rmit-nuaa epuav reportaircraftdesign.nuaa.edu.cn/pd-2007/report/2010-b.pdf · 2010 chua yee leing...

2010

Chua Yee Leing

Wing Tat Christopher Li

Lucille Temporal

David Boktor

Nita Wiroonsup

Nanheng Lv

George Burry

RMIT-NUAA EPUAV Report

2 | A e r r o w s - E P U A V R e p o r t

CONTENTS

CONCEPTUAL STAGE

Configuration design ......................................................................................................................................... 9

Initial sizing....................................................................................................................................................... 15

Fuselage layout ................................................................................................................................................. 15

Wing design ....................................................................................................................................................... 17

Airfoil selection ................................................................................................................................................. 17

Empennage design ........................................................................................................................................... 22

Landing gear ..................................................................................................................................................... 23

Aircraft performance ....................................................................................................................................... 24

Weight estimations .......................................................................................................................................... 25

Avl analysis ........................................................................................................................................................ 28

Analysis .............................................................................................................................................................. 32

PRELIMINARY STAGE

Structural layout and sizing ........................................................................................................................... 33

Integration of the propulsion system onto the airframe ............................................................................ 33

Propulsion Analysis and Test Results ............................................................................................................ 35

Wing structure Analysis .................................................................................................................................. 39

Control surfaces ................................................................................................................................................ 44

DETAILED DESIGN

CAD Definition ................................................................................................................................................... 46

FABRICATION

Fuselage Fabrication ....................................................................................................................................... 52

Wing fabrication ............................................................................................................................................... 55

Vertical Stabilisers Fabrication ...................................................................................................................... 57

Control Surfaces ................................................................................................................................................ 59

Landing Gear Assembly ................................................................................................................................... 60

Skinning ............................................................................................................................................................. 61

Accessibility ....................................................................................................................................................... 63


FLIGHT TEST

Attempt 1 ............................................................................................................................................................ 68

Attempt 2 ............................................................................................................................................................ 68

Attempt 3 ............................................................................................................................................................ 70

Attempt 4 ............................................................................................................................................................ 71


Nomenclature

Aspect Ratio Wetted Aspect Ratio Wing Span Span of Vertical Tail Wing Chord Mean Aerodynamic Chord Mean Aerodynamic Chord of Vertical Tail Coefficient of Drag Coefficient of Drag at Zero-Lift Drag Equivalent Skin Friction Centre of Gravity Coefficient of Lift Coefficient of Lift at

Maximum Lift Coefficient of airfoil

Maximum Lift Coefficient

Lift to Drag Ratio Coefficient

Moment Coefficient Wing Chord at Root Wing Chord at Tip Vertical Tail Volume Coefficient Vertical Tail Chord at root Vertical Tail Chord at tip

Drag Oswald Efficiency Factor Height of Fuselage Length of Fuselage Length of Fuselage Nose Width of Fuselage Height of Vertical Tail Lift Moment arm of Vertical Tail

Maximum Lift to Drag ratio

Dynamic Presure

Reynold’s Number


Reference Area of the Wing Reference Area of side of Fuselage Reference Area of top of Fuselage

Wetted area of Fuselage

Takeoff Distance Reference Area of Vertical Tail

Wetted area of Vertical Tail Wetted Area of the Wing

Wetted Area Ratio

Maximum Thickness

Thickness ratio

Maximum level flight Velocity Minimum level flight Velocity Stall Velocity Fuselage Weight Vertical Tail Weight Takeoff Weight Wing Weight

Wing Loading at Stall

Wing Loading at Cruise

Maximum Angle of Attack Taper Ratio Taper Ratio of Vertical Tail Leading Edge Sweep Angle

Quarter Chord Sweep Angle


EXECUTIVE SUMMARY

An Electric Powered Unmanned Aerial Vehicle (EP-UAV), the ‘Aerrows’, was

developed to fulfill the requirements of a design project for the subject ‘Course Project

of Aircraft Design’, at Nanjing University of Aeronautics and Astronautics (NUAA). The

team consisted of RMIT students undergoing the RMIT-NUAA Exchange Program 2010.

It was decided that the main objective of the Aerrows mission would be to create

a light-weight UAV. The Aerrows design was developed based on the desire to unveil an

unconventional and light-weight aircraft within the given criteria that was given. The

team (lead by Chua Yee Leing) encountered various difficulties during design due to the

high level of complexity as a result of using a duct-fan, rather than a commonly used

propeller. The team devised six initial design configurations, although only two were

analysed in detail. Initial sizing and calculations were based on the design requirements

communicated to the team at the beginning of the course. These calculations were

completed with the assistance of information extracted from past RMIT-NUAA EP-UAV

project reports and methods specified in the book ‘A conceptual Approach in Aircraft

Design’ by Raymer (1992).

Various problems such as: air inlets, wing positioning, weight and landing gear

configuration were encountered while working to satisfy these requirements. After

analysing the two possible configurations, following on from innovative proposals from

professors, a duct-fan powered single engine tailless design was chosen as the final

aircraft configuration. The design process was iterative with a corresponding increase

in detail and accuracy, as well as frequent modifications to design in order to achieve

better performance, throughout all stages of design process.

In the later stages of the design process, the aircraft’s components were

designed using CATIA in collaboration with the appropriate software to import the

desired airfoil profile. The airfoil section for the wings and vertical stabilisers was

chosen using ProfiliV2, and then the aerodynamic coefficients were calculated using the

software AVL. The centre of gravity was calculated manually in the earlier stages of the


design, as it was prone to change every time the team decided to modify the design;

however in the later stages CATIA was used once again to determine the weight of the

aircraft’s components and its centre of gravity.

