Making a Boom

What follows are instructions on making a Carbon Boom for a Cherub, it is based on the techniques used at a Class Association organized Sticky Weekend where a boom was made for Paul Croote this was laid out step by step by phil kirk in an email and with additional information and tips from Dave Chisholm (aka Carbonology Dave) and from other build projects.

These instructions could be modified to build any other light weight tubular structure, however the reliability and suitability of this is dependent on the builders skill with the techniques described. The chemicals and tools used in building anything out of Carbon composite can be dangerous and appropriate safety measures should be taken.

This boom is build on the class three piece boom mandrel, which has been designed so that after construction the centre section can be slid out allowing the other two sections to be removed easily. It is not necessary to use a multi piece mandrel, however it can be difficult to get the finished tube off a one piece mandrel and there is more than one boom still attached to its mandrel.

1. Make sure you have all your materials, tooling, gloves and safety glasses to hand. Along with plenty of time and assistance as it always takes longer than you think and an extra pair of hands is useful.

1. rub mould release wax over all the surfaces of the mandrel especially between the three parts.
2. wrap mandrel with Mylar film and secure to itself with parcel tape. The Mylar moves easily over the waxed mandrel which will help you get it apart. To hold the Mylar film tube in place neatly Spiral wind parcel tape on over the top overlapping the previous turn.

• Alternative 1: Neatly cover mandrel with parcel tape sticky side outwards. Then apply a second layer sticky side inwards. This should also slide off the waxed mandrel but with the greater number of spirals of tape can be difficult to keep neat.
• Alternative 2: Try using polythene instead of Mylar and spiral wind parcel tape over it again keeping it neat can be difficult.

Note: you can't cover the three pieces of the mandrel directly with tape sticky side down. This will stop the centre section from coming out.

1. Wax the taped mandrel thoroughly.

Do not skimp on these stages as you will waste much more time trying to get a stuck mandrel out of your boom and may even ruin both the mandrel and the boom in the process.

The Lay up is done so that the thickness and so the strength of the laminate increases as you move forward along the boom. There is also extra cloth on the upper and lower sections of the boom as this is where the strain will be greatest so the extra cloth here be most effective in increasing the stiffness of the boom.

Depending on how the fittings are attached, the second highest load in a boom after the bending moment is torsional. The clew and mainsheet are rotating against the kicker, and the tube can twist itself to destruction. This means that as well as the Uni directional fibres along the length of the boom you need some off axis fibres to take the torsional loads.

The lay up is as follows:

1. 200 gram plain weave full length of intended boom plus a bit (ensure each cloth overlaps itself by about 15-20mm when it is wrapped round the mandrel.) This layer can be cut so that the fibres run at 45 degrees helping to take the torsional loadings.
2. 300 gram uni around the whole tube for the full length of intended boom plus a bit. - lay it up on the zero axis first, then tape one end so it can't rotate, then 'squew' it from the other end). 30degrees would be ideal, but 20 will be enough. You will still get a lot of compressional/tensional strength from it so you won't need to replace it add extra on axis fibres. Tape the far end so it will not untwist when you do the next layer.
3. 300 gram uni full length of intended boom plus a bit as previous layer then squew but in the opposite direction.
4. 300 gram uni strip for the full length along top an bottom sides of boom
5. 300 gram uni strip for the full length along top an bottom sides of boom
6. 300 gram uni strip along top and bottom of the boom to mainsheet take off point. (Round the outboard end of the cloth to avoid stress raisers) This would be variable depending on mainsheet setup i.e. with an aft mainsheet it may not be necessary
7. 300 gram uni strip along top and bottom of boom to kicker take off point.
8. 200 gram plain weave to give some hoop strength and to prevent the uni from peeling. (This can be added on a second laminating batch to give a factory shiny finish.)

To wet out cut some polythene sheet into strips wider than the circumference and slightly longer than the boom, the cloth is then wet out on the sheet which is then picked up and used to transfer the cloth over to the mandrel. The sheet is then peeled off the wet cloth layer and used again for the next one. The cloth can then get the last few tweaks in its position and the next layer added finally the lay-up needs to be consolidated.

With the lay-up in place it now needs to be consolidated to remove any air pockets and any excess resin. There are a number of different ways to do this depending on what you have available and what you plan to do next with your boom.

1. The proper stuff is “Heat shrink Tape” it will shrink by approx 10% when heat from a paint stripper heat gun is applied. Applied the right way around (outside of roll to the job) it has its own release agent and comes off so easily it's a joke. It should be applied in a tight spiral with quite a large overlap between each of the wraps with the tension kept as even as possible. It needs less heat to contract fully than parcel tape and therefore causes less damage to the matrix. It is typically sold in 100m rolls of 30mm wide tape.
2. Peel ply best cut into strips and wound around the tube in a spiral, you can get peel ply in rolls of tape 50mm wide which is ideal for this and other applications. Peel ply is good if you want to bond the tube to something else or if you want a grippy finish e.g. for a tiller extension. You can cut your own strips or lay it on length ways but it is difficult to keep it neat when doing this.
3. Use parcel tape as heat shrink tape. (Remember to make this up before hand by sticking two layers together stick sides together. make this in lengths and stick together with more tape. You will need quite a lot!

Length = circumference of boom * (length of boom / 1/2 width of tape). Stick one end of the heat shrink tape to the mandrel beyond the boom and wind firmly and careful until you reach the other.

When you heat the laminate up with the heat gun the tape will shrink removing air pockets and the now warm and runny resin will be squeezed out, sometimes at high pressure so wear eye protection.

When doing the laminate all in one go unless you are incredibly lucky and careful with the tension of the heat shrink tape you will end up with a few wrinkles and bumps in the surface. If you want to get the bling factory finish then you should leave the outer 200g plain weave layer till after you have filled and faired the surface.

Another tip if you want to do it all in one go is to use some sort of breather fabric or padding between the peel ply and the shrink tape on the first consolidation. We use cheap woven glass fibre (300g) or old vac bag breather. This prevents the shrink tape marking the structure at all, and spreads the loads so the way you apply the tape is less critical - most importantly you have less defects to sand out/fill before you put your top layer on. And if you use Glass then all your sanding is done on the glass so you do not turn your expensive structural Carbon into not very useful dust.

Once you have filled and sanded your base structure to perfection it is time to add the final layer of 200g cloth, getting the best finish is tricky and you need to be careful with the cloth so there are no pulled threads or wrinkles. This layer also needs to have UV protection or it will quickly dull and turn white. This can be done either by using a resin with built in UV protection such as SP115 or a couple of coats of clear PU varnish to provide the protection.

As they say on Blue Peter 'Here's one I made earlier'