important to monitor both the dynamic stability as well as the load safety factor in order to safely operate the model at increasing critical test points.
Figure 2: ATTILA Wind Tunnel Model
Accelerometer locations and measurement directions
A Ground Vibration Test (GVT) was performed before wind tunnel testing to measure dynamic characteristics such as natural frequencies, damping ratios, and mode shapes. The GVT ensured that the model’s dynamic properties matched design specifications and identified discrepancies before exposure to aerodynamic loads. A critical stiffness-adjusting component, the downstop, was found to be less effective than anticipated, prompting design modifications. The GVT also revealed significant system non-linearities affecting dynamic response measurements. A picture of the GVT can be seen in Figure 3, and the first four mode shapes of the test bed can be seen in Figure 4.
Online Monitoring System (OLM) and Wind Tunnel Experiments
The purpose of the online monitoring system OLM is to identify and track the modal parameters during wind tunnel operations. When the wind is on and the rotor is spinning, the structural dynamics interact with the aerodynamic forces - resulting in an aeroelastic system. Since it is not possible to measure the aerodynamic forces directly, a system identification method called Operational Modal Analysis OMA is used to identify the modal parameters. By accurately identifying and tracking these modal parameters in real time, trends can be observed, and the stability margin (i.e. distance to flutter critical velocity) of the system can be determined. When damping trends decrease towards zero, the structure approaches the stability boundary. Slight increases in the wind speed could result in rapid vibration growth and catastrophic model failure. During wind tunnel testing a team of engineers monitored the modal parameters as seen in Figure 5. The OLM system was able to robustly identify and track critical whirl flutter modes during wind tunnel testing. Finally, an overarching goal of operating the model in multiple configurations on the stability boundary with less than 0.2 % damping was achieved.
Figure 5: OLM team from DLR monitoring aeroelastic stability in real time
The ATTILA model showed the onset of whirl flutter during wind tunnel testing, with the piloting and actuator system functioning effectively. The OLM system was capable of tracking modal parameters in real time, and detecting when the structure approached instability. Simulation models predicted whirl flutter and were updated with test data. Ongoing comparisons of experimental results with simulation models are underway and will be published in upcoming conferences. The collaboration between NLR, DNW, POLIMI, LHD, and DLR contributed to a successful outcome, and an incredibly valuable whirl flutter data set.
This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 863418. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union
Authors:
Keith Soal and Marc Böswald, Department of Structural Dynamics and System Identification, DLR Institute of Aeroelasticity
