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Old 01-11-2009, 12:21 AM
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1/8 scale Foam RC model (card model inspired)

My dad won a RC plane back in 1989 but after putting in a few hours on it never did anything else. About 9 years later I did some work on it, then let it sit for another 9 years or so before I finally decided it needed to be finished and did it. Then it sat for another 2 years before I finally ran into a guy who was into RC aircraft and I asked him to help me fly it.

Long story short I got a little more interested in RC aviation since then and came up with an interesting application of card modeling principles. I came up with the idea of using a hotwire foam cutter (not very original I know) to cut out the sections of fuselage out of blocks of foam by tacking the formers from paper model kits to either end.

I am currently designing a airplane I one day hope to build and fly (1:1 scale). I think I posted something about over on the Z place. Anyhow it's a small single engine elliptical inverted gull wing hot rod. I am building a 1/8 scale model of it and here is what I have so far.

The first pic is part of a sheet of 2 in thick foam insulation purchased at Lowes, the second shows a hotwire cutter I bought at a local hobby shop with a template of the front half of a wing section (remember elliptical so I have to segmet it) the third shows the main spar combined with the fuselage former at that station (1/16" plywood) along with the outer main spar (1/16" basswood) and the sections of foam that I have cut glued in place. The fourth is a somewhat dated Rhino screen capture of the aircraft. There have been some design changes to the design since then, mainly the lengthening of the tail and enlargement of the tail surfaces to promote better stability.

I was in the hobbyshop earlier today and they had some glue specially made for joining foam together but like a dummy I decided to wait. I needed it to glue 2 layers of foam together so it would be thick enough to cut fuselage parts out of. But I can still cut out the wings. The hotwire cutter has no temp control on it and it gets really hot really quick so if you dilly dally cutting it will melt a large swath and ruin the part. If I gave a closeup of the leading edge sections cut so far you could see how they improve as I've learned a little better technique. Still not very good but these are probably just test pieces for now.

I am not sure yet if I will use a balsa leading edge ( I will definately use balsa on the trailing edge) hence the section with the leading edge cut back. That's it for now.
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1/8 scale Foam RC model (card model inspired)-000_0229.jpg   1/8 scale Foam RC model (card model inspired)-000_0230.jpg   1/8 scale Foam RC model (card model inspired)-000_0231.jpg   1/8 scale Foam RC model (card model inspired)-frontleft.jpg  
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Old 01-11-2009, 12:40 AM
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You can build a larger hot-wire cutter really easily.
You'll need some scrap lumber, a length of NiChrome wire (although .025 safety wire will work in a pinch), a small tension spring and a model train transformer (Lionel works great) wire, alligator clips

Build a U-shaped frame out of 1X2 big enough for whatever you're going to cut. Attach one end of your wire to a nail or screw on one side - attach the other end to the spring and attach the spring to the opposite side of your frame, making sure the spring keeps the wire very taut. Connect one lead from the transformer to the fixed end of the wire. The other lead goes to an alligator clip.
By adjusting the location of the second clip (ie. the heated length of wire) and the 'speed' on the transformer you have wide latitude in adjusting the temperature of the wire - which determines speed of cut and indirectly smoothness and accuracy of cut.
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Old 01-11-2009, 01:44 AM
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He's Diseased!...,

Will,

You've got one of the worst cases of "homebuilder gottas" I've ever seen. Not to worry, it's one of the greatest of all diseases and has only one cure...,

I suggest Jack Lambies book on composites if you haven't got it already:

Amazon.com: Composite Construction for Homebuilt Aircraft: The Basic Handbook of Composite Aircraft Aerodynamics, Construction, Maintenance and Repair Plus, How-To and Design Information: Jack Lambie: Books

I built a thermal regulator for hotwire cutting out of 12 V battery charger and a 1500 Watt light dimmer switch. Wire the dimmer into the primary winding of the transformer. Dimmers can be used on battery chargers just as long as you don't try running any inductive loads on it (inductive as in dc motors) so you still have a battery charger when needed. As per Shrike you'll need the "U" shaped cutter nichrome or a good substitute is aircraft safety wire. Also get a shop vac otherwise you'll have foam bits all over everything you own. Quick prototyping with foam is very addictive. Don't even bother with covering just spray it with latex house paint an go fly it (electric powered, right?).