INTRODUCTION

The aim of ‘Course Project of Aircraft Design’ was to design and manufacture an

EP-UAV over a period of almost four months. A group of thirteen exchange students and

one post graduate student were divided into two smaller groups; each group

developing an EP-UAV, resulting in two aircraft. The goal of the Aerrows project was to

create an unconventional, yet aesthetically stylish, light-weight aircraft, aiming to

collectively apply each member’s knowledge and capabilities in achieving the

requirements related to the aircraft’s design and construction. The purpose of this

report is to analyse the processes involved and present the project.

The following course objectives have been addressed:

To understand the process of the aircraft design and developments

To integrate the knowledge of: aerodynamics, aircraft structures, aircraft

propulsion, and flight performance

To develop engineering student's problem solving skills

To cultivate a teamwork environment within the aerospace engineering

discipline

In addition, the project has specific design requirements, as listed below:

Performance Requirements

Endurance ≥ 11 minutes

Maximum Level Flight Speed (Vmax) ≥ 18 m/s


Minimum Level Flight Speed (Vmin) or Stall Speed (Vstall) ≤ 9 m/s

Takeoff Distance (STO) ≤ 18 m

Gross Weight (WTO) ≤ 2.8 kg

Payload (WPL) ≥ 0.5 kg

It was initially anticipated that Aerrows might exceed the gross weight specification due

to the decision of using a duct fan rather than a propeller. Thus, for most of the

estimation the gross weight stated was reversed.

Operational Requirements

Wing Span ≤ 2.3 m

Fuselage Length ≤ 2.2 m

Cost Requirements

Airframe Cost ≤ 2000 RMB

TEAM COMPOSITION Seven exchange students from RMIT University (Melbourne, Australia) collaborated to

develop the Aerrows:

Chua Yee Leing

Wing Tat Christopher Li

Lucille Temporal

David Boktor

Nita Wiroonsup

Nanheng Lv

George Burry


CONCEPTUAL DESIGN STAGE

CONFIGURATION DESIGN

The team was collectively motivated to develop an unconventional aircraft based on

past RMIT-NUAA student EP-UAVs. Consequently, a proposal to use a duct-fan was

highly favourable. After, deciding that the plane will use a duct-fan for propulsion, the

plane has more thrust available than what a propeller would yield. As a result, the plane

is then comparatively faster than it would be if a propeller were used; and for because

of this addition thrust and speed, the group ambitiously decided to design an aerobatic

aircraft. After a group discussion and referendum, the decision was made to name the

aircraft 'Aerrows'.


Various design options were put forward in an attempt to meet the previous

requirements, as well as the general aim of the group. These design options are detailed

below:

Designs 1 and 2

Two designs were devised. Design 1 and Design 2 were similar in design, with the only

difference being either the use of a propeller mounted at the nose, or the use of a rear-

above mounted duct-fan. As the group were not sure if using a duct-fan would be

available for use with this project, having a second prototype provided redundancy. Not

having a duct-fan would lead to only a minor-compromise and not an inhibiting hinder.

Design 1 later became obsolete, after the team was able to both ascertain duct fan’s

suitability and availability.

Design 3

After looking at Design 2 in more detail, and comparing it to previous designs

developed by past students, the group decided that the plane was too basic and

seemingly bared resemblance to the past EP-UAVs. As a result, Design 2 was also axed

from the list. Design 3 was then created to add a ‘greater’ purpose for the plane. This

design was influenced by agricultural aircraft.


The configuration had slots for stowage and release of: fertiliser, water, and seeds so

that it can be utilised in big farms. An issue with this configuration was that there would

be many installations which need to be incorporated in order for the plane to fulfil its

purpose. Further still, the design would most likely need to be propelled by a propeller,

which was not an express interest of the group, where the duct-fan is a point of

difference. For these reasons, Design 3 was removed from the table along with the first

two designs.

Design 4

Design 4 was suggested and influenced by the fighter aircraft configurations: F14 and

F18. Such an aircraft initially aroused much interest and excitement amongst all

members of the group, as it would be aesthetically and functionally impressive, should

it be possible for the team to pull it off. However, after looking further into the design


and fabrication requirements, it became apparent how complex the notion of

developing a model fighter jet indeed is.

This was overwhelming when comparing the level of knowledge and expertise required

to the actual knowledge and experience existing throughout the team. The

configuration was modified to a more realistic and practical version. This was

manifested through designs five and six.


Designs 5 and 6

Designs 5 and 6, present similar complications in terms of manufacturing – the air

inlets proving to be the biggest pre occupier of thought, as it would be difficult to

connect the wings to the fuselage, and there would be a notable increase in overall


weight of the aircraft. CAD configuration designs for both Design 5 and 6 also show that

manufacturing will be either extremely difficult or impossible, given which materials

would be available.

Final design configuration

Finally, the group achieved a consensus that it would be simpler and cheaper if the

aircraft only had one duct-fan as opposed to two, as was previously considered. Yet, if

the single resulting duct-fan is placed on the bottom of the fuselage, manufacturing is

still a problem. As a result, the group decided to mount the duct-fan above the fuselage

and consequently, this approach meant that the tail configuration needed to be altered,

as there would be a conflict between the duct-fan and/or its inflow/outflow, and the

vertical stabiliser. Ultimately, a tailless aircraft with two vertical stabilisers was decided

upon, and this became the Aerrows’ final configuration.


