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Author Topic: Working with and Finishing Foamboard  (Read 88 times)

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Offline Deerslayer

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Working with and Finishing Foamboard
« on: May 02, 2017, 08:12:51 AM »
The following is being offered in the hope that other builders of foamboard flying objects can utilize one or more of the described techniques.

I am completing #7 or #8, not sure which (I have a good memory but it is rather short), in my series of foamboard-based deltas. Some have been pushers, a couple are tractors (one converted from being a pusher, the latest being created as a tractor). I have experimented with vertical vectored thrust, vertical + horizontal vectored thrust, etc. A couple of the earlier pushers carried a Mobius, most recently with the 5.8GHz Tx, to do some FPV.

 Whenever I get bored, an uncontrollable itch to hack out something shows up.

 The FPV version worked so well (Wilf flew it and even landed it FPV, so you know it is a magical design!), that I decided to make one primarily for that purpose.

My design criteria, particularly with this plane, is to make things from foamboard that:
a) fly well (stabilizer assisted)
b) are as cheap as possible (not allowed more than 3 foamboards, no exotics such as carbon fibre, i.e., a $5 aircraft structure)
c) doesn't look like it fell off the $Tree truck
d) applies everything I have learned so far (if my incredible memory holds true)

This particular delta is constructed from the Elmers foamboard (sold by Dollarama), rather than the Adams product (sold by $Tree). While this will be slightly heavier than some of my previous ones, it has terrific structural strength without the need for any CF, etc. The paper covering, which is very well bonded to the base foam on the Elmers product, gives high strength and adheres very well, following the particular treatment that I use for strength and for water-proofing. Of course, the weakness with any foamboard is in compression, as it will buckle easily. Hence the choice of a KFM type of airfoil, i.e. multiple (3 in this case) layers at the leading edge, reducing to 2 at perhaps 40% chord and then to single layer a bit further back. There may be some aerodynamic advantages to this airfoil, too, but the jury is still out on that one.

Here is the basic technique. Note: some of the first 3 steps are rather specific to my specific design):

1. I mark and cut the various parts from the foamboard. This includes the 3 wing section layers and the 2 vertical fin layers. In this latest model, I have a multiple layer slab fuselage (3 layers at the front, reducing to double layers aft of the "cockpit"). The wider front fuselage has a real purpose; it allows me to set in a solid, wide base upon which to set the FPV camera/transmitter and at the optimum angle (roughly 10 degrees below horizontal, to start with. If subsequent adjustment is required, the knife and hot glue gun will take care of that nicely!

2. As I intend to create rounded leading edges and need to be able to sand easily, I lightly score just through the paper back perhaps 1 cm from the leading edge on the sides which will be joined together, only. I do this freehand, with a sharp Exacto or cutoff knife or razor blade. This will also be done on the outside leading edges at a later stage of construction.
 After scoring, pick at one edge and gradually peel off the paper strip. The key here is to ensure that you have cut completely through the paper. I find that holding the paper rather flat to the board and slowly pulling at a slight angle toward the front works well. Whatever works for you! If you have a little bit of paper residue here and there, don't worry, it will not prevent gluing and won't be visible anyway

3. To make it easier to more precisely mate the wing surfaces, I pre-fit them and use about 3 round toothpicks down the center line to be my temporary alignment dowels. Poke then through, then remove them until after the contact cement is applied to the surfaces,
Spray the surfaces to be joined with contact cement. I use 3M Super 77 these days, as it is quick and simple. I would likely use brushed on water-based contact cement if I were building a foamy with the paper stripped. Do not overdo it, just a nice even coating, and do not get too close or heavy on the areas you have stripped. Then, after a few seconds, give another light coating near the edges of the joining surfaces; the initial coating will protect the foam from being eaten away.

Do not be alarmed that your foamboard may have bowed somewhat. If you do a good job on mating the sections and then weight them down for a couple of minutes, they should end up perfectly flat.

 Stick your locating dowels (toothpicks) into one of the surfaces.

 Within perhaps a minute of spraying, mate the surfaces. You only get one chance with contact cement, hence the locating dowels!

 Now, back to completely design-independent aspects! Treating all surfaces as follows will greatly strengthen the structure and make it water-proof. As well, you can potentially tart up your $5 aircraft so that it can stand proud out there in full view of the Peanut Gallery. (No one really cares, but, flatter yourself anyway.)

4) I use Minwax Polycrylic Finish Protector, bought at Home Depot in a 1 litre can, to protect/strengthen/finish a surface . This is a water-based clear polyurethane product. I dilute a small amount of it with water in, say, 1:5 ratio and keep it in a small bottle.

