gboulton
Posts: 1862
Joined: 5/28/2005
From: La Vergne,
TN, USA
Status: offline
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Heh...got a little anxious. 
Thought I'd go ahead and start the top right wing panel today, and share info/details as I went along. not much chance of getting it "done" before going out of town but should be able to at least get a bit into it today. As such, expect the next several posts to be rather long and detailed, and contain lots of pictures.
The Bulldog's wings, as mentioned before, are sheeted foam core wings. They are not, however, completely sheeted, but rather sheeted with a LE and TE sheet, and then "cap strips" between the two. This is being done primarily to preserve the scale look of separate wing bays, and it has the added benefit of reducing weight.
What I'll detail here is a "step by step" process for doing one of the wing panels. Each wing panel has been done in a similar fashion. I won't include any measurements since, obviously, mine are specific to this particular set of wings. The method, however, should work well for ANY set of foam wings.
Before I get too heavily into things, allow me to say first of all that I was once told "once you know how to do foam wings, you'll never build another one". I found this rather difficult to believe at the time, and found it downright WRONG after doing my first foam wing on the small bulldog. After complaining rather loudly to Mike Dunphy (he who offered the original prediction), I learned that what he had in fact said was "ONCE YOU KNOW HOW"...and I most certainly did not.
After a bit of education, I gave things another shot on another wing for the small bulldog...and found things going much more smoothly.
Now...I've also heard a GREAT NUMBER of people say that foam wings are heavier than built up wings. This is simply NOT TRUE...or, at least, they certainly don't HAVE to be. They sure as heck CAN be, though. My own opinion is that this belief is the result of mass produced foam wings available on many larger ARFs. Since it is impossible to expect the "assembly line" to take the same care with our wings that we do it seems inevitable that many of the painstaking, but worthwhile, "tricks" will be left out of the mass production process.
Somewhere in the middle of building wings for the small Bulldog I was lucky enough to receive another rather lengthy education from Mike's son, Scott. Scott Dunphy is, arguably, one of the more knowledgeable builders ANYWHERE when it comes to wings. Some of you long time pattern guys probably recognize the name. To be certain, a fair number of world class pilots do. By no means did he teach me everything he knows...but I HAVE picked his brain enough to have learned a few of the tricks for keeping foam wings light, but strong. I'll share these as I go along.
So...let's begin:
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First picture is of a brand new wing core as it comes out of a wing cutting machine. For those who are completely unfamiliar with foam wing cores, they are made simply by passing a hot wire through a rectangular block of foam. the wire is guided either by hand or by a machine (the latter being preferable imo) to cut an airfoil out of the center of the block. The top and bottom pieces (called shucks) are left together, with the wing sandwiched between them, to protect the wing during transport. They also provide nice bases when working on the wing, and will aid with alignment, as we shall see later.
Note that i have marked the wing, indicating that thi is the top of the right wing, and showing leading and trailing edge. It will become important down the road to know which way the wing should be oriented, and also to keep it in its original shucks. It might not seem critical with a symmetrical airfoil, but it is. Again, we'll see why later. For this reason, I also mark the ends while the wings are still in the shucks, indicating which way is up, and which end (root or tip) it is. Note that drawing a complete line, top to bottom, will also help with alignment down the road.
Once the wing was out of its shucks, the first thing I did was weigh it. Clearly, foam wing cores don't weigh much at all...what makes them "heavy" is all the stuff we'll be adding, especially glue. So, i wanted to see the weight of the core itself, so we could compare later and see just how much weight we add during this process.
The next thing I do is to start preparing the core to receive its spar(s). In this case, I'm using 1/4 x 3/8 Spruce for spars, and running one top and one bottom spars. Since the top wing of the Bulldog is not straight, I've had to engineer a joint where the wing sweeps back. This has been done by placing two horizontal spars in the center section (pictured at top) which extend a ways into the swept wing section. About 6" inboard of the section's root, they will join to a spar that runs parallel to the LE of the wing section. So, the first task is to draw out the lines to cut the groove for the center spar sections. We're doing that here:
The longer spar section of the wing panel simply runs parallel to the LE of the wing from the joint to almost the wing tip. I just use a straight edge to mark that line.
