Anyone done this to make a Supra spar?

[davekra]

I'm contemplating the process of cutting the cores for the Supra spar.
I was thinking I'd cut it with the hot wire to keep the cuts straight, but what if I make the spar cuts first. Then glue the blocks back together with a light mist of 77. Then cut the airfoil like normal.

The way I figure it the hotwire cuts are a bit too wide and the wraps of glass wouldn't make up kerf. I could use a knife to cut it after the airfoil but keeping the knife from wandering is my concern.

I've got to wait till the stuff from ACP get's here to get the spar width right so right now I'm just cleaning the basement and watching Blazing Saddles on AMC.

Thanks,
davidk

[jirvin_4505]

Hi David

Seeing the various methods of cutting the cores I have wondered the same myself.

I was thinking cut cores with hot wire after determining the knerf allowances. The core web can than be cut from some offcut. I was thinking this is possible as only the bottom angle of the web has to have some resemblance to the airfoil and when I think it through it really has to be flat to glue against the bottom web. The top of the foam web gets sanded.

I cannot recall reference to allowing correct allowance for the glass wrapping but I have seen cores cut by knives and bandsaws. Surely widely differing knerf allowances.

Cutting a web out of a foam core with hot wire certainly makes a simple opperation of the job.

Cheers jeff

[flyonline]

David
I've used 3m77 to hold various things together while cutting and sanding - mainly templates and formers, but also for a slab sided fuse. I don't think you'll have much trouble getting them apart if you only mist enough to hold it gently together, though you could always use something like a piece of mylar sanded nice and smooth to open it up. However I'm not sure how a hot wire would cope with the glue. It may cause some difference in the cutting and could lead to troubles if your wire isn't hitting it at exactly the same time across the core so a test piece would be in order first.

Let us know how you go.

Steve

[davekra]

Thanks guys,
I'm sure if it came down to it I could use an off cut if I mess up. Your right about the difference in kerf's of the various methods. I'm not sure it really matters about the thickness of the glass. So the wing ends up .05" narrower, no big deal.

I'm just thinking about the jig's I'd need to cut it. The simplest I could come up with are combination squares on either side of the foam. As far as cutting through the 77 with the hot wire...it's not a problem. I've done it before on something cant remember what right now.

Getting ahead of the project, I bought some of the System Three water based poly paint from Nelson Hobby. I have hopes of it giving an "epoxy paint" like finish. Sounds like it should be a very hard paint that's resistant to solvents.

Thanks,
davidk

[jirvin_4505]

I have used a jig made from combination squares in a similar fashion.

I had the cores sitting on the wire in the beds and pulled up - pull some tension and switch on. This was done to avoid putting a significant wire burn in the beds. Probably could be avoided if cores were sitting on waste foam.

>3m77 .. agree hot wire cuts straight through it. I have used it when desperate to make up a core blanks to size before cutting.

cheers jeff