 There are a couple of other items which are part of my processing, all of which are water-soluble and compatible with each other. Light spacking compound can be used either by itself or mixed with your water-based craft paint to fill or repair gouges, etc. The edges of the paper covering can be well sealed to the foam, by using just the polycrylic or, as I now prefer, by painting the edge area with Mod Podge (I had some lying around for a decade or more and finally found a good use for it. there are other things you could use, too.

I brush on one or two coats of the thinned polycrylic. This stuff dries in minutes, no need to sand between coats. As with any of these products that I use, you can come back a day or a week later and apply more stuff, it will adhere to the previous application. Nice feature!

 5) Now that you have sealed and strengthened the whole structure, it is time to shape the leading edge, etc. as was done before joining flat sections, I now freehand cut the paper, back perhaps 1 cm from the leading edge and peel off the strip. You will find that it goes much easier now, as the polycrylic treatment has  greatly improved the paper' strength.

 After rough shaping of the leading edge, if you notice that there is some separation of layers, now is the time to fix it. I brush on Mod Podge or whatever and work it into the joint, wiping off excess. Weight the leading edge down (no need to be too aggressive) and leave it alone for an hour or so. If there is still any separation, you can deal with that later with filler and/or paint.

6) You can now carefully sand the leading edge. I use 150 sandpaper, Sand right up onto the paper, if you wish - just try to mostly sand in the direction away from the paper and/or parallel to its edge. You will still have a detectable little ridge at the paper. What to do? Option 1 - don't worry, no one else cares, it won't affect the flight characteristics and you can inform the Peanut Gallery that this is a boundary layer trip strip. (They may not know what you are talking about, but they might even think that you know something about aerodynamics, Just try to keep a straight face. Besides, it may actually be true!). Option 2 - get serious and really clean it up, per the next step, so that the joint will be practically invisible.

 With the shaping of the leading edge, etc., complete, let's treat and finish that area. I use thinned spackling compound brushed or scraped on, optionally mixed with whatever paint I may plan to use, and smear it on. Let it dry for several hours, then sand to shape. Repeat as you see fit.

 When I really think I have wasted enough time and effort and beers on this shaping effort, it is time to do the finishing.

7) Apply a couple of coats of polycrilic to the leading edge area, at least past the paper joint and perhaps over the rest of the structure. It will be an insignificant weight, as it is probably 90% water, which evaporates.

8) It's paint time! I use the craft paint (little bottles, from Michaels, Walmart, etc.) and either a small foam brush or decent sized artist's brush. It is cheap and you use surprisingly little. This has previously been thinned down with water, and I keep a small container of water handy to dip my brush into, as needed. I put a puddle of paint on a plastic credit card, dip the brush into that as needed, perhaps dip the brush into water occasionally to thin down the puddle, etc. Take it easy, build up several thin coats, allowing a half hour or more between applications.

 If you now notice some dings or defects, mix up a bit of spackling and paint, then apply it and wait a couple of hours before sanding. Then re-apply paint to entire structure.Do not be alarmed if the paint looks really crappy after applying. Provided that it was well-thinned, it should smooth out and obliterate small brush marks as the water gradually evaporates.

 Apply whatever trim you choose (trim tape or brushed on paint). There, you are done! ... Or perhaps not? If you want a shinier surface, and if you are not completely sick and tired of this project, you can go over the whole thing with another coat of polycrilic.

Now, we are done ... I think!

Aside: I have patched up "real" foam structures, such as some dings on a Fun Cub, using the spackling compound and paint technique. Generally, rattle can paints have a solvent that will attack foam. You can use them - clean the foam with alcohol, then paint it with polycrylic and let dry. Now, it is safe to spray with your rattle can. You can usually avoid the polycrylic step if you are very careful and only lightly spray, holding the rattle can at least a foot away from the foam, let it dry, repeat, etc. If the first coat was evenly and lightly applied, your plane should now be protected from additional spray damage.
« Last Edit: May 03, 2017, 04:26:55 AM by Deerslayer »
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Offline ganguy

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Re: Working with and Finishing Foamboard
« Reply #1 on: May 02, 2017, 08:29:53 AM »
Can you post the executive summary?
Churchill said: "Success is a series of failures during which one does not lose enthusiasm!"

Offline Deerslayer

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Re: Working with and Finishing Foamboard
« Reply #2 on: May 02, 2017, 09:55:20 AM »
Can you post the executive summary?

 :'( Grrrr ...
Oh well, here it is, just for you:
Grab hunk foam. Make marks on foam. Cut foam on or near marks. Grab spray can. Spray paint on foam. Wash spray out of hair, off the cat (or dog), etc. Declare job done!

An old Ontario Land Surveyor whom I worked for used to say, " That's close enough for railroad work." 
(Note: I no longer will set foot on a train.)

 Can you take it from there?  ;D  ;D  ;D

My purpose in Life is to serve as a Warning to others