Aerrows’ final configuration: top, side, front, and bottom views

INITIAL SIZING

Initial sizing was quite challenging as the Aerrows' configuration was not within the

scope of the reference book given for the project. Therefore, opinions from the

professor and a fair amount of research were required for the initial dimensions of the

aircraft. The reference book, Aircraft Design: A Conceptual Approach (Raymer 1992)

was still used for formulas, estimates and criterions for the wings, empennage and

landing gear geometry calculations.

FUSELAGE LAYOUT

The fuselage layout for the Aerrows was chosen so that it would be aerodynamically

beneficial for the purpose of reducing drag; it is feasible and easy to manufacture, easy


to assemble other components of the aircraft, and at the same time aesthetically

tailored.

Square Fuselage

A square fuselage, is easy to manufacture and will provide an easy assembly for the wings.

However, a square fuselage is not good aerodynamically as air flow will not be smooth thus

creating more drag which is not desirable for the aircraft especially for an aerobatic aircraft.

Circular Fuselage

A circular fuselage similar to most aircraft is proven to have a superior aerodynamic

performance in comparison to a shape with sharp edges. However, for a student aircraft

project, manufacturing a circular fuselage will result in greater complexity when attaching

the wings into the fuselage.

Listing the pros and cons for a square fuselage and a circular fuselage, the group

decided that for the purpose of the Aerrows, a combined configuration for the fuselage

is the best approach. The round/curved top and bottom surfaces will provide the

desired aerodynamic performance while the flat side surfaces will provide the ease of

attaching the wings onto the fuselage.

Fuselage cross-section shape


WING DESIGN From observing the previous aircraft configurations for the Aerrows, it was clear that a

tapered wing would be ideal for the group mainly for aesthetic reasons. Hence, the tapered

wing design was adopted. Since the Aerrows has a tailless configuration, it was advised

that a larger sweep would lead to better flight performance in comparison to a smaller

sweep. Thus, the wing appears, to some degree, similar to a delta wing.

Initial calculations were made for designs five and six; however, since the final

configuration for the plane has a similar design, apart from the empennage, the initial

values calculated did not change dramatically.

AIRFOIL SELECTION Initially, the group was considering a thin airfoil as it was proven that this would be more

suitable for high speed. However, given that the plane will still lie within the subsonic

range, a slightly cambered and thicker airfoil is a more desirable option. The selection was

Wing design with dimensions


based on factors such as: airfoil cruise drag, stall characteristics, pitching moment, and

ease of manufacture.

Airfoil characteristics are strongly affected by the Reynolds number. The reynolds number

influences whether the flow will be laminar or turbulent, or whether flow separation will

occur. Maximum camber is an important factor, as it allows the flow to remain attached

and thus increases the lift and reduces drag. It also increases the lift by increasing the

circulation of the airflow. The thickness ratio has a direct effect on drag, maximum lift and

structural weight. The drag increases as the thickness is increased due to increased

separation and should be in the range of twelve to sixteen percent.

After reading previous reports and conducting some research, a group of five potential

airfoils for the Aerrows configuration were shortlisted during the airfoil selection process.

ProfiliV2 was used to examine some of the airfoil’s characteristics and values, allowing

individual comparisons between the airfoils.

The four airfoils which were shortlisted after using the direct method selection are

listed below:

NACA 0012 – was considered due to its wide usage in aircraft design, especially

for ‘first-time’ aircraft building. Professor Yu also suggested that this airfoil is

worth looking at for the aircraft’s aim.

NACA 1412 and NACA 2412

- NACA 1412 and NACA 2412 were considered as they have the same

thickness as the NACA 0012 and therefore it shows a similar curve in

ProfiliV2, with a few different characteristics due to different camber

values. This then gives the group an idea of how much camber the

aircraft needs to fit its aim.


Clark-Y 11.7% smoothed – was considered due to its use in tailless aircraft.

The airfoil is also still relatively thick; hence it remains within the requirements,

set out by the group.

ProfiliV2 Cl vs Cd graphs of the shortlisted airfoils,


The first value, pitching moment coefficient, about the aerodynamic centre (where the

value does not vary with the change in angle of attack), is a function of the pressure

distribution (camber line) along the chord. In general, it was noticeable that the higher

the maximum lift coefficient of the section, the higher the pitching moment. During

cruise, the horizontal tail therefore must provide necessary down lift in order to counter

the nose-down tendency caused by the pitching moment value, hence the higher the

pitching moment the higher the trim drag. An additional effect of too much pitching

moment is that if the trim requirement due to the wing pitching characteristic is high,

the forward CG limit may have to be positioned further aft to assure that the elevator

has sufficient power to provide flare control at minimum speed during landing.

Since, Aerrows’ has a tailless configuration, the ailerons on the wings act as both

ailerons and elevators (elevons). For this reason, it was desirable that the airfoil used

with the Aerrows have a low pitching moment in order to reduce the nose down

moment produced by the wing, leading to greater stability. Comparing the values

attained from the graphs for the short-listed airfoils, it was clear that Clark Y satisfied

the requirements for the aircraft configuration – high lift coefficient, high lift-to-drag

ratio resulting in better climb performance, and low pitching moment.

