Shop our Airplanes Products Drone Products Sales

The Scale Builder's Workshop Video How to -- August 2005

This month, Jim Young walks you through the process of vacuum bagging a foam core wing.

Article Thumbnail

Welcome back to the Scale Builder's Workshop. The initial response from the first installment has been very positive and there are a lot of great ideas out there for future videos. This month, we'll walk you through the process of vacuum bagging a foam wing. Dave Campbell posted an excellent how-to article here on the E-zone a while back, but even after reading it many times I was still uneasy about trying it myself. So, I invited Dave to one of my club's meetings where he showed us all how simple it could be with a little planning and preparation. I also posted an article on vacuum bagging, but now using my wife's FoodSaver. Using a FoodSaver has many advantages over a traditional vacuum pump system, including price.

There were a few comments on the video format last month (.WMV). The WMV file format is the RC Group's preferred format for posting video. I did investigate several file converters available online, and the result was that the file size grow to almost 10 times the size of the WMV files. This is just not practical to post and download online. If you are unable to view a WMV file, I would suggest that you do a search for video file converters and do the conversion on your PC after you have downloaded the WMV file.

With that said, let's get back to the fun stuff...

Vacuum Bagging a Foam Core Wing

Foam core wings have many advantages, and can be built very lightly if you go easy on the glue. However, to keep the strength up, you need to get a good bond between the wing skins and the foam. Vacuum bagging provides a method to firmly press the wing skins to the foam core ensuring a strong bond with a minimum of adhesive. The alternative would be to pile on an enormous amount of weight to press the wings.

There are two types of lay-ups that can be vacuum bagged; what I am going to call wet and dry. In a wet layup fiberglass or carbon fiber cloth is wetted out with resin and wraped around a foam core. This type of lay-up is typically used for sleek gliders where airfoil accuracy and strength are a necessity. In a dry lay-up, wood wing skins (balsa, obechi, etc...) are bonded to a foam core. This is the type of lay-up that will be demonstrated here.

When you start talking about vacuum bagging, most people immediately think that you need to have specialized pumps and equipment. While this is true if you want to bag large pieces, I've found that you can get good results with a FoodSaver. A FoodSaver is designed to vacuum seal food for long term storage and is available form most department stores or online for less than $50 for a basic system. The only real limit to the FoodSaver is the width of the wing you can bag, about 9", but the length is pretty much unlimited.

This is a rather involved subject, so I've split this month's presentation into two pieces. The first part covers the equipment and preparation, and the second part demonstrates an actual lay-up.

Downloads

Downloads

This Month's Quick Tip

While working on a model, there is nothing worse than dinging it up while you work on it. Here are a few tips to avoid hangar rash while you're working on your next masterpiece...

Downloads

Wrap Up

I would also like to use this column to feature your building skills. So, please send me some photos of your latest project (scale or not) along with the building techniques you are using. For this month, here are a couple of shots of my Weddel-Williams Model 44:

The specs on this model are:

Wing Span: 84"

  • Length: xx"
  • Wing Area: 1100 sq. in.
  • Weight: 15 lbs.
  • Wing Loading: xx oz/sq. ft.
  • Motor: MaxCim MegaMax 3.7/4Y and controller
  • Battery: 10S LiPo

There appears to be a lot of interest in vacuum forming, and I am researching this topic for a future video. As always, I welcome your comments, suggestions, and ideas for future episodes of the Scale Builder's Workshop. So, dust off that workbench and start making some new dust.

Thread Tools
Sep 01, 2005, 11:32 AM
Registered User
just watched your video as the last one EXCELLENT and very informative.
I built a cnc foam cutter a couple of years ago so I will have to now go out and get a seal a mealer . Quick question I have herd of people using PROBOND to glue there skins to there cores have you tried using it if so do you think it would work with vacum bagging?

Tom

Keep up the good work
Sep 01, 2005, 12:32 PM
Designer/Builder
Jim Young's Avatar
Thread OP

ProBond???


Hi Tom,

I'm not sure about ProBond (Gorilla Glue, etc..) with vacuum bagiong. These glues foam up as they cure, and the vacuum is just going to make them foam up more. They may work fine, or you could end up with a real mess.

