Investigation into the flight performance of an autorotating Samara with varying flight conditions

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Samara seeds are one of nature’s most outstanding fliers. Using their light wings and the use of a special flow mechanism known as the leading edge vortex (LEV), samaras are capable of generating high lift whilst descending at extremely low speeds. Such efficient flight characteristics observed in samaras led to many samara-inspired Micro Air Vehicle (MAV) designs. However, the performance of samara, in the wide design envelope, were not fully understood. Furthermore, in recent decades, emphasis has been placed on visualising the LEV of the samara, but there has been very little progress on characterising their flight performance.

The primary aim of this thesis is to investigate the flight performance of autorotating natural samara with varying flight conditions. The investigation focuses on understanding how certain characteristics of the samara affect its thrust generating capabilities and hence affect its descent speed. In this process, we attempt to shed some light on why samaras adjust their flight parameters in the way they do. To achieve the objective of the thesis, the flight performance of the samaras in natural descent were first characterised via a series of droptest experiments. In parallel, an analytical model based on the Blade Element Momentum theory was developed, which incorporated different lift and drag coefficient models to estimate the aerodynamic characteristics of a samara with a leading edge vortex. Finally, series of different wind tunnel tests of natural samara were conducted at various wind speed and flight conditions. This was made possible by using a novel experimental Samara rig mounted on a specially designed wind tunnel and a bespoke Samara hub, designed to allow the control of the coning and the pitch angles. Wind tunnel and droptests results were then combined with the numerical analysis to obtain better insights into samara aerodynamic performances.

For the first time, a comprehensive set of data of autorotating samaras was obtained. This was novel in that the thrust and rotational rates of the samara were measured with changes in the wind speed, coning and pitch angles. Furthermore, the experimental analyses revealed that moderate coning angles can actually increase the thrust produced by the samara which to the author's knowledge is novel. Additionally, it was found that samaras operate at extremely low pitch angles to maximise the thrust produced. The numerical analysis also investigated the most appropriate lift and drag coefficient models to describe the LEV of the samara and gave estimations for the aerodynamic characteristics of the natural samara.

It is foreseen that the novel wind tunnel results obtained in this research will be used in validating samara models of various studies and contribute to the design of more efficient MAVs.
Date of Award22 Mar 2022
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorDjamel Rezgui (Supervisor)

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