Tuesday, April 7, 2009
"Okay," the newbie sez in his message. "What do I have to do to get started? I've got almost a hundred dollars saved up, two junked VW engines I'm taking apart, and I've got access to about a half-car garage."
He goes on to explain that the "half-car garage" is just that. It started out as a two-car garage but one bay was devoted to storage, which took up about half of the space. He's allowed to use the remaining space which -- wonder of wonders! -- contains a Craftsman 10" table saw, a floor-model drill press and a small work bench. There's even a vintage home-made air compressor abandoned by some past tenant -- probably because the thing isn't working.
The air compressor becomes the focus of our messages, which wastes a lot of time and space until he gloms onto an older digital camera declared surplus by a family member who has up-graded to more pixels. With a picture of the recalcitrant compressor the problem is quickly tracked down to the pressure-regulated switch. I had been telling him to look for a black or gray metal box. And there is such a thing... but it is nothing more than an on-off switch. The pressure-regulator switch had lost its box years before probably during an attempt to repair it. All that was left was a leaking diaphragm and a set of adjustable points but not wired.
This wasn't something in need of repair. What it needed was replacement.
"I can't believe it actually works!" he says a few posts-- and about $30 later -- after replacing the regulator switch. Looking back, it was probably a very worth-while bit of work since I was able to refer back to it whenever his confidence needed a boost, which it did in returning the table saw to service.
The saw was rusty, of course. Not the flakey, pitted rust we see here so near the ocean, but the softer, powdery rust that comes from sitting un-used through a decade of mid-western winters.
He didn't quite believe me when I said the cast-iron table of the saw had to come down to a bright finish but after working on a small section he admitted it might be possible, although I'd be long gone before it ever happened. But he kept plugging away, not only on the table saw but on the drill press, restoring them to rather elegantly with the idea of selling. Along he way acquired a zero-clearance shoe for the saw and a vise for the drill press table, for he had come to see the need for such things when building from wood.
The air-compressor acquired a pneumatic brad-driver which, when paired with a jug of foamy glue, allowed him to build things in minutes that would normally take hours. I saw evidence that the half-car shop was becoming more of a home.
An important part of Getting Started was to select a design. Initially he was hot for the VP because he understood it could be built very cheaply. As we spent more time together, electronically speaking, he began exploring other designs and comparing their different methods of fabrication, the need for a zero-clearance shoe to prevent the saw's blade from binding when cutting 1/8" and 1/4" stock, and the need have some organized storage for such stock, since it's small size often leads to problems.
He starts collecting wood and I end up sending him pictures of what's acceptable and what's not. He finally decides to jump in; to build a dummy fuselage based on the FRED but with a number of differences.
There are several First Steps since many of these tasks must be started in parallel with each other, a fact having to do with setting up the saw, then storing what comes out of it.
Table/Building Frame is a major step. It is five selected 2x4's and four selected pieces of 1x12 shelving, eight feet in length, plus half a dozen 1x4's. By 'selected' I mean each piece has been examined for straightness, grain-per-inch, grain run-out and a basic 'feel' for moisture content.
To make the Table/Frame two of the 2x4's get chopped at five feet and butt-spliced to 8' 2x4's, giving a length of 13'. The butt-splice is aligned with 1x4 scab-plates about two feet long, assembled with foamy glue & 6d nails. The basic alignment tool is the floor, which is now carpeted with cardboard. An old comforter becomes the top half of an 'oven with a small light bulb for heat. This produces a pair of 2x4's 13' in length. The remnants of the cut 2x4's are cut to 22" lengths. This is among the hardest of the jobs because the wood is sawn by hand.
The 22" pieces are the ends of what is to be the building frame. One face of the frame is declared to be UP, the other is DOWN. The down face gets a pair of carefully fitted 1x4 diagonals, inlet into the 2x4's with saw & chisel, assembled with 6d's and foamy glue.
The Table/Frame is put aside to cure and the saw is set-up for slicing the shelving into square strips, 5/8" on a side -- the thickness of the shelving. Some of the sticks break across a knot when cut but nothing is thrown away.
