How to make full depth spars for foam wings

This is the method that I use for making strong (not light) spars for foam wings. A spar in my open class plane, 50 inches long, weighs 7.5 oz with 0.060 top and 0.030 bottom CF spar caps. These spars are easier to add to a foam wing core than a carbon tube, and cheaper too.

Pick a design safety factor depending on the proximity of the parking lot to the winch turnaround and how spectacular a zoom launch you want. Do your cantilever beam calculations and design the layup schedule for the wing skins. I use the Joe Wurts Excel Program(tm), adjusting for a fixed beam moment carried by the spar.

Materials:

1. 1/2" thick balsa wood, light is good.
2. 1/2" by 1/8" spruce, 2 pieces, each as long as the spar or see splicing instructions below.
3. 1/2" wide carbon fiber (CF) top spar cap, twice as thick as bottom, more safety factor. Buy the preformed laminates, this method won't work with CF tow.
4. 1/2" wide bottom carbon spar cap, same thing. I've seen tow spars fail due to poor resin saturation on layup.
5. Glue - thick CA and slow epoxy with Cabosil (thickener).
6. 3 oz fiberglass cloth, bias cut, or braided CF sleeve.

Tools:

1. Vacuum bag equipment and aluminum I beam.
2. Normal hobby tools for working with balsa and spruce.
3. Large flat workspace covered with mylar for gluing.

Prep work:

1. Sand and clean the carbon fiber caps, top and bottom. I'm sure that carbon dust is carcinogenic.
2. Lay the CF on a flat surface and spread a layer of thick CA.
3. Press the spruce onto the CF and using a roller to get even contact, while keeping the CF flat on the workbench. The CA should set up in about 5 minutes. The only reason we use the spruce is to bond to the CF, i.e. end grain balsa is difficult to glue to the CF. I have used a woodworking jointer to shave the spruce down to 1/32" thickness after gluing, if you want to save weight and risk your fingertips. The spruce carries no load due to its low modulus. Trim the CF so that it does not overhang the edges of the spruce. Round off the long edges of the CF, this prevents the bias FG from being cut at the corner. You want the spruce/CF caps to be 1/2" wide.
4. Measure and cut the balsa to the desired height of the shear web. Test the final spar depth by squeezing the components between a pair of measuring calipers. I squish the caps and balsa down to the exact depth required, knowing that the vacuum bag will squeeze it to a little bit smaller depth. The spar is to have the balsa grain running vertically for maximum strength. You can taper the spar, but make sure the total spar depth is a few hundredth's less that the foam core depth all the way. I use a bandsaw with a fixed cutting guide to cut the balsa webs from sheeting that is 4" wide by 1/2" thick.
5. Plan any inserts such as joiner tubes. For longer spars I will splice the caps in the center, and imbed a gigantic round CF rod in place of the balsa shear web. This works well for the center panel of three piece wings.
6. The aluminum I beam comes as a cutting guide for circular saws. Two 4 1/2 foot sections can be bought at Home Depot for $20.00 and they have a huge number of other uses. Prep the I beam by waxing it to prevent glue sticking and tape over all the sharp corners to prevent punctures. Alternatively, include the aluminum I beam in your stress calculations of the spar, and it will know not to stick properly.

Assembly

1. Get the vac bag stuff out and mix up the resin. Put on the gloves. Make up the sandwich of vertical grain balsa and the spar caps with the CF on the top and bottom surfaces. The resin will hold the mess together. Make sure the end grain balsa does not soak up all the resin and make a dry joint.Use enough Cabosil to keep a layer of epoxy on the end grain balsa.
2. Coat the outside of all surfaces with resin and lay on the bias ply fiberglass cloth. Wrap the cloth around the spar and overlap on one face. If you're finicky, use kevlar cloth wrap at the joiner tubes/splice rods. CST suggests CF braided sleeve, and this sounds great but I have never tried it.
3. Use masking tape to attach the gooey mess to the I beam. Lay the spar on its side with the bottom CF cap against the straight edge. I like making messes, that's half the reason building planes is fun.
4. Pick up the spar and I beam, and vac bag the whole thing. Unfortunately you cannot peel ply the spar because the spar will not be straight. Well, I suppose you could peel ply one side and the top of the spar, but you and I fly so well that 1/4 oz. is a handicap we will spot the other pilots. As the vacuum is pulled, help the bag to crease properly against the flat sides of the spar and I beam, and the spar will get sucked dead straight. You can work the excess resin to the ends by rubbing the spar lengthwise. Make sure the spar is lying on a flat surface and tight against the I Beam and let the resin set....

