Chapter 21 - Strakes
In a Long-EZ, the strakes are the part joining the fuselage and the wing. They hold baggage (in the front part, adding much-needed elbow room and baggage room to the fuselage). They also hold fuel (towards the rear, and outboard).
The Long-EZ strakes are "kinked" solely so that the inboard section would have the same angle as the Vari-Eze. (In the Vari, the strakes are much smaller, and hold only fuel).
Early on, I decided to eliminate this design backward-compatibility. By building the strakes with a single, straight leading edge, I would increase volume within the strake, for effectively no drag. Of course, this also moves the center of pressure forward very slightly, but if I also move fuel slightly forward, it will compensate. Which is exactly what I will do. This mod is hardly unprecedented; the "Big-EZ" N3R was built this way. So is each and every Cozy and Aerocanard. It works.
The strakes are one of the hardest parts of the airplane to get exactly right. The leading edges are notoriously difficult to get straight, and waves and dimples in the top surface are easy to notice, too. Switching to Cozy-sized strakes opened a new possibility; I could order strake skins from Aerocanard. This would save construction time and ensure a better-looking strake all at the same time.
The strakes arrived in a BIG box; I hauled them out and got a good look. No instructions, but since what I'm doing is nonstandard anyway, what did I expect?
The skins have an outstanding exterior finish, a smooth interior finish, and appear to be laid up with EZ-Poxy. The exterior will need refinishing and paint, though, after I've put BID tapes to attach the edges, etc.
MY MISTAKE. Okay, so I sit down, think about it a bit, measure a lot, and start cutting a strake. This is nice, I say. The new Fein Sander, by the way, makes straight cuts easy and fun. Too easy, perhaps, and too fun, because it turns out I'm cutting in the wrong place!
What I did was, I cut off about 8" off the OUTBOARD end of the strake. This was a mistake, because that meant the new outboard end no longer matched the curve of the wing! (I was also mistakenly cutting up a top skin instead of a bottom skin, but never mind). Yikes! Cut mostly complete, I discover my mistake, and panic. What to do?
THE CURE. I did what any self-respecting composite builder would do: I glued the pieces back together...and the faster the better! Then I sanded down the plies in the area of the joint. And then covered that area back up with two plies BID. In other words, I repaired the area per stock Rutan techniques. Whew! Problem solved. Cost: 3 or 4 ounces and ten years' worth of worry lines. But once painted, it should be about undetectable...In the words of a Cozy builder, "No one will ever know".
Ya gotta love working with fiberglass, though. Try fixing an aluminum part that way!
Okay, enough fooling around. I cut the INBOARD ends of the strake skins, and sand the final 1/4" or so to butt them against the fuselage, and micro in place:
Here I run into another problem. The foam sandwich doesn't reach all the way back to the spar; it ends a bit short of it. This is, again, because I'm trying to adapt a Cozy strake to a Long-EZ; it's certainly no fault of Aerocanard's strake skins. An easy fix, too; I just lay a bit of blue foam in there:
And glass (2 ply BID) in place, lapping onto the spar.
And now the hard part: I have to CUT HOLES IN THE FUSELAGE! This is not easy; this beautiful bathtub, thus flawless canoe, needs some big holes chopped in it! It takes me a while to work myself up to this act of madness. Of course, I'll use the Fein sander, not an axe, but still....
I measure twice, cut once, and still decide to make the holes undersize on top so as to allow me to sand to a perfect fit.
Now there's another problem; with the strake attached firmly at root and spar, it still wants to curl a bit too vigorously at the tip. You can sorta see what I mean in this photo, where only the barbell weight is holding it straight:
Nice joint, but I'm sure I'll have to remove that barbell weight before I fly. Or even attach the top strake. What to do? Then I realize I have ALREADY provided the answer when I added the wing attach pin in the Wings chapter (Ch 19). With that 1/4" bolt sticking out from the wing inboard front to attach to the strake, I can put that rib in first thing, holding the rest of the strake skin in place. By the time the wing gets removed again, the strakes will be a completed structure, with ribs and top skin holding the strakes in their proper shape. Hooray!
I make a pair of ribs, drill and attach. With some effort, everything aligns!
Okay, time to build the ribs. Here, due to the straight-edge shape of the strake skin, I've made a few modifications:
- There is no urethane cap at the leading edge. Hence, no front-edge baffles. Fuel tanks are wet all the way to the leading edge; baggage area is empty all the way forward, too. Careful sealing at the leading edge will be called for.
- Aft end of the baggage area is unmodified, as is the butt line (BL23) of the outboard baffle. Because of the longer distance to the leading edge, B23 is 7.5" longer.
- To trap slightly more usable fuel volume at the same CG, I moved the leading edge of B45 outboard by 5". It's now diagonal, starting at BL45 at the spar but moving out to BL49 at the leading edge. B45 is actually smaller than per plans.
- This necessitated extending the diagonal baffle (DB) by 3".
- I didn't like the way stresses at the leading edge were transmitted to the spar end of B45, so I retained the outer diagonal (OD) to take some of the load.
The result you see here:
I installed the tank vents (the plans vent line, plus an additional one running along the front face of the spar, teed into the same vent line). Then wire-brushed the tops of the fuel baffles and filled them with micro, so that a pinhole leak wouldn't destroy the foam. I also installed a pair of capacitance fuel probes from Wayne Lanza at Composite Design. These required some tube bending and drilling a new vent hole at the very top. It was a LOT easier to do this with the strake tops off than it would have been as a retrofit!
