TommyWatson

My Feedback: (1)

I need some help!!.

I want to do some foam wing, tail and and fuse parts with balsa skins. The kit is a 35% Carden Extra.
While I have skinned many wings before I have never used Vacuum bagging before and would like to know what type of bag do I need. The wings are quite large.

Can someone tell me what type of bag material I should buy. I will probably buy a vacuum kit, but I am sure I will need larger bags. I will do the wings etc in their shucks.

I have looked at the various companies but i am still confused as to the correct material to buy.

Any assistance or comments would be most welcome.

Thanks

DMcQuinn

My Feedback: (13)

I bought a large roll of plastic sheet sold as painters drop cloth. I make a bag by cutting a rectangle twice as big (plus extra) as the wing. Then I fold it over the wing and seal the other three sides with the cheapest caulk I can buy. I stick a tube thru the seam somewhere along one of the edges and put the vacuum line thru this tube. The caulk is used to seal around the tube also. I put the caulk on the plastic when everything else is ready to go. It seals the bag well and does not need to set at all. The next morning, you pull apart the seams to remove the finished wing. I cut off the caulked together seams and I am now left with a smaller rectgangle of plastic that I can use to bag smaller parts. Only think about the caulk is that is is messy. You need to have a vaccuum pump with a regulator on the line. Otherwise a good vacuum pump will pull so hard as to crush the piece. I like to use poluurethane wood glue, spread thin on the balsa. I only tape the balsa sheets together (no glue) to save time. For wings, you must put in the receiving tube and its plywood end supports before covering and bagging. Don't cut your servo wire hole until after the bagging. If you cut any holes for servos or servo wires before bagging the pressure of this process can crush the balsa where it is not supported by foam. After it is bagged, I drill my servo wire hole from the root end using a three foot long piece of copper tube. this takes a while, but it works. I'll try to find the picture of the wing I crushed because I drilled the servo wire hole first.

DMcQuinn

My Feedback: (13)

This is a picture of what can happen when things go wrong. Firstly, I used too much vacuum. I think I was over 10 psf on this project. Somewhere closer to 5 - 7 is plenty. Also, note the crushing about 6 inches from the trailing edge, parallel to the trailing edge. This is where I had pre-drilled a hole from the root about 75% of the way out for the servo wires to run thru. This leaves a hole in the foam of course and this is where is crushed the worst. So ever since, I always drill the hole after the skins are glued on. Same with servo holes. Cut these out after the skins are on. I put two strips of plywood under the skins, recessed into the foam before the skins are glued on. These plywood pieces are about 2 inches long and 1/2 inch wide. I glue them parallel to each other and as far apart as the length of the servo. I carefully measure where these are located. After the wing is skinned, I cut out a hole for the servo between these two plywood reinforcements. The plywood is good for the servo screws to hold in to. These of course typicaly go on the underside of the wing -- typically two per side.

Note the blue masking tape holding all the balsa sheets together. In more recent projects, I run the masking tape over the complete seam, the entire length of the balsa wood. This seems to work even better, although either way you go through a bunch of masking tape. Still this is easier than pre-gluing every sheet together.

TommyWatson

My Feedback: (1)

Hi, Dmcquinn,

Thanks very much. This is exactly what I was looking for. I have an old fridge compressor. It looks like I will have to experiment with a regulator or buy a comercial vacuum pump. The kit cost me a lot of money and Idon't want to wreck the cores, So I will probably go with the comercial pump. The plastic sheet is an excelent idea.

Do you leave the cores in the shucks?.

Regards

Ed Smith

Tommy,

I did my first vacuum bagging with a starter kit from Aerospace Composites, in the US. http://www.acp-composites.com/
The Aerospace web site has a lot of usefull information.

Since then I have made my own vacuum unit using an old refrigerator compressor. It works very well, I did fit an adjustable vacuum switch to the system. If you are using the usual white insulation foam then 6" of mercury is the max safe vacuum. any more and the foam will likely crush. I have not used the other pink, blue or grey material. They are too heavy for model aircraft when trying to save weight. Aerospace sell all of the necessary materials for bagging. It sells nylon tube/sock that is 18" wide, it can be purchased of the roll at any length. It comes with two plastic clips for sealing just the ends. I do not know if this is all available in your location.

I do make up the skins first and glue the edges with a sandable glue. If you do not the resin will creep through the seam and there will be hard ridges all along the wing and you will never sand them out. In fact I go a little further. Most sheet edges rarely are the same thickness. After I make up the skins I sand both sides with one of those rectangular orbital sanders. My skins are perfectly smooth both sides before I glue them to the core.

