Theoretical and Practical Limits on Multi-Rotor Manoeuvrability

  • Ethan Bond

Student thesis: Phd

Abstract

This thesis contributes to the state of the art in the design and evaluation of high thrust-to-weight (T/W > 5) multi-rotors in horizontal axial flight through the development and experimental validation of novel theoretical models for predicting manoeuvrability. The models are validated using wind tunnel and flight testing. Key manoeuvrability metrics of interest are maximum airspeed and available acceleration, with utility metrics of range and endurance considered secondary. The literature on multi-rotor subsystem modelling is relatively mature, and reliable endurance prediction methods exist for multi-rotors in hover. However, significant gaps exist in understanding the systems and aerodynamic limits on the manoeuvrability of multi-rotors and the tools and models required to predict them. A novel flight testing method for measuring multi-rotor drag based in axial flight has been developed and evaluated. Using existing flight control sensors for experiment instrumentation is highly convenient experimentally but was found to significantly limit the achievable measurement accuracy due to susceptibility to the powertrain generated electromagnetic interference. In flight tests, the measured drag coefficient values were within an accuracy of ±6% of the wind tunnel measured values. Wind-tunnel experiments show that rotor interactions and aerodynamic download reduce rotor thrust coefficient by -15% for a given advance ratio. A multi-rotor manoeuvrability dataset is presented. The dataset includes drag coefficients, available acceleration, achievable airspeed, and powertrain performance data. The dataset improves on existing studies by covering a range of multi-rotors with significantly different aerodynamic configurations. A piecewise drag build-up model using primitives for estimating multi-rotor drag is validated with a 95% confidence interval of ±20% of the wind tunnel measured values. A multi-rotor maximum airspeed model based on the power equation, a powertrain efficiency parameter and the typical drag equation is found to overestimate maximum airspeed by up to +80% compared to flight test data. A second model based on momentum theory is validated to estimate multi-rotor acceleration with a 95% confidence interval of -5% to +10% of the wind tunnel measured values. By using rotor aerodynamic data and a download correction, the model accuracy is increased to have a 95% confidence interval of -5% to +4%. A theoretical study of multi-rotor manoeuvrability improvements uses the validated models to analyse the effect of improvements in battery specific power, body aerodynamics, powertrain efficiency and the use of variable pitch rotors. Near-term multi-rotor manoeuvrability improvements will likely come from the development of multi-rotor body aerodynamics and long-term from improved battery-specific power. The key contributions of the work are 1) Validated low-order models for multi-rotor manoeuvrability assessment. 2) A flight testing method for measuring multi-rotor drag coefficients. 3) A comprehensive multi-rotor manoeuvrability dataset. 4) An assessment of multi-rotor manoeuvrability performance improvements.
Date of Award1 Aug 2024
Original languageEnglish
Awarding Institution
  • The University of Manchester
Supervisorwilliam crowther (Supervisor) & Ben Parslew (Supervisor)

Keywords

  • Wind Tunnel
  • Manoeuvrability
  • Performance
  • Flight Testing
  • Multi-Rotor
  • Modelling
  • Quad-Rotor
  • Drone
  • Aerodynamics

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