British
Human
Power
Club

 

Zen and the Art of Fairing Construction

Introduction

There follows instructions on constructing a GRP fairing from scratch, based on my experiences building a streamliner and over a decade of experience in composites and vehicle prototyping.
 
If you are considering building an HPV shell in GRP then I advise that you first ask yourself some questions. To finish my fairing took around 300 hours. If you have little or no experience of such activities then it could easily take you 50% longer. Be realistic about how many hours you can devote to the project each week and divide this into the figure above.
 
Do you end up with a timescale that is measured in months rather than decades? Remember that the above figures do not include time for designing the fairing or to clear up after yourself or source materials or chat with your friend, who turns up every other day to 'see how it's going'. If you are to enjoy doing this (it is after all meant to be a hobby) then you won't want to be getting stressed out or working all night, every night.
The second thing you should ask yourself is do you have the facilities? Do you have the room? A streamliner is likely to be upwards of 2.5 metres long. Can the space be economically heated? Some materials require a generous room temperature for them to work, as do many humans. How will the people who share this space feel about dust, noise and chemical smells? If you have a spacious, fully equipped workshop then there is no problem, but the chances are that you haven't.
 
I believe in attainable goals. Unfinished projects are rarely of any use or satisfaction. If you are giving some unfavourable answers to the questions above then maybe you should consider another method of construction.
 
There are a number of very effective ways of building a fairing which are much less work than a GRP shell and, if done well, almost as effective. The materials that have been proven to work are plywood (Velocars), fabric combined with a rigid nosecone (Kingcycle, Lightning F-40), 'Corriboard' (A corrugated plastic) and finally camping mat type foam. You can combine all these with commercially available mouldings or make your own small mouldings by laying up glass or carbon fibre reinforced epoxy over Styrofoam and then scooping out the foam once it has cured. Alternatively, make an MDF former and get it vacuum-formed in plastic. There is plenty of information on The Web about these methods.
 
If, after reading to here, you still want to build a GRP shell, then welcome aboard! I admire your courage and determination.
 
First, you need to design your fairing. I had a slight advantage here, in the shape of a £6,000 CAD system. This meant that I could plot out accurate sections, making the pattern making process much quicker. This requires an A0 plotter, which I was lucky enough to have access to at the time.
I believe there are simple surfacing programs on the Web. Mike Burrows used one to design the Windcheetah shell. The best alternative is to make a 1:5 scale model in Styrofoam and then slice it and scale up the resulting sections. Beware of high street copy centres - their copies can be as much as 0.5% out.
 
Before doing this you need some dimensions to work to. I dressed in cycling clothes and attached small sticky-back discs to myself at my leg and arm joints. I then sat on the bike and asked my beautiful assistant to measure the distance between the discs, the width across feet, knees, thighs, elbows, hands, shoulders and head, with and without helmet. I then asked her to take photos from dead side on and dead head on. Use a long focal length lens to reduce the perspective. Make sure the neighbours aren't watching when you do this; otherwise you may never be invited to dinner again! This should give you the information you need to design your fairing. For information on suitable wing sections and other aerodynamic advice see the BHPC Website, in the links section.
 
It will help if you have a good idea about where panel splits are going, how you are going to get into the machine, how it will be mounted to the chassis, how you will get at the mechanicals etc. before you start making the pattern. This is called 'DESIGN'.
 
Making the Pattern
I should point out here that I built my fairing in a single garage. The only fancy machinery I used was a band saw with a 300mm throat.
 
The process we are using requires us first to make a male pattern (or plug or buck), that is: a solid of the outside shape we require. We take a mould (female) from this and then from the mould we can make thin mouldings. We can make as many duplicate mouldings as we like until the mould finally falls to bits. But the most important advantage of this long-winded process is that the mouldings can be light and have a good surface finish.
 
I decided to make the pattern in two halves. This gives you an accurate centre section, is much easier to work on (each half sits flat on a bench) and makes the moulding process much simpler, as we will see later.
 