Enough procrastinating. Now it's time...to attack the upper skin!
This is a big step, because one false step here can lead to months of aggravation. Flox is troweled on the tops of the baffles, and the upper strake skin lowered into place. Too much flox makes the plane heavier, but too little means a leak in the fuel tank that can be very very hard to find and fix. Any twisting or jiggling of the upper strake skin as it's put in place can also move flox out of the way and cause a gap in the joint.
I attacked this problem in two ways. The first, I call the "Flox Roll-up". In essence, I make a very small tube of 1 ply BID, wet, to hold some flox. This stays on top of the baffles much better than a simple glop of flox. It also allows a slightly wetter flox, and is thus less likely to leak. This is because only very dry flow will stay on top of those baffles. Of course, those roll-ups had flox troweled all around them...I wouldn't want the only joint to be a narrow tangential one with 1 ply BID! No, we need every baffle to join solidly with the upper skin.
The other way I attacked this problem was simply that the pre-fab nature of the Aerocad strake skins helped me in several ways. Since I could ensure both skins' leading edges matched up, I had better control of position than the per-plans setup with its curved foam block which is covered on one side and doesn't go all the way to the leading edge. In fact...looking at the problem further, I realized that by joining the two skins at the leading edge with duct tape, I could make a "hinge" and prevent all unwanted movement.
So I joined the skins this way, pulled the top skin off, did my business with the flox, and put the top skin back on with the "hinge". No muss, no fuss! What the plans say requires 3 or 4 people now requires 2 at the most, and with less likelihood for error. But this trick only works, of course, if you take the kink out of your strake; leaving it in makes this stunt impossible.
A 2-ply joint at the fuselage and a 2-ply leading edge, plus interior joints, complete the structural assembly of the tank and strakes. Whew! I just hope I didn't miss any major leaks in the fuel tank....
It looks like an airplane, doesn't it?
Blue foam inboard of the wings (inboard of BL 23) are marking guides to aid engine accessory and cowl placement by marking the inboard edges of the wing. With the wings off, I should still be able to emplace foam and lay-up a cowling, eventually.
Leak checks took forever and a week, but eventually came to an end. Once the obvious leaks are found, check your fittings again...and again...and again, because they move, and that was always the source of my leak. Well, except for the pinhole leak that was at the inside of the upper fuse-strake joint, where it can't be seen. Took me weeks to try brushing that spot. Oh, and try using a manometer setup...I broke an altimeter through overpressure. Expensive mistake.
Update Jan 2002 -- More buffoonery!
More buffoonery, indeed When I mounted the engine mount extrusions, I measured twice and cut once. Really, I did. Looks like I should have measured thrice. At least. Because the right lower extrusion came out 3/16" too high. No way to fix it without removing and re-mounting that extrusion.
Okay, don't panic. You put them in once before; you can do it again. What's the big deal?
The big deal is that NOW the holes are drilled in the spar and longeron, so the new extrusion location will have to use those holes. Okay, so we need a new extrusion; I have some. Let's get started....
Oops. The bolts that run through the lower longeron were floxed in place against just this eventuality. Unfortunately, they weren't floxed well enough; they just turned when I tried to remove them. So I was going to have to grind out the flox so I could get at the nut, and remove the bolt.
Oh, now you see the problem. Because this happened AFTER I leak-checked the strake, plainly, the sump blister is on! How do I get to the nut, which is buried under the sump blister??!?
Surprisingly easily, once I mustered the courage to do it. I simply ground holes in the sump blister (the aft one was right at the aft corner), and again in the fuselage. I dug out the flox, removed the bolts, pounded out the longeron (it came out depressingly easily), checked the PROPER location of the new longeron, built a wet six-ply layup to support it, and, when it was dry, drilled through the existing holes into the aluminum extrusion. Then REALLY flox the bolts, vacuum the inside of the sump out very very well, and repair the sump blister per Rutan techniques.
At the outset, this project would have been beyond me. But now it's easier to do than it was to say! I must be learning something.
Here's the heartbreaking sight: I had to cut a HOLE in a perfectly good sump blister!
This shows the repair after I've put the new extrusion in. The aft hole is already filled with flox, though it later got a sanding and a BID overcoat. Inside the forward hole, you can see I've floxed the nut. That hole in the sump is about to get sanded layer by layer, and patched.
Which, in an amazingly short time, I did. Whew. Breathe easier, everything is okay now....
But all things considered, it would've been nice if I'd test-fitted the engine mount against ALL extrusion hardware much earlier in this process.
Well. If we learn by doing, we learn twice as much by doing it twice. Or something like that.
If you're putting your extrusions in, fasten the fuselage side with nutplates, rather than nuts. You'll be glad you did, should you ever need to service this area.
The fuel caps went on before painting, in defiance of the manual's advice. I wanted something that seals better than the stock Brock caps, so I blew some bucks on a set of racing caps from ACS. They have a machined aluminum ring they fit into; the caps themselves are fiber and thermoplastic. I attached the cap with a chain, which also has an extension which stays submerged in the fuel, thus ensuring grounding at all times. I floxed the aluminum ring in place in the strake, used a bit of filler to smooth the top, and voila!