I think the Probond was the answer for people without a vacuum bagging system. The foaming action of the glue expands into the foam for a good bond without needing a lot of weight piled on top to press the wing.

If you are going to vacuum bag, I don't think you can beat the strength and consistancy of epoxy.

-Jim
Sep 01, 2005, 05:37 PM
Registered User
gavoss's Avatar
Caveat:

I don't want to take any thunder away from this thread or it's writer. There is a lot of work that goes into an article and even more into a hands on "how-to." I know because I've written quite a few in my 38 year career! Jim should be proud of his accomplishments here on RCG and I for one salute him!

Onto my comments.

Jim Young gives you some very good information for what I would refer to as entry level vacuum bagging. His system works well for the modeler wanting to vacuum bag balsa onto light (white foam or tecnically, expanded bead poly styrene) foam only, and for something up to say three or four a year.

My comments are broken down into steps in the system.

Vacuum pressure:
Note: I have both a food vacuum system simialar to the one used in the review and I also have a professional system that pulls 20 Hg.

Without measuring, I'd guess the home vacuum system is only good for about 3-5 in. Hg. White foam can only be pressed with about 7 in Hg. (Hg means inches of mercury) before it starts to crush under the vacuum. The vacuum system used in the article doesn't pull very much vacuum; certainly not enough if a person wanted to use a composite skin of fiberglass, Kevlar or Carbon Fiber. The method he uses is very capable within its element.

The food vacuum systems are only designed to operate for a very short period, literally seconds. There is a warning on many units that the unit will auto shut-off if it gets too hot. This means you you have to have a bag that is 100% leak free. Trust me, after bagging litterally hundreds of wings, I can tell you there is no such thing as a 100% leak free bag! If the bag looses pressure before the epoxy cures, you may have a real mess on your hands, depending on how much weight you have on the bed and how much pressure you lost. To solve this, commercial pumps are rated at 100% time on. This means you leave the pump on while the items are in the bag.

If you plan on bagging quite a few wings a year, you should get a decent vacuum pump. They can be made from refridgerators or freezer vacuum pumps or you can buy commercial units. Sometimes you can find a vac pump free at the local repair shop and then make a setup for about $50. You can also buy a ready to run setup for about $250. Both of these methods will allow you to handle balsa-over-foam and composite bagging. (actually, balsa over foam is technically composite bagging but "composite bagging" is usually referred to when fiberglass/Kevlar/CF and epoxy are used as the skin.) Composite skins require around 15 Hg to have the wing come out properly.

Epoxy System;
Personally, I only use and recommend epoxy for vacuum bagging wings balsa or other wood product onto foam. The brand of epoxy matters little, but you do want a laminating epoxy. They typically have a 20 minute pot life and cure in about 4-6 hours. Now, DONT PANIC! Pot life means little to us in this scenario. The pot life of an epoxy is how long the epoxy takes to cure in a mixing cup, if stirred up properly and left to cure. Since we are going to be spreading it on the wing skins, it will take much longer to cure. You will actually have plenty of time to mix, pour and spread epoxy. Personally I like West Systems from CST or Z-Poxy from Zap. Most epoxy systems that the hobbiest would use don't cure properly if the temp is below 70 degrees. This means if you live in a 4-season climate, don't leave it out in the garage if the room temp will get below 70 degrees. In other words, NO WINTER BAGGING unless the room is heated. One way to get around this is to make a "hot box" out of foam and use a standard light bulb to keep the core warm enough so it cures properly. Another way to keep things warm is to put a heating blanket on top of the beds.

Preparation:
The only thing I do differently is that I spray the wood that is going to have the epoxy applied to it with hair spray or spray laquer. I usually apply one wet coat and allow it to dry before putting the epoxy on. I do this to keep the epoxy from soaking into the wood as much as it would with bare wood. Try it yourself. Spray say the top skins with hair spray, but don't spray the bottom skins. You will need to put more epoxy on the bottom skins than the top. Next, my spreader has little "V's" cut in it so it leaves a little extra epoxy on the wing. From the surface layer it looks something like this:
___________/\________/\________/\______

In the bag.
I only put the core in the vacuum bag, not the beds. Wood on the LE has a tendency to be slightly pulled away from core if you put the beds on the core before putting it in the bag. If you just put the core in the bag and not the beds, the wood gets properly adhered to the core.

