HP-24 Project

Update 21 February 2005: More IBS, little VP

Lately I've been doing a lot of the itty bitty stuff that it takes to keep a program like this going, but without much in the way of visible progress.

One thing I've been doing is recovering from a minor error in the specification for the fuselage molds. Harald had asked me if I inteded to vacuum bag the fuselage halves, and I told him no, figuring that I'd just do ambient wet layups. Since then, I've changed my mind and decided to go with bagged fuselage halves, and that means that the fuselage mold needs to be airtight. Of course, most of the fuselage mold is airtight; it wouldn't be much of a mold if that weren't true. However, there's a lot of little places where that isn't true. There are holes through the mold where we have locating features for the lift carrythrough hardpoints, and also holes through the parting flange of the left shell mold for bolting down the removeable mating joggle features.

If I'd originally specified vac bagging, all of those holes would have been made either blind of with sealing features. But I didn't, so they're not. So now I have to.

For the joggle feature bolt holes, what I've been doing is putting the bolts in place, secured on the bottom with sealed acorn nuts. Then I'd take half a ping-pong ball, put a patch of saturated fiberglass cloth into it, fill the hemisphere with flox, press the half ball to the bottom of the mold with a C-clamp, and then stipple the perimeter of the cloth onto the bottom of the mold half. This approach is a slightly awkward approach to the task, but is one that I can do it without having to turn the mold over. And that's pretty important, since it takes three people and two engine hoists to invert the mold half.

For the carrythrough hardpoint features, it's a bit more complicated, since I need to be able to use those holes to demold the fuselage half. It turns out that the mold topology is slightly constricting in one spot near the forward carrythrough, so I need to be able to get a jackscrew into the forward hole to drive out the fuselage half shell. For those holes, I'll be gluing in some sort of neck for a sealed cap. I'm attracted to the idea of just gluing in the threaded neck of a plastic water bottle, but I'll continue thinking on that for a few more days.

I'm also working on building what I used to think of as vacuum distribution manifolds for the two shell molds. But about halfway through, I realized that that's not the right way of thinking of it. Vacuum equals nothing, and you can't distribute nothing. So what I'm really working on is an air collection manifold. The utility of the manifolds is that they will substantially speed up the job of getting the vacuum bag closed up over a fuselage shell, and yield better flow rates for more quickly removing the air from under the bag.

Anyhow, the manifolds are lengths of 1/2" schedule 40 PVC pipe that are semi-permanently mounted to the steel frames of the shell molds. They'll have barbed fittings for 1/4" ID vinyl tubes that will penetrate the vacuum bag and remove the air from under the membrane. They'll also have union conections for the vacuum pump, and also have their own vacuum guages.

Last weekend I also carted up to the shop a 150 lb roll of Knytex DB170 double-bias cloth. Ever since doing the one left-side layup, I've been studying ways of simplifying the fuselage layup. One of the things I'm going to do is use full-length swaths of knitted double-bias cloth instead of fussy little diagonal swaths of woven cloth. That drastically reduces the number of pieces to emplace, and so speeds up the layup process.

Next weekend I figure to finish off the air collection manifold, and also hopefully finish off sealing the through-holes in the mold shells. If that goes well, I'll also try to do a test drape of the DB170 cloth so I can make swath templates. I know right from the start that I can't just use the swath templates I made for the 0/90 cloths - the nature in which the +/-45 cloth drapes results in a differently shaped swath perimeter. The swath templates are sort of important, since I want to try to get two fuselage swaths (one each right and left) out of each 50" width of cloth.

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page updated 21 February 2005 all text and graphics copyright (c) 2005 HP Aircraft, LLC