Sealing the trailing edge

The stripper arrived about a week ago. I put the traling edge wedge from the rudder in the portable spray booth, now pressed into service as a fume hood. (As it turns out, the the stripper doesn't seem to give off a lot of fumes, but I was taking no chances with stuff capable of removing epoxy.) I didn't need to strip the rudder's skins because the epoxy adhered much better to the trailing edge than it did to the skins.

Stripping was a matter of brushing the epoxy on, using a disposable paintbrush, waiting a while for it to bubble or show signs of softening, and then cleaning it off. The stripper dissolves in water, but it took a while to find a suitable scraper. I eventually settled on a green Scotchbrite pad, which did a pretty good job of taking off epoxy without itself dissolving. I went through three coats of stripper. The epoxy came off just fine--in fact, it came off pretty well with the first coat--but I'd also primed the wedge. To make sure the fuel tank sealant adhered well, I also wanted to strip as much of the primer as possible, so the sealant would be gripping aluminum on both sides of the bond. Except for a couple small patches, almost all the primer came off with a combination of the stripper, scrubbing with the Scotchbrite pad, and a bit of wet/dry sandpaper.

The intervening week was a busy one with no opportunity for workshop time, and we were out of town this weekend. When we returned this evening, I mixed up the batch of sealant. If you've never seen the stuff, looks like sticky, somewhat runny black tar that gets into absolutely every place it shouldn't. You can remove it with MEK or acetone, at least before it cures, but it's a challenge to keep it under control. It won't stick to the non-sticky side of packing tape, though, so I put a piece of packing tape the length of the aluminum angle. After squirting sealant between wedge and skin on top and bottom, assembling the trailing edge, and clecoing it to the aluminum angle, I spent a fair bit of time with paper towels cleaning up all the sealant that oozed out and trying mighty hard to get the sealant off the aluminum instead of spreading it further.

Also, to ensure the trailing edge came together fairly tightly (without the wide gap I ended up with before), I decided to clamp a second piece of aluminum angle along the narrow space between the rivet holes and the very trailing edge of the rudder. You can see that here.

The next step is to wait. The sealant has a 30 minute working time, but it'll need to stay there at least 48 hours to cure properly before I can mess with trying to rivet the trailing edge.

Disassemble, reassemble

I spent some time last night taking apart the rudder's trailing edge.

The process of drilling out rivets is pretty simple. If you haven't seen one before, a rivet has a broad head and a cylindrical body. You drill partway down the body, use a pin punch to pop off the head, and then push the rest of the body through the hole. The tricky part is you don't want to make the hole any larger, or the rivet you put in there when you reassemble the thing won't hold properly. I usually have a problem with the drill bit trying to walk a bit off center when I start drilling, so I use a center punch to make a starting dimple in the center of the head. It's not perfect, but it helps. I also check often while drilling.

After a lot of careful drilling, I eventually managed to drill out all the rivets without enlarging the holes. Here you can see the wedge that goes between the trailng edge skins. What you can't really see in the photo is that the wedge has dried T-88 epoxy on it. (So do the skins, but not as much.) The web site for the epoxy manufacturer says you can remove dried epoxy using a suitable paint remover. So there's now a quart of Certified Coatings Sure Strip paint remover on its way from Aircraft Spruce.

By the way, Aircraft Spruce was most helpful with this order. According to the epoxy maker, "suitable" means the paint remover must contain methylene chloride. I couldn't tell from the Aircraft Spruce web site whether Sure Strip contains methylene chloride, but the folks at the Aircraft Spruce order desk went into the warehouse, pulled out a can, checked the contents, and called me back to confirm that it does.

Follow-up on the rudder

I heard back from tech support this morning. They recommend drilling out the rivets, cleaning it up, and redoing it with fuel tank sealant. It'll be a pain, but I agree with them.

