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Old Feb 11, 2013, 05:16 PM
Theoretical Modeler..
plane_tech's Avatar
Joined Oct 2007
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Question
Built up or foam core???

Greetings all...

I know this is a matter of personal opinion to some extent, but I am in a quandary and figured I would ask the masses..

I am in in the middle of a new design of a 140" span electric glider and I can not decide on whether I want to make a foam core wing or built up.

Foam core, sheeted is by far easier. I have a hot wire set up and a vacuum bagging setup and could probably get all the panels (all 14 of them) cut in a day, prep another day and have them sheeted within a week.

On the other hand the built up wing would be a week or two of design work, an hour or two on the laser cutter, and a good couple of days worth of building. (Not that there is anything wrong with that.)

But all differences aside, I am still stuck on which way to go. Any suggestions? Examples?

Or maybe I will just get all crazy, build both and see which one flies better...
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Old Feb 11, 2013, 06:01 PM
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Foam core is the way to go...

Easier to build, less susceptible to hanger rash, probably less expensive, all for just a little bit of a weight penalty (maybe).

If you like building go with built up and enjoy the journey. If you want to fly foam core is the way to go...
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Old Feb 11, 2013, 06:10 PM
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Illinois
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Foam core always seems to take me so much longer. I prefer built-up because it's easier and faster to build, doesn't have the problems with cutting out and facing all the control surfaces, looks nicer, and is often lighter.

The only reason I could justify a foam-cored wing is that it would never have any sags between ribs so the airfoil would be more accurate.

Which might be all the justification you need for a 12' span bird.

Andy
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Old Feb 11, 2013, 07:15 PM
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If you have the templates, foam core wings can be built in hours if you have the right technique.
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Old Feb 11, 2013, 07:55 PM
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templates are easy to get / make... so would the rest. Having a readily available laser cutter has its advantages..
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Old Feb 11, 2013, 11:19 PM
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Having just completed another built up wing, I do like them, but yes they're more fragile, but they're lighter, unless steps are taken to lighten the foam wing, then they're almost line ball. That said, the amount of effort you'll spend lightening the wing, you'll almost be at the same point as built up wing.

Personally I like to work with balsa, and so will generally err on the side of built up. If however the plane is likely to have a hard life, or I just can't be bothered cutting ribs, I'll do a 'brick' foam wing, with no lightening and just cringe daily at that added (or more precisely, un-removed )weight.
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Old Feb 11, 2013, 11:24 PM
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I am too hard on planes to really enjoy a built up wing, so that's the reason for my bias. Plus each rib has to be straight, and I am NOT an accurate builder.
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Old Feb 11, 2013, 11:27 PM
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Then foam is perfect for you, but don't go pushing you innacurate ways on me buddy!

I was gonna have a quick dig at you for being too hard on your planes, but I remembered that it takes a big man to realise his foibles, and if you're a hangar rash king, you should probably build to suit. Me,I'm a complete klutz, but i'm all about the build. I take (some arguably OTT) steps to ensure my planes stay pretty, no matter how much I spaz out in the shop with them.
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Old Feb 12, 2013, 12:14 AM
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Curare, I am tending to lean in your direction. I like to build. It is as relaxing, if not more than actually flying to me. And I am finding that I am thinking on the same lines as far as the weight goes. The only foam I have readily available to me locally is white foam, which works, don't get me wrong. But after using the blue foam on previous projects, just does not compare.
Cutting the ribs is actually the easy part. Between Profili, Rhino and DraftSight, is where the harder part comes in. 140" is a lot of ribs, spars, shearwebs, etc. Its just a matter of time.
The accuracy comes as a part of the laser cutting, jigs, and an OCD issue with having a copious amount of clamps, squares, pins, and trial fittings. I like to design my wing so that it is almost self aligning to the point you could assemble the entire panel on a building board, adjusting every piece for an accurate fit, then go back and secure the joints.
But this is where the labor intensive part comes in. The last wing took almost a month to draw up, and I am not just getting it to the laser cutter... And with nothing airworthy in the hanger right now, I am itching to fly something besides Real Flight...
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Old Feb 12, 2013, 02:27 AM
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Thanks for everyone's input..

Its going to be combination... Main parts of the wing will be built up, with the fairings into the fuse foam core and sheeted... Should be an interesting project..
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Old Feb 12, 2013, 06:59 PM
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When I rebuilt the plane in my avatar (H9 ARF), the original wing was built up. I cut a new foam wing and weighed it as I went along. It came out about 1 oz. lighter than the original. Selection of materials could have swayed that figure. The whole plane is glass & epoxy. Color by Krylon (never use again) and clear coat of automotive polyurethane. 7 1/2 pounds RTF

I always thought a foam wing was / should be significantly heaver than a built up. Guess I was wrong in this case.

