Solid balsa wing questions

[MSB]

Hi all,

I built a solid balsa wing for a pylon racer - 6mm thick with 1/16" sheet on the bottom. The bottom is flat.

The wing had a slight warp in it, and I misted the bottom with water and weighted it down. Now it looks fine, but will it stay this way? This leads to my second question...

Finish. I was planning on covering the wing with Ultracote, but could also just use a clear poly finish. How much weight would the poly finish add, and would this be more stable?

Thanks much in advance - this isn't really a science question, but seemed to fit better than the other forums...

- Mark

[SheldonYoung]

One advantage of using Ultracote is that you can use it to pull the warping out of your wing later on, assuming it's not severely warped. Simply twist the wing to remove the warp, or slightly more, then shrink the covering. This technique also works to add wash-out to a wing after it has been built.

[Sparky Paul]

Something with ammonia in it might be better to get a permanent set into the wood. Ammonia loosens the lignin fibers so they can stretch and keep the new set when the liquid evaporates.
Film is a smoother (faster, as is speed wise) finish than paint.

[El Pe]

Mark,

If you really don't want the wing to warp, make sure it is exactly the way you want it to be and put 2/3 oz glass cloth on it with either glass resin or thin Cyanoacrylate glue (my personal favorite). The glass adds practically no weight and if you use Cya, practically none there. Over balsa I use the standard Cya and over foam, the odorless type (which doesn't melt the foam). If you want the details of how to do this, post it in a response.

I have used this for years, since I saw it in a Satellite City video, and it works great. You can fill the glass weave with lite spackle, paint it with spray can paint or cover it with plastic covering.

By the way, if you use regular Cya, make sure you have adequate ventilation and a fan blowing the fumes away from your face. I seem to have developed a sensitivity to it.

Good luck

Lee

[MSB]

Yes, I would like to hear your glass/cya procedure!

TIA,

- Mark

[El Pe]

Okay Mark, here goes:

To glass with Cyanoacrylate:

Assuming the surface is prepared to your satisfaction (dings filled in, smooth surface, etc.)

Cut the 1/2 to 3/4 ounce fibreglass cloth 1 to 2 inches oversize all around for the area you are covering.
If the area is relatively small, fix the cloth to the target (wing half, fuselage, rudder, cowl, etc.) by brushing the cloth onto the item with a soft bristled, clean, flexible paint brush. This will cause the cloth to electrostatically adhere to the surface.

If it is a large surface, spray on a low tack adhesive such as those used to adhere photos to cardboard. It works best to have two people lay the cloth onto the surface if you use the adhesive as this lightweight cloth does not want to lift off with out becoming a mess. It is better to toss it and cut another piece.

Once the glass is flat to the surface, tilt the surface so the glue will be able to flow down, but not too fast (about 45 - 60 degree angle). Use thin Cya.. Start just below the top and move the glue tip horizontally back and forth quickly two or three times going down the glass as you scan. Quickly take a paper towel and swipe across the surface which has the glue on it to absorb the excess and spread the rest evenly. Continue doing this until the surface is almost complete, not quite to the bottom. Make sure to continue the paper towel wipe as you go. Turn the paper towel and/or replace it frequently so it is fresh. Some people use an old credit card, but I don't seem to be able to use one without pulling the glass cloth.

When doing the edges, pull the excess cloth around the edge, trickle the glue on the edge, wipe with the towel and continue down the edge until done. With thin glue, it should be cured enough within 5 minutes to sand the edges to remove the excess cloth. When doing this part, you should have on a rubber type of glove to which Cya will not stick (or you and the glass cloth will become inseparable). Once that is done, switch it over and do the other edge. You should be able to do the opposite side (in the case of the wing and tail parts) within 5 minutes.

When doing a fuselage or other round parts, you can use the same technique as above and cut darts in the glass cloth to eliminate overlap, or just allow the overlap and sand off the high parts.

