Foam wing cutting question

I'm currently cutting out a foam wing. Built a couple of cutters (big one and a small one) and a suspension system (pulleys, cord and a weight) and it seems to have worked pretty well overall. Got the tranformer and it seems to work great (built from Radio Shack parts. Got the plans for this sometime back...pretty easy) . Had one bad core (my first try) that I had to pitch, but then sort of got the hang of it and came up with some keepers.

So, the question. I need to cut out the wing core to accept a spar, support ribs and landing gear blocks. What sort of jigs do I need to use as guides for a hot wire to cut the wing core up to accept the spar and ribs and such for the gear blocks? I can't seem to come up with a way to satisfactorily hold the wing core in place, keep the guides in place and still use both hands on the cutter. I've been noodling it all evening and have a couple of ideas (none I particularly like) so I thought I'd ask if there's some standard procedure for doing this before going off on a lark.

Thanks for any help.

You're absolutely correct. There are some standard procedures and Jigs for accomplishing what you want to do.

I don't know what width bows you have but you may need to make a mini-bow for cutting spars and spar slots.

To cut the spar, you'll need a pair of wood or aluminum guides, that had been milled to the correct width to give you a 1/8" cut. Remember, you have to allow for 'foam loss'. If you start out with a 1/8" gap in your guide, the slot in the foam will be substantially wider.

It is possible to use the same guides to cut the slot for the landing gear clips, if you measured carefully and don't put too much pressure on the taped-down guide.

Make very careful measurements to assure that your spar slot is cut 90° to the wing root. If you don't get that right, you'll know it as soon as you test fit the cores to the spar.

The gear clips aren't such a big deal as far as squareness goes but neatness still counts. If you're handy, you can bend up a music wire frame that will fit into a soldering gun, for cutting away the foam where the basswood mounts clip in.

Remember, the spar slots are cut with the wing cores stacked together. The gear-clip slots can be cut either one wing half at atime or both at once. The latter is the prefered method. (Be brave. It's only foam!)

If you do everything right, you should always have both hands on the bow (When cutting wing cores) or one hand holding the work in place while the other cuts with the bow (When using a mini-bow for slots or trimming offsets.)

To do a good job, you'll also need several different sizes of weights. I like steel plates, the heavier the better. For starters, get a flat steel plate about the size of your average wing core and glue a wooden handle in the middle. That one is for hold holding your foam blank on the bench. After a while you'll find it so useful around the shop, that you'll wonder how you ever lived without it.

There are lots of little tricks that you can use. Everyone seems to have their own techniques. I may (Or may not!) still have some videos on cutting foam wings. If I can locate them, I'll loan them to you.

Bob Hunt is probably the most talented foam cutter that I've ever met. I've never met anyone who cut nicer wings than he did. Crist Rigotti cuts nice wings these days. His cores are very clean and I am very, very happy with the ones he's done for me.

I'm sure that either one of those guys might be willing to give you a few hints.

Bigs,

So, tape on the guides and weight the core down sans cradles?

Formica makes nice templates.

All of the cutting must take place with the cores in the cradle. (Now, repeat that 20 times! LOL!!)

Don't EVER cut a core thats 'Bare' unless you are removing the foam where the landing gear block goes. The cradles are what keep everything square. Flatness and squareness = goodness!

PM me and we can talk via phone. OK?

Mr. 1969 NATS
Bob Lampione

Randy,

You might want to give Bob Hunt a call, he's been doing this stuff for years. I'm sure he could help.

Regards,
Mr. 1969 NATS
Bob

Hi Randy:

What Bigs said....and one of the things that has worked for me.

To make cutting templates for the spar and gear rib slots, go to an office supply place and get a few 12" rulers with the metal ruling edge that is along one side. The smooth and narrow edge is perfect for the wire.

I use a similar edge for my vertical guides for blanking the foam.

Cutting the slots in plywood or phenolic templates is very time consuming and the result is not so good. With the rullers, you can spike them down just like the foil templates. Once you have the cutting allowance figured out, it is very quick.

Just for reference--I found that cutting the spar and gear rib slots to be the most difficult part of the whole process.

LAST EDITED ON Oct-14-04 AT 08:50 AM (CDT)
 
Randy,

One of the ways us old converted RC types cut out the foam for things like servos, landing gear blocks, spars, and such, is to make a little bitty foam cutter the exact shape and size of the cut out or slot that you want to make.

For instance, for a landing gear cut out; I cut a piece of 1/4" plywood to a size of 2" X 4". Then bend a piece of .027 music wire to the width (with square corners) of the LG block. With a drill press, I drill .027 holes through the plywood at the same spacing. Now, epoxy in the U shaped wire with the U sticking out the bottom of the plywood the depth of the LG block. Hook up your power supply across the wire ends and practice on a piece of scrap foam to get the right setting for heating the wire. Use a balsa guide pinned or taped to the foam and after the wire is heated up, just drop it down on the end of the landing gear location and slid it along the balsa straight edge to the marked other end of the landing gear location and pull it straight up and out of the foam. You now have a perfect square cutout for the LG block. I undersize the U a little so I get a tight fit.

Of course, once you make up the little LG cutter it can be used for future ones and the same for the spar, etc.

I make a note on the plywood what the setting on my power supply is, so in the future I don't have to retest to get the correct heat. Each individual cutter will have a different setting.

Feel free to contact me directly if you want to discuss this further.

Dee

Whoa! Wait, Randy, what's this?

You can't cut an elliptical wing out of foam!

