HighPlains
Posts: 2715
Joined: 3/2/2003
From: Over da rainbow,
KS, USA
Status: offline
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John,
It used to be said, ‘walk gently and carry a big stick’ or something to that effect. But I guess for web forums it should be “stir gently with a big stick” ;-) Anyway, anything to keep it interesting, eh? Well this is way long but then I realize that a lot of readers are new to building. This only concerns general construction, another post will take up the other questions…
I build wings the old fashion way, circa 1980’s, of balsa and foam. I usually use common white foam, though I got a hold of “spider foam” from aerospace composites and plan on trying it soon. It is a directional white foam that has about 5 times the compression strength in the vertical direction as in other directions. It is slightly heavier than white foam.
Anyway, I have a set of wing core templates that are built into a nail bed table. The lower core templates are mounted directly to the table, while the upper core templates are cleco’ed to the lower core templates for the upper cut. The reason for the cleco (a temporary aerospace fastener) is because I have a large series of holes drilled in both templates (upper and lower on each end). These holes were spaced in regular intervals on both templates, but different spacing on the top template vs. the lower. What this does is allow me to alter the thickness of the cores precisely so that for a 424 wing it could be sheeted with 1/16” balsa and for 428, as heavy as I feel necessary.
After the cutting of the cores, a bit of sanding with 320 paper and cleaning with the vacuum. I then prepare the sheeting skins by joining them with model airplane cement. I don’t use white glue or ca, because they are a lot harder and difficult to sand. After they are joined, I do most of the sanding while they are on a flat surface, both inside surface (not so good), and the outer surface (this shows). Since I like to use carbon on the bottom of the wing inside the skins, I figure out where to place it, and sand a slight groove into the bottom skin for the thickness of the carbon and glue it to the balsa with Ca. Again I vacuum the skins. Then I apply 3M tape (the tape is 3M 465 acrylic transfer tape) on the skins. This tape is a contact cement with a backing paper that come on a roll. I use the 2 inch wide rolls that are 60 yards long. Enough for about 5 quickie wings. This tape is only 0.002” thick and adds about ¾ oz. to a quickie wing. I put the tape directly onto the balsa in strips and then burnish it into the wood with the backside of a 3M sanding block (while the backing is still on the tape).
After skinning the top and bottom of the wing in the core shucks, I’ll weight the assembly for a day or two if I got time, otherwise, I just sand the leading edge down to the foam and glue on the balsa LE with white glue and masking tape. Later I do the same with the trailing edge. I don’t use shaped stock for either, preferring to carve both to shape. That gives me greater control of the firmness and weight of the balsa used. For skins I like 6-8 pound balsa, though I like 8-10 pound wood for the leading edge. The trailing edge is 1¼” wide, and the ailerons are cut out later at 1” wide.
If you are serious about building, I highly recommend that you buy a triple beam balance. I have an Ohaus gram scale. There are 28.35 grams to every oz. This balance’s scale reads to the tenth of a gram. I can place a freshly glued (white glue) assembly on the scale and watch the weight change as it dries.
So much for the easy stuff, shaping the wing is where the rubber hits the road. Quickie wings are relatively easy, because the airfoil is the same down the length. The first thing is to make a very accurate template of the front of the airfoil. Mine only gauges the first inch or so of the top and bottom and the leading edge radius. Draw the leading edge and the trailing edge lines on the balsa. Start carving and using a razor plane until you are close. This isn’t a race, so take your time. I usually break up the sessions over several days, so that I have a critical eye to look at where I am at in the process. As you keep getting closer to the final shape, use you leading edge guide. I have found that if the leading edge is done correctly, the wing will usually be pretty fast. I once sanded without using much of a guide, got the LE too high on the airfoil, and the plane was a complete dog.
Now sanding the rest of the wing, you must be concerned with having the trailing edge located right for the airfoil, and building it straight with no unintended wash-in or wash-out. For me, this is the hardest thing, so I block the wing on a flat surface and do a lot of measurements. For a lot of the sanding, I cover the skins with masking tape so that I can sand to the surface without changing it.
I bury the wing hold down blocks so that the flat head bolts are flush with the wing surface. For the front of the wing, I use ¼-20 nylon bolts, and for the rear I use 10-24 nylon bolts. The front bolts go through ¾ hardwood dowels that are drilled and countersunk. The rear bolts go though a 1/8 plywood strip that is sunk into the trailing edge balsa. It too is drilled and countersunk for the flat head bolts.
I glass the center section with multiple layers of medium weight glass, usually about 2 oz. cloth. I usually use two or three layers, depending on how fast I want to go. Actually, a 424 quickie can use two layers, while a 428 might need three. You make the glass wider at the leading edge, and narrower at the trailing edge. The load on the wing is greatest at the leading edge which is why I can use less glass and smaller bolts at the trailing edge. I use polyester resin for bonding the glass. I know it stinks, and the catalyst is dangerous (the drops of catalyst cause nearly instant blindness if it gets into your eyes). Be careful. The reason I use it is because polyester cure very hard and rigid. It also sands well, though I avoid sanding the center glass with a trick that I developed. Once the glass is on (widest layers first) and wetted out with the resin, I take a sheet of wax paper and place the shiny (waxy) side down onto the wet resin. I just smooth out the bubbles under the paper until they are gone in all directions. The wax paper is globally flexible, but locally stiff. Surface tension will attract the resin to the surface of the wax paper and the results when you remove it are a smooth surface, even to the edge of the glass were it meet bare balsa. Throw the wax paper away, it is only good once, or you will find out what tedious sand really is!
Cover the wing, place on the airframe, go race…
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All Forums >> RC Airplanes >> Pylon Universe - RC Pylon Racing >> Q-500 Racing >> RE: slightly slower 
RE: slightly slower - 
) You've also said that it's important that the wing be sanded correctly with no flat spots or waves along the surface of the airfoil and that it is important that the leading edge not be "too high". Could you explain these statements some please? Maybe you could hit on some of your building techniques too. 
Theory is not a theory, 