Ken
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Old Feb 12, 2013, 08:02 PM
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Quote:
Originally Posted by plane_tech View Post
Curare, I am tending to lean in your direction. I like to build. It is as relaxing, if not more than actually flying to me. And I am finding that I am thinking on the same lines as far as the weight goes. The only foam I have readily available to me locally is white foam, which works, don't get me wrong. But after using the blue foam on previous projects, just does not compare.
Cutting the ribs is actually the easy part. Between Profili, Rhino and DraftSight, is where the harder part comes in. 140" is a lot of ribs, spars, shearwebs, etc. Its just a matter of time.
The accuracy comes as a part of the laser cutting, jigs, and an OCD issue with having a copious amount of clamps, squares, pins, and trial fittings. I like to design my wing so that it is almost self aligning to the point you could assemble the entire panel on a building board, adjusting every piece for an accurate fit, then go back and secure the joints.
But this is where the labor intensive part comes in. The last wing took almost a month to draw up, and I am not just getting it to the laser cutter... And with nothing airworthy in the hanger right now, I am itching to fly something besides Real Flight...
Ok, there's a couple of things to consider here.

1. a 140" wing is a LOT of area, which also equates to a lot of volume.

You're right in that a foam wing would be less work, but you'll need to find (or make) a foam cutting table that can handle 70" panels. This can be a really hard task, not to mention that the longer the cut, the more succeptible you are to having problems like wire sag, and lagging wire (look it up if you're not sure).

Also, sheeting a 70" span is a freakin' pain!

If you do do it, I'd highly suggest that you honeycomb the wings to get the weight out. Use the lightest foam you can, as you'll need to sheet a wing that big, probably with a minimum of 3/32" to get enough strength.
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Old Feb 12, 2013, 11:47 PM
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Originally Posted by Curare View Post
Ok, there's a couple of things to consider here.

1. a 140" wing is a LOT of area, which also equates to a lot of volume.

You're right in that a foam wing would be less work, but you'll need to find (or make) a foam cutting table that can handle 70" panels. This can be a really hard task, not to mention that the longer the cut, the more succeptible you are to having problems like wire sag, and lagging wire (look it up if you're not sure).

Also, sheeting a 70" span is a freakin' pain!

If you do do it, I'd highly suggest that you honeycomb the wings to get the weight out. Use the lightest foam you can, as you'll need to sheet a wing that big, probably with a minimum of 3/32" to get enough strength.

The wing is a three piece wing, and with the taper breaks, the longest panel is 25". Easily done if I were to do a foam core.

This is one of the other reasons why I went with a built up wing. If I were to do this as a foam core, I would want to have access to a 4-axis cnc foam cutter so that I would be able to accurately and correctly add lightening holes to the foam.


On that note, I already have the center section 50% done... Easiest panel as it is constant chord. The outer panels on the other hand are going to be a pain.
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Old Feb 13, 2013, 12:19 AM
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Curare's Avatar
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Ahh, ok this is sounding like sailplane-ish to me.

In that instance, I would suggest you do a little research into wet bagged wings, if only to gain a nice smooth airfoil surface but also to gain the fidelity to the original airfoil that is required, to ensure efficiency.

Back in the day we did built up sailplane wings because that was really all that was available to us, we made compromises with the airfoil to make it easy to build and strong. This meant the airfoils were always thicker than we'd like, to allow enough structure to stop the wing folding, and we'd bear the issues caused by having an open bay wing (the aifoil between the ribs isn't the same as AT the rib) for lightness.

Nowadays we can build a strong wing (with a weight penalty) that is thin and has the correct fidelity to the original airfoil, for maximum efficiency.

Finally, there is an easier way to honeycomb a wing other than 3 or 4 axis milling ,which will produce a lot of mess.

check this.
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Old Feb 13, 2013, 12:48 AM
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Twin Boom, Three lifting surface, Electric sailplane to be exact....
I have a vacuum bagging setup, and that was a consideration with the foam core option.
It is really a toss up... still.

I think the airfoil can be kept thin for the required efficiency thanks to laser cutting, carbon spars, and the latest and greatest of the micro servos that have the same torque output as the old standard size units of 10 years ago.

Even if it is a built up wing, a fully sheeted wing will give the required smoothness. Add in some lightening holes in the sheeting behind the spar, and the strength has not been compromised, and the sagging effect of iron on covering has been minimized... at least in the theory that is in my head.. still gotta test that one out.

and that is a dead link.
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