Design, Modelling and Simulation of Maritime UAV-VTOL Flight Dynamics

Jun Cao1,a and Amir Anvar 1,b 1The University of Adelaide, School of Mechanical Engineering

SA 5005 AUSTRALIA

Emails: aJun.d.cao@student.adelaide.edu.au, bamir.anvar@adelaide.edu.au

Keywords: UAV, VTOL, Structure, Maritime, Aircraft, Design, Micro-sonobuoy, Model, Flight

Dynamics

Abstract. An Unmanned Aerial Vehicle (UAV) is referred to as a remotely piloted aircraft or an

unmanned aircraft which can be navigated with human operator in the loop. This paper discusses

the design of a UAV with Vertical Take-Off and Landing (VTOL) capability for Maritime

applications. The design provides a means for surveillance and communication in Maritime

applications. The VTOL-UAV utilises a five-rotor propulsion system that can be launched from

confined platforms such as ship-decks. The intended applications of this M-UAV drove the

development of an innovative landing system which in the case of emergencies can allow the M-

UAV’s soft and potential landing on the water surface. The other functionalities of this UAV are its

capabilities to deploy communication Micro-sonobuoys which allows for communication with

underwater Robot(s) via Operator in the loop. In this paper we also discuss the process of structural

design modelling and evaluation of the development of the Maritime Unmanned Air Vehicle.

Introduction

A VTOL-UAV is an aerial vehicle that does not require an onboard crew to operate. It is therefore

a useful tool in many operations, including search and rescue missions, environmental assessments,

possible sonobuoy deployment, and remote communications with operator in the loop and so on.

However, the UAVs can also be used to help people perform secure research and environmental

assessment within a natural disaster zone, (for example, earthquake and high radiation level

environment) (Anderson 1981). Additionally VTOL-UAVs have better maneuverability than other

types of UAVs and this will enhance the feasibility of the application. One example is the use of

UAVs’ after the earthquake and tsunami in Fukushima-Japan. By using VTOL-UAVs’ rescue

search processes could be enhanced as UAVs can be designed in micro size to access tunnels and

other gaps in demolished buildings (Eisenbeiss 2004) where conventional search methods cannot

reach. Additionally with the advantage of UAVs’ remote control capability people can undertake

environmental assessment without entering high radiation areas, thereby improving search safety.

Literature review

A review of the literature reveals that previous and existing designs of UAVs with vertical takeoff

and landing capability use a similar propulsion concept. The conventional configuration utilises a

certain amount of rotors for driving blades to produce sufficient lift for the aircraft. However, using

a different type of rotors will lead to different airframe design configurations. Several design

configurations have been selected for review such as single rotor and quadrocopters.

Applied Mechanics and Materials Vols. 152-154 (2012) pp 1533-1538Online available since 2012/Jan/24 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.152-154.1533

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 144.32.128.14, University of Newcastle, Callaghan, Australia-09/10/13,23:20:47)

Single rotor configuration is the most conventional VTOL UAV configuration and has been

investigated for nearly 80 years. In single rotorcraft the main rotor is used to generate sufficient lift

for the whole aircraft. The small tail rotor is used to counter the rotation torque produced by the

main blade. As a result of their mature design, the flight dynamics of this configuration is well

understood.

Single rotor configurations usually have high stability due to their large main rotor in size. The

other main disadvantage of a single rotor UAV is that a large rotor is required to be designed in

order to meet the design specification. For example the Joker 2 helicopter has a total weight of

4.89kg however the main rotor has a diameter of 1549mm (Stingu, 2010). The design specification

of the UAV-VTOL for this project needs the capability of carrying a sonobuoy with the weight of

2kg as payload. Based on this data the size of the expected rotor to be designed would be within the

vicinity of 1500mm in diameter, which would cause design difficulties, such as in manufacturing as

well as limiting the possibility of purchasing from the local market.