As opposed to the past RMIT-NUAA EP-UAVs which made use of non-airfoil stabilisers,

the group decided that it would be better to use airfoil vertical stabilisers for improved

aerodynamics. The aim for selecting an airfoil for the plane’s stabilisers is quite separate

from the wing airfoil selection; this is for the reason that the stabilisers are not required

to generate lift, just as the wing does; and thus, the vertical stabilisers do not require a

high-lift coefficient. Based on lift-coefficient not being important, it was decided that the

airfoil should be reasonably thin and symmetrical. Since the NACA 0012 was already

being considered, it was more expedient for the group to choose from the 4-digit NACA

series, which are slightly thinner than the NACA 0012. As a result, the NACA 0010 airfoil

was chosen for the vertical stabilisers.


EMPENNAGE DESIGN

The Aerrows has similar design characteristics to a flying wing design, which results in

the omission of an empennage. Therefore, horizontal stabilisers are not required, and a

vertical stabiliser is only required where there is a need for additional stability in the

yawing plane. Practically, the vertical stabiliser(s) can only be attached between the

wings; or one on each wing, in the case of a twin configuration. Two vertical stabilisers

were decided for the plane due to the fact that the aircraft’s configuration is tailless

which means that the plane would not have enough control for manoeuvrability.

Consequently, the placement of the propulsion system and the degree of the available

stability and control were considered.

Empennage(vertical stabilisers) initial design


LANDING GEAR

Due to the configuration of the aircraft, it was decided that the most suitable landing gear setup would be

a tricycle configuration. Landing gear geometry was determined using the estimated CG found using the

AVL software. As the duct-fan is mounted above the fuselage, towards the rear of the aircraft, the vertical

stabilisers must have enough clearance from the duct-fan; yet, not so far apart as to not leave enough

room, outboard on either side, for the elevons. Also, due to the Aerrows’ configuration, the main landing

gear must be aligned with the vertical stabilisers, as reinforced ribs will be placed in this position. This

provides a starting value for the landing gear geometry. However, since the distance between the two

main landing gears are the only value that is known, an excel spreadsheet was used to estimate the height

and position of the landing gear.

The following diagrams show the criterions used for the estimation:


The stability requirement formula was rearranged to set the height of the landing gear

(h) as the unknown, because the distance between the main gears (B) is fixed. A trial-

and-error approach was then applied to determine the distance between the main

landing gears and the nose gear (b) based on the estimated CG position of the plane.

After calculations, the height of landing gears is 155 mm.

AIRCRAFT PERFORMANCE

Giving the thrust provided by the duct fan the approximate value for the ground friction is

0.35 the take-off distance was calculated to be


Using the specified current for the duct fan and using two batteries, endurance was then

calculated to be

Note that the endurance of the aircraft is not as long as propeller-propelled aircrafts.

This was expected as the duct fan did add a much large weight than if a propeller is

used. Also, the duct fan requires more current to keep its blades in rotating. However,

the 7 minutes endurance span is agreed to be enough for the course.

WEIGHT ESTIMATIONS

Empirical formulas were used to estimate initial weights of the aircraft and its

components. These formulas were taken from equations used for a program analyzing

electric powered UAVs at low Reynolds numbers (Yu, 1998):

Wing:

Fuselage:

Vertical Tail:

Landing Gear:


The group has calculated these initial weight estimations through researching past

EPUAVs and their usual weights and also reading the past RMIT-NUAA EPUAV reports.

This was an effective approach for initial sizing as it gave the group an idea of the

average weight of each of the component and thereby the overall plane which further

helped in deciding the number of reinforced ribs and plates the group can use.

PART WEIGHT (kg)

0.2123

0.2647

0.100

0.21

0.50

0.40

0.20

0.15

TOTAL EST WEIGHT 2.037 kg

The above weight estimations set the aircraft’s initial take-off weight to be ~2 kg. After

discussing with Professor Yu, it was agreed that the Aerrows’ configuration and mission

is complicated enough and adding a payload might just hinder the plane to perform its

designed purpose. As a result, the group decided that Aerrows will either have a very

light payload (<0.5kg) or no payload at all.

Aircraft Performance

Lift

Drag


t/c >0.05

PART

Wing 0.38664 0.7885 0.0106

Vertical Tail (x2) 0.0680 0.139 0.0236

Fuselage 0.25012 0.39289 0.00911

Following the empirical calculations for the drag predictions, the analysis was then

carried out using software called AVL which utilizes the vortex-lattice method for

analyzing the aircraft’s aerodynamic parameters. To first work with the program a text

file must be created, which the program can read and use to calculate the aircrafts

parameters. This text file is unique for every configuration, however a sample text file is

provided to the group for references.


AVL ANALYSIS


AVL input data


Aerrows’ AVL data


AVL graphs according to the input data


ANALYSIS The concept of a tailless aircraft was approved and revised by Professor Yu, however

uncertainty surrounding the manufacturing process still existed. The main problem for

the Aerrows’ configuration is the duct-fan. Installing the duct-fan onto the fuselage was

also challenging, as the group have a limited range of materials to use – i.e. structural

materials are limited to woods.

PRELIMINARY DESIGN STAGE


STRUCTURAL LAYOUT AND SIZING It was decided that the plane would be a smaller aircraft in comparison to past RMIT-

NUAA aircraft. This is due to the fact that the group was unable to ascertain the use of a

duct-fan. The group attempted to order a type of duct-fan which the group believed

would be suitable for the aircraft configuration. However, as it turned out, there were

readily available duct-fans in the laboratory. Upon opting to take advantage of what was

available, the team decided to make use of the smaller of two duct-fans, which is rated

to produce 1.5 kg of thrust.

