Roll Your Own Tailbooms

This pages describes a method of making your own tailbooms for RC sailplanes. The technique is pretty easy to master and does not require any elaborate tools. If you are already working with composite construction for model planes, you will probably have everything already. This is the Jelly Roll method as explained in the Allegro Lite Yahoo group. Some of the methods come from Mark Drela's description of making booms, though he uses a different layup stack and a metal mandrel.

I have made over 20 booms with this method and it works. Booms have ranged from open class TD planes, the 2m Badger, several Bubble Dancers, and over 12 DLG's. Why would you want to make your own booms? Well you can vary the layup and diameter for any size glider and function. The cost is about 1/2 of what you would pay commercially, and just like anything else, the satisfaction in producing exactly what is required is fun. The commercial booms are higher strength to weight than these home made booms but you can come close.

Tools

What you will need for this project is not unusual:

Mandrel - the fabrics will be rolled onto a mandrel that is the inside diameter of the finished boom. I use graphite golf club shafts for HLG's and light 2m planes, pool cues for bigger than 2m. Golf shafts are 0.61" diameter at the big end and 0.335" at the small end, 45" long. This gives a good range of tapers to choose from when making your boom. If you are buying a golf shaft, buy the cheapest graphite driver shaft you can find. Maybe you can beg a used one from a repair shop.

The mandrel needs to be tapered along it's entire length, reasonably stiff, straight, and have a smooth finish. Look around and I'm sure that there are many other items that can be used as mandrels. A golf shaft clamp is very nice if you use mandrels about 0.4" dia., see the last picture.

Layup tools - You will need to have regular tools for composite layup like mylar, rollers, cutters and a flat surface.

Wrapping - It is really useful (necessary?) to have a bench vice to solidly hold the mandrel while wrapping. Clothes pins can be used as clamps, a tape dispenser is also useful.

Materials

Carbon Fiber (CF) - I use the flat Uniweb carbon from CST or Aerospace in the 2.9 oz/sq. yd. weight for HLG's and the 4.9 for bigger planes. The unidirectional CF comes in several widths, the 12" width is wide enough to do any boom, the 1" wide tape is too small.

Fiberglass (FG) - Bias cut fiberglass will give the boom resistance to splitting and torsional strength. My HLG's have 0.74 oz FG and the bigger planes use 1.4 oz. The fabrics will be laid flat and rolled up together like a jelly roll, resulting in alternating CF/FG layers.

Nylon Bagging film - the 1 mil nylon is used to cover the mandrel and 4 mil nylon is used to spiral wrap the boom. The width of spiral wrap tape should be about the diameter of the boom and five times as long as the finished boom. Since you need varying widths of nylon tape for the spiral wrap depending on the boom, cut the strips of tape from sheet nylon.

Tape - Masking tape is good for general uses, but you will need Zagi tape for anywhere you do not want the epoxy to stick.

Epoxy resin - I use the West System resin with the 205 (fast) hardener, sometime MGS 285.

Cabosil - a small amount of thickening agent will be required.

Vaseline - for coating the mandrel.

Steps

Choose your layup - I don't know any method of calculating the layup required for a boom, so you have to go by trial and error. Booms are usually sized for stiffness not strength. Any boom which is stiff enough to prevent flexing under normal flying loads is plenty strong enough to not fail in flight. For my DLG's, they have 3 layers (big end) and 2 (small end) of 2.9 oz Uni-CF and full wrap of 0.74 oz. FG. The Bubble Dancers have 4 layers of 2.9 oz. Uni-CF and full wraps of 1.4 oz. FG. The Supras have 5 layers of 4.9 CF at the root, 4 layers at the end. You can use these as starting points since they have not broken under flying conditions.

Calculate the required width of the pieces for the number of wraps and add 1/8 of a wrap to account for diameter growth and some overlap. Remember your high school formula C=Pi*D. Pi is approximately 3.1415926535897932384626433. Cut your CF to the exact size needed and the bias FG bigger all around to allow for some distortion during layup.

Here I am making a DLG boom that is 28" long and will be used on a Supergee. The pieces have been laid out after cutting. From left to right we have yellow Zagi tape, the 4 mil nylon film for spiral wrap, fiberglass trapezoid, CF trapezoid on top of the FG, 1 mil nylon film for the mandrel, and the mandrel (golf club shaft). The mandrel has been coated with a thin but noticeable layer of Vaseline to assist in adhering the 1 mil nylon and it makes it much easier to remove the boom after curing.