Once that's done, it's time to cut the slots. There are dozens of ways to do this. Many folks use a soldering iron method, or even a plain ole X-Acto knife. I've found, however, that I prefer using a Dremel. Many dremels come with, or you can separately purchase for little money, a router attachment. Mine has a "fence" that can be installed instead of the point used to route circular paths. With this fence, I can simply select a bit of the correct width (1/4" in this case), set it to the right depth (3/8" ) and run the fence right down the leading edge of the wing. (the diagonal slot coming in from the center section was a bit tricker...but simply clamping a piece of wood in place, and running the dremel along its edge did the trick)
Once this is done, it's then simply a matter of cutting the spar to the length you want, and epoxying it in place. One tip...I draw a few X's on the top surface of the spar so I know not to epoxy that side. No sense in adding the extra weight there.
A few words now on epoxy:
I use 30 - 60 minute epoxy when working with foam wings (depending on how big it is, and how much time I think I'll need to complete any given step. 30 is more than strong enough for any task we have, but 60 sometimes gives me a few extra minutes if I'm coating a large amount of sheeting or something).
Epoxy is, probably, the single biggest factor in adding weight to a foam wing. it's certainly one of the most controllable, and most frequently "abused". Certainly you can use lighter foam (many wings are cut out of 1 or even 2 oz foam, when 1/2 oz will do just fine) but, unless you're cutting them yourself, you may not have a choice. You can also be careful when selecting balsa sheeting, and look for competition grade balsa, though it is indeed pricier, and at times hard to find.
Epoxy, though, can make a HUGE difference, and control of the amount used is entirely within your hands. It's deceptive how quickly an otherwise light wing can become heavy just by being careless with it. Think about it this way:
Let's say on the top side of this wing panel, I use a mere 1 oz more epoxy than I needed. Very easy to do, you must admit. An ounce of epoxy is no more than a couple good sized drops.
Now...presume I do the same thing on each side of each wing panel. 4 wing panels, 2 sides each....8 ounces.
Yep...we just added A HALF POUND to the wings alone.
I'll share some tips in future posts, as I get to the sheeting stage (where the tricks are more useful, and the difference can be more easily demonstrated)...but suffice to say, be OBSESSIVE about removing excess epoxy.
More posts coming later today, and next week, as the process continues.
_____________________________
The free man will ask neither what his country can do for him nor what he can do for his country. - Milton Friedman
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All Forums >> RC Airplanes >> Giant Scale Aircraft - General >> RE: Jim LeRoy Pitts Bulldog - Scratch Build Thread 
RE: Jim LeRoy Pitts Bulldog - Scratch Build Thread - 
 and, as a reuslt, PREVENT THE EPOXY FROM SOAKING INTO THE WOOD. <br> <br> Yes, clearly we want the epoxy to bond with the wood...but we DON'T need it to soak completely through. Try it some time...spread some epoxy very thinly over a scrap piece of sheeting, and then look again in 5 minutes...it'll appear to be GONE. the effect is even MORE dramatic with softer, lighter wood, like contest grade balsa. What's happening is the wood is absorbing the epoxy, just as it would water. How much? I've seen 4"x36" 1/16th sheets drink 2 whole ounces of epoxy. Try adding 4 ounces to each wing of your bipe sometime, and see how it likes that extra pound. <br> <br> So...we spray the balsa with a light coat of hairspray...we don't want to soak it, just "seal" it a bit. Once that's done, mix up some epoxy, and just sort of goop it on in a few places... <br> <br> [image]http://img.photobucket.com/albums/v119/bluesconvert/airplanes/IMG_0008.jpg)