INTEGRATION OF THE PROPULSION SYSTEM ONTO THE AIRFRAME Because limited materials were available for use by the group, installing the duct-fan

atop the fuselage exposed some difficulties. The duct-fan needed to be secured onto the

fuselage using a base plate, with the initial decision to make use of 3mm plywood.

Consequently, the duct-fan needed ‘housing’ so that it could be mounted up onto the

plate. There were two approaches discussed: (1) surrounding the duct-fan with several

aligned and displaced plywood rings, and slotting tabs on the bottom of each ring into

the respective slots cut into the base plate; or (2) wrapping a sheet of aluminium

around the circumference of the duct-fan, so that two tabs can be formed where the two

ends of the sheet meet, squeezing these tabs together to create tension around the duct-

fan, and inserting them into base plate (as shown in the figure below).


Option 2 was viewed more appropriate in terms of stability and strength, however due

to the Aerrows’ fuselage configuration – the fuselage gets smaller in cross-section, going

progressively back towards the tail cone. As a result, the base plate for the duct-fan

attachment does not allow much room on either side for use of aluminium sheet.

Therefore, the group decided to go ahead with propulsion testing for the duct-fan, using

option 1. The results of this propulsion test determined if the plywood housing passed

for both strength and stability. Due to the thrust that the duct-fan can produce, it was

discussed on the likelihood of appreciable, or even catastrophic, longitudinal

displacement occurring. In response, a ‘truss-like’ support at the front and rear of the

duct-fan was devised, based on the propulsion test results.

Duct fan housing pieces


PROPULSION ANALYSIS AND TEST RESULTS On the EP-UAV a light-weight duct fan was used. Tests were conducted on the duct fan

in order to find the maximum thrust it produced compared to the theoretical value, also

the amount of time the duct fan was able to run when placed on full power and to see if

the structure made for it was strong enough. The duct fan was first attached to the

testing apparatus without screws but only a tight fit to make sure that it was in working

conditions and the wiring to the battery and speed controller were correct. After

making sure the duct fan was in working condition it was then attached to the testing

apparatus through the correct procedure. Firstly 2 correctly measured 3 mm ply sheets

were glued together making the part which will be directly attached to the fuselage by

sliding it in as shown below, also correct sizing slits were cut out for the duct fan to be

placed in.

Ductfan plate fuselage position


Furthermore mushroom shaped 2 mm ply sheets were cut out in order to be screwed

together around the duct fan to keep it upright and held in position without movement.

The stem of the mushroom cut outs were then placed inside the slots that were made in

the piece that was then attached to the fuselage, as shown below.

Propulsion Testing (Ductfan attached to the propulsion testing rack)


For extra support, an L shaped support was screwed underneath the part attached to

the fuselage and also to the stems that were slotted in, giving the structure much more

strength, support and stability. Once these pieces were constructed and assembled

together the completed structure was attached to the testing apparatus and the duct fan

was tested.

L-shaped support

The testing revealed the duct fan produced a maximum thrust of approximately 1.5

kilograms and lasted for approximately 4-6 minutes on maximum thrust using one

battery. The structure was unharmed and showed it was strong enough to support the

duct fan and its forces. Pictures of the duct fan are placed below in order for a visual

representation of how it was attached.


WING STRUCTURE ANALYSIS

The Aerrows' has a mid-wing configuration, that is, the wings are secured halfway up

the fuselage; hence the need for a strong enough material to connect the wings into the

fuselage. It was initially decided that a 1.4mm diameter carbon rod would be used for

this purpose. Since bending moments occur at the wing roots, it was necessary to add a

second carbon rod to be conservative in ensuring adequate resistance to this bending.

The vertical stabilisers are attached to the wings, thus reinforced wing ribs were

necessary where the bases of the vertical stabilisers meet; the landing gear boxes are

attached between the reinforced rib pairs.

Carbon rods attached in the fuselage


Reinforced rib parts for vertical stabilisers and landing gear box attachment

The wing structure was carefully designed based on the observation of previous aircraft

models. Designing the wing structure was quite challenging as the wings cover a large

area. Not as a result of a long wing span; however, due to the large ratio of root chord

length to tip chord length.


CAD modelling of the wing's internal structure is considered to be the most difficult

part of the preliminary process. As the wings are tapered, it was necessary to calculate

the angles, positioning, and dimensions of each rib. Specifically, the placement of the

first rib on both sides took the longest as the angles are based on the tapered nature of

rear fuselage section.


Additionally, designing the spars CATIA was also time consuming due to the same

reason – which is that the Aerrows' wings are tapered. There are 10 ribs in total (each

side); 3 reinforced ribs for the areas that require additional strength (i.e. attachment of

the vertical stabilisers and the landing gear box). The maximum and minimum spacing

between each rib is 97mm and 39mm respectively. This was desired to make sure that

the wing structure has enough strength.

Furthermore, since the aircraft has a large wing area, it was essential to keep the wing

structure as light as possible. This was acheived by cutting ‘circles’ out of the non-

critical areas of the ribs. Ribs 1-5, require holes for the 60mm carbon rod; and ribs 1-4

require holes for the 45mm carbon rod. Moreover, fabrication was taken into

consideration during the wing structure design; for instance, the balsa skin (1 mm

thick) needs to cover the leading edge and the trailing edge of the wings. Hence, the ribs

were designed and cut, leaving a 1 mm reduction in both leading edge and trailing edge

to maintain the smooth shape of the airfoil.