The longest sticks are selected for the straightest grain and fewest knots. From the sixty 8' sticks there are several with only one knot and some of those have the knot near one end. These are laid aside for scarfing.
The scarfs are cuts made at an angle of 1 in 15 using a jig built for that purpose. The resulting cut is about ten inches in length. The sticks are marked so their grain is properly oriented when cut, since an up cut must match with a down cut. The goal is to get four longerons at least fourteen feet in length and having just one scarf joint. Despite having 60 pieces to chose from, this happens only once. In all other cases it takes two or more scarfs to make up the required length.
Actually, the fuselage needs only two of these near-perfect longerons: the ones that go on the bottom. The top longeron can have half a dozen or more scarfs since it always lays level. But the lower longeron must be bent and since a scarf joint tends to stiffen the piece, the two sides will have different curves if one is significantly stiffer than the other.
The scarf joints are fabricated with epoxy and great care is taken to ensure the joints are precisely aligned. The pneumatic brad-driver, waxed paper and some scrap wood ensures the sticks are aligned.
USING THE PNEUMATIC BRAD-DRIVER
A major problem with nailing is that the part(s) must have adequate back-up, otherwise much of the energy of the hammer's blow causes the parts being nailed to bounce out of alignment. With the pneumatic brad-driver the brad is driven so quickly that the inertial mass of the parts is usually sufficient to keep them in alignment. The first-time plane-builder must learn all of these things or their labor will be wasted. Indeed, there are so many ways to do things that that it is impossible to simply tell the newbie 'Do it like this...' because his situation may not match that of the person giving the advice.
Once the advantages of using the brad-driver are understood the work begins to go faster.
This is also true with regard to adhesives -- to the glues we are using.
Epoxy is used to splice the longerons because it needs only the pressure of alignment to achieve a good joint, whereas Plastic Resin, Tite-bond III and the urethane glues needs a significant amount of pressure to ensure the adhesive will flow into the capillary structure of the wood. Without adequate pressure to hold the pieces together, the glue will simply force them apart as it cures. When the area of the contacting surfaces is small, you may get sufficient pressure by simply fastening the parts together using brads or pins, but in most cases you will need a couple of brads to keep the parts in alignment plus addition pressure, as provided by a clamp or wedge.
This gluing pressure turns out to be another of the many things the newbie must learn because what appears to be an obvious solution, such as using C-clamps, turns out to be horribly wrong for certain types of glue.
Glues that contain water, such as Weldwood's Plastic Resin or Tite-bond III, cause the wood to swell. This swelling will force the glue-line apart unless there is sufficient pressure to prevent it from doing so. That pressure appears to be about 70 psi. In a similar vein, glues which expand as they cure need a similar amount of pressue although for a different reason. Epoxies do not cause the wood to swell nor does the epoxy expand as it cures. This makes it a near-perfect adhesive, offset by it's lack of convenience (ie, the need to mix components) and its high cost.
Epoxies appear to go through phase of greatly reduced viscosity shortly before hardening. During this hyper-viscosity phase the adhesive appears to be drawn into the micro-structure of the wood by capillary action. While brads or pins are needed to ensure alignment, there is no need for clamping pressure.
In using epoxies without clamps we are taking advantage of the characteristics of that particular adhesive. In later stages we will do the same with the urethane adhesives, hollding small bamboo struts in position with clothes-pins. The adhesive will then expand to completely surround the fixed ends of such struts, forming its own fillets.
In carrying out these exercises our goal is not to build a fuselage, although the finished product will be more than safe enough for that role. Instead, our role is to learn how to build a fuselage using tools, methods and procedures that are unique to our particular situation.
The simple truth is that one set of rules or methods can not be applied to all situations.
BENDING THE LOWER LONGERON TO A 'NATURAL' CURVE
The upper longeron forms our longitudinal reference line. When we start out, it is perfectly horizontal. (And we hope to keep it that way.) But the lower longeron prescribes a graceful curve from the bottom of the firewall, to the bottom of Frame D, and to the bottom of the stern-post. (It really isn't a stern post but we'll call it that to keep from getting it confused with anything else.)
