3/22/2007

Prototype Part Production

It has been a fun time working on the Sled in recent months. Progress has been quick. Relatively speaking, on a 5 year + project that is. :-)

At this point all the parts are done, except the big fuse. If the initial test flights go well, there will be additional parts and molds required for turbine airflow ducting and the inlet spikes, but other than that all the rest is done. Of course when I say done, what I really mean is that I'm now up to the point if this were a kit, you would have just pulled them from the box. I've still got to do the servo mounts, hinging, etc.

Take a look below at some of the construction photos of the pieces and parts.

After PVA is shot, then Dupont automotive gray primer is put down, and it's time to start cutting glass to make some parts. Cutting the glass is not as simple as it would seem. There are many things to keep in mind here. The fibers do not want to lay into a square corner, so the cloth should ideally lay in such a manner that the fibers hit those parts of the mold at an angle. Secondly, since I'm using 2 layers of 6 ounce cloth, I want to take advantage of the cloth direction, and make the 2 layers at different angles to each other as well. I've found that having the original plug parts handy, works great to set on the cloth, at various angles to assist in cutting out what's needed.

Even with trying to help hit sharp edges at angles, you'll see here I was still having trouble getting the cloth to stay put in the corners. Since I used clamps and such to help, it produced a bit of print through on the finished part. Subsequently, I've learned that if I take a few runs of carbon tow, and run in the corners before putting the cloth in, it gives it a bit of a radius, and also a really strong edge to the outside of the part too. This is a good example of why to refine techniques on the small parts first.

Internal structure being glued in here. Mostly 1/2 inch foam, and 1/2 inch balsa leading edges that will later receive hinges.

After the wood and foam has dried, I'll sheer the foam off flush with the mold with a hot wire, sand it a bit, then be ready to glue the 2 halves together. What you see here is the first parts being removed from the molds. You'll note that even as tough as the flange is, it bows quite a bit next to the mold wedges, before breaking free.

Once one half of the mold is free, the part is still stuck in the other half. I'm using a carbon tow laid in along the seam, and a small amount of that squeezes out onto the flange when it is clamped together. If I can get a wedge under this, it makes a great lever to pry the part out, as shown here. I've come to learn that 99% of the sticking a part may have to a mold is at the point it is seamed. The rest of it pops out with ease.

On to more parts here. You'll see the 2nd rudder, and the 2 wing tips having just been laid up. I added a full 5.7 oz layer of carbon in on the wing skins. Considering the leverage of the big elevator and the torque of both the wing tube mount and the droop in the wing tip, it seemed like a good idea to go a bit overkill here.

Detail of the rudder innards. The solid foam here is where I drill out a hole and insert a ½ inch carbon tube, that will be what it pivots on.

Inner wing detail here. I made a tongue and groove arrangement to hold the wing tube in place. There are 1/8 inch ply plates top and bottom of the wing anchor points to ensure the pressure is spread evenly. The trailing edge is ½ inch balsa, set ¼ inch in from the back. This will allow the elevator to be hinged up in the wing a bit, closing up the hinge gap from air bleeding through, which should help prevent flutter.

Here is a better shot showing how the edges are seemed with the carbon tow. 90% of it is against the inside wing skin, and about 10% sticks up above the edge. This is done on both halves, so that when they are clamped together the entire seam is really solid. I don't know the term for this, but I call it a 'blind seam'. I use it when you need to glue two halves together and cannot access it while clamped to lay fiberglass cloth across it, which is the traditional way of doing a seam.

After wetting out this carbon tow by hand on a table for many sessions, I took an industrial idea and shrunk it down to a really slick tool. What you see here, is my carbon coming in from the right, going into a paper cup up fairly high, down under a dowel in the bottom of the cup, then back up the other side and out. I put epoxy in the bottom, so that when I pull it through, it automatically wets it and I just cut it to length as it comes out. You'll even see a ruler clamped on the output side, to aid in measurement.

The wing tips are my first chance to see how the corrugations have come through the process and how they look. I am really pleased that all the detail had held up so well, and think it looks really sharp so far.

Here's a look at the droop on the wing tips that I was cussing about several years ago, when making the plugs.

And finally, here is the stack of completed parts. 2 rudders, 2 rudder bases, 2 wing tips, 2 outboard elevators, 2 inboard elevators, and 2 turkey feathers. Weight is decent so far. All of what you see here, has added up to 5 lbs and 14 ounces. My goal was all these parts and the fuse to come in at 20 lbs, any my limit is for it to all come in at 25 lbs. So... That means that the fuse now needs to come out at under 19 lbs, and the lighter the better. I think that will be doable, but will be close. Next up, will be laying up the shells for the top and bottom fuselages, and starting to design all of the internal formers and structure. Stay tuned...

Head back to the Main Page