Next, put the trailing edge of the item your bagging the crease of the bag. Then, put the vacuum bag with the core in it and lay it on the bottom bed. Make sure there are no wrinlkes in the bag and put the top bed on top of the bag/core and apply weight. NOW apply the vacuum. This all but guarantees an absolutely straight trailing edge!

Well, I think that's it. I'm sure I'll think of something else. I'll repost when I do.

Again, nice work Jim.
Last edited by gavoss; Sep 01, 2005 at 08:17 PM.
Sep 02, 2005, 04:41 PM
Registered User
Jim Phelps's Avatar
I've used Pro Bond several times with and without vacuum bagging for EDF jet wings and it works great. It's very light and makes a really stiff wing.

BTW, moisture causes the urathane glue to expand. All you need is a very thin layer, makes no more mess than epoxy. Some people recommend spraying or wiping the cores with water but don't do it, you'll have a warpped wing.

Jim Phelps
Last edited by Jim Phelps; Sep 02, 2005 at 04:51 PM. Reason: another thought
Sep 02, 2005, 09:49 PM
Designer/Builder
Jim Young's Avatar
Thread OP
Gavoss,

Thanks for adding your years of experience to this article, I truely appreciate your comments.

I presented the FoodSaver system since I have had good results using it and it an inexpensive way to try this technique out. So, you are right in that it is a beginner's system. While the FoodSaver does have some warnings on it about not sealing a bunch of bags in rapid succession, this is do to the sealing mechanism over heating and melting the bags. My wife uses this quite a bit and we have bags that have been sealed for several months with no lose of vacuum. So, for holding the vacuum overnight while the epoxy cures, I would consider this a "leak free" bag.

The largest wing that I have done in the FoodSaver is my 80" wing span B-29. It has a 9.5" cord and uses white foam cores and 1/16" balsa sheeting. This model has been flying for a few years now, with no signs of fatigue, so I think the vacuum generated by the system is sufficient to achieve a good bond. The previous FoodSaver had a vacuum adjustment on it, and I tried both extremes with no noticiable differences (your mileage may vary).

Jim,

Thanks for chiming in with your ProBond experience.

-Jim Young

What I'm hoping to achieve with these videos, is to show people that building can be fun and is not that hard. I am somewhat limited (by file size) on how detailed the presentations can be, so if any of you can chime in with your experiences it is greatly appreciated. I'll also try to add some more text in future columns.
Sep 07, 2005, 07:39 AM
Proud member of LISF and ESL
Great article and great videos. I have wondered how vaccuming bagging was done. I have used the magazines on wax paper method to do some minor repairs. Now I know how the REAL process works!
Sep 21, 2005, 11:55 AM
12th Pursuit Squadron
TheAeronut's Avatar
My two cents... A friend and I have bagged a number of wings (white expanded bead foam and balsa skins) using PU glue - Gorilla, Pro-Bond and Liquid Nails.. In our experience, the Liquid Nails once cured does not get hard. This is OK for the glue joint but is a REAL BEAR to sand any glue that seeps through joints or the wood grain. Gorilla and Pro-Bond are both good. We prefer the Gorilla by a slight margin - better bottle life and slightly more consistent bond results.

One warning.. There is a comment in post #5 about adding moisture to the layup before bagging. Jim says not to do it. We have had varied results with this. We have had several layups that did not cure until the bag was opened. Bad news, as once the bag is opened the uncured interface loses its bond integrity. Apparently the atmospheric humidity was rather low when the layups were bagged and there was not enough moisture in the layup to catalyze the PU glue. After this experience, we ALWAYS lightly mist the skins and cores. Very lightly mist, then wipe down with a dry, or nearly dry paper towel, that is. You can also just wipe the core and skins with a damp paper towel. Yes, you do need to be careful not to leave too much moisture in the wood, or you can have problems with warping unless you are EXTREMELY diligent about how you bag the layup (core beds outside the bag, but well weighted down before pulling vacuum). We have found that this also reduces the amount of PU glue that seeps through the grain of the wood during the bagging process. I will admit that there is a bit of an art to this, but again - we have found that it is critical for the layup to have some moisture in it in order to catalyze the PU glue. On a humid day, no problems. On a dry day, add moisture. We found it easiest to always add some moisture. Cheap insurance policy, more consistent bonds, and less glue seepage through the wood grain....

