HP-24 Project

Update 3 October 2003: Another disappointing (though minor) setback

Last night I went back to the garage and unbagged the elevator plugs on which I'd done the layup on Wednesday. Everything went fine until I went to separate the cores from the top shucks. It turns out that the cores had gotten bonded to the shucks along the trailing edge, and I ended up destroying the parts by breaking the core away from the layup near the trailing edge.

Epoxy had seeped in between the two parts further than I'd expected. I had put down a strip of packing tape (which, because epoxy won't stick to it, makes great release tape), but the epoxy seeped in past the taped area. I think that some of the epoxy may have actually seeped through the foam itself from the layup side. Back at the trailing edge the foam core gets very thin, and probably more porous.

Oh well. I didn't like those parts much anyhow. As an experiment, I'd had them cut from blue styrofoam instead of the beaded polystyrene I'd been using before, and they were troublesome from the very start. Because of the extra heating where the wire cut the very shallow trailing edge, the trailing edges shrank spanwise. That shrinkage bowed the parts so that they swept back about 1/4" at each tip. I was able to straighten most of the sweep out by staking them down to the shucks with toothpicks, but I was never certain that they would come out straight enough. Well, actually, they did come out straight enough, but that's irrelevant now.

I happened to get a call from the foam cutting guy in Riverside right after I'd finished breaking the freshly unbagged parts. He called to say that he'd just finished cutting the wing cores. He commiserated about my blunders with the elevator, and said he'd cut a new set out of the friendly beaded styrene, and that they'd be ready when I arrived to pick up the wing cores.

In other news, I made good progress on the fin-to-fuselage fillet last night. I laid on several swipes of bondo over the rough microballoon surface, and finished creating a smooth extension of the fin surface right down to where it intersects the fuselage surface. The process was actually a lot friendlier than I'd anticipated. I think the key was having a lot of different grits of sandpaper to choose from. I did all the rough shaping with 36 grit, using a steel wire brush to unclog the paper every dozen strokes or so. Then in with 60 grit, and a little clean up with 80 grit. That's as smooth as I got both sides before starting in with the filleting. To spread the Bondo, I used a stiff 8" taping knife, which I carefully oriented so that its edge was always parallel to the fin surface ruling lines.

I started the fillets using a paint can lid of about the right radius to start squeegeeing in the bondo, and that's where I left off on the project for the night. When I return, I'll bring some 4" ABS pipe (OD 4"), which I'll use to fill in the fillets some more. I'll also cut some 4" long sections of the pipe into three 120-degree sectors each to use as sanding blocks.

And if you're wondering why there's such a big difference in approach between the fin/fuselage fillet and the wing/fuselage fillet, it's because it matters a lot less at the tail. By the time fuselage boundary layer gets back there to the fin, it's pretty tired and grumpy, and there's almost nothing I can do with the fin fillet that won't piss it off a bit more. So far as I'm concerned, the major function of the fin/fuselage fillet is to provide a smooth structural transition for the fuselage laminations.

Tonight, Doug and I head for Riverside. We're overnighting in Bakersfield, and then diving over the Grapevine in the morning.

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page updated 3 October 2003 all text and graphics copyright (c) 2003 HP Aircraft, LLC