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first time vacuum bagging: leading edge not perfectly straight!

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first time vacuum bagging: leading edge not perfectly straight!

Old 01-03-2010, 09:13 PM
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le fou
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Default first time vacuum bagging: leading edge not perfectly straight!

Hi,

I sheeted one of my laser's wing (from Lanier) using a home made vacuum pump. Vacuum was kept between 8 to 12". I used polyurethane glue for the sheeting and my building board which is straight. I let it cure for about 23hrs before to remove it. When I removed it from the bag, I noticed two things:
1) The sheeting superior to the wing tube and the aileron servo cutout was more compressed than the rest of the sheeting, so there is like a dent. I can fix that with some putty and sanding.
2) The trailing edge does not seem to be perfectly straight. It appears that the middle of the wing has sagged in relation to the root and the tip. I have read that I can wet the sheeting and put it back into the press to make it straight.

Can anybody give me some advice on how to fix the problems? Is it better for me to try to cut another wing core and start over? If yes, where to find a 3" EPS foam block?

Thanks a lot,

Le fou
Old 01-04-2010, 09:08 AM
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Ed Smith
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Default RE: first time vacuum bagging: leading edge not perfectly straight!

It is little things that make a difference. 8"-12" of vacuum is a bit on the high side for just skinning a wing. I can suggest only what I do. You may already do it this way.

I use a max of 6" of vacuum. I skin both sides at the same time. I rest the core to be skinned in the bottom half of the cut out block which is on the outside of the bag. I rest some heavy steel straight edges along the L.E. and T.E. I rarely have straightness problems. If there are cavities uinder the skin the vacuum willl pull the skin down into them. I do all cutouts after skinning.

I hope this helps.

Ed S
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Old 01-04-2010, 11:41 PM
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JohnW
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Default RE: first time vacuum bagging: leading edge not perfectly straight!

If you're going thru the effort to bag wings, you probably won't ever be happy with any fix on your existing wings, depending upon how warped they are. You might be able to press it out, but normally once the skin is set to the foam, that is pretty much how it will stay. As much as it sucks to pitch them and start over, that is what I’d probably do.

Next time, what Ed said, reduce your vacuum and brace the wing with the shuck. I personally prefer the weight method when sheeting wings with poly, but it too has shortcomings, such as needing a building table that stays flat with hundreds of lbs on it.

As for 3” foam, you can special order it from contractor suppliers, but you won’t have much control over the density. What I have done is glued two 2” sheets together as they can be sourced from local hardware stores. Still don’t have much choice in density, but it is cheap and locally available. In theory, the center line of the wing should not see any sheer stress, so this works fine if you are worried about strength at the glue line.

-John, Just west of you in Lincoln
Old 01-05-2010, 08:33 AM
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Default RE: first time vacuum bagging: leading edge not perfectly straight!

Double post. Sorry. Not enough coffee yet!
Old 01-05-2010, 08:37 AM
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Default RE: first time vacuum bagging: leading edge not perfectly straight!


ORIGINAL: CloudyIFR


ORIGINAL: le fou

Hi,

I sheeted one of my laser's wing (from Lanier) using a home made vacuum pump. Vacuum was kept between 8 to 12''. I used polyurethane glue for the sheeting and my building board which is straight. I let it cure for about 23hrs before to remove it. When I removed it from the bag, I noticed two things:
1) The sheeting superior to the wing tube and the aileron servo cutout was more compressed than the rest of the sheeting, so there is like a dent. I can fix that with some putty and sanding.
2) The trailing edge does not seem to be perfectly straight. It appears that the middle of the wing has sagged in relation to the root and the tip. I have read that I can wet the sheeting and put it back into the press to make it straight.

Can anybody give me some advice on how to fix the problems? Is it better for me to try to cut another wing core and start over? If yes, where to find a 3'' EPS foam block?

Thanks a lot,

Le fou
Le fou,

1) If you make your cutouts prior to bagging this will happen with excess pressure. I suggest cutting out your servo pockets etc... after bagging and use MORE pressure.
2) This is due to the lack of pressure and/or adding too much pressure without the wing aligned in the bottom shucks. I place the bottom shucks on the outside of the vacuum bag with say 2-3'' of pressure, align everything so I know the airfoil shape is correct, then weight everything down as previously shown, then apply full pressure. I use 15'' when sheeting balsa over 25psi blue/pink foam. If you're using white bead foam I suggest no more than 6". If you want to determine your crush point for the foam your using take a blank piece and throw in the bag and pull pressure till you start seeing it compress, then back off an inch or so. I also mist the glue after applying the glue and they are pretty much dry in 3-5 hours. That's for Gorilla Glue.

Here's a website I suggest you reviewing, it's quite good:
http://www.mackrc.net/patternwings2/index.htm

and then I spent many, many hours working on a DVD movie that shows how to do everything you're speaking of and more, see this:

http://www.rcuniverse.com/forum/m_92...tm.htm#9385823

Best of luck.
Curtis
www.TailwindGliders.com




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