The largest airfoil and smallest airfoil of Aerrows with 1 mm reduction for skinning,

respectively


CONTROL SURFACES The ailerons take up 30% of the wing chord length, and are designed to follow the

shape of the wing rather than producing straight cut ailerons.


Manufacturing the ailerons and the rudders was planned to be completed separate to

the entire wings. Therefore, a separate section to connect the aileron structures is

necessary for skinning and attachment.

Attachment piece for ailerons

Attachment for rudders


CAD DEFINITION During the CAD definition process, there were some attaching and manufacturing issues

that were predicted and therefore some minor alterations were made on the initial

sizing and positioning of components. Additionally, the wing design, empennage design,

landing gear calculations and CATIA drawings were allocated throughout the group and

since communicating with group members was a problem during this process, some of

the dimensions used are different (as changes were not passed on adequately and at

regular intervals). These differences were critical in making the final defined CAD

drawings. For example, the wing design/dimensions submitted by the respective

individual, did not match the dimensions required for the landing gear positioning as

determined by the respective individual for this task; it also did not match the carbon

rod positioning required inside the fuselage. This was then dealt with through some

minor modifications to the wing rib designs. However, this lead to problems with

attaching the vertical stabilisers. During the CAD drawing process, it came to attention

that the vertical stabilisers would extend below the wing rear spars and thus resulting

in manufacturing complications; as this particular section of the spar would first need

to be cut at the top, for the vertical stabiliser be installed, and then the top portion of

the spar would need to be reset for equal stress distribution. This also meant that the

vertical stabilisers would be part of the wings, once fixed, and skinned with the wing

during fabrication.


DETAIL DESIGN STAGE


Front wing webbing

Rear wing webbing


FABRICATION

The detailed designs modelled in CATIA were required to be exported in the draft

application, and saved as a .dxf format for printing using a 2D-laser cutting machine.

This process was not particularly complicated, however it was noticed that the ‘cuts’ are

sometimes a little inaccurate due to the woods’ original state/shape; also it was noticed

that the machine produced cuts are bigger than it actually is as defined in CATIA. This is

because the machine uses a laser of non-zero width to cut the wood and hence the

product is always a little bigger. As a response, the CATIA designs for printing (i.e. holes

for the rods and slots) were decreased by about 0.5mm. This was decided upon to

ensure that parts fitted/meshed together with minimal play, and also decrease the

amount of glue required to fill in the gaps.


During the fabrication of the aircraft, various difficulties and errors were encountered.

Therefore, minor modifications were implemented. For instance, there was a limited

quantity for each type of wood between the two groups; and thus, there were changes

in the thickness and type of wood used in many components of the aircraft.

Additionally, since the CATIA designs were completed with dimensions and thickness of

materials included, decreasing the thickness/dimensions to fit the available materials

lead to errors in some of the components. As a result, a significant amount of

recalculating and re-printing was required.

The order of fabrication was fairly simple. CATIA files had to be converted into .dxf files

and then parts rearranged in AutoCAD to fit the given dimensions of various materials.

The data was then sent via a programme to the laser cutter, and the laser then cut the

exact part as desired. At the time, it was believed there was a potential issue with this

process, as some of the planks of wood were not entirely flat, so there were minor

errors when the laser machine was cutting the parts. Those errors were small enough

Wing pieces showing errors in lengths and angles


to neglect so it did not cause a big problem. It was necessary to ensure that tiny parts

which were required to be cut, were secured, as they might fall through the gaps

between the laser cutter supports. It was found best to slide a thin sheet of used balsa

underneath, to prevent this from occurring.

There were three types of glue specified for usage: two component acrylate glue,

cyanoacrylate glue, and PVA glue. The two component acrylate glue was used to glue all

of the joints in the aircraft as it provides a better wood-to-wood connection than the

other two glues. While the cyanoacrylate glue or super glue was used more often in

assembling as it is fast drying.

Laser cutting machine


FUSELAGE FABRICATION As oppose to previous EP-UAVs, the Aerrows’ fuselage is not symmetrical from nose-to-

end.

Moreover, as mentioned previously, all other components of the aircraft are at an angle;

therefore careful fabrication of the fuselage was crucial to ensure that all of the other

components are in the right positions.

The cross-sections of the fuselage are made with 3mm and 2mm thick plywood, where

the 3mm plywood is used for the sections where the wings and nose landing gear are

attached.

Fuselage front cross-section showing nose landing gear attachment


Some of the cross sections have slots put in place for connecting plates where the wings

are attached. However, due to the fact that there are limited materials and with an aim

to have a lighter aircraft, the initial 5mm wide slots were changed to 3mm.

Two carbon fibre rods are installed through the fuselage, as part of a design to connect

the fuselage and the wings. These rods however, need to be fixed at an exact length and

position to make sure that the moments are balanced on both sides. It was discussed

that a ‘stopper’ is necessary to hold the rods in place. Initially, these stoppers were

going to be screws, screwed through the carbon rod; however, as drilling holes through

the rods would affect the material’s integrity and strength, doughnut shaped balsa and

paulownias parts were instead implemented.

Chua Yee Leing assembling the fuselage showing the 3mm ply side plates


Stoppers

First part fuselage (notice the stoppers used to hold the rods in place)


The nose was later attached, once the landing gear attachment and servo were in place,

on the nose plate.