PU glue is an excellent choice for bonding skins to a core using the vacuum bag method. Light weight, no mixing, and the expanding action forces glue well into the expanded white foam bead structure. Works well on extruded (blue or pink) foam too.

J.P.
Sep 22, 2005, 05:39 AM
Designer/Builder
Jim Young's Avatar
Thread OP
Hi J.P.

Thanks for adding your experience to this thread. You bring up another point about blue foam. All of my experience has been with white EPS foam. It is what I have avaialable and I have never needed anything denser. If you use epoxy on blue or pink extruded foam, it is difficult to get a strong bond. A friend of mine has actually sheared the wing skins off of a blue core wing. His solution was to score the surface of the foam to allow the epoxy to penetrate further into the foam. This was on a larger glider.

-Jim
Sep 22, 2005, 11:36 AM
12th Pursuit Squadron
TheAeronut's Avatar
This is true. The cell structure or grain of extruded foam is generally very fine and glues tend not to penetrate farther than the first cell from the surface. Truth is that I do not know that PU glues will penetrate any farther than epoxy, but they may due to the expanding nature of the glue. I can see that texturing the surface could help adhesion. Maybe using a tool like the 'Woodpecker' that is used to perforate solid surfaces for covering with iron on films could perforate the surface of the extruded foam and allow a better grip. This is definitely something to investigate.

I suppose that I need to do a few test pieces to see how far various glues penetrate into extruded foam and what kind of treatment is easiest to use to enhance the bond.

J.P.
Sep 23, 2005, 06:03 AM
Designer/Builder
Jim Young's Avatar
Thread OP
Hi J.P.

If you do any tests, I sure we would all appreciate your results. My friend ended up using a credit card shaped the opposite of what Gvoss uses to spread out the glues. Think of it as a comb and he would do a cross hatch pattern on the foam. I like the woodpecker idea as well.

-Jim
Sep 30, 2005, 01:53 PM
Registered User
3dben's Avatar
Im wondering, has anyone tried using those "space bags" out there? Google for "Space Bag" and you will see what im talking about.

Thanks.

-=ben
Sep 30, 2005, 02:00 PM
Registered User
gavoss's Avatar
My wife has several of the "space Bags" and for what they are, they are OK. I wouldn't however, use one for a wing. The method they use to seal the bag, causes some of the vacuum to be lost between the time the nozzle is removed and the bag sealed.

Other than the typical holes and things you get in CST's ore ACP's bags, they seem to be the best. gv
Mar 05, 2006, 11:47 AM
Cant fly enough
Greg Beshouri's Avatar
Jim, I just discovered this thread. All of my wings have been built with the 20 lbm of Encyclopedia method sitting on top so I am very interested in your approach.

One simple question. Do you pin through the saddle, balsa and core?

Thanks for the great video

Greg
Mar 06, 2006, 06:21 AM
Designer/Builder
Jim Young's Avatar
Thread OP
Hi Greg,

Yes, I run a few pins through everything to help keep it lined up while stuffing it in the bag. Just make sure your pin are short enough to not poke a hole in your bag.

-Jim


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Article The Scale Builder's Workshop Video Column - January '06 Jim Young The Builders Workshop 17 Sep 22, 2011 10:48 AM
Discussion Good package deal to start a builder's workshop? twotacos The Builders Workshop 6 Mar 21, 2007 02:24 PM
Article The Scale Builder's Workshop - March 2006 Jim Young The Builders Workshop 14 Mar 10, 2006 11:09 AM
Article The Scale Builder's Workshop Video Tips -- October 2005 Jim Young The Builders Workshop 5 Nov 16, 2005 07:04 AM
Article The Scale Builder's Workshop Video Tips -- July 2005 -- Planking Compound Curves -- b Jim Young The Builders Workshop 18 Aug 24, 2005 11:57 AM