Foam Cores

Step 1 CUTTING FOAM CORES AND BONDING TO TORQUE TUBES

I used my bandsaw to cut out the elevator templates, then, with a sanding belt installed, sanded them to exact size. To make the templates, rather than cutting up the M drawings (or buying a spare set) I start by scanning the plans diagram with my PC then printing out the resulting image. I adjust, as needed, the size of the image, using a bright window to look through the scanned image to ensure it's an identical match to the M drawing image. I then trim the image from the paper, spray glue it to a piece of aircraft plywood and then saw and sand it to shape.

However, when I did make both templates, and put them side by side to compare them, I realized they were not the same size. After checking my scanned images to be the correct size, I realized the plans M drawings were off by about 3/32 of an inch.

I measured the left side one and it was almost exactly 5" in this dimension.

The right side one was almost 1/8 of an inch shorter. I was pretty sure these were not supposed to be of unequal length (I checked with my buds on the Cozy mailing list to be sure) so I made alternate plans. I suspect these are off due to duplication errors or uneven shrinking of the paper the drawings were made on.

After making a mirror image of my left side image, I re-printed, re-glued and etc. to make a new right side template. Yes, now they match exactly.

Here's all of my elevator cores, hot wired and ready to be used. I drove down to Aurora and visited Dan Pettinger, we hot wired these together one Saturday. I have borrowed the hot wire saw he has before, but, he still needed it so I just drove down and hot wired these cores. The hot wire saw is the best I have ever seen - click this phrase to see pictures of it.

Ok, so I like to make gadgets. Some of them might not be necessary, but, they sure seem to help get some of these parts made more easily. After hot wiring the foam for the elevator cores, the plans say to sand the inside until you get a good fit. I made a sander out of a couple pieces of scrap PVC pipe and fittings, 3 pieces of sandpaper taped together with packing tape and a piece of duct tape to hold the paper to the tube. The tubing is 1/2" PVC thick wall, which ends up about 7/8" in diameter, just about right, wrapped with half a sheet of sandpaper, to sand the insides of 1" tubes. Just roll the paper around the tube, insert, then spin it and slide it in and out. Remove, check for size, sand more if needed.

Here's the sander in action. It was pretty easy to insert if you left the paper "unrolled" and passed it through the slot at the front. Then gently roll the tube, the paper gets pulled into the hole, and rotate with the roll until you get the size you want.

None of the NC-2 inserts don't fit without some adjustments to the milled slots in the tubes. I went ahead and labeled mine once they were installed and fit properly, however, this was probably unnecessary.

I had to file the corners where the milling corners were made to get these to fit. The archives say not to disturb the radius, but frankly, you could not get the insert in without making the corner (at least on the inside of the tube) more square. I'm not too worried about stress cracks here, if these flutter that might be a problem, or, if I had hydraulic flight controls, but I don't think either of those issues should arise here...

I used a piece of wood, narrow enough to fit in the milled slot, to prevent the tube from rolling and keep it square on the drill press bed. This made drilling these holes a bit easier.

I then used a square piece of aluminum scrap to hold the insert into the milled slot. The scrap also, again, kept the insert and tube square to the drill press bed while drilling. A drill press vise would probably have worked, too, but I don't have one and didn't like the idea of marring or flattening the tubes.

The plans don't say this, but, given the way the tubes are oriented you need a left and right. Of course, because of that, you will need to change the orientation of the NC-2 inserts in the torque tubes. The hinge pin hole is offset, in the NC-2 insert, from the center. That means, when mounted in the torque tube, it's either towards the top or the bottom. To prevent ending up with one hinge pin up and one down, make sure both inserts in each tube match (orientation wise) and that the other tube has them the other way. It will be clear which way goes down when you micro on the foam cores, but, if you're riveted the inserts in the same way in both - you'll have to drill the rivets and reinstall them. Hooray! I didn't mess this step up and installed them right the first time!

Here's the foam cores for one side trimmed to length (close anyway) and the torque tube all ready for inserting into the core. I have put the CA glue into the slots around the insert, per plans, and am waiting for that to dry thoroughly until I do the next step. It was time to quit for the day so this will wait until I'm back in the shop.

When I was ready to install the torque tubes into the cores, I mixed up some micro and filled a syringe to make it easier to get the micro into the slot in the core and evenly distributed.

I used a small piece of plywood to keep the hinge pins up off the table and at the right height. This was a pretty messy operation altogether, but I spent a few minutes with a paper towel and some cotton swabs and cleaned up all the extra micro that managed to get onto the hinge pins.

I guess I mixed the micro a little too loose because a bit of it ran out the end. I think I needed a bit thicker of a mixture to prevent this from happening. I think I will be OK, though, the fiberglass layups that go over the cores will help the torque tube stick to the cores and avoid any possibility of rotating inside the foam core.

Here's the core with the the micro curing and the hinge pins in and rotated around the NC-7 pieces to set the right angle. I gently weighted the cores down and left the workshop for a couple of hours to let the micro set and not be disturbed.

Click here for installation of the Bottom Skin