Self Pulled Canopy

I've done this a couple times now using the "bake a 2-liter bottle" method and it has worked out quite well. What I did:

1. Form the canopy shape out of something. Anything that you can make an impression with in sand. I have used wood (overkill maybe) and foam (much quicker). Leave the edges extended, so you have plenty of smooth material to cut off.

1. Fill up a big coffee can with sand and make an impression of the plug in it. The container should be METAL (it will get hot), and the sand should be just damp enough to hold its shape. Too much moisture in the sand will steam up and ruin your casting.

2. BE CAREFUL. MOLTEN LEAD IS DANGEROUS! Having said that, melt some lead in a pan or a pot (that you will no longer use for food service). A propane torch is usually sufficient, with patience. Lead wheel weights are usually free from the auto-repair shop (you have to melt/pour a few times to get the steel bits out). Have it right next to the sand as you melt it, so there's no un-necessary movement of the molten lead.

1. Once the cast lead cools, you can wipe Bondo over it, to fill the cast surface, and sand it down smooth. Imperfections show up in the finished product. A bare, cast-metal surface works, but makes the canopy look foggy.

2. Now, cut the ends off of a 2-liter bottle (I've used small water bottles for micro-models) and construct a jig from some scrap wood that will hold the canopy up tight inside the bottle. Support it in the oven, and bake the whole thing. I have found that 450 for 10 minutes works for me. Watch it closely - shortly after it's "perfect", it's "ruined". I've used a clear bottle, and a green Mountain Dew bottle - the green one looks really good against the yellow on my GeeBee.

It sounds like a lot more trouble than it really is. I can do the whole thing in a day now.

Dan.

I've had decnt luck skipping most of your steps. Just carve a plug the shape you want out of balsa and sand it smooth. Cut one end off a pop bottle of the appropriate size and wedge the wood plug inside. Then carefully heat it with your heatgun. May take you a couple times until you get the hang of it but after that it's less than an hour from starting to carve until finished canopy.

remove my-wife to reply :-)

Tried, yes.

I have actually built a box out of wood, with a frame to hols the plastic, and a bity of waste pipe to put the vacuum cleaner hose in, and holes in the top to let the suck out, amd actually, I did pull a pretty decent canopy off it, but there is a way to go yet.

Then I got sidetracked.

Tips I got given were

(i) leave the first canopy on the former, as it becomes a better surface to pull teh real one over

(ii) temperature and materials is everything.

(iii) shrinking pop bottles doesn't work. Urban myth I reckon ;-)

I've done two, one largish and one quite small.

It was fun. The UltraSport 40 canopy is still flying. The little Wonder got drilled by a big Ugly Stik and is awaiting a summer bonfire.

There is a limit to how much I was able to persuade the pop bottle plastic to conform to my plugs using a heat gun. I can see where baking the thing in the oven might work better.

Marty

Guess i missed who posted this. If you really think it doesn't work then you need to quit thinking before you hurt yourself. Next time you're in northeren Indiana give a shout and i can show you. remove my-wife to reply :-)

I had somewhat the same problem - the plastic from pop bottles only wants to shrink so much, and then melt. I can forsee problems trying to pull a really tall canopy.

I got around the problem by tipping the nose up, so the plastic didn't have so far to shrink from the very top to the base of the nose. You can also use the rounded top of the bottle to get kind of a head start in that area.

You really can't rely on just the shrinking. I used to do a fair bit of molding and the plastic in the bottles (PET-G IIRC) can be cut and laid flat and then clamped with a couple pieces of wood. Heat it and pull it over the mold. Best if you mount the mold on a short post smaller than the mold. Stretch the plastic down past the mold, allow it to cool and then trim.

AH! That may be where I made some of my mistakes.

I stapled the pop bottle to the bottom of the mold. I didn't apply any pulling power to it at all.

I can see how your technique would produce better results than I got.

I tried the technique twice and got two canopies out of it anyway. Both planes looked and flew OK even though the canopies didn't match my plans.

Marty

Its ok. I built a vac forming box, that does work!

Maybe the UK pop bottles are made out of recycled cowshit or something.

Probably constipated ones. No wonder they're so mad.