Written by Elliott Nurcombe
I am writing this from the perspective of a student on a student-led project with very little industrial experience in this field; so don't believe everything I say and do your own research.
View more about Avis Drone Labs here.
A brief history of my experience
I joined project HEX in October 2023, initially starting off in the manufacturing subteam even though I had experience in other areas of UAS design. At this time, management was still coming down from Vulcan V2’s (a 20kg quadplane with a 3.2m wingspan) crash that prevented the team from attending SUAS in the summer of 2023. The crash, primarily caused by a failed elevator servo, was complicated by rushed manufacturing. Delays in production pushed deadlines, resulting in the foamboard-clad fuselage being completed just mere nights before the maiden flight.
Following the crash there was an emphasis on completing Vulcan V3s manufacturing as fast as possible in order to allow more time for integration and testing. This, along with a shift to a fully carbon fibre construction, forms my initial exposure to composite manufacturing. The rushed nature of Vulcan's design did not play well into its future; large questions were left unanswered in order to start manufacturing immediately. This led to parts that should have been rejected making their way too far along the manufacturing process to be rejected and start again.
Project Hex and the other UAS teams at the University of Sheffield disbanded at the end of the 2024-25 academic year and united under the Avis Drone Labs team which has 7 projects operating within it:
- Two international competition projects (SUAS 2025 and ImechE 2025)<
- Two research-based projects (Athena and Lightning)<
- Two beginner projects (Avinex and FPV racing)<
- One ground control station project (FGCS)<
Currently, I am one of the acting directors at Avis and a co-technical lead on project Athena. I have met with many other technical leads and discussed their experiences with using composites and experienced first-hand some of the pitfalls.
The goal here is not to point fingers and tell anyone “You're doing it wrong” but to share my experiences and (my tiny bit of knowledge) to prevent future engineers from making similar mistakes.
Perhaps what I say is useful, or perhaps I'm talking a load of rubbish.
Introduction
Composite materials are an obvious choice for drone manufacturing and the aerospace industry due to their high strength-to-weight ratio. In my brief experience, composite use often relies on a 'brute force' method. Teams apply excessive amounts of resin and carbon fibre, aiming for a finished product that not only has a visually appealing surface but also achieves uniform weave distribution, complete resin saturation, minimal voids, and strength superior to a 3D-printed alternative.
In the following article, I will discuss some of the errors that I have seen as well as mistakes and oversights that I have made. I will also discuss some of the manufacturing techniques that we (Avis) have used in the past and techniques that we have not yet tried.
What are composites?
Composites are basically a mix of 2 materials, however, this is not the same as an alloy or a chemical compound because the constituents of the material can be mechanically separated from each other. This means that the fibre and matrix are not necessarily chemically combined.
The goal of composite material design is to create a part that combines the optimal properties of its constituent materials. Composites consist of fibres, high aspect ratio strands with high Young's modulus and flexibility, and a matrix, typically a thermoplastic or thermosetting polymer, which is characterized by low Young's modulus and brittleness.
The high aspect ratio of fibres makes them inherently flexible, behaving similarly to strands of string or hair. They exhibit high tensile strength but are prone to buckling under compression. To effectively utilize the desirable properties of these fibres, they are embedded within a matrix. This matrix binds the fibres together and facilitates the transfer of stress to the fibres, enabling them to bear the majority of mechanical loads.
THE LENGTH OF THE FIBRES MATTER. The very high aspect ratio of the fibres allows them to distribute loads through the part where there lies more fibres to distribute stresses even more. Shorter fibres rely more on the matrix to distribute loads, since the matrix has a lower yield stress the matrix will fail way before the fibres.
When buying rolls of woven fibres you will see different materials, twill counts, and weave patterns. It is important to buy the right fibre for your application so research is required into what these values mean.
TLDR: fibres are strong, the matrix is weak and composites get stronger with longer fibres
Common mistakes
Over-reliance on cured carbon fibre products
The “carbon will fix it” mentality.
