daven

My Feedback: (1)

I vacumm bagged a couple v-tailed stabs in a vacumm bag yesterday, and had some problems with slight warpage.

The stock is 3/16" contest balsa, with 1/64" ply inset into the edge all the way around the stab. The stab has a sanded airfoil, that is pretty sharp all the way around, utilizing the 1/64" ply as a stiffner. I have a layer of 2oz on the top, and 2oz on the bottom, with a 1" wide piece of 1.4 oz Kevlar on the hinge line. The fiberglass / kevlar was layed up on Mylar, and the wood stab sandwiched between the mylars and placed in the bag.

The first tail, I did at about 15 lbs, and the second one at 10 lbs. The first tail definately has a cupping effect to it, I will be taking the 2nd tail out of the bag tonight. I am hoping by going to less vacumm that this will help, but I am not sure.

Any ideas? Any way to straighten the one tail??

davidfee

Apply gentle heat with a heatgun and slowly bend it into the position you want. You'll likely need to go a bit past where you want, since the stresses in the skins will make it want to relax to the original position. It may get close enough, depending on how badly warped it is. I usually try to press the part between two core beds to avoid this problem, but I know you were using balsa so it's not so easy.

Good luck,
David

Darrinc

My Feedback: (5)

On pieces like this, I pull the part down with vacuum, then release the vacuum and let the part settle and make sure that it's square. Then I pull vacuum again for the cure. You might need to repeat this a couple of times. Hot wireing out some cradles will help. If useing cradles, I put the cradles on the outside of the bag and apply weights after the part has settled just to keep it square.

Experimentation is the key!

daven

My Feedback: (1)

I'll try to take the slight warpage out with a heat gun. It is very minor, but enough to bug me. I had done a couple tails with a little heavier balsa, and don't remember this problem, so it may be the contest balsa is just a little to light. You are right about experimentation, it does take some practice and "hard-headedness".

It sure has been fun though. I'll keep working on it, and will try and post a picture or two tonight.

I'd sure like to hear other comments if you have them.

Thanks,

CAPtain232

My Feedback: (40)

One of the big misconceptions in this hobby is that if you vacuum bag, it takes away the need for weighting the parts down. Vacuum bagging is great and it does not require the great amounts of weight of the traditional non bagging method, but you still need to use a little weight to hold the parts down on your FLAT SURFACE. There was a guy that posted on here a while back that you could bag a part and then hang the bag from a hook to free up table space in the shop.......It may not always work out to be so bad, but he will definitely have issues with warping from time to time.......

The best thing to do is to DRY FIT everything and make sure it all looks good and then add your adheasive and bag the parts, and then add a a LITTLE weight to hold everything in place on your FLAT SURFACE...........Notice how I keep refering to a FLAT SURFACE......very important

daven

My Feedback: (1)

I started another thread in the pylon forums.

There is a couple pics you can see here:

http://www.rcuniverse.com/forum/m_14...tm.htm#1471025

Darrinc

My Feedback: (5)

Why not just layup directly to foam?

patternwannabee

My Feedback: (1)

Daven,

I too had this problem a year or so ago. It made me revisit all my techniques, and all it did was raise more questions.

I was using 1/16" contest balsa on white foam. Cradles on the outside of the bag on a flat table, weighted down with sandbags. Vacuum was 10" HG. Used Probond.

When I took the wings out of the bag, the trailing edge was straight as an arrow. After ab 20 min, there was a very pronounced cupping effect.

I experimented some and came to these conclusions:

Contest balsa is much more prone to warpage than the heavier balsas.
ANY moisture can effect the straightness when bagging with balsa. Initially, I sprayed some water to the foam to ensure the cure (as often recommended when using probond). This turned out to be the culprit (mostly). Anyway, of all the samples I tried, I could get the contest balsa samples to warp every time with application of moisture. I could only get the heavier balsa samples to warp some of the time. Water on the Balsa vs water on the foam made no difference. Epoxy, vs Probond also made no difference.

This raised questions about how a warp is created in this fashion and what I could do to avoid it.

Seems to me that balsa shrinks and expands due to humidity changes. It doesn't have to shrink much at all to create a noticeable warp. Different grades of balsa shrink at different rates. So if the top and bottom balsa skins are not well matched, once the humidity level changes from that when it cured, a warp could develop.

Does that mean mismatched skins have a greater tendency to warp?
Does it also mean that I have to air out all the balsa sheets for some time so they can stabilize before bagging them?
Also, would it make a difference to skin the wings only at the same humidity as flying weather?

I'll have to try Darrinc's suggestion of releasing the vacuum a couple of times.

Anybody else with more thoughts on this?