Keep us posted often...,

+Gil

P.S. Nice little Sky Cycle design...,
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Old 01-11-2009, 09:55 AM
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Thanks for the encouragement guys. I do have that book and 2 others on composites besides plus Dan Raymers simplified aircraft design for homebuilders and his college textbook on aircraft design. I also have a number of other book on aircraft construction. Unfortunately I think it will probably take another 10 years to finish the design and another 10 after that to build it.

For now I'm going to stick with the hotwire I've got but when I build a bigger version (after the design is more finalized) I will definately build a bigger temp controlled hot wire so thanks for the tips.

Yes Gil this will be electric.

Just a little more background on the design. I was inspired by Mike Arnolds AR-5. He broke the world speed record for aircraft weighing less than 600 lbs (pilot included) by flying about 213 mph with a 65hp engine!

He wasn't an an engineer he just studied books and did everything he could to make his plane as aerodynamically clean as possible. His plane was only 20-22 inches wide at the cockpit and since I'm a little claustrophobic mine is going to be a little bigger. I am shooting for 300+ mph so I decided to put in a 250 hp engine. It's going to be a modified Subaru enigine built by Crossflow Aero Corp. The engine normally has the radiator mounted on the bottom but my design is so small that I'm going to stick the radiator behind the pilot P-51 style. I'm hoping to get some thrust out of it like the P-51. I'm also using P-51h airfoils on it.
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Old 01-11-2009, 03:21 PM
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Gil thou king of everything that fills or gets sanded (or anyone else who knows). I am probably going to need to do some surface prep of the foam and add some wing root fillets so do you know a good light weight filler I can use on this foam?

Thanks in advance
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Old 01-11-2009, 04:12 PM
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Lightweight Filler

Quote:
Originally Posted by Willja67 View Post
Gil thou king of everything that fills or gets sanded (or anyone else who knows). I am probably going to need to do some surface prep of the foam and add some wing root fillets so do you know a good light weight filler I can use on this foam?

Thanks in advance
Try Red Devil Lightweight Spackle. It's the electric foamies filler of choice...,

If not already a member of RC Groups I'd recommend it:

RCGroups.com: The ABCs of Radio Control - Aircraft, Boats, and Cars!

+Gil

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Old 01-11-2009, 06:12 PM
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Mix-it-yourself microballoon slurry was what we used to use on sailplanes. (The sign shop I used to work at did glider repair as well)

You'd be amazed at how much glass microballoon you can mix into a little bit of epoxy.
A thin mix works as a surfacer and a really really thick mix will stand up a few inches on it's own.

Before working with epoxies, be absolutely sure to get some non-powdered nitrile gloves. Life-threatening epoxy allergies can come on with zero warning, even after you've been exposed to lots of it with no ill effects.
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Old 01-11-2009, 06:49 PM
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The general plan at the moment is to hotwire the fullscale version then glass it, so maybe I should start getting familiar with the epoxy and glass now but your warning echoes some of the reasons I have been avoiding that. Maybe when I build the bigger RC version I will glass it and start getting some experience. Thank you both for the advice.
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Old 01-11-2009, 07:36 PM
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It's not that scary, but it's like a great many other 'hazardous' materials in that a little bit of precaution goes a long way towards safety.
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Old 01-31-2009, 11:41 AM
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It's starting to look like an airplane

This foam stuff is a lot of fun to play with. In true ADD fashion I have been designing parts and then having to come up with innovative ways to fit the adjoining parts instead of designing it all then building it. But it is actually coming out quite well so far.

The first pic shows all the pieces so far created. I'm going to cut off the top of the main spar and make an access cover that goes from the firewall back to cockpit.

The second pic shows the little slot in the bottom that the wood frame member goes in (though it's a little hard to see on the nearest piece. In the middle piece you can see the tunnel that the wires for the wing servos go through. I'm going to put a servo in each wing instead of making a fairly complicated linkage mechanism with the aileron servo in the fuselage.
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1/8 scale Foam RC model (card model inspired)-000_0232.jpg   1/8 scale Foam RC model (card model inspired)-000_0233.jpg  
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