Only the skinned core goes in the bag. I place the whole assembly in the bottom shuck. I place long heavy weights on top to keep the wing straight.

Ed S

Terryoc

Tommy
Like Ed says it is easy to build a pump set up using a fridge pump and vacuum switch , I have a out of date LPG bottle as a tank and 2 ball valves from that to bags .
The easiest plastic to buy is the brickies plastic that they use under cement which you can buy from Bunnings for about $15 for 20m or so . Seal along the length with packing tape and to seal the ends and keep the bag reusable I use flyscreen frame and spline cheap and easy.
Terry

TommyWatson

My Feedback: (1)

Hi Ed,

Thanks for your great information. I have just purchased a pump kit form ACP. The fridge compressor is a bit ordinary so I decided to get a good unit. I am glad I found out about using too much vacuum. I will practise on the tail parts before Itackle the wings.

How good is the RCU, lots of good advice from blokes that have actually done the job.

Thank you all.

Regards

Newc

G'Day!

There have been a large number of similar posts on these boards, so you may want to research them as well. I bought and used the ACP system fror my 104" project and am very happy with the results. You don't need to end glue the sheets of balsa prior to bagging as was suggested. When bagging the adhesion is great enough that the edges are taken care of. There's a good video at the following link that will give you info on another vacuum source, and it gives very good general information regarding bagging...http://www.rcgroups.com/links/index.php?id=4901

As has been reported in an earlier posting of mine, after some experimenting I used Gorilla Glue instead of epoxy and it worked great! I spread the GG on the balsa (pieces cut to shape and taped together) and then scrapped as much off as I could, then gave the foam core a spray of water from a spray bottle and put the assembly in the bag and let it cure over night. I also practiced on the tail parts, and think that this is worthwhile. One real benefit of the Gorilla Glue over epoxy is that you have much more time to 'play' wiith the assembly because the Gorilla Glue doesn't really start setting up for many minutes.

davidfee

I agree that the foaming polyurethane adhesives do work well in this application but, what kind of epoxy are you using that sets up faster than Gorilla Glue?

Ed Smith

I use a variety of glues for model applications. I use polyurethane glue as well as others. I always experiment with different "Stuff". It
is always a question of using the correct material for the required result. I have impregnated fibreglass with gorilla glue. Compared to epoxy resin there is no comparison in terms of stiffness and strength using gorilla glue. There is a very good reason why epoxy resin rather than polyurethane glue is used for glass layups.

So, yes. gorilla glue will glue the wingskins on. However for the same weight of glue it will come no where near the strength and stiffness of epoxy.

Ed S

Newc

First of all to respond to davidfee's question, almost any epoxy will 'set up' faster than Gorilla Glue, since it takes at least 3 hours for the GG to get set dry to the touch, but 12 hours is what I recommend for full set. The 12 hour figure is what is also recommended by ACP when vacuum bagging using their resin, so there's really no difference in working time.

To Ed Smith - I don't understand what you are trying to say relative to the question of sheeting a wing when you report that GG is certainly not something that I would ever use for fiberglass adhesion as it sets up as a porous foam, not what is wanted for a fiberglass surface. It does provide at least the degree of adhesion between the balsa and foam as does epoxy, and at less weight when applied properly. (see http://www.rcuniverse.com/forum/m_3815201/tm.htm) Remember tht epoxy doesn't provide strength or stiffness; the combination of the foam and balsa sheeting - if they are well adhered - is where these properties come from. If one is just doing a fiberglass layup, it's the fiberglas that is contributing the strength, not the resin.

Keep in mind that, when vacuum bagging, the key to a strong completed part is to apply the bonding agent (glue, resin, etc.) completely over the balsa surface and then scrape as much as you can off the surface. I, and much more experienced others with whom I have compared techniques, scrape the glue off the balsa with a credit card until we can get no more of the glue or resin to show on the edge of the card. This provides plenty of adhesion while minimizing the weight gain. When not vacuum bagging more resin (or GG or other glue) is needed, contributing to the weight and not helping the strength at all.

ptxman

My Feedback: (2)

I see no benefit to skipping this very easy step. A wing sees torsional load as well as bending so having a uniform strength skin is very important. If the joints are pre-cut tight as they should be, it is more difficult for the laminating adhesive to fully penetrate the joint during press up. If you glue the joints beforehand, you know for certain they are structurally mated as opposed to 'hoping' the adhesive will seep in & do the job. Also, a pre-glued joint will not allow the adhesive to leak out & cause a big mess. Yes a plastic barrier sheet should still be used to prevent sticking the wing to the shuck, but you can still get ugly glue lines along the joints inside teh barier sheet if the joint is not sealed, especially so with expanding type glues. The other advantage of pre-jointing is you can make up larger panels of wood sheeting, pre-sand them to consistent finish & thickness, then lay down templates for the various flying surfaces to match grain orientation & get the most use of irregualrly shaped surfaces on (rectangular) sheeting layups.