A quadrocopter, also called a quadrotor or quadrotor-helicopter, is an aircraft that utilizes four

rotors to propel the vehicle. Generally quadrocopters have the capability of stationary hover and

omnidirectional flight, whereby pitch angle, roll angle, yaw rate and thrust can be controlled

independently. This capability solves the issue raised by single rotor and tandem rotor UAVs

regarding the relationship between the stability and maneuverability. Recently quadrocopters have

become more and more popular due to their large payload capacity made by the large amounts of

lift that can be generated by those four rotors (AirRobot 2007, Stingu 2010, Goss, T. Nguyen, T. et

al, 2010) presently. AirRobot AR100-b is the largest commercial quadrocopter. It can fly silently

through the air or hover while transmitting live images to the operator at the ground station within

1km. However the maximum payload weight of this aircraft is limited below 200g (AirRobot

2007). Additionally its agility and position-holding precision have made that type of UAV widely

used for many tasks such as surveillance, rescue missions and payload delivery. However

quadrocopter control theory has become a popular research field in many Universities. According

to Sikiric 2008, most of these problems focus on the control of small, lightweight quadrocopters.

As most small scale quadrocopters use fixed pitch propellers to generate lift. This type of

configuration will ease the navigational control system flight dynamics as the orientation of the

aircraft can be controlled by changing the propeller’s rotational speed.

Design

The literature review introduced different types of existing design concepts which were discussed

in detail. In this section the overall design concept for the

maritime VTOL-UAV project will be introduced and

evaluated.

An important issues related to the VTOL UAV design is

how to balance the relationship between the stability and

manoeuvrability of the aircraft. The designed UAV needs

to be relatively stable both in forward flight and in hover as

well as the need to be agile, capable of

hovering in low altitude and meeting the

Figure 1: Maritime UAV-VTOL, Design Model

1534 Mechanical Engineering and Materials

requirement of fundamental control manoeuvres such as slow turns, forward flight. Additionally

innovation is an important issue to be considered. If similar designs can be found within the local

market the design will be considered as less innovative.

According to Salazar et al (2009) a rotorcraft will become more stable and efficient when the rotor

area is increased. Therefore the designed UAV needs to have more rotors in order to increase the

stability of the aircraft. However, this rotor leads to another issue which is the limitation of the

aircraft’s manoeuvrability. In order to tackle this issue, quadrocopter flight dynamics will be

applied to the chosen design as this type of configuration will ease the control, as the orientation of

the aircraft can be controlled by changing the propeller’s rotational speed. Thus the prototype

design needs to have more than four rotors to power the aircraft.

Hence a five-rotor configuration will be considered in the first step of design. Figure 1 shows the

conceptual design for this UAV. An air tube will be used in the design for the landing gear, as this

device acts as a shock absorber during hard landing. By using this feature, electronic components

which are located inside the ring shaped cage can be protected from shock.

Five-rotor and quadrotor configurations employ rotor speed control for orientation control.

Therefore the flight dynamics of a five-rotor configuration is similar to that of a quadrotor system.

This means that manoeuvring can be achieved with no extra mechanical complexity, weight penalty

or energy loss.

A review of the existing landing system in the market indicates that most landing systems are

designed to be hard landing. One of the major disadvantages for such designs is that if an aircraft

lands in an emergency scenario, the UAV will crash into the ground instead of landing safely.

Consequently both the airframe and electronic components will be damaged severely if the

approaching speed is high. By utilizing an air tube to construct a landing gear this problem will be

resolved.

Additionally air tube incorporation into the landing gear design, will minimize the landing shock

generated during landing.

Alternatively air tubes will enable the possibility of water surface landing if the air tube is large

enough as mentioned in the project definition. This

type of capability will be an additional innovation to

the five rotor configuration for the maritime UAV-

VTOL. However the ability to land on a water surface

will depend on the size of the landing tube selected

and the total weight of the aircraft. The air tube will be

fastened on the UAV by attaching the cable tires so

that there will be no extra components to be designed

for mounting landing system. One of the major

advantages of an air tube landing system is the

flexibility of the assembly.