This is becoming more common, especially with less experienced engineers. Maybe this is partly because of the hype around carbon fibre. When you hear about a new aircraft or car having carbon fibre parts it feels like those parts are the best in the world, like carbon fibre is some sort of wonder material that has all pros and no cons. But it does, this is true with everything in engineering. Everything has pros and cons that must be evaluated to make the best choice for your given situation or application.
Obviously, I understand that sometimes there is no substitute for such a product but I do not believe that changing the material from a 3D print to a carbon plate (for example) should be the first step. In my opinion, it reflects a lazy, almost cheating attitude towards engineering design. Using brute force to solve an issue instead of working on the problem or considering design changes. I have found that more often than not, having a deeper look at a design will reveal some oversights not previously noticed.
Strength should not be the only consideration when choosing a material for your design, factors such as ease of manufacturing, lead time, availability of material, material cost, and available facilities should also be factored into your design. For example, the unreliability of a waterjet cutter would justify spending additional time redesigning a part to use a different material.
While carbon products certainly have their place in the aviation and RC industry; they are no substitute for good engineering practices and standards.
Lack of understanding of what composites are
This is simple, you must have some kind of working knowledge of the materials you are working with. How can you expect your part to function the way you envisioned it without an understanding of what you're working with? You don't need PhD level knowledge here, a heuristic overview is more than adequate for a good working knowledge at a hobbyist RC scale.
Composites are not just carbon fibre
There are many, many different kinds of fibres and matrix/resins, each with their own applications. I will leave resins out of this because without further research I can only tell you the differences in viscosity.
However, there is some useful information in regard to fibre choice.
To summarise:
- Carbon has high Ultimate Tensile Stress (UTS), very high stiffness, and high fatigue resistance but is less resistant to impacts and a brittle/immediate failure. Generally, carbon has the highest strength properties but factors such as its failure mode or cost may make it difficult to justify.<
- Kevlar or aramid fibres have high UTS, high impact resistance, and high toughness, but they are expensive, difficult to process, and lack compressive strength<
- Fibreglass is cheap and has good tensile strength, high impact resistance and good fatigue resistance, but it is heavier than other options and overall less impressive than other materials. BUT IT’S CHEAP.<
- Diolen and Innegra are polymer fibres that offer similar properties to Kevlar but have softer failure modes as well as additional flexibility.<
Lacking practical layup experience
A primary issue with the Vulcan V3's carbon wings was inadequate debulking. This likely resulted from a deficient vacuum seal, stemming from insufficient experience and practice during the layup process.
Not through any fault of our own, those of us who were left to perform the layups had only done a maximum of 1-2 layups previously, so moving straight to Vulcan's 3m wings was bound to create issues. To be clear I am not blaming anyone here there are many other reasons out of anyone's control that Vulcan did not fly. From this past experience it’s clear that hard-earned knowledge, skills and lessons learnt must be passed down to future engineers; to ensure mistakes are not repeated and the organisation progresses without known setbacks.
FDM carbon fibre filaments are just as good as the real thing!?
NO, THEY’RE NOT!!!!!!!!!
Most FDM CF filaments (excluding the Markforged continuous fibre machines) are a matrix with fibre particulates which have a very low aspect ratio; so the additional properties gained by the presence of the particulates are minimal at best. The main reason why carbon fibre has a high UTS is because of its long strands, spanning across the length of the material; but this is not possible with most FDM CF filaments. The low aspect ratio means that less load is distributed through the fibres and more load is distributed through the brittle matrix.
See this video for strength testing conducted by easy composites with continuous fibre filament and particulate filament.
Manufacturing techniques that we have used
Wet layup
Wet layup is commonly used for large parts in the aerospace industry as there is no need for your part to fit inside an autoclave (a big oven that has a vacuum attached to the side).
