Micro-Aerial Vehicle (MAV) Research: The Development of a Biplane Ornithopter

Prepared by Kin Yuen Leung
Supervisor : Prof. Mark Bradford & Dr. Zora Vrcelj

Ornithopter – Noun – A machines that flies by flapping its wings. Word root: Greek: ornitho for bird and pteron for wings.

1 Introduction

1.1 Background

Due to the current interest in MAVs, various designs for highly agile aircrafts are being explored. While flapping flight is a relatively unknown territory for man, it is one of the most prevalent form of locomotion in nature. The possibility of mimicking the flight capability of small birds and insects offers much potential for researches into the area.

1.2 Objective

As a part of MAV research, the possible advantages of flapping aerial vehicles are to be explored. The project could be divided into the two part:

1.) To gather and summarize current knowledge in flapping flight
2.) To build a prototype flapping flier as a testing platform for future research into the aerodynamics and control aspect of flapping flight.

Fig 1. The prototype ornithopter MAV

2 Existing Theory of flapping flight

The analysis of the aerodynamics of flapping flight can be broadly separated into two categories:

2.1 Quasi-steady state analysis

Acceleration is assumed to have no effect other than changing velocity; the forces on the wing at each instant is simply given by

where V(t) is the instantaneous velocity, CL(t) is the coefficient of lift which varies with the changing angle of attack. Due to the wing position dependence of V(t), the wing is divided into strips for analysis.

2.2 Unsteady flow analysis

Most results of current research into the effects of unsteady flow on flapping flight are qualitative. Various high lift generating mechanisms have been observed, although little has been attempted in quantifying these effects:

• Clap and fling (or clap and peel)
• Delayed stall
• Leading edge suction
• Wake capture – wake from previous strokes are used to generate more lift in subsequent strokes
• Rotational lift – the wings a rotated at the end each stroke to generate circulation

3 Design and construction of a prototype flapping MAV

3.1 Approach/Method

Due to the difficulty in analyzing the aerodynamics of flapping flight, we decided to modify an existing rubber band powered design with an electric power source.

3.1.2 Testing method
Small weights (Australian coins) were attached by sticky tape to the longitudinal center of mass of the model. The models were than flown to see if they can maintain altitude.

3.1.3 Results

Based on the above result, Luna was chosen for electrical modification.

3.2 Electric Motor selection

3.2.1 Torque requirement
In order to choose an electric motor capable of delivering the required torque to flap the wings, the stall torque provided by the original rubber band motor was tested. Small weights were tied using electrical wires to the crank of Luna, the torque at which the rubber band stalls was found to be less than 1.2mNm.

3.2.2 Pager motor
A toy manufacturer called Didel (www.didel.com) was found to supply miniature electric motors and gear boxes for ultra light fliers. The maximum power torque of their 6mm and 4mm pager motors at 3V was used to find the required gear ratio to achieve 1.2mNm. The motor that requires the least gear reduction was used.

3.2.3 Electronics/Battery
The radio control system of the autonomous blimp project was dissembled and refitted to provide 3 channel receiver plus electronic speed control.

4 First Flight

On Wed 9 Feb 2005, the prototype ornithopter MAV flew for the first time. Due to insufficient space in the laboratory (it flew for 5m till it crashed into the wall) the full capability of the vehicle is yet to be tested. There is much potential for further development, such as adding a servo actuated rudder, and non-hand launch mechanism .

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