Experimental identification of blade stall in axial and edgewise flight conditions

Croke, Alexander Daniel (2025) Experimental identification of blade stall in axial and edgewise flight conditions. PhD thesis, University of Glasgow.

Abstract

The recent resurgence of the airscrew propeller, growth in both military and civilian tiltrotor technology and successful operation of a rotorcraft on Mars has led to a reinvigoration of research surrounding blade stall in axial and edgewise conditions. Modern tiltrotor blades are designed to operate in both propeller and rotor mode, thus operating in underexplored regions of the operational flight envelope, where aerodynamic and aeroelastic instabilities may occur, leading to unfavourable and potentially fatal operational conditions. Despite several experimental investigations focused on blade stall over the past century, there is a clear lack of a comprehensive aerodynamic and aeroelastic experimental data set of sufficient resolution which can provide further insight into these complex phenomena and be used to validate high-fidelity numerical models. Therefore, within this body of work, state-of-the-art experimental test rigs were commissioned to develop a novel experimental methodology, primarily based on blade strain measurements, that can be utilised to identify operational conditions at which blades are stalled, for both axial and edgewise flight regimes.

Axial blade stall was investigated using the United Kingdom National Rotor Rig situated at the University of Glasgow De Havilland wind tunnel. A blade stall operational boundary was constructed for the MENtOR tiltrotor blade set through the use of key markers within the blade strain, wind tunnel velocity and motor data. It was shown for stalled conditions that the flap bending and torsional strain measurements departed from linear behaviour when increasing blade pitch with substantial growth in their standard deviation, up to twice the pre-stall value. Stall was also apparent in the strain spectra, exhibiting a significant non harmonic content and large distribution of signal energy. Unsteady flow generated by the stall acted as a broadband forcing term for the blade structural dynamics resulting in the identification of blade eigenmode frequencies, particularly the first flap bending mode. To support the validity of the developed stall boundary identification criteria, a multi measurement approach was implemented to identify blade stall, using independent measurement techniques including stereoscopic Digital Image Correlation and load measurements. Flap bending deflection measurements and load remeasure-ments were shown to correlate strongly with the identified stall conditions obtained using the stall boundary identification criteria.

Subsequently, an investigation using a single bladed rotor was performed at the University of Maryland towing tank to assess the viability of utilising measurements of the blade strain distribution to identify regions of reverse and separated flow in edgewise conditions. Reverse flow regions were identified through phase resolved measurements of rotor thrust, torque and pitching moment around the rotor azimuth. At large advance ratios where strong reverse flow effects occurred, phase resolved measurements of blade flap bending strain exhibited a strong agreement with load identified azimuthal locations at which the blade entered and exited the reverse flow region. Averaged and unsteady strain measurements displayed an identical trend to load measurements, demonstrating the applicability of strain distribution to identify stalled and reverse flow regions.

The results of this work have demonstrated the ability to use strain measurements to identify the presence of blade stall in both axial and edgewise conditions. Highlighting the capability to quickly, reliably and cost effectively develop blade stall boundaries when compared to more conventional experimental methods. Finally, the novel criteria developed within this work can be implemented across many technical readiness levels supporting academic and commercial blade testing activities.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Supported by funding from Dowty Propellers.
• Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Funder's Name: Dowty Propellers
• Supervisor's Name: Green, Dr. Richard and Barakos, Professor George
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85317
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 08 Jul 2025 15:17
• Last Modified: 08 Jul 2025 15:19
• Thesis DOI: 10.5525/gla.thesis.85317
• URI: https://theses.gla.ac.uk/id/eprint/85317

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