I drew a 100mm grid on two 8' x 4' sheets of 12mm thick MDF and marked out the side profile (longitudinal centre section) on each, one being a mirror of the other. I then temporarily doweled the two halves together, cut out the profile with a jigsaw and smoothed the curves with abrasive paper. The two halves were then separated.
 
NB: Always wear a good mask and use a well ventilated area when working with MDF. It contains phenol formaldehyde, which ain't very good for you. The same precautions apply for spray mount (I think it's banned in some countries!).
 
It is very important to keep these bits of MDF flat. I screwed and glued a length of 4" x1" to each, longitudinally and end on. I got some distortion later on, so it would be best to use two lengths. Make sure they extend right to each end - you will have to profile them at the end to sit below the surface of the pattern. You may need to do the same vertically. I then sealed all exposed surfaces with polyurethane varnish.
 
Now you will need some Styrofoam or similar to bulk out the shape. This usually comes in 50mm thick 8' x 2' sheets. The stuff I used is called Glascofoam from Trent Insulation in High Wycombe - Phone 01494 462002. It was about £12 a sheet and I used 12 sheets in all. Spray mount your paper sections onto the foam. Don't forget to subtract the thickness of the MDF. I used transverse sections every 100mm, which have the advantage of being a manageable size. Some people use longitudinal sections. The sensible way to cut Styrofoam is with a hot wire, as it doesn't produce dust. You will however have to do the final smoothing with a Surform and abrasive paper, so it's not a total solution. The foam dust is terrible stuff. It picks up a static charge and sticks to everything. Be prepared to find it even inside your underwear!
 
I'm told a hot wire cutter can be made from a battery charger but I have no experience of this. Not having one of these gizmos, I used a band saw to cut out my sections. If you have areas on your shape where part of the section is diminishing and part is increasing then you will have to chamfer the sections to leave sufficient material to carve the shape from. Don't forget to cut slots in each section to clear the 4" x 1" ribs. I ran a red marker round the edge of each measured section to give something to aim at when carving.
 
 
Because I had only drawn sections every 100mm and the foam was 50mm thick, I had to cut another set of foam sections. These don't have to be as accurate as the first set, so I just drew round the first set and cut them fairly roughly.
 
Now we have to glue the foam sections to the MDF side profile. From here on I did one side at a time, to save space. Once the first half was nearly finished it was wrapped in polythene and left in the garden while the second half was done. This isn't ideal - the temperature change will probably cause some movement in the pattern and water may get in.
 
To keep the sections vertical, I took four evenly spaced sections and glued them at the correct stations (your grid should be numbered) with 5-minute epoxy glue, using a setsquare to check they were vertical.
 
I then glued the other sections in place using special Styrofoam glue from Trylon (01933 664275). This is a contact adhesive that stinks of ammonia. It is dry enough to join in about 30 minutes. 2.5 litres will be plenty. To stick the end of the sections to the centre profile (MDF) I used the type of spray mount sometimes called "double-sided tape in a can" (it comes out stringy), sprayed directly onto the MDF and left to go tacky. Every now and then you will have to thin down one of the infill sections as each section inevitably adds slightly more than 50mm.
 
Once all the sections were in place I trimmed of any excess foam with a serrated kitchen knife. Always cut away from you - I know this seems obvious but it is so easy to forget. I ended up in A&E with a 30mm deep stab wound in my arm one evening after carving Styrofoam!
 
I then worked down to the sections with a Surform. Don't go mad at this stage. Work down slowly over a large area to give yourself a chance to "feel the shape". Finish off with some 80 grit production paper on a sanding block (use a rectangle of neoprene foam on the tighter curves). Make sure there are no lumps sticking up. It is better to have a few slight depressions than lumps at this stage as the latter will require you to cut into the foam later on or add a corresponding amount of filler over a large area.
 
 
The next stage is to cover the foam shape with a layer of 280 g/m woven glass fibre mat wetted out with epoxy resin. DO NOT USE POLYESTER RESIN - It will dissolve the foam very quickly. Place the sheets of mat at 45 degrees to longitudinal direction of the fairing. This allows the mat to drape to the shape of the foam in the same way that a bias cut dress accentuates the shape of the wearer :-). Do not overlap the mat, as this will cause high spots. It is easier to fill slight gaps. The epoxy needs a generous room temperature to cure. Leave it at least 18 hours. Don't worry if the surface remains slightly sticky.
 