Addendum Nov. 2003

I've made several dozen spars using this method and there are a few variations that are worth noting.  The first is to eliminate the spruce/hard balsa in the spar cap and glue the CF directly to the vertical grain shear webs.  This works fine and will save a little weight, but it will require a better fit up of the shear webs to prevent steps and voids in the spar/shear web joint.  Another possibility is to eliminate the bias cut FG skin and hand wrap the spar with Kevlar tow or thread.  This will result in a spar with high strength but it will be less stiff than the FG wrapped version. I did this on the Bubble Dancer and it requires more work to grout the spar into a foam wing.

For the ultimate in spar stiffness, go with the CF braided sleeve, I'm using the 1/2" diameter 3k sleeve from CST.  While the cost of the braided sleeve is high, in the total cost of the wing it's not that much more to get a very, very stiff wing. It's difficult (impossible) to get the assembly into the bag using the braided sleeve, so vac bag the spar caps to the shear web in one step, then add the braided sleeve in a second bagging operation. This gives you the advantage of sanding the shear webs to width after they are glued to the caps. When applying the CF braid, you can vacuum bag with peel ply to suck some of the resin out.

If you use the braided carbon sleeve, the stiffness seems to be now limited by the shear modulus of the balsa web.  To overcome this, I sometimes use a laminated shear web with 1/4" plywood in the center and balsa outer. I would imagine that a bias plywood shear web would be the stiffest possible combination. Now the spar assembly is becoming a little complicated. The last spar I made with plywood webs, CF sleeve and a stepped thickness carbon cap took three vacuum bag operations and about 10 hours to build. The satisfaction will be that you are approaching the engineering  limit of strength to weight ratio for a home made structure. 

Pictures are worth 1k words

	This spar will have a 5/8" center joiner rod and is 24" long per side. The spar will be built and bagged in one piece to ensure that the tubes align with each other. Here we see the main parts (l to r) joiner tubes, top spruce caps, 0.060" top CF spar cap, 0.030" bottom spar cap, lower spruce caps, and joiner rod with dihedral. The CF joiner tubes are too large in diameter to have spruce caps, so a filler strip is made to get the right fit.
	The spruce pieces are CA'd to the Carbon Fiber. and the parts measured for thickness. 1/2" balsa shear web is cut to exact depth for final sizing, this spar will be tapered so it is full depth in the first panel of the wing. A little trial and error gets the spar to taper from 0.920" at the root to 0.830" at the ends. Try that with any other method!
	Resin has been added to the end grain and spar caps. The aluminum channel is wrapped in cling wrap to prevent glueing to the spar.
	After assembly of the components, bias FG has been wrapped around the spar. The spar is laid next to the aluminum channel and then....
	the entire mess is taped together. Alignment is not at all necessary, the parts will be straightened out in the bag and vacuum will hold it in place.
	The spar is in the bag. Dark green felt in the foreground is breather to connect the vacuum connection to the part.
	14 inches of Hg is pulled and the individual pieces are squished into place. Press the shear web parts even with the caps and get the tubes flat on the table.
	A few bags of lead shot will hold everything flat until cured. Total time from start to finish, 2 hours.

The spar is cut in half for insertion into the wings of my latest and greatest world beater sailplane. This picture shows the core prepped for the spar slot. I have bagged a strip of 3 oz. unidirectional CF onto the top surface strictly for the purpose of the spar slot. The slot will be cut by hand from the bottom surface, the fit needs to be close, and a little on the looser side of fit.

Here the slot is cut into the bottom surface, and you can see where the slot is a little wider to accomodate the joiner tube. The joiner tube on this spar system is 5/8" inside diameter, so it is wider than the spar which is 1/2" wide.

Lastly, glue the spar into the slot, and grout the bottom surface flush. Resin will leak through the top cap so tape over the uni-CF to reduce the leakage. The end of the spar is abrupt, so the the skin layup will have patches to help distribute the load and prevent the spar end from punching out of the wing core. A 1/16" plywood root rib will help on the fuselage end of the spar.

Epilogue

This method results in a really nice spar that you can control the dimensional accuracy to within a few hundredths of an inch and, more importantly, is perfectly straight. You can vary the CF thickness and strength to whatever loads you wish the spar (versus the skin) to carry. Without good engineering data on shear strength of vertical grain balsa, my guess is that the shear web with glass cloth facing is overbuilt, but the ease of adding it into the foam wing is worth building a rectangular spar shape. By the way, I build the spars as constant strength, and use tapered CF cloth layup to decrease wing strength towards the tips.

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Copyright 1999, Warren Man-Son-Hing