How to make Carbon Propellers
The stuff mentioned in this article is “R&G” brand products. For illustrating pictures refer to the German text or here for the Photoalbum
1. Preparing the mould
Apply 10 coats of R&G paste wax. Let dry for 15 - 20 minutes. Polish with a soft cotton rag. Don’t ever use paper towels, since the fibres are very hard and might damage the wax coat. Then add one coat of PVA release agent with a brush ( before any future use apply two wax coats and one PVA coat ). For markings on the mould use colour markers (see below). The mould can be locked tight with commercial G-clamps or homemade clamps (see pictures).
We are using R&G resin L with hardener EPH 161 (VE3261). This resin system can be made heat resistant till 130 degree Celsius by tempering. This might possibly be detrimental when trying to repitch the propeller, but the tendency to turn back to original shape at high temperature (like summer heat in your car) seems to be greatly reduced, compared to commercial props. The resin is mixed according to manufacturers recommendation. We add a few drops of dye to be able to detect spots where there are not enough fibres, plus some defoamer from R&G which prevents air bubbles in the prop.
2. Carbon fibre
We use those widely known unidirectional carbon stuff which is usually sold at contests by Ukranian team race flyers. However you can use any other carbon fibres, even single fibres out of heavy carbon weave can be used. For industrial purpose rovings are mainly used. You’ll also need a mix of chopped carbon and resin to fill the hub area. This can be homemade by cutting short pieces from surplus carbon or glass fibre strings.
3. Distribution and amount of fibres
The correct resin-fibre combination can only be found by trial and error. To get an idea we check the weight of the original prop (the pattern) or - if this isn’t made out of carbon - of a very similar carbon prop. According to R&G we can achieve a volume portion of fibres of 35 % at best, which means a weight portion of 50 %. Now when weight checking the carbon stuff you know which amount of fibres you should use. This amount is to be imbedded evenly in the mould. A propeller is thin at the tips and thick at the centre, so we have to use more fibres at the centre. For a 32 cm diameter prop we usually cut down all of the (calculated) carbon stuff into pieces of 10 cm, 16 cm, 20 cm, 24 cm, 28 cm, and 32 cm length until all is used up.
4. Making the propeller
Carefully cover the mould with release agent. Put some marks at the edge of the mould according to half of the length of fibres; in our case that means at 5 cm, 8 cm, 10 cm, 12 cm, 14 cm, and 16 cm. This will help you to place the fibres. Now one coat of resin is applied. Don’t cover the side areas, only the area where the prop is placed. Put some additional chopped carbon-resin-mix around the axle. Start with half of the longest fibres and put down from tip to tip, then add the second longest ones. Those marks will help you to place them precisely. Continue until you have used up half of the fibres. Fill each layer carefully with resin. Make sure that all fibres are fully saturated. On a three blade prop the fibres will pass the axle on the outside of the centre.
Now you’ll see some empty space close to the axle, running to each blade tip. This space is filled up with some resin mix. The rest of the fibres is applied now, short pieces first, long pieces at last. Again empty spots are filled, especially around the hub. If there are any air bubbles enclosed in the resin, you can carefully remove them by pushing them out with a resin filled brush. Begin at the centre and work towards the tips. Use a small brush and/or small sticks and push any fibres away from the blade edge (around 0,5 mm). This will keep the fibres within the propeller blade shape and keep them from getting to the blade surface. Now the male mould is covered with resin and the mould can be closed. Do this SLOWLY to allow surplus resin to flow out. Take your time, 10 - 15 minutes is quite adequate. Let the resin cure. When the gel phase is finished you can place the mould on an oven to accelerate curing. The propeller should be removed from the mould after no less than 24 hours.
Now the flashings are roughly cut off, release agent removed, and the prop is ready for tempering. If no special oven is available the kitchen oven can be used, but the prop should be packed airtight threefold in aluminum foil. First cure for 12 hours at 60 degree C, then increase every hour by 10 degree up until 90 degree for another 12 hours. Let cool down before finishing the prop - remove flashings, check diameter and pitch, balance.
Making a Carbon-Prop