Sunday, March 13, 2011

Fabulous flaperon fabrication

This was the kind of weather that would have frustrated me to no end in the days before I had an airplane to build. Tauntingly blue skies were calling for me to either join them in the RV-6, or at least marvel at them during a long, destination-less drive in a convertible. Alas, winds that where blowing at 26 knots when they weren't really letting us have it, and at 34 knots when they really wanted to drive home the point, kept me on the ground, while temperatures just a few degrees below the level needed for even moderately comfortable top-down driving precluded an open-air ride.

But these days, weather like that is just fine. I was able to spend a comfortable five hours working at the hangar.

Having passed over the fabrication of some parts in favor of the more entertaining flap counterweight drilling, I had to back track to step one. As I was looking at the drawings of the part that I had to make first, it became much clearer to me why The Jackson Two had asked a local machinist to knock these parts out for them. As I mentioned before, there is something intimidating about all of those different measurements and radii and offsets and dimensions measured in 1/23" and, last but certainly not least, the critical nature of the part in question. You see, I just had to look ahead to see what this part is used for.

The explanation is somewhat lengthy, but the crux of it is that the flat "paddle" part will fit into a slotted part emerging from the side of the fuselage. That slotted part will rotate when the pilot enters roll commands via the control stick and/or moves the flap handle. The rotation of the slotted part will be transferred to the "paddle," which will in turn transfer motion to the flaperon. So, at the end of the day it is this part that I am fabricating that will determine whether or not I can turn the airplane, or even maintain level flight for that matter.

And yes, I will be making two of them, one for the left side, one for the right.

The parts will be cut from a length of angle aluminum. I did a series of useless measurements and markings before realizing that the only line I needed for the first step was the one that would cut the raw material to the total length of the part. As you can see in the drawing, that is 2 29/32". I can only measure to 1/16" on the ruler I have, so I convert the measurement to something like 30/32", and then simplify to 15/16". I measure the part to the 15/16" on the ruler, then mark the part at the little gap between 14/16" and 15/16".

I do that at two places on the part, then draw a connecting line between the marks to use as my cut line on the band saw.

To form the paddle, part of the other bend in the angle will get sawed away. There's going to be lateral tension against that upright part, so Van's wants a stress-relieving 1/4" radius at the junction. They have us do that by measuring to a certain spot and drill a 1/4" hole. When we cut the two lines to remove the unneeded metal, we will cut them so they intersect at the tangent of the hole, thus leaving a 1/4" radius on one quarter of the hole. Or something like that. The pictures should explain it better than I can.

I was trying to get the drill bit right flush up against the vertical part; you can see that I got it too close. Most of that part is going to get cut off anyway. I also cut off a thin strip on the horizontal part to narrow it down to the 1 5/8" width (surprisingly, it wasn't 1 19/32") called for in the drawing.

Another hole as to be placed and drilled. This one is 3/16". I used a center punch to mark the location, then used a small bit on the drill press to get the bit perfectly centered on the mark before clamping down the part for final drilling. I think even with those precautions, I got the hole just a smidgen off from where it's supposed to be. Time will tell.

There's a 3/8" radius around the center of that hole as shown on the planes. I used a little math to lay out the curve. Two times 3/8 is 3/4, so I measured 3/4" from the base to the top of the radius above the hole and marked the diagonal cut line from there back to the aft end of the part.

I used the band saw to cut the diagonal (as is evident from the not-so-very-straight line) and to round off the corners of the 3/4" radius. Then I buffed the whole thing up with the Scotchbrite wheel.

As I was working on the second part, Al the Tool Junky stopped by to see what was going on. He has a nice bench metal shear down in his hangar and offered to do the diagonal cut on the second part in the shear, the theory being that it would result in a cleaner, straighter line.

It did.

I was ready to quit for the day and was reading ahead to the next step when I realized that the holes that would have to be tapped didn't use the same size tap that I had bought for the wing tie downs. I said something to Al, and before you know it he not only had gone down to his hangar to get the tap but was fully engaged in the fabrication process. I had him cut the four 2" lengths of tie-down extruded aluminum on the band saw while I got the parts ready for the steps I will work on Monday night. There were eight more of those "narrowing" cuts to do (one on each side of the four 2" parts) and those too were beneficiaries of the metal shear.

We worked out a process of getting the parts properly aligned for drilling in the drill press, then Al demonstrated the proper way to tap a hole that deep. I had been using Boelube to help lubricate the tap, but he showed me something that works better: he dripped a few drops of MEK (paint thinner wold have worked too) down into the hole and used a little candle wax rubbed onto the tap. It went much more easily than the wing tie-downs had for me!

He took off after the first was done and I finished the remaining three. The first step is to align the part. We did that by putting a size N bit in the drill press and lowering it into the part. That forced the part into a good vertical alignment before tightening the hold of the vise.

The next step was to replace the size N bit with a size Q bit. This is the bit called for in the plans for drilling the hole. A little MEK was used here too for lubrication. The drilling was very easy. The hole had to be drilled to at least a one inch depth; I put a black Sharpie mark at the one inch depth of the drill bit. The MEK washed most of it off, but it was enough to make sure that I get the hole deep enough.

I like the way the shavings spiral up the bit.

With the hole drilled, the tapping is simple.

Here's the day's worth of fabrication. The four parts that needed to be tapped will get those tie rods screwed into them after they are mounted to the flaperon spars, two to a spar. The bearing in each of the tie rods will have a bolt put through it after being centered in the bearing brackets hanging off of the back of each wing. So, these fabricated parts are what will hold the flaperons onto the wing and allow them to pivot in the bearings in response to the command inputs transferred from the pilot through the paddle pieces (called "actuators")!

Pretty heady stuff, isn't it!! These are some critically important parts.

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