Is it possible to remove the foam in a composite wing to save weight?

I have finally shaved all the weight that I can from my wizard glider including shortening wires and eliminating connectors. The next step is to replace the canopy with lighter stuff.

Anyway, I was looking at the wing and thinking that surely all that foam is not necessary, especially at the wingtips. Is it possible to gouge out some of the foam and if so, where can it be removed?

From Don Stackhouse

Warren, your zeal to minimize weight is commendable, but I think you are fast approaching the point of "too much of a good thing", if not already past it. When you get into making major modifications to primary structure like the ones you propose, the possible adverse side-effects are potentially devastating. You are re-engineering the basic structure, without benefit of the design and test data that went into the original design. The probabilities of making a fatal error are very high. You also need to consider the efficiency of the weight reductions you intend to do, they may not save as much as you think, and may in some cases actually increase weight. The cost to the model in utility, durability, aerodynamic efficiency and in repairability may far outweigh any possible benefit of a few more grams of weight savings. You need to proceed with EXTREME care in this area, if you proceed at all.

The foam in the wing is very necessary. Just as the epoxy in the wing skins supports the load bearing fibers and keeps them from buckling on a micro scale, the foam core supports the skin and keeps it from buckling on a macro scale. It also holds the skin in the proper airfoil shape. In addition, it helps transfer shear loads between the upper and lower skins, which contributes significantly to the wing's torsional stiffness among other things. Without the foam you would have oilcanning and buckling of the skin, airfoil distortions and loss of torsional stiffness in the wing panels and flaperons, with performance loss, flutter and structural collapse as possible consequences. Coreless structures are certainly possible (our Spitfire pylon racer is one example), but such structures need to be designed for that from the outset. In the particular case of a structure like the Wizard's, a coreless structure might actually be heavier.

Even if it was possible to save weight by removing some of the foam, the difficulty of removing foam from the already built wing must still be dealt with. If you simply cut it out, you must also rebuild the wing skin over that area. This will probably add more weight in lap joints and stray epoxy than the weight of the foam you removed.If you try to use heat to remove the foam you will not save any weight, and will cause distortions to the airfoil shape in the process. Remember, to save weight you must actually remove the styrene plastic that forms the foam from the wing. Melting it in place will only convert it into solid styrene lumps instead of gas filled styrene bubbles. The weight will still be there. Trying to use solvents to melt the foam in place has similar problems, plus the weight of retained solvents in the structure afterwards. Mechanical cutting from one end might work, but will be difficult to control and has a high risk of weakening and distorting the structure. Since the total weight of foam in the entire wing is only slightly more than an ounce and a half, the potential for weight savings here is not very good. You would probably spend a huge amount of time and effort, probably ruin an expensive model in the process, and have very little to show for it even if you were successful.

The canopy is a low-risk area strucurally, but also a very low payoff area. The distance from the c/g of the canopy to the model c/g is about 2/3 of the distance from the end of the nose to the model's c/g. This means that for every ounce you take out of the canopy, you will have to add 2/3 ounce to the nose to keep your model's c/g in position. The net result is that you will ultimately save only 1/3 of the actual weight you remove from the canopy. This is a good example of a "low efficiency" weight savings method. You will spend a fair amount of effort developing a new canopy design and have almost nothing to show for your trouble.

If you take the effort you were going to spend on the canopy and wing and spend it on the tail instead, you will probably see a much better return on your investment. Make sure you use the minimum amount of glue possible when attaching the tail assembly to the fuselage, while still making a sound structural joint. Use epoxy instead of c/a to attach the tail and apply the fiberglass tape. Wet out the tape on a flat surface with low-viscosity epoxy resin and squeegee it out thoroughly to remove all excess epoxy before applying it to the tail. Cut the 1" tape in half and use 1/2" wide strips for the reinforcements along the edges of the tail saddle. Use one steel pushrod for your antenna, and consider using a carbon pushrod for the other ruddervator (just make sure you don't add all the weight back in with a heavy coupler on each end of the carbon rod). Make sure you do a good job of sanding off the flashing along the parting line seams along the top and bottom of the tailboom, but be careful not to sand through or weaken the skin on either side of the seam. When you epoxy in the rear wing hold-down plate and finger peg, mix some micro-spheres with the epoxy so you can get a good solid joint without adding a lot of weight.

Remember, every ounce you save in the tail is 3-4 more that you don't have to add to the nose.

A better approach for taking weight out of the wing is a 'Lite' version such as the technique we use to make the Monarch 'D-lite'. Joe and I are debating the possibility of a Wizard 'lite', but until we've tested it ourselves we don't want to make any promises or take any orders. One of the problems with this approach is that it's contrary to the intended use of the Wizard. The Wizard is meant for windy conditions and turbulence, where you need huge launch heights, gobs of penetration and maximum control authority. A certain amount of well-designed structural weight is good in that situation. Rough and tumble landings tend to be fairly common in that environment as well, so durability is also important.

If you're really looking for maximum light air performance you should be flying a 'D-lite', which is designed for maximum float and super thermalling ability without giving up much in launch height or penetration. Do the best you can to eliminate any unnecessary weight, but keep your Wizard structure the way it is so you'll be ready for those March winds.

Don Stackhouse @ DJ Aerotech