Stjohn's RV-7

2 hours.

Drilled the left wing. Everything was lined up, everything went perfectly, everything was easy as pie. Then I went over to the right wing. I measured and checked level again, then locked in my drill guide. I almost got through when the 12″ drill bit broke off in the guide block. Then the remaining nub snapped again when I tried to extract the guide block and drill bit from the spar. Oops.

There’s that feeling you get, I know everyone’s felt it at one time or another, and if you haven’t, you haven’t lived. It’s that sinking feeling in the pit of your stomach, when the world does a traveling zoom, where the only thought running through your head is ‘I am so utterly, completely, well and truly f**ked.’ Before the urge to throw breakable things takes over, it’s good to breathe, look at what’s going on, and come up with a plan. The facts: I have a drill bit broken off, pinning 3 layers of aluminum together. I have enough edge distance to completely drill around the broken bit to facilitate extraction.

First step was to drill alongside the broken bit. A relief well, if you will, to see if that would allow me to wiggle the broken bit out the way it came. I tried drilling out a #40 hole to see if that would loosen things up. No joy. Next was to see if I could just go up a couple of drill sizes (#3) and pulverize the stuck bit while drilling through along the same path. I got just past the first layer (aft part of bracket) when the #3 bit started to spin and wouldn’t cut into the broken bit at all. Back away. Back away before you make it worse.

So I pondered, and then had a hunch. What if I can move the wing spar relative to the bracket? I unclamped everything, making sure to double check that my sweep and incidence positions were marked and gave the wing a jiggle. YES! The spar end slid inside the bracket, which meant that the broken bit wasn’t pinning all three layers, just the first two. So what to do now?

Take the damn wing off. With the wing off, I can get at everything and backdrill to the broken bit from the forward side, then pound it out with a drift punch and a hammer. The broken bit will be right below the #40 hole, so with a good guess, I can put another #40 hole below that one and hit the tip of the broken bit, all without compromising my edge distance to the sides of the bracket or the spar. I have to tread very, very carefully now. This has the potential to be a nasty incident pit.

And if I fail completely, I can have the bracket and spar welded up and redrill it. I think. Might have to contact tech support on that one. Best policy is not to mess it up.

This is truly a “come back victorious, or on your shield” moment.

4 hours.

Before we go on, a little lesson in aerodynamics, because it’s important that you understand just how important this step is. When you’re talking about wings and aircraft, you need to be concerned with three angles: Dihedral, sweep, and incidence. On this aircraft, The wings make a very shallow “V” shape about 3.5 degrees up from horizontal. That’s the dihedral. If they had a downward angle, like a Harrier or a Colonial Viper (either mark, pre- or post-war), that would be the Anhedral. Dihedral gives an aircraft horizontal stability, like the hull of a boat, and has various effects on roll rate, maneuverability, etc. At the factory, the wing spars and the center section are drilled together in a jig, so the angle’s dead nuts, and you’d have to try very hard to screw it up. Sweep is the horizontal angle of the wing relative to the aircraft’s intended direction of flight, the long axis of the fuselage. This is important. The RV series has a big Hershey bar for a wing shape, so the sweep should be zero, or as close to it as you can get. If it’s not zero, both wings better be the same. Most light aircraft don’t have swept wings. At the speeds we go, they’re not necessary, wave drag isn’t a big issue for us. Even the A-10 and the Incom T-65 X-Wing have zero sweep. The last angle is incidence. This is the vertical angle of the wing relative to the direction of flight, and this one is probably the most important one of these to get right. It’s especially important to have the wings at the same angle of attack as the horizontal stabilizer.

If you screw up these angles, the plane flies badly, and in extreme cases the aircraft is unsafe to fly. Messed up incidence will result in pitch or roll artifacts, sweep errors will make it yaw funny. Put it this way: unless everything is flat and symmetrical, the plane turns without you asking it, and your autopilot will detach itself from its mount, skitter up the back of your chair and strangle you with its own wiring.

This step is one of the biggest stressors about the whole project, especially on a quickbuild kit, because you have to make sweep and incidence just as dead-on as the factory-built dihedral. So you wind up using a lot of levels, plumb bobs, string, lasers (if you have them), and unless you’re very lucky, you might be mounting and unmounting the wings multiple times to test fit.

What you do is get 4 plumb bobs and hang them off the leading edge of the wings. You then wiggle and jitter the wings until all 4 of them line up on a string, chalk or laser line betwen the two outboard ones. This tells you your wings are straight relative to each other. Then, you use a piece of safety wire, tied to a bolt through the tailwheel mount bolt hole and measure a reference point on the wing. The last rivet on the outboard end of the rear spar, for instance. Mark that on the wire with tape. Then, find out where the tape winds up on the other wing. If it lines up, hooray for you, you have a fuselage that’s perpendicular to your wings. Mark the spot on the rear spar, and drill, baby, drill. Or, you could be like me and not have anything work at all.

See that angled bit being covered up by the square bit? That’s the aft spar, and even after cutting it down per plans, it still didn’t go in far enough to give me zero sweep. I had about half a degree of forward sweep. Not like the X-29 or the Su-37, but enough to irk me. So I had to unmount the wing and file the spar back. Even with my rough rasp, it’s a time consuming process. I was careful not to take off too much metal, because if you do that, it’s gone, daddy, gone. You can’t glue it back on. After two or 3 iterations per side (ask me sometime how much fun this is) I got them to zero sweep, which I then marked on the spar and the bracket (the square bit).

This shot is pulled back a bit, and it doesn’t show much except how little room there is to do anything in there.

Then it was time to do incidence. To do this, it’s necessary to level the fuselage so you can level the wing relative to that. The plans have you using the main longerons as a datum to set level. One small problem. My main longerons aren’t on the same angle, because there’s an ever-so-slight twist in my fuselage. The PDF from Van’s says to average the values. So I leveled the fuse as best I could and got the incidence set on both wings. But I still haven’t drilled anything yet. Tomorrow maybe, after another round of measuring, checking, and realigning.