How To: Tubular Composite Airframe Layups

Fully composite layups are similar to overwrap, except the tube (called a mandrel, because it is used to form layups) comes out to leave a fully composite tube.Our procedure needs to be improved (see suggestions below), but the current process is outlined below. This process could also be used for coupler tubes.

Materials:

• Aluminum mandrel with outer diameter (OD) equal to the desired inner diameter (ID) of the composite tube
• Mold release (we're currently using Stoner)
• Fabric (carbon fiber, E- or S-glass fiberglass) - enough for about 5 layers (called plies)
• Double stick tape
• High temperature vacuum bag
• Vacuum tape
• Epoxy resin system
• PPE: gloves, lab coat, safety glasses
• Rubber scrapers for spreading epoxy
• High temperature perforated release film
• Breather/bleeder fabric

Procedure:

1. Assemble the mandrel. The coupler mandrel does not require assembly, but the tube needs to be slid together with the two smaller ends of the tapered mandrel in the middle. To ensure a good friction fit, put a couple of small pieces of painter's tape along the coupler to keep the mandrel from bending at the seam.
2. Slide the tube mandrel onto one of the aluminum rods that store the fiberglass rolls if you're doing a tube layup; slide the mandrel endplates into the end of the coupler mandrel for a coupler layup
3. Slide the aluminum rod or mandrel endplates onto the layup jig
4. Generously coat the mandrel with mold release
5. Cover the mandrel in a layer of high temperature vacuum bag (mandrel length x 19" wide). To do this, wrap the tube in a vacuum bag of the appropriate size. Using double stick tape, tape the vacuum bag to itself at the seam, making sure it is tight around the tube and does not allow air bubbles. Make sure it is not in any way adhered to the tube, because it will not come off
6. If the width of the fabric is wider than the section of the tube, you can just roll it off the roll onto the tube. If it is too short, or if you are using +/-45 degree plies, you will need to cut the fabric to the correct size
   1. When cutting the fabric, make sure you aren't cutting into a table, and be careful not to stretch the fabric when you're cutting. To avoid this, you can cut slowly with a scissors, and hold down the fabric on either side of the cut to prevent it from pulling
7. Mix the resin system according to the directions
8. Wear the appropriate PPE (safety glasses, gloves, respirator, and probably a lab coat - composite dust and epoxy are skin irritants, and the epoxy is harmful to breathe)
9. Begin the layup as if it were an overwrap - place one end of the fabric along the tube axis and tack on with epoxy
10. Using rubber scrapers, spread epoxy over fabric to fully coat it without adding too much excess
11. Turn mandrel to keep wrapping fabric around the tube, and cover with epoxy. 
    1. Make sure to wrap the fabric tight
    2. Remove all air bubbles as they appear by spreading towards the edges of the layup or towards the dry fabric
    3. Be careful not to touch the wet fabric (even with gloves) because it will pull and compress the fabric, which will result in air bubbles
    4. Spread epoxy in the fiber direction to avoid distorting the fabric. Ideally, everyone should spread the epoxy along the circumference of the tube
    5. If you need to use more than one sheet of glass, make sure to offset the next layer by an inch or two from the seam
12. Continue to add epoxy and fabric, spreading the epoxy so the fabric is fully wetted, but no excess epoxy is added. 
13. Wrap enough layers of glass around the tube to withstand the flight loads. You can determine the flight loads and test samples of composite tube by following yet to be written wiki articles.
14. After all of the fabric has been placed, do one final check to make sure the fabric has been wetted (covered with epoxy), has no excess epoxy, and has no air bubbles or pulls in the fabric
15. Wrap a layer of peel ply (layup length x 20") around the tube to provide even pressure and aid the surface finish. The epoxy should keep it well attached to the tube, but using some rags in order to apply even pressure, smooth out the peel ply around the tube. Make sure there are no air bubbles that could harm the surface finish
16. Wrap a layer of breather fabric (layup length x 22") around the tube
17. Wrap a layer of vacuum bag around the tube (layup length x 24"), and attach to the bottom vacuum bag with vacuum tape.
18. Pull vacuum to 0.7 barr
19. Cure in oven based on epoxy system guidelines