Safe Flutter Determination for Wings Undergoing Large Deflections

Safe Flutter Determination for Wings Undergoing Large Deflections: Experimental Application of the Parametric Flutter Margin Method to a Highly Flexible Wing

de Boer, Stefan (TU Delft Aerospace Engineering)

Sodja, J. (mentor)
Karpel, Moti (graduation committee)

Delft University of Technology

Aerospace Engineering

2023-01-12

The flight tests which are part of the certification procedure that prove that no flutter occurs within the flight envelope are fraught with risk. To reduce the risk, numerous numerical analyses, wind tunnel and ground tests are performed, which together with the flight tests result in a long and costly test programme.

To reduce the risks associated with flutter flight testing, a new flutter test version of the Parametric Flutter Margin (PFM) method, specifically applied to wings undergoing large deflections is presented. The PFM method adds a stabilising parameter, such as a stabilising mass, to the model such that the flutter velocity is increased. By exciting the stabilising mass in one of the primary (i.e., x, y and z) directions while simultaneously measuring the response in these directions and repeating the excitation in other directions, the flutter margins that are associated with the original model can be determined. To demonstrate the method a wind tunnel test campaign was performed at TU Delft using the Delft Pazy Wing which can exhibit large nonlinear deflection. The wing was equipped with a flutter pod consisting of a shaker and stabilising mass that was placed at the mid-span position at the leading edge of the wing.

During the test campaign, three test series were performed. The first identified the flutter boundary through direct flutter tests, with the flutter onset and offset velocities being determined by actually hitting flutter that turned into an LCO. The second and third test series were the PFM measurements, where both SISO and MIMO PFM were applied to obtain the nominal flutter boundaries without actually hitting flutter. The different PFM results showed a maximum difference of 4.4 % between each other at an angle of attack, α, of 6°, with the difference found to be minimal at α = 0° and increasing with increasing angle of attack, which was as expected.

Compared to the directly measured flutter tests, the MIMO PFM results identified a flutter velocity of 4.8 % lower than the directly measured flutter offset velocity at an angle of attack of 4°, and the SISO PFM results identified the flutter velocity to be 8.2 % lower than the offset velocity at 4° angle of attack. The PFM-identified flutter frequencies showed a difference of less than 2 % compared to the direct flutter test, with the difference in identified flutter frequency between the SISO and MIMO PFM reaching a maximum of 2 %, which was also increasing with increasing angle of attack.

However, even with the successful application, several research areas remain open, and several points of improvement for the performed test campaign have been found. Although besides the points of improvement, the acquired data shows the potential of the PFM method for performing safer, shorter and consequently cheaper flight tests for the certification procedure of new aircraft configurations.

Aeroelasticity
Aeroelastic flutter
Parametric Flutter Margin method
Flexible Wing
Delft Pazy Wing

http://resolver.tudelft.nl/uuid:b0e12214-ca2a-4dde-8499-955d2da08937

Student theses

master thesis

© 2023 Stefan de Boer