So now I need to order some sealant. Proseal at Aircraft Spruce is tres expensive, $78 for the smallest quantity, but it turns out Vans has it, or a newer incarnation of it, in 3.5 oz quantities for around $16. Horray for Vans!

I'm still working out how to clean the epoxy off the aluminum parts.

Rudder issues

I'm having rudder issues. One of the more ticklish bits of building this particular airplane is the trailing edge of the rudder, one of the last things to touch the air as the plane goes by. You have to get it straight, and ideally you need to have it nice and, well, together. The trailing edge consists of two rudder skins and a wedge-shaped piece of aluminum that all meet at a point. The instructions say to put fuel tank sealant or some kind of glue on the wedge, glue it all together, clamp it, leave it alone for 48 hours, and then go through the ticklish process of riveting it so it comes out nice and straight.

Now, this is the second time I've built this rudder. The first time around, I got it all finished and hung it on the wall of the workshop to get it out of the way. (I was in a rather small workshop at the time; space was precious.) A few weeks later, the string I'd used to hang it broke, the rudder took a catastrophic fall, and the trailing edge got crunched rather badly. After much wailing and gnashing of teeth, I ordered new parts.

I recently finished the rebuild, and it was time to do the trailing edge again. The last time I did this, I used fuel tank sealant and had a whole bunch of the stuff left over, that I wound up tossing because, let's face it, how often are you going to use fuel tank sealant around the house? (Fuel tank sealant comes in two parts. Once you mix them, it'll eventually set. I ordered a small quantity, but it came in a container where you had to mix the whole thing, and the whole thing was enough to do a whole fuel tank and quite a bit more than I needed. You can store mixed sealant in the freezer for up to about six months, but at the speed this project's progressing, six months won't cut it.)

So this time I tried T-88, an epoxy used in aircraft construction. Here you can see it clamped together so the glue will set. That little aluminum bit at the left is a test piece. All seemed to go reasonably well. I gave the glue a full week to set and the test piece seemed pretty solid, so I started working on it. That's where I ran into problems.

Here you can see a shot of the new trailing edge, on top, and the old one on the bottom. The epoxy separated as I set the rivets. It turns out T-88 on aluminum just doesn't have the peel strength fuel tank sealant does. When I tried peeling the test piece apart, I was able to pop it as well. I've written the tech support folks at the kit manufacturer to find out how serious a problem this is.

The trailing edge itself came out pretty straight. It wasn't perfect, but there weren't any major problems. I've contacted tech support about the width of the trailing edge. We'll see if I need to drill out the rivets and redo it.

I tawt I taw a bomber

While cooking dinner, I happened to glance outside just now and saw an airplane silhouette that looked, well, unusual. A quick check with binocs confirmed it: round tail, four engines, big, and not moving all that fast. A B-17 maybe? A few minutes later, it turned and flew over the house, making that distinctive, rumbling big radial engine sound that only World War II warbirds make. And an H-tail. Not a B-17, but a B-24 Liberator. Probably taking people out for rides on a beautiful flying day. Pity there was no camera handy.

RV-7A meet Toyota Prius

One of today's tasks was transporting the horizontal stabilizer for the airplane that's under construction, a Van's RV-7A, to the new house. The horizontal stabilizer is the horizontal part of the tail. On an RV-7A, both sides of the tail form a single unit. A rather long single unit. And like most airplane parts, it's extremely strong, but only in the ways that it needs to be strong, which means I really wanted to move it myself rather than letting guys who are used to moving furniture do it.

Now, the last time we moved this thing, we put down the top on the convertible and tied the stabilizer across the seats, with one end on the dashboard and the other hanging over the back of the car. That worked OK for moving it less than a mile late enough at night so there'd be no traffic, but it wasn't going to cut it for moving the thing twenty miles at highway speeds.

After puzzling over it for a bit, I decided to try it in the Prius. Amazingly enough, it just fit:




With room to close the hatch, even! With the front seat reclined, and the back seat dropped, one tip of the stabilzer's about an inch from the dash, and the other comes right up against the tailgate. Mission accomplished!