The firewall is twenty-two inches deep, including the thickness of the lower longeron. Using a framing square or other suitable tool, lay-out the centerline of Frame D at 40-1/2" aft of the firewall. It is twenty-four inches deep, including the thickness of the lower longeron. The aft face of the stern-post is 152-3/4" aft of the forward face of the firewall and the stern post is one foot deep, including the thickness of the lower longeron.
There are eleven vertical or diagonal members connecting the upper longeron to the lower. These members are connected to the longerons using a comination of gussets and corner blocking. Using a tape measure and square, we want to transfer the information from the drawing onto the Table/Frame. We want to do this as accurately as possible so that the two side-frames will be as close to identical as possible. Once the fabrication is complete, the Table/Frame deserves a coat of thinned varnish. An area of WHITE paint goes onto varnish at some point. Once the white patch is cured we use an ink marking pen to identify the purpose of the fixture, when it was assemled and by whom. This information patch is usually given a full-coat of varnish.
With rungs on one side and a pair of diagonal braces on the other, the Table/Frame will be quite sturdy. But with a depth of only 3-1/2" it is subject to twisting due to changes in the local temperature & humidity. To straighten it out simply clamp it to your saw-horses.
LIFE WITHOUT SAW-HORSES
Life without saw-horses is a hard life. You must learn to make saw-horses from whatever is available. Such tasks are tests, constantly placed upon you. You must welcome them and offer proof of your worth by solving such problems quickly.
For those of you fortunate enough to find your local Box Store carries a good selection of cedar, suitable not only for the Table/Frame but for the means of supporting it, you will find a Table/Frame made of cedar to be light in weight and easily stored by hanging it on a wall or under the eaves of a building.
To erect the Table/Frame lift one end an fasten two legs to the corners. The keep the legs aligned tack redwood plaster lath between them from head to tail. The result is the Trinity and will not fail. Then go to the other end, raise it up and do the same. Now add additional lath from the rails to the legs so that the Table/Frame is supported to & fro.
Using a heavy mallet or hammer, drive each leg into the ground. Between the ends divide the length by two and put down a leg on the front beam and a leg on the rear beam. Use your brad-driver and plaster-lath to triangulate each of the new legs, first from the front, then from the back. When those triangles have been created, divide the rail on either side of the middle legs in to two and put down another pair of legs.
When you are done the Table/Frame stands on ten legs.
The forward part of the fueselage side will be plated with plywood. The fuselage-side shown at the opening of this article is for the FRED and is plated with 3/32" plywood. But you could use the fuselage for a different airplane and use a different plywood schedule... or none at all.
Notice that the straight top longeron aligns with one edge of the Table/Frame. This alignment is facilitated by adding blocking strips -- short pieces of 1x2 for example -- at a few places between those areas where the struts join the Table/Frame.
Notice that this particular fuselage side-frame uses a Warren Truss. Others may call for a Pratt Truss, a Howe or any of a dozen others.
While the side-rail may seem a bit casual the only practical test is: Does it work? This Table/Frame passes such a test with flying colors.
Once we have assembled a fuselage side we simply make the other side to match. One of the frame becomes the Lefty, the other becomes the Righty and we move on to the next step, which is to join the two side-frames together starting with the cockpit. Once the cockpit area is joined we focus on the nose, then the tail, at which point the fuselage is turned over and the cross-members that constitute the top of the fuselage are installed.
At some point prior to completon of the fuselage we will have fabricated either a wheeled cradle a set of false landing gear so as to make the fuselage mobile. Or, since some of you are simply building the fuselage for the experience, once you have extracted all the data, the fuselage may be stood on its nose in some out of the way corner of the shop.
One very nice advantage of the Table/Frame is that, having produced a fuselage we simply clear away the cross-members that were specific to the fuselage, we can begin fabrication of the spars.
(To Be Continued)