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"And we were poor too. Why, if I wasn't born a boy, I'd have nothing to play with!" -Rodney Dangerfield

>Whoa! Wait, Randy, what's this?
>
>You can't cut an elliptical wing out of foam!
>
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>
>"And we were poor too. Why, if I wasn't born a boy, I'd have
>nothing to play with!" -Rodney Dangerfield

Paul Smith

I've seen it done, and quite nicely at that.

Step 1 ---- cut the elliptical plan form into the foam blank. Press-fit the offcut back to the "good" blanck.

Step 2 --- Cut tapered core as usual.

Step 3 ---- A little technique and finnesse with a sanding block.

Wakkerman's FAI combat jobs were built this way, quite nicely at that.

Larry,

And how did you get in the elliptical taper? I suppose if you were willing to basically carve a core by hand, you could vacuum bag it. No need for cradles.

I'd rather use StuntRib and build it up.

Stop by the house and we'll cut the foam. Bring the cradles.

Howard,

I may do that if I don't come up with a satisfactory method at my shop. I like the idea of the metal edged rulers as guides. I came up with a jig to do this, I just haven't tested it yet. I'll run some tests tonight on some scrap foam. I want to be able to do this again, so I'd like to figure out how.

Curt,

I came up with an effective jig for making the blanks. Worked really well. I don't see how you can cut the spar and rib stuff with the cores in the cradle without cutting up the cradles. Seems I'll need those.

Larry,

Yea, yea, that's right. and elliptical foam wing. That will work.

That's plane comes next.

Yes...I havent figured out how to do a good job of cutting the slots without cutting the shucks as well. I have just made a second set for one wing. THe other I vacuum bagged so I didn't care about the shucks that much.

LAST EDITED ON Oct-14-04 AT 07:51 PM (CDT)
 
Oh come on..!

When you cut the slots for the gear clip you are going to make a 1/8 inch wide slot from the LE back to the already installed spar stub. One slot on outside edge end of the spar stub and one near the middle.

You use a guide and make two passes with your wire. You brace the wire against one side of your guide and cut in till you hit the spar, then you slide the wire to the other side of the guide and pull the wire out, cutting the other side of the slot.

If you do your spar and gear-clip slots with the cradles on, it's easy to keep everything square and the cradles are NOT RUINED. If you draw yourself a diagram, you will see what I'm talking about.

If you want to cut with the wing uncradeled, you are completly on your own. It's a pain, it's almost impossible to keep the guides secure enough to rely on because of the shape of the wing and squareness becomes a matter of chance at best. All in all, you'll probably screw things up and revert to the method that I have described. Thats MY experience anyway and I threw out a lot of junk wings.

If I were you, I'd probably take my friend up on his offer and go get a foam cutting lesson. You'll save some time and a bunch of materials.

After you've cut a few wings, you'll probably understand why some of us think that having them cut FOR us, is such a bargain!

Well, I got the spar and rib slots cut tonight. Came out unusually well. I'm still a bit spooked that I had zero problems with it.

Thanks for everyone's help. Wasn't as tough as I thought it would be. I used a jig made from a bunch of 1/2 square poplar I had hanging around. Just built the guides into a sort of rack that sat on the bench and held the core stable. Took a bit to make this contraption, but it worked better than I expected.

Thanks again.

Good show!!

Yes this exactly true!

BUT..if you try to cut all of the slots at once, without installing the spar into its slot before cutting the gear slots, things get flimsy.

I've done it your way too.

I agree that its not good to do it without the shucks.

I think thats what I tried to say above.

Curt,

Well, I cut all the slots at once. Guess we'll see how it comes out. They are pretty straight. I'm building up the sparr and ribs now. I'm using a composite spar made from balsa and 1/64" ply along with some carbon. Should be light and strong.

Hi Randy:

I'm sure you will take enough care to make it come out better than all right--your reputation for detail supercedes (sic) you!!

;o)

Curt,

Thanks for the vote of confidence.

Being from the school of ultra low tech, I use a bread slicing knife to cut my spar slots and gear clip slots. It's thin bladed but not *really* serrated.
Don't laugh all you experts out there, it's quick and very accurate.
Bill <><

Yep--I've been known to use an electric carving knife on foam.

I've used it to carve any number of prototype car and boat parts to some 2xsize foam and paper machei Canadian Geese!

Hi Curt,
Any word on the lacquer? (at work, can't access my home email)
Bill <><

Not yet--I'm going to the PPG place in the AM..

Work?? (spoken in my best ever Maynard G. Crebbs voice)

This note is for anyone reading this string who might be interested in cutting their own foam wings. Good-To-Go Models has produced a video on cutting foam wings that runs two hours and covers everything you need to get started. (Sounds like we need to do a follow-up on putting in landing gear blocks!) Anyway it covers things like making a bow, cutting templates, power sources, finding foam locally, coring wings and even a few lame jokes. It was shot in my shop so I have copies available for sale. They are $15 postage included. I will take a personal check or a money order. Money order I ship same day. Personal check I wait for it to clear the bank.

Bob Furr
15872 Farnam Street
Omaha, NE 68118

I should probably say that I've cut a lot of foam wings in the past, I just hadn't done it in a long time and had never done one with a wing mounted gear. I didn't have much problem cutting the blanks, it was just jigging up to cut the spar slot and gear clips that was the question. Wasn't actually pretty easy once I'd made a jig to do it.

BTW, the spars and gear clips are in the cradles now drying. Tomorrow I'll put in the actual gear blocks and such. This plane is going to have a rather different sort of gear setup, so putting in the gear blocks will be interesting. Hope to sheet the wings tomorrow or Monday.