Red Bull Air Race

The Red Bull Air Race had its penultimate race today. My video clips seem to be giving Google indigestion, but here are some still images.

realizing just how far out of touch I am

I'm slowly realizing just how far out of touch with current events I've become. For example, I'm only now learning about the propellant explosion at Scaled Composites that occurred back in July. I've been following progress at Scaled because they're one of the companies leading the charge for significant technological progress in space travel. And because Burt Rutan's work is, well, just plain cool. Now, I have sort of an excuse--July 27 was the day after the bar exam, so I was slowly crawling out of my little legal foxhole and beginning to pick up the many things that fell on the floor during the studying process--but it's not a very good excuse because I learned about Japan's Kaguya lunar probe mission on September 14, the day they launched it. So now I'm faced with the need to figure out how to better plug into current events in the space industry. And probably in the rest of the world as well. I guess I have a lot of learning to do.

Do they make Cliff Notes for current events? (I guess there's Google News, but somehow it lacks the breadth of coverage that I'd like to have.)

surfing

Coppertop was just telling me about her co-worker who broke her foot surfing. Fell off a board, foot went between a couple rocks. Then earlier today a friend was telling me about a mutual friend who fell off her board, landed on a coral reef, and got cut up pretty badly. Now I'm really starting to wonder about surfing--maybe it's a good thing I chose a nice, safe hobby like aviation.

flying qualities report: Micro Mosquito helicopter

I'm doing everything I can to procrastinate working on a paper. Part of the procrastination includes playing with a new toy, Radio Shack's Micro Mosquito radio controlled helicopter.

Yeah, I know, totally geeky. So sue me. I'm having fun playing with it till I can afford the ten grand it'll take me to get a real rotorcraft rating. :-)

The Micro Mosquito is an indoor radio controlled helicopter with counter-rotating rotors and pretty amazing engineering. It weighs maybe a hundred grams, fits on the palm of my hand, and has survived repeated, ahem, "rough landings" from as high as ten feet with no permanent damage, though I do occasionally have to pop a piece back in place.

Helicopters are, by nature, unstable. Most airplanes want to fly--take your hands off the controls and most of them will right themselves and keep going. In contrast, you have to fly a helicopter constantly. The MM's counter-rotating design makes it more stable than most choppers and particularly easy to hover, but you still have to stay on top of it. If it bumps something while moving, or you give it too sudden a burst of forward thrust, it can start swinging around the rotors like a pendulum. If it swings too much, the rotors will eventually bump into each other and you're looking at one of those "rough landings."

The MM doesn't have a vertical stabilizer, which means its directional stability isn't so hot. While flying forward, air currents or friction in the drivetrain might suddenly kick the nose left or right. You have to watch and correct for it, but correct fairly gently or it'll start swinging around like I mentioned before. Our house is a bit drafty, leading to occasionally rough flying conditions, especially near hallways. In fact, the MM will sometimes get caught in up-, down-, or side-drafts that I can't even feel, sometimes too strong for the rotor system to overcome. I've installed the included "for extra forward speed" kit to help punch through some of the drafty areas.

My only complaint with the toy so far is that there's sometimes some odd control lag. For instance, I might tell it to thrust forward, but the tail rotor won't respond for a second or two, and there's an occasional lag in throttle response. The lag can lead to pilot-induced oscillations, where I tell it to go up to avoid the couch, it eventually responds, and by that time I'm saying "no, too high," then later it starts coming down and winds up bobbing drunkenly towards the coffee table. I don't know whether it's in the control system or maybe we just have a noisy radio environment. Otherwise, it's been a blast buzzing around the living room, avoiding the air currents from the wall furnace, and trying to land it on the helipad.

jet man

Four small jet engines. A set of folding carbon-fiber wings. No control surfaces whatsoever. A parachute landing at the end of the flight. You really ought to see this video.