UWE Bristol Engineering showcase 2015

Tom Willis
MEng Aerospace Design Engineering
Project Supervisor
Dr David Richardson
Design and Manufacture of a Composite Spar for a Flying Wing UAV
Composite Materials
A composite material is any material which is made up of two or more
elements with properties different to those of the individual elements.
Common examples include fibre reinforced plastics (FRPs) and concrete.
Composite materials are an excellent choice for aircraft structures as they
have the ability to withstand high loads for a much lower weight (Specific
Strength) than currently used materials such as aluminium alloys.
Design
One of the key design requirements for this
project is that the UAV should be able to
breakdown into three main components (the
fuselage and two wings) for transport and be
easily re-assembled for flight. Because of this, the
spar length should be a maximum of 400mm.
The spar is initially modelled as a cantilever beam
with a constant cross-section. The effect of taper
and sweep is looked at later in the project. The
cross-sectional geometry could take many forms
such as circular or a typical I-beam shape.
Finite Element Analysis
FEA is used to perform analysis on tapered
versions of the cantilever. It is particularly useful
for highlighting areas of stress concentrations.
Optimisation
It is found the ‘C’-section and circular section spars are
most applicable to this project and therefore these
cross-section spars are taken forward to the
optimisation stage.
The ‘C’ section spar is split into five equal sections along
the length. Each section is then analysed in bending
with a 283.3N load (15g load at UAV MTOW)
Each of the five sections then has the number of layers
varied until the length of the spar has the same failure
stress. This process is repeated with the spar split into
four and three sections.
It is found that the best solution is available when the
spar is split into three sections with the number of
layers being six at the spar root and reducing to four and
two layers in the next sections.
The circular section is also optimised similarly but by
tapering the spar such that it theoretically fails in all
locations simultaneously. It is found that best solution
lies with a spar that has a 36mm diameter at the root
and 11mm diameter at the tip.
Presented with the possible options, the customer
decided to proceed with the circular cross-section spar
due to manufacturing considerations within the wing.
Other advantages include superior durability, higher corrosion resistance and
reduced part count due to the more complex part geometry achievable
through manufacture.
Current Usage
Current usage of composite materials for wing spars can be seen on the
Airbus A350 XWB and A400M. This project is studying wing spars on a much
smaller scale, suited to a UAV of a wing span of 3.4m
Manufacture
Initially, a parallel carbon tube is manufactured
using two different methods. One tube is made
using a wet layup with vacuum consolidation and
one is made using a pre-preg with heat shrink tape
consolidation.
Manufacture using the wet layup process was
performed a few times allowing some to made
with two layers, and some with four. This helped
to identify problems faced during manufacture for
the final product.
It is found that pre-preg with heat shrink tape is
the most appropriate method due to a better
product finish and the simplicity of the process.
This method is used to produce some test tapered
spars.
Project summary
An investigation into the use of composite materials
in the design and manufacture of a spar for an
existing flying wing unmanned aerial vehicle (UAV).
The work in this project is carried out alongside other
undergraduate projects which are working towards
improvements to components within the same UAV.
The spar is produced alongside the fuselage which
provides some restrictions in size and geometry.
Project Objectives
The overall aim of this project is to design, optimize
and manufacture a composite spar for a flying wing
UAV. To achieve this, several key aims are set out:
• Literature review into existing use of spar structures
and composite materials
• Initial designs & CAD (Computer Aided Design)
models
• Structural calculations - Will it work?
• FEA, optimisation and materials selection
• Manufacture of test component(s)
• Testing of component(s)
• Re-design (based on test feedback)
• Manufacture of final product
Project Conclusion
This project successfully investigated the use of
composite materials for a spar on a flying wing UAV.
The optimised design provides a component which
meets all design requirements and has a factor of
safety of three along the length of the spar.
Methods of manufacture have been investigated and
the most appropriate processes have been
determined.
Future work on this project includes investigations
into bladder molding, testing of the spar to validate
theory/FEA and considerations into larger scale
design or larger production volumes.