The mandrel has been wrapped in the 1 mil nylon film at least 2" longer than the cut fabrics. It is wrapped at the diameters we want along the taper. One wrap of nylon film plus 1/8" is sufficient. Then the nylon is held in place with tabs of Zagi (packing) tape. The nylon film is only attached to the mandrel at the ends! You cannot tape the film to the mandrel along the length because the finished boom will not come off, I have spiral wrapped the film onto the mandrel but prefer the lengthwise method for removal of the boom. Note that I have marked the direction of the wrapping of the nylon film on the end tape. All processes will proceed in the same direction.

The CF is wetted out on a strip of mylar (which you cannot see). I use a vast excess of epoxy and work it back and forth into the cloth with a scraper until the resin is worked through to the other side.

After wetting, the CF is aggressively rolled to remove excess resin.

The CF is laid onto the FG as shown, with one wrap of FG to start the wrap. If your lay up has more than 2 wraps of Uni-CF, lay out the carbon to align the fibers along the mandrel as shown, with the long edges butted up to each other.

The CF has been peeled off the Mylar and laid down on the bias FG as above. Then it has been rolled again to get the FG to stick to the underside of the CF. The FG is not wetted out at all, there is more than enough resin in the CF sponge to wet out the FG. A small batch of thickened epoxy is mixed up and dabbed in a line onto the mandrel along it's length. Anywhere on the mandrel is fine so long as it is not the seam.

Place the boom onto the FG so that the thickened epoxy line picks up the tab of FG and roll the mandrel towards the CF. You did remember to roll in the same direction as the nylon wrap? Trim the excess FG at the top end of the wrap within 1/8" of the CF.

Here the full rolling is complete. Get the wrap as tight as possible, but minor puckers and loose tabs will be corrected in the next steps.

Clamp the mandrel in a vice for wrapping, I bought this vise just because it has the notch in the jaws for gripping round objects. Because I am using the mandrel to the maximum of the big end, the mandrel is clamped from the small end. Either way (end) is not important. AARRRGG, the fabrics are starting to unroll! Not a problem.

Starting at the small end of the boom, spiral wrap the entire length using as much tension as you can muster. This is where the vice helps a lot. When you need a rest, clamp it off. Overlap the nylon strip evenly, I try for 1/2 strip overlap. Again... which way are you wrapping the strip?

Here the wrapping is done, put a couple of insurance wraps of masking tape on the ends. You can see the strip of nylon tape is joined in the lower right, I cut the nylon strips from 60" sheet material. The boom goes into the warmest room in the house to cure hard.

Now to get the boom off the mandrel. Unwrap all of the outer spiral wrapping and untape the ends of the boom.

Clamp the mandrel in the vice again and twist with your palms. You will feel the boom break free of the mandrel. Work your way along the length to release the full length.

OK, what if you cannot break it free? We need to apply more torque, so get some double sided sticky tape and wrap the boom. Then take some nylon webbing from your F3J towing strap and a screwdriver. Wrap the webbing onto the sticky tape and use the screwdriver for leverage. This will break the most stubborn boom free. This method works every time, you can apply so much torque that the boom will fail. I guess the epoxy shrinks and really tightens up the fit. Use more Vaseline next time.

Now it's time to remove the inner nylon wrap. Clamp the nylon into a Vice Grip locking pliers and twist merrily away. The wrap will bunch up and pull out with satisfying popping noises. Oh noooo, the nylon film broke! Double sided sticky tape to the rescue again. Chuck up a long dowel in your drill, and put a 2" strip of sticky tape on the end. This will grab the loose film remaining on the inside of the boom and twist it out.

Addendum: Another method to get the inner wrap out, twist the loose nylon at the small end and tape together. Push the loose twist into the end of the boom, followed by a long dowel. The inner nylon will peel back on itself and can be pulled out from the large end.

This is the finished boom, weighs 12.8 grams and is 28" long. The part in the foreground is a rubber golf shaft clamp (www.golfsmith.com part #913) that is extremely useful if you are using golf shafts as mandrels. It allows you to really clamp the shaft solidly without crushing the tube. Buy 3 of them.

So there you have it, a nice boom with the strength/stiffness you want and you are the envy of the club with your elegant pod/boom fuselage.

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