Rub the whole pattern down with 40 grit production paper and then cover with a thin layer of body filler (about 1mm thick - keep it as even as possible. Rub down again with 40 grit. Fill the low spots and rub down again. Repeat until you are happy or bored. Concentrate on the surfaces that do not abut the centreline.
 
At this point I put the two halves together and blended the join.
 
 
I then used a layer of polyester spay filler, but applied with a paintbrush and rubbed down with 60 grit wet'n'dry. You should not spray this stuff without professional facilities. Do not breath the dust and be VERY careful with the catalyst.
 
Once this is done the two halves can be separated again and the first half waxed and polished. I used about 4 layers of mould release wax for this (from any GRP supplier)
 
Making The Mould
 
You will now have to make up some 'walls' to mould against, thereby forming flanges in the mould. This process is called weiring-up by pattern makers. At the appropriate moment these are removed, the exposed mould surface is waxed and the adjacent part of the mould laid up against it. Once cured, holes are drilled in the flanges, so that the mould sections can be bolted together later.
 
First I made a 'wall' for the centreline. I cut another sheet of MDF that was at least 50mm bigger than the centreline profile all round, sealed it with polyurethane and waxed it. I made some indents with a countersink in the 50mm flange. These will help locate the two mould halves accurately later on. It was screwed to the back of the first half pattern. If you want other panel-splits with returns, as I did, then you will have to make up some more 'walls'. I used melamine-faced chipboard for this. Mark the position of the panel-splits on the pattern. You can get the shape right by using a profile gauge to make cardboard templates. These can be stapled together to form a template to cut the chipboard. Wax the walls.
 
Stick some 50mm parcel tape or flash tape (from composite suppliers) to the pattern along the join lines, with most of it the side you are going to put the wall. Attach the walls with a hot glue gun and then run a scalpel along the split-line, to remove the excess tape. Use plasticene to fill any gaps. Sheet wax filler is best for this. It is bright yellow and available from Burnco in Birmingham.
 
I then applied a coat of wax suspended in solvent, which is available from Alchemie Ltd (01926 641600) and is called R7. If you are making a GRP mould then use PVA release.
 
I decided to use a mould-making system called Polydur. It is made by Denaco (01283 520777) and is a water-based acrylic resin combined with a synthetic gypsum filler. It can be used in conjunction with chopped strand fibreglass mat or their own purpose made mat. It is a bit like thin polyfilla and consequently not very strong, but quite rigid. I wouldn't, however, expect to get more than 2 or 3 mouldings from it. Its advantages for the HPV homebuilder are that it's quick (two layers of 300g/m mat make a very rigid mould), you can wash the brushes etc in water and, above all, it doesn't smell! Use a 3" brush to apply the resin.
 
Once the first section of mould has cured you can remove the redundant walls and wax the exposed mould surfaces. Give the next section a coat of PVA or R7 as before. Continue until you have laid up half the mould.
 
You will now need to remove the centreline wall, turn the first half over and rejoin the second half of the pattern. You should be able to reuse the panel-split walls from the other side. Once the mould is cured, don't forget to drill the holes for the bolts (I used M8s) before you carefully remove the mould sections from the pattern.
 
 
Making the Moulding
 
I used epoxy resin for the mouldings, partly because I had some (!) and partly because it doesn't smell too much. It also gives you plenty of time before the resin starts to cure but needs a generous room temperature to cure properly, otherwise it will forever be like warm toffee - not very structural. It also benefits from a 'post-cure' at 40 - 60 degrees after the initial cure. I used strong sunlight (I was lucky). If you do not have any of this then a fan heater can be used. Make a lid over the mould to keep the heat in and make sure you don't set light to anything! I moulded the shell in two halves and then joined it down the centre. This makes laminating much easier - GRP mat doesn't like sticking to upside-down surfaces!
 
