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Vacuum Bagging a High-Performance Wing


Would you like to vastly enhance a future model's performance? Are you interested in competition technology and performance? Are you lazy and want to build a strong wing in 3-4 hours work? Read On!

The wing that is being made in this how-to is for a 400 racer that most of you know, the Sliver 400. The methods I show you here are the real deal when it comes to what is done by the fastest racers in the country. The current national record fast time for 400 pylon was set in 2000 by Pete Bergstrom using a plane I built and set up. The methods I show you here are exactly the same as I used to make his wing with one exception: his wing didn't have glassed skin hinges or servo-mount reinforcement. This costs .2-.3oz and I wanted to have all the help I could give him.

With this in mind, if you follow the instructions I give here you can have wings that are stronger and lighter than any that can be bought commercially. In fact, wings made with Balsa and Foam are half the weight of the best hollow molded wings on the market and just as strong.

When people hear "composite wing" these days, they immediately think of fiberglass cloth over blue or pink insulation foam or hollow molded wings. A bagged Balsa wing is still a composite wing, just not as glamorous because Grampa used to make his airplanes from the stuff.

Balsa has several benefits:

The first is that they stay looking nice longer than a glass/foam wing and are repairable with scrap balsa when you break them.

The second is weight. While the glass over foam wings can be built light, they are very easy to dent and ding when you are being weight concious. The hollow molded wings are really strong and beautiful, but are very heavy. This is because they have to be cosmetically "pretty" in order to sell and because they are making them as fast as they can to fill orders. To prove the point, one of the commercial 400 racers out there has only 75sq. in. of area and the wing itself weighs 1.8oz. before adding torque rods and hardware. Compare this with the 163sq. in. wing built in this article that weighs 1.7oz. After covering and torquerods, the bigger wing will only be .1-.2oz heavier. Balsa may be low tech, but it is light!

All you need for this project is a set of foam cores, glue, a Vac-Bagger, and modeling odds and ends that you probably already have in your workshop. Once you make a wing this way, you'll never want to build up a ribbed wing again. Bagging a wing is so fast and easy that it feels like cheating!

Major items that you'll need:

1) A set of cores!

These can be from a commercial kit or ones that were cut from your own templates. There are many cheap ways to make a foam bow, and even exotic machines such as Tekoa's FeatherCut. One other option is to have a core cutting company make them for you. Don't forget to account for sheeting thickness when drawing your core templates!

The cores you need for bagging the wood wing I show here are made from 1lb. Virgin white foam. Do not buy 'regrind' foam, it wire cuts like a hot knife through gravel. For glass composite wings, 2lb blue or pink foam is used, but here the balsa skins are a lot of the wing's strength so 1lb is more than sufficient. The spar is only a 9" long by 1/32" thick piece of plywood. The foam's main job is just to maintain the airfoil shape, that's it. On your wing, make sure that the spar extends 3" past the root of your aileron-this is where wings like to fold. For those that worry, this wing will take up to a medium size brushless motor, i.e. 250-300w. To exceed this, you'll need a more substantial spar and glass/carbon capping it on top and bottom.

2) Epoxy Glue!

You'll need a quality laminating epoxy for this job, the 5, 15, and 45min. hobby shop slop won't work here! I've been using West Systems 105 resin and 206 hardener with good results and recommend them because they are inexpensive, easy to find, sold in smaller quantities, and there are pre-set ratio pumps available for them. Don't buy a lot of glue as it has a shelf life! You are going to use it an ounce at a time and be throwing most of that away….

3) A Vacuum Bagger!

There are ready made systems available in the U.S. from Aerospace Composites and Composite Structures Technologies. I bought ACP's bagging machine on advice of a friend and am very pleased with the results I have seen with it. Up until then, I had been using his home-made bagger, but didn't want to try to source the components and get bogged down with troubleshooting my own. The first wing I made with the ACP system turned out perfect, best one I had made up to then. My first wing was in the bag less than 10 minutes after the bagger was pulled out of the box. They really do provide a turn key bagger!

There are a lot of details that make this the Cadillac of baggers-the wide range of vacuum adjustment is a plus. Probably the most advantageous part of the ACP setup is the large plenum(tank). This is the large cylindrical object in the pic. It allows the air pump to run only briefly and less often to maintain vacuum. This is a good thing if you have sleeping children or make airplanes in your apartment bedroom's walk in closet like me!

4) Wood!

As mentioned above, the wood is the primary source of strength in this wing. If this wing were to be used strictly for competition, then Ultra-Light contest balsa could be used. The only problem with this is that you can't use a high heat iron-on covering or you'll transfer too much heat through the wing, melting the cores underneath. Silkspan and dope, LightSpan, or UltraCote Lite are good choices if you use lighter skins.

For more strength and ding resistance, you should select medium weight balsa skins. Medium weight skins were used in this wing and it was still very, very light. In the case of the 400 racer, there is always plenty of this to be had at the local hobby shop because it's all that's left after the Free-Flight guys picked through the lightest 1/32" sheet Balsa.

5) Fiberglass!

This is only a luxury.. You'll be placing strips of light (1-2oz) cloth at the ailerons and the center section. None of this is for strength-the piece in the center goes under the top sheeting so that you have a reinforced area to mount the aileron servo when buried in the wing. For a seriously stressed wing, fiberglass and unidirection Carbon Fiber should be sandwiched in strips and placed on the top and bottom of the spar. This creates an "I" beam underneath the skins making the wing extremely strong, yet still very light. A typical application would be in a long, thin wing such as those used in F5D and B. Remember that the thickness and aspect ratio of your wing will affect the strength it needs when completed. If you are bagging a Sp400 wing that will only be .3" thick when finished, it would be a good idea to add light glass spar caps.

6) A sanding stick!

Meet you new best friend, the extruded aluminum sanding bar. You'll be using this to prep your skins and sand the wing to shape once out of the bag. This tool is what gives the commercial results that you are looking for. You'll be able to perfectly radius the leading edge and sand the trailing edge to a razor sharp point on your first try. Very handy, you'll wonder what you ever did without one before.

7) A 36" metal ruler, masking tape, squeegee, pens and markers, Exacto blades, wax paper, and paper towels.

Place the top of the core on the top skin. This is where the skin gets closed up and you start getting ready for the bag. Make sure that the cores are placed properly on the skins-remembering the overlap. Slip the cores into the beds, again, checking that top is top and bottom is bottom. Slide the wax paper dividers between the beds and skins and give the cores a final line-up in the beds before taping it all together with masking tape.
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