WING FABRICATION

Wing part cut outs

Printing the detailed airfoil designs was not difficult; however as mentioned, the wings

and their inner components are set at angles, and thus wing assembly was found to be

the most challenging part of the fabrication process. Ribs were specifically labelled as

‘for-left wing’ and ‘for-right wing’ as the holes for the rods needed to be sandpapered

enabling the wings to attain the shape required. Similarly, the webbing needed to be cut

according to the angle it is set at. Wing assembly was extremely difficult and took four

members of the group to achieve.

It was discussed during the preliminary stage of design, that it would be easier to skin

the aircraft by skinning the parts separately. Therefore, it was intended that the wings

should be able to slide on-and-off the rods attached to the fuselage. However, during


manufacturing it was more desirable to have firmly fixed wings rather than an easy

skinning process. So during wing fabrication, each rib was glued onto the rods, and for

ribs 1, 3, and 4, stoppers were also used.

The ribs are connected to each other through the use of webbing and spars, ensuring

equal stress distribution. As was discussed during the final CAD definition process, the

rear-top spar at ribs 3 and 4, needed to be cut at the top for the vertical stabiliser

assemblies.

Nita Wiroonsup and Lucille Temporal assembling the wing


For aesthetic reasons, a half airfoil cross-section was attached at the end of the wings

(refer to the figure below). This gives a curved surface at the wing tip rather than a

straight cut.

VERTICAL STABILISERS FABRICATION Similar to the fabrication of any other parts, the vertical stabilisers were created by

gluing all the airfoil pieces, spars and webbings. Printing the airfoil pieces and other

components of the vertical stabilisers, it was noticed that it looks very small for the

aircraft. Upon this observation, the group seek advice from Professor Yu’s Master

student. It was advised that the area of the vertical stabilisers are enough for the given

size aircraft. Thus, fabrication was carried through.

Half airfoil shape cut out


As was mentioned earlier, the vertical stabilisers are meant to be inside the wings.

Enable to do this, the spar was needed to be taken off and put back again once the

vertical stabilisers are inside. However, while in the process of doing so the group’s

pilot saw the plane for the first time and commented that the Aerrows’ vertical

stabilisers area are too small and thus have the tendency to just ‘circle’ around when

flew. For this reason, and also for lack of enough time, a truss like vertical stabilisers

Initial vertical stabilisers

Vertical stabilisers attachment position


with a larger area was created. As was advised by the pilot, it was not necessary for the

aircraft to have rudders as well, as it is very small and light already. Therefore, a new

set of vertical stabilisers made of balsa woods were fabricated.

CONTROL SURFACES The control surfaces were fabricated in a similar way as any other parts. The only

difference was since, the group were advised to ignore fabricating rudders, and the

aircraft only has set of ailerons. This was not particularly hard to do as it only needed a

small block of wood to insert the connection piece to connect the ailerons to the servos

attached on the wings.

Similar to the vertical stabilisers issue, at preliminary design stages the group were

advised that that the area of the group’s control surfaces were adequate enough for

stable flight. However, after the pilot checked it, unfortunately it was a bit too small and

might encounter difficulties in flying. Since, these errors were not pin-pointed on the

earlier stages there were not much ‘quick-fix’ the group could have done before the

flight test.


LANDING GEAR ASSEMBL From previous EPUAVs it was notable that most landing gears either detach from the

fuselage or break upon ground test and landing. Hence, it was crucial to ensure that the

landing gears are placed strongly within its allocated positions. A piece of layered 2mm

ply was used to create a block to support the landing gear stick.

Landing gear block and Landing gear positioning

The landing gear steel rods were cut according to the specified length that was initially

calculated. These pieces were then put in place and glued using the AB glue for stronger

connection.


SKINNING Balsa skins were done to the fuselage, leading and trailing edge of the wings and control

surface

Balsa skinning


After all the servos were attached and wirings along the aircraft were done, plastic

skinning was done. It is done by ironing a special plastic type onto the surface of the

aircraft. This process was quite tedious due to the fact that Aerrows’ wings are fixed on

the rods that are attached on the fuselage, hence the wings was needed to be skinned

along with the fuselage. Moreover, Aerrows’ fuselage shape makes it hard to skin it

smoothly.

Plastic skinning


ACCESSIBILITY The top section of the fuselage of the aircraft was made that it can be easily being

opened for accessibility. The top part and a small side of the fuselage nose was also

designed to have a hatch for accessibility for the nose landing gear, in case it might need

some adjustment in height for easier take off.

Fuselage hatch for accessing batteries and wirings

Fuselage hatch for nose landing gear attachment


FINAL PRODUCT

Aerrows final aircraft


Aerrows with finishing touches


GROUND TEST Ground test was necessary to do to check if every servos and controls installed are

working properly. Aerrows successfully passed ground test and was approved by

Professor Yu, Mr. Yang , and the pilot. Thus, send it off for flight test.


FLIGHT TEST

ATTEMPT 1 The aircraft had difficulty taking off. As a response, the battery was pushed further back

to lessen the load the nose is carrying.

ATTEMPT 2 The aircraft successfully did lift off from the ground, however due to the cold weather

the right wing aileron’s connection pieces was brittle and broke, causing the aircraft to

experience instability. Moreover, since the aircraft only have ailerons as its control

surface, it was extremely difficult to manoeuvre it while one of the ailerons is not

working. This caused the aircraft to hit the ground twice before it stopped and hit the

ground violently.

It was impressive enough that the aircraft was still in good condition after the second

attempt. However, the aileron piece was needed to be repaired for the third attempt.