Figure 2 shows the concept theory of the flight

dynamics for a five-rotor configuration UAV. Ascan

be seen, while the central main rotor rotates clockwise

the other four rotors rotate counter clockwise. This arrangement allows the rotation torque

produced by each rotor to be balanced. In terms of aircraft manoeuvrability, by increasing the

rotation speed of the right assistant motor and reducing the rotation speed of the left assistant

Figure 2: Flight Dynamic Diagram

Applied Mechanics and Materials Vols. 152-154 1535

motor, so that the rolling dynamics of the aircraft can be achieved. Additionally increasing the

RPM of the rear assistant motor and reducing the rotation speed of the front assistant rotor aircraft

can achieve pitching dynamics. This is discussed in the dynamic modelling derivation section in

more detail.

Dynamic modelling for hovering

The dynamic altitude model for the five-rotor UAV was derived from Newton’s Laws (Nelson

1998).The rotation torque produced by the main rotor in the center is cancelled out by four counter-

rotating rotors on the side, which removes any coupling between the pitch and roll dynamics.

However the total collective thrust, , is the sum of all five rotor forces, as given below:

(1)

This collective thrust will be equal to the total weight of the aircraft. Otherwise, when the vehicle is

in the takeoff and landing hovering phase, the dynamic control can be varied even with the pilot’s

initiative inputting towards variance rotation speed of each rotor. By reviewing the power plant

design, total thrust can be calculated by using the equation 1:

As the main rotor can provide 7500g lift for the aircraft therefore:

However, according to the calculations, the assistant motor can generate 1900g lift during the

operation:

Then total thrust would be

Test results

Designing a UAV will involve the selection of different components such as motors, propellers and

batteries. Although the performance of each component has been announced by the manufacturer,

conducting a test to prove the performance will be necessary before installing them onto the real

airframe.

Thrust capability analysis is of primary importance to ensure the thrust generated by the UAV is

sufficient for flight and meets the flight requirement. A propulsion test will allow the experimental

determination of the maximum thrust that can be achieved by a chosen propulsion system and

hence the thrust to weight ratio.

Figure 3: Testing equipment

1536 Mechanical Engineering and Materials

thru

st (

kg

)

Rotation Speed (RPM)

throetical

data

test data thru

st (

kg

)

Rotation Speed (RPM)

theoreticl

a data

test data

In this project, a static thrust test was conducted. The test rigid includes a pivoted beam with the

motor mount on one end and a strain gauge on the other end. Therefore the thrust generated by the

rotor is measured by reading the voltmeter that has been connected to the strain gauge. Additionally

in order to control the rotation speed of the motor, the speed controller is connected to a signal

generator. A tachometer is important for the propulsion test as the rotation speed is one of the most

important data to be collected during the operation. In order to simplify the test layout, a laser

tachometer is used as it has the simplest setup. Figure 3 above shows the laser tachometer used in

the test, the test rigid layout, electronic components and the overall test layout respectively. The test

results for the main motor test are shown in Figures 4 and 5.

Reviewing the graphs indicates that when the rotation speed of the main motor is 5500rpm the main

motor will provide sufficient thrust according to the theoretical calculation. However, the rotation

speed of the assistant motor will be 5000rpm in order to provide the sufficient thrust for

manoeuvring the aircraft.

Conclusion

This project illustrates the capability of five-rotor VTOL-UAVs for Maritime applications through

the design and build of a prototype. The UAV has been manufactured and tested according to the

time frame constructed in the early stage of the project. It gave strong proof of the concept and a

basis for future work. This project is also unique in the design concept employing a five-rotor

configuration, Micro-communication Sonobuoy deployment, soft landing system and a water

surface landing capability. The high level of innovation shown in this project will be present in the

development of the system.

Acknowledgement

The Authors would like to thank Mr Amir Parsa Anvar for his editorial comments on this paper.

Figure 4: Main Motor Test-results Figure 5: Assistant Motor Test-results

Applied Mechanics and Materials Vols. 152-154 1537

References

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[4]Coleman, C.P., (1997), A survey of theoretical and experimental coaxial rotor aerodynamic.

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[10] Nelson, R., (1998), Flight stability and automatic control second edition, McGraw-Hill Higher

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1538 Mechanical Engineering and Materials

Mechanical Engineering and Materials 10.4028/www.scientific.net/AMM.152-154 Design, Modelling and Simulation of Maritime UAV-VTOL Flight Dynamics 10.4028/www.scientific.net/AMM.152-154.1533