Make any necessary repairs to the moulds and rub down. You now need to wax the moulds so that the mouldings do not stick to them. I used 4 - 5 coats of R7, polishing with a duster between each but not after the last coat.
 
I added returns to all the panel-splits. This gives greater rigidity to the shell and provides surfaces to attach mountings and fastenings. I laid up one side at a time. I started with the central panel (the lid) and bolted pieces of waxed, melamine-faced chipboard to the mould flanges, extending them inwards. In order to facilitate a better location between the panels in use I added MDF protrusions to the flanges. In Motor Racing circles these are called 'chocolate fingers' on account of their shape, although the ones I used to locate the lid were more like Tolberones...
 
If you require a rebated return (a 'joggle') then use some sheet wax (from Alchemie or Burnco) applied to the mould. You can build up layers to the required thickness. I did the screen rebate and 'bomb door' apertures this way.
 
We are now ready to lay up the first panel half. I applied a thin layer of epoxy resin (do not mix more than about 800g at a time) with a 2" brush and put GRP chopped fibres (from Alchemie or GRP suppliers) into any sharp corners and around awkward shapes. Make sure they are well 'wetted-out'. The chopped fibres fill areas that the mat will not push into.
 
I then applied two layers of 280 g/m woven GRP mat, rolling between each layer with a 3" paddle roller (Trylon do them). I used a single sheet to cover the whole area, where possible. Slit the mat anywhere where you are having trouble getting it to stay down. With an awkward detail, it is often better to cut a hole around it and add in a local patch. If you push too hard into the corners with the brush you will force out the chopped fibres and create a void. I added 2" Carbon Fibre tape along all the returns and as ribs in any large flat areas. I put extra thickness anywhere there would be a fixing. Try to use the minimum amount of resin. Roll out the mat first and then add resin where it is needed. You can add a lot of weight by liberal use of resin...
 
When the moulding has cured to a toffee-like consistency then you can carefully trim off the excess mat with a knife. If it has gone too hard then use a small saw. There is nothing more dangerous than trying to trim hardened GRP with a knife! Don't do it!
 
After about 18 hours remove the chipboard flange extensions and the 'chocolate fingers' and wax the exposed surfaces of the moulding. Bolt on the next section of the mould and add extensions to any flanges where you want a return on the moulding. Lay up the second panel as the first. Lay up against the returns on the first panel to give a perfect location. Continue until you have a completed half shell and then do the same for the other side.
To join the two halves together you will need to get inside, so unless there are any large apertures you will have to remove one of the mould sections and its attendant moulding on each side and join it separately later. Make sure the moulds aren't flexing when you do this, otherwise the panel will be distorted and will not fit to the rest of the shell. I would run a couple of 2" strips of woven GRP mat along the join. Strengthen the join at edges and the edge of any apertures you are going to cut; otherwise the moulding will split under load.
 
 
Once this has all cured properly you can release the mouldings from the moulds. As the mouldings are so thin great care is required at this stage. Use a thin strip of ABS to slide between the mould and the moulding if it proves stubborn. You can now clean up the mouldings and trim them where necessary. Because I didn't use a gelcoat, in order to save weight, then the surface had tiny pits in it. I had to fill these with cellulose stopper before painting. This is was very time-consuming process!
 
Your bodyshell is now finished - Congratulations!
 
 
Below is a list of all the materials I used and their approximate costs to help you budget for your own project. It adds up, don't it!
 
MDF (3x12mm) and battens - 70 GBP
Styrofoam 12 Sheets - 145 GBP
Foam Glue - 20 GBP
Printing - 20 GBP
Polyester Body Filler 2 cans - 25 GBP
Polyester Spray Filler 2 cans - 40 GBP
Epoxy Resin 10 litres - 110 GBP
Polydur Mouldmaking Resin - 160 GBP
280 g/m Woven GRP Mat 20m - 46 GBP
300 g/m CSM 15m - 20 GBP
2" Carbonfibre Tape 20m - 60 GBP
Moulding Sundries - Wax, Roller etc. - 20 GBP
Paint - 100 GBP
Sundries - 120 GBP
TOTAL - 956 GBP