Aerrows’ snapshot from attempt 1


Attempt 3 snapshots


ATTEMPT 3 Now that the CG is properly placed and the ailerons were fixed, the aircraft was again

attempt to fly. Unfortunately, due to the strong wind hitting a very light aircraft with

only two control surfaces, as the aircraft lift off the ground it was blown away sideways

by the wind hitting a tree. This fatally damaged the aircraft. Its nose came off the

fuselage body and hence the nose landing gear was also damaged, one of the vertical

stabilisers came off and the right root wing broke.

Attempt 3 snapshot

Repairing Aerrows after attempt 3


ATTEMPT 4 With the desire to actually see the plane fly, major repairs were done to the aircraft.

However, some of the damage from attempt 3 was beyond repair. The last resort is to

launch-throw the aircraft and hope that the repairs were good enough to still generate

lift. Unfortunately, since the damage in the right wing was severe, the aircraft just went

straight down the ground after throwing.

POSSIBLE CAUSES AND

MODIFICATIONS Since all the calculations, references and regular consultations from Professor Yu and

Mr. Yang were done from conceptual design until the flight test, possible causes of

failure were synthesised during and after flight test. These are listed and briefly

discussed.

The duct fan positioning

o Would be beneficial if it was tilted downwards, however this was

impossible to be modified during the flight test as the duct fan was

securely and firmly attached to the fuselage (as was initially advised the

group to do)

Small area of the ailerons (as discussed previously)


Lack of control surfaces (as discussed previously)

Lack of familiarity in flying duct-fan powered UAVs

o It was surprising enough for the group to find out that there were not

much duct fan powered aircraft has been flown among the EPUAVs, and

therefore it was difficult to predict the problems that comes in using it.

It was later theorised by the group along with other professors, that the aircraft design

would probably fly if a propeller at the front was used, replacing the mounted up duct

fan configuration. Additionally, the following modifications were also suggested.

H-tail configuration would probably provide better stability

A canard configuration for added lift

Longer nose landing gear

Stronger materials for landing gears

Larger servos

These modifications could have been implemented to the plane for next flight test.

However, due to lack of time that the group and the rest of the RMIT exchange students

are staying in China, the group only had one flight test. Professors are really keen to

make the aircraft work; in fact they have various theories for its modification (as listed

above).


List of tasks and assignment areas

Nita

W

Lu

cill

e

C

hri

s

N

an

he

ng

D

avid

G

eo

rge

Y

ee

Lein

g

Co

nc

ep

tual

Desig

n

Synthesis of concept

Configuration design 3 3 3 3 3 3 3

Initial Sizing 0 2 0 0 0 0 2

Fuselage layout 1 1 1 1 1 1 1

Airfoil design 2 3 0 0 0 0 3

Wing planform design 0 2 0 1 0 1 2

Empennage design 1 2 0 0 0 0 2

Geometry for landing gear 0 0 0 0 2 0 0

Analysis of concept

Analysis of propulsion system 0 0 0 2 0 0 0

Weight and C.G. estimation 0 0 0 0 0 0 3

Aerodynamic performance analysis 0 1 1 0 0 0 3

Flight performance analysis 0 0 0 0 0 0 2

Stability analysis 0 0 3 0 0 0 3

CAD definition of the concept 0 0 3 0 0 0 0

Pre

lim

ina

ry

De

sig

n Structure layout , initial sizing, and internal layout

Wing structure 3 0 0 0 0 1 0

Fuselage structure 0 1 0 0 0 0 3

Empennage structure 0 0 0 0 0 2 0

Control surface structure 0 0 0 3 0 0 0


Landing gear 0 2 0 0 2 0 0

Integration of propulsion system 0 0 0 2 2 0 0

Payload arrangement 0 0 0 0 0 0 0

Structural analysis

Wing structure analysis 2 0 0 0 0 0 0

Deta

il

Des

ign

Fuselage detail design 0 2 0 0 0 0 3

Wing detail design 3 0 0 0 0 0 0

Empennage detail design 0 0 0 0 0 2 0

Control surface detail design 0 1 0 2 0 0 1

Landing gear detail design 0 2 0 0 2 0 0

Control system design 0 0 0 0 0 0 2

Structure analysis for key parts

Fab

ric

ati

on

Preparation for materials 1 1 1 1 1 1 1

Fuselage fabrication 0 0 0 0 0 0 3

Wing fabrication (inner section) 2 2 2 2 0 0 1

Wing fabrication (outer section) 2 2 2 2 0 0 2

Empennage fabrication (inner section) 0 0 2 0 0 2 2

Control surface fabrication 0 0 0 0 3 0 0

Landing gear fabrication 0 0 0 3 3 0 0

Installation propulsion system in to

airframe and test 0 0 0 3 3 0 0

Control system installation and test 0 0 0 0 2 0 2


Assembly and test 2 2 2 2 2 0 2

Measurement of Weight and C.G location T

es

ts

Test plan 3 3 3 3 3 3 3

Ground tests 3 3 3 3 3 3 3

Air test (1) 3 3 3 3 3 3 3

Air test (2) 3 3 3 3 3 3 3

Note：The number is the indication of a student’s contribution to a specific task.

3 – primary contribution; 2 - secondary contribution; 1 – minor contribution; 0 – no contribution.

rmit-nuaa epuav reportaircraftdesign.nuaa.edu.cn/pd-2007/report/2010-b.pdf · 2010 chua yee leing...

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