The basic process is as follows:
- Decide on what weave you need<
- Pre-cut the weave so you have enough to cover the part (be generous)<
- Mix up an appropriate amount of resin (per the weight of fibre used) usually aiming for a 60-40 fibre-to-resin ratio<
- Wet out the fibre with the resin being careful not to disturb the weave, use a brush or roller to evenly distribute the resin<
- Cover with a breather, peel ply or perforated film<
- Form a vacuum bag around the part being careful not to get resin in between the bag and sealing clay<
- Set up the vacuum pump and attempt to pull a vacuum in the bag<
While it is a difficult and stressful process, it is necessary for medium to large-sized parts. There are some techniques to achieve a decent debulking and desirable volume fraction of fibre but nothing beats experience.
‘Forged’ carbon
Creating forged carbon is basically compression moulding with short fibres.
You create a 3D-printed split mould with one open end and extend the final mould part slightly. Then pack in the appropriate amount of resin and short-strand fibre and slowly close the mould, squeezing out additional resin.
Some tests done on the Easy Composites YouTube channel show that forged CF parts can be stronger than aluminium. HOWEVER, THESE RESULTS ARE TO BE ENTIRELY DEPENDENT ON THE FIBRE ORIENTATIONS DECIDED DURING THE MANUFACTURING PROCESS.
3D-printed moulds must be treated with a mould release and also must be designed in such a way as to promote easy de-moulding via multiple parts and draft angles.
Pre-cured carbon products
This includes carbon rods, tubes, box sections, strips, angled extrusions and sheets. These are all products that can be purchased via the Easy Composites website. Being fairly easy to use, these products can replace materials like plywood or balsa. However, they should not be used by default. Thought must be had on whether it is appropriate to use such products; for example, I could replace a plywood plate with a carbon plate of the same thickness but will the CF be heavier than the ply? Will the ply be strong enough anyway? Will the additional strength justify the added manufacturing difficulty and therefore time needed? These are all valid questions.
It is important to note that there are many kinds of pre-cured products and each of these are made for different applications. Make sure you have considered all options before settling and placing an order. This video discusses the different kinds of carbon tubes
Manufacturing techniques that we have not yet used
Pre-preg
Pre-preg is a form of composite that comes ‘pre-impregnated’ with a matrix. This is by far the most common practice in the aerospace industry due to the ability to achieve a dimensionally accurate part with <1% void content (with good technique a wet layup will typically produce ~15% void content). Pre-preg is easy to work with as there is no mixing of resins, and the weave will not fall apart as it is held together by the matrix.
However, it is expensive; not so much the material but the tooling. Pre-preg must be cured in an autoclave because the matrix will only cure when it is exposed to a specific heating cycle. This means that your mould must also survive the curing cycle with temperatures between 110 and 180 degrees C without deforming under atmospheric pressure (due to vacuum).
The common choice for this is an epoxy tooling board, but this is expensive and must be CNC’d to act as a mould. MDF can also be used but it is not as good and also requires CNCing as well as treating with some kind of release agent or film.
But, it is possible to create a prepreg part from a 3D print. The process is time-consuming though, it involves pressing a fibrous clay around a 3Diprinted part to create a mould. The mould must then be dried and cured. Now it can be used to create pre-preg parts; here’s a video showing the process.
Resin infusion
This method is capable of achieving parts as professional as prepreg.
Most similar to wet layup, resin infusion involves a similar set-up. A dry weave is laid in a mould, the part is then encased in a vacuum bag and a vacuum is drawn. Resin is then introduced in one end of the bag and the vacuum draws the resin through the fibres ensuring an even distribution of resin throughout the whole part. The volume fraction of fibre will typically be higher with this technique as it is very difficult for excess resin to accumulate in parts of the mould. It is also possible to achieve a very good surface depending on your mould surfaces.
This technique is typically used with large parts and really sees its benefit when wet layup becomes impractical with the size of a part. See this video for more information.
Conclusion
To wrap up, composite manufacturing is naturally challenging. But there are plenty of techniques and methods to make the process more reliable. Hopefully, this article has made you think about materials beyond carbon — or at least made you aware of them. Learn from our mistakes, do your research, and don’t just assume carbon is the answer. I hope this has been helpful, even if it’s just a student’s take on things.
Avis Drone Labs are sponsored by the RS Student Fund
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