Quadrotor Flight Performance

  • David Langkamp

Student thesis: Phd

Abstract

The aim of this thesis is to develop improved understanding of the effects of configuration choice on the forward flight performance of quadrotors, in particular on endurance and maximum flight speed. Configuration choices include rotor arrangement, fixed vs. variable pitch rotors and fuselage geometry. The work is distinct from previous research on large-scale helicopters not only in the rotor arrangement and fuselage geometry, but also because of area-volume scaling laws, lower Reynolds numbers causing a stronger CL/CD variation along the blade radius, trim without cyclic control and variable-speed electric propulsion.A numerical blade element method using nonlinear aerodynamic models was developed to provide the six components of hub forces/moments for any level flight condition of practical interest. A novel numerical method is presented to stabilise the blade element iteration schemes. A hingeless flapping model is included and several induced velocity models are compared. Wind-tunnel experiments on isolated fixed-pitch and variable-pitch rotors at a broad range of operating conditions were conducted as validation cases. It was found that a local-differential blade element momentum method could provide an acceptable and robust low-order solution over a large range of practical flight conditions.A simulation model for trimmed level flight was created by combining a semi-empirical fuselage aerodynamic model with the blade element code and a first order electric motor model. Design methods to improve endurance and maximum flight were evaluated through a number of configuration design case studies. Wind tunnel experiments were conducted to measure the aerodynamics of two different fuselages and rotor-rotor interference as a function of spacing and flight speed. A novel closed-loop trim experiment in the wind tunnel was shown to be an effective method to obtain quadrotor forward flight power and trim curves in a controlled environment.Results show that current quadrotors require large negative vehicle angles of attack for forward flight trim which causes a steep rise in power demand and limits forward flight speed. This is largely driven by the fuselage drag and downforce, which are widely ignored in quadrotor literature. There is also a trim limit due to rear rotor saturation that restricts maximum flight speed and efficiency. This arises due to the need to compensate the strong nose-up pitching moment from the hingeless rotors. Rotor-rotor interference appears to be similar in nature to tandem rotors, but effects for typical rotor spacings are small and mainly fall within the noise of the force measurements.The design case studies show that the use of collective pitch on quadrotors brings power benefits over a wide range of velocities if both collective and rotor speed are adjusted. However, if approximate mass penalties are taken into account net power benefits could only be shown for speeds above 80% of the top-speed. It is shown that a design optimised for forward flight should be in an x-arrangement to maximise pitch/roll control authority. Teetering rotors and a rearrangement of the vertical centres of gravity and pressure were found to be effective at reducing the net pitching moment and thereby increasing maximum speed in the order of 5-10%. A rotor shaft angle with respect to the fuselage can be used to align the fuselage to practical high speed angles of attack, minimise fuselage forces and reduce forward flight power by up to 20%, depending on the fuselage design.
Date of Award1 Aug 2012
Original languageEnglish
Awarding Institution
  • The University of Manchester
Supervisorwilliam crowther (Supervisor)

Keywords

  • trim
  • fuselage drag
  • local-differential inflow
  • power requirements
  • airframe aerodynamics
  • blade element theory
  • windtunnel
  • maximum speed
  • endurance
  • performance
  • interference
  • forward flight
  • quadrotor

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