If you make an error and need more coverage, cut a small piece of cloth and glue it over the bare spot. Sanding with a 100 to 200 grit paper will quickly feather it level. I definitely would wear a mask when sanding glass cloth.

This technique does not fill the weave as some resins do, which is both good and bad. Bad because you will probably want to do it, good because this whole program adds a negligable amount of weight.

I use lite spackle which I get at Lowe's or Home Depot or most hardware stores. Thin it with a tiny bit of water so it can be brushed on and here is where you can use that old credit card to smooth out the application. A thin application can usually be sanded after 12 hours. You can dink around with this until it is very smooth or take the 10 foot stand-off scale approach and not spackle the thing at all.

Lately instead of getting out the paint gun, I just spray with the commercially available model paints. Cleanup is a lot easier.

This technique really strenghthens the airframe and is 99% impervious to warping and hanger rash. I took my Yak 23 from 50 feet up straight into the ground (grass soccer field) and only the nose ring was slightly crumpled.

Again, wear a mask, have good ventilation and a fan. Have some acetone around to clean up the Cya and get it off your fingers. Acetone also works well to wipe your newly covered plane before you put on the spackle or the paint. Incidentally Nitromethane works very well as a debonder. It was originally sold as an industrial solvent.

I am sorry this is so wordy. It is an extremely easy and rapid way to glass anything, but takes a long time to describe.

If this isn't clear, hit me again.

lee

[El Pe]

Mark, one other thing.

After seeing that thing in print, it is hard to believe I left something out, but this technique works well with balsa surfaces and the same technique works for foam. The difference being you must use the odorless Cya over the foam and it costs about 3 to 4 times as much as the stinky stuff.

Lee

[Pierre Audette]

Any idea how the FG soaked with CA compares to using water based polyurethane? It would be cheaper and easier to apply (and clean up), but is it as strong? My guess is yes, as most of the strength should be coming from the FG. The liquid should just be a bonding agent.

[Viper Pilot]

Pierre,

Correct. The glass provides the strength regardless of the bonding agent. It's amazing to watch the CA while doing this (outdoors). Smoke actually rises, and the surface become very warm.

VP

[tom_yellowblue]

Thanks for the explanation Lee, that was very helpful!

Has anyone done a strength and weight comparison of CA vs polyurethane for this? I've done this with CA, but I've always just used wax paper to spread the CA around. This results in the fiberglass fibers being moved sometimes, and I think I have to use too much CA. I'll try the paper towel method (which should also work to soak up excess polyurethane).

[bearsa]

i would also like to know if anyone has used diluted pva with fibreglass or wall paper paste i know pva is used with brown paper could this be used to apply fibreglass to foam. i think wall paper pasteis very light not sure about its adhesive strength

[Sabrejock]

Balsa has lignin?? I read here that it has none or very little. I just use water. Tex.

[Mike Rolls]

The only time I tried using the paper towel method I got in a real mess - my preferred method is to use a thin disposable plastic glove on one hand and just rub the CA with a fingertip. I would agree with what folk are saying about the essential nature of good ventilation when using CA like this - a lot of fumes are generated.
Mike

[Slope Nut]

You guys would be amazed and delighted at how light, odorless and easy it is to just use thinned epoxy on the surface rather than CA. It provides a great deal of strength and is pretty darned light.

Mix two part epoxy (I use Envirotex Lite found at Michaels or Hobby Lobby) 1 ounce of epoxy and one ounce of hardner, then mix in one ounce of denatured alcohol. You can get the alcohol at the hardware store. DO NOT USE RUBBING ALCOHOL....it has water in it and really makes a mess of it.

Once this is all mixed up the consistency is that of water. Just use a paint brush and brush the thinned epoxy into the cloth laying on your wing and your done! When its dry do the other side of the wing. Let it dry for about 24 hours, and then fill the weave with auto primer. Sand nearly all of it back off and your ready for paint.

Very permanent and nice paintable finish. So much easier than sucking CA fumes all nite :)

Greg