Numerical study of morphing helicopter rotors

Steininger, Rinaldo (2025) Numerical study of morphing helicopter rotors. PhD thesis, University of Glasgow.

Abstract

This thesis shows high-fidelity numerical simulations for combined fluid, structural and servo-systems of a helicopter, to make predictions and validations of helicopter rotor on blade-actuators. This aero-servo-elastic approach forms the main novelty of this thesis. The physics of the active rotor blade aerodynamics and structural dynamics are simulated, and the resulting knowledge can then be implemented in comprehensive rotor suites. Assessing the physics is important for the introduction of new rotor concepts, something which low- and mid-order methods can not do. The Helicopter Multi-Block 3 (HMB3) simulation suite developed at University of Glasgow was expanded to include the servo system in the aeroelastic models. The software does not include a rigid body model for the rotor hub. Both hub and blade structural dynamics are simulated in the commercial FEM solver MSC NASTRAN.

Expanding the flight envelope with higher top-speeds, while avoiding retreating blade stall, has been a challenge since the inception of helicopters. A promising innovation is the variable twist rotor blade, which through intrinsic actuation, can optimise the twist for the given flight condition and exploit aeroelasticity to modify the rotor blade path for vibration control and possibly higher trim-able thrust. This thesis only considers twist actuators, due to their benefits in weight, simplicity and airworthiness over other actuation methods. However, the aero-servo-elastic modelling can be applied to any rotor blades with piezoceramic actuators, such as active flaps or Gurney flaps, if the actuator is sufficiently resolved in the structural model.

The aeroelastic method is validated against the Helicopter Validation and Acoustic Baseline rotor (HVAB). Performance metrics, structural deformations and flow physics are compared and confirm the accuracy of the method. Additionally, rotor blade structural models of 1D beam and 2D/3D-finite elements are modelled and compared, including a mesh study. Finite elements allow the modelling of on-blade actuators, instead of simplifying the problem to only include their effects in simulations. The structural models of the Smart Twisting Active Rotor (STAR) are compared to experimentally obtained data, for validation of the methods used in this thesis. This also includes a study of the actuator modelling via a thermal analogy method, where the voltage on a piezoelectric material is equated to temperature in a thermally expanding material.

Using the developed aero-servo-elastic method, improvements in the forward flight vibration metric could be predicted for the STAR rotor blade in two flight conditions. The largest vibration improvement was observed at high-speed level flight. In this condition, increased passive blade twist has a large vibration penalty. In a level flight at maximum thrust, experiencing blade-vortex interactions (BVI), the active twist was shown to improve the moment trim, leading to conclusions of higher thrust capacity in this relatively high-speed flight case. The observed effects on the rotor lift-to-drag ratios were negligible.

For the STAR, a 1D beam model and a mixed 2D/3D finite element model were compared. The beam model overpredicts the blade mode frequencies of the measured rotor blades, because the sectional properties of the received dataset were slightly overpredicted. A modified set of properties, yielding more accurate mode frequencies is also presented. The finite element model, built from available geometric and material data, was within 10% of the measured frequencies. The strong 3-dimensional structural coupling of the finite element model is showcased, which could not be observed in the beam model. A small blade-untwisting under centrifugal force is found for the finite element model. The actuator effectiveness degrates under centrifugal tension and this mechanism is discussed. A hovering simulation of the rotor, with a loosely coupled fluid-structure simulation shows the differences between the models. It concludes, that especially for blades with torsion actuators, 3D finite element model approaches are necessary to obtain correct results.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > T Technology (General); T Technology > TL Motor vehicles. Aeronautics. Astronautics
• Colleges/Schools: College of Science and Engineering > School of Engineering > Autonomous Systems and Connectivity
• Supervisor's Name: Barakos, Professor George N. and Woodgate, Dr. Mark A.
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85237
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jun 2025 14:24
• Last Modified: 30 Jun 2025 14:26
• Thesis DOI: 10.5525/gla.thesis.85237
• URI: https://theses.gla.ac.uk/id/eprint/85237

Related URLs

• Article DOI
• Article DOI
• Enlighten Publications Record
• Book section
• Enlighten Publications Record
• Conference proceeding
• Conference proceeding
• Conference item