What I do to prepare the sheeting is: cut 1 edge (only) of each sheet set with a straight rule & sharp xacto, mate the joints dry & cross tape the joint at each end (meaning accross the joint, pulling them tigh to one another) & also in the center with scotch tape. Then run a continous tape all along the joint. Continue onto the next edge in this manner. This way if there is any bowing or stress releivng in one joint, its not transferred to the next. Do as many sheets as practical so they are all assembled dry. Now for each joint, bend them over the corner of the workbench (ie hinging on the tape) & bead in some glue. I agree 100% with the rec for sandable glue, makes for easy sanding & there are no joints to show in the case of iron on covering. Wipe the excess glue, allow to dry, sand the whole sheeting assembly, cut the appropriate sized panels for the skins.



You can get high quality structural epoxy with a pot life time between 5 minutes & 24 hours depending on teh hardeners available & you can modify its viscosity from near water thin to peanut butter with strengthening fillers. So in fact you have much more control of epoxy properties vs out-of-the-bottle urethane glues.

FWIW Ive used expanding urethane glues vs epoxy in this manner & tried a lot of similar techniques. Properly applied epoxy is every bit as light as urethane glues. The gorilla glue does penetrate as it expands & appears to anchor, but so does epoxy. In fact in 'gram counting' high performance applications, the whole trick is to prevent epoxy from wicking into low density 4-6 pcf contenst balsa & into the foam cells/pores (thus creating extra weight). But in a normal application this just means better bond. Thats why things like lacquer pre-sprays are used, to act as a permeability barrier. Ive actually dyed epoxy, done a layup, cut the section & looked at it under a microscope. You can see fingers going right through to the outer surface of 1/16" balsa under conventional (non vacuum) layup.
I used to do a lot of F3A pattern planes & I thought urethane would be a good way of glueing in sockets for plug-in wings. There were still structural spars & ribs of course, but the idea was to fill the void with an expanding high density (urethane) glue vs heavier epoxy. Well it appeared to work OK, but guess what. After about 200 flights the urethane eventually started to loosen under repeated load & stress. This never happened on the epoxy. When the section had cut you could seee the glue (foam) was kind of sugary, IOW it was breaking down. And the annular space was not excessive, maybe 0.020" at best. having said that, i do like GG for bonding foam panels together at the joint to make up for anything less than perfection. But thats only because I know there is reinforcing composite material going over it.

There was a good woodworking article, wished I still had it. It compared the tested strengths of various glues & putt hem on a bar graph. The urethanes (under all brand names) were a diamal fraction of epoxy & their reference was West systems which really isnt the best anyway. But the caveat was gap thickness. If the uerthane gap was anything other than very tight, the strength went for crap as it essentially becomes a high density foam.

Cost: have a look at what a bottle of GG costs on a volume basis compared to buying a good grade structural layup resin (which every modeler should have a gallon of lying around!). It might surpise you.

Ed Smith

ptxman,

By chance, were you somehow watching me make up wing skins? My procedure is exactly as yours, even to the folding over the edge of the bench to apply the glue. I am in total agreement with your post. It is comforting to know that there are some, as well as myself, that offer information based on the experience of actually doing it and not just guessing.

Ed S

TGoodwin

So which of the above posts are real world advice and which are pure conjecture?

Ed Smith

It is up to the reader of this stuff to decide that which is credible and that which is conjecture. As a rough starting point treat with suspicion those that disagree with the majority.

Ed S

TommyWatson

My Feedback: (1)

Since I started this thread, I will comment. I will probably go with Epoxy. I have used it a lot in the past (Just weighing down the cores) and had great success. I feel that using a Vacuum bag, which I have never done before, will give me a better job. I intend to practice on some trial pieces and make sure I am comfortable with the bagging process.

Thanks for all the great suggestions and advice. This is what makes RCU so good.

Regards

Ed Smith

Vacuum bagging will yield far better results than weighing the skins down.

6 inches of vacuum will give a pressure of app 3#/sq". That vacuum is good for the white foam. On a 500 sq" wing for example that is 1500# pushing on top and 1500# pushing on the bottom, evenly over the whole wing surface. That is equivalent to 3000# of weight resting on top. I think not too many people lift that amount of weight from the floor to the work bench when skinning wings.

Ed S