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Old May 18, 2001, 09:51 PM
Registered User
memphis, tn
Joined May 2001
24 Posts
Building Scale From Scratch

Hello,

I was wondering how people build scale from scratch models like the XB-70 in that column on this website. Do they use balsa?

I am interested in building a dual or triple mini fan 480 powered s-71 blackbird, but I am not sure what the best way to construct it would be.

The material I am leaning towards now is EPP foam. I was thinking about getting a big block and just start wittling with a dremel and maybe a hot knife or something, then maybe drill a few holes in it if it were too heavy, but this could get a bit tedious. Secondly, I was thinking about making some sort of a fiberglass mold and investigating getting liquid foam that would harden when cooled or something such as this. Does this exist in a feasible price range (I am guessing it has to exist to mass produce zagi's, etc.)?

I also saw a picture of someone who had built a scale B-2, and it was really cool. What construction technique did they use?

Thanks,
Austin Carpenter
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Old May 19, 2001, 11:37 AM
Registered User
Canada
Joined Nov 2000
6,980 Posts
a few points... the sr71 seems to attract beginners (no criticism intended) but there are few worse choices for a first attempt. witness the fact that mention using 3! minifans.. what's the third one for ?
EPP foam is crash resistant, being very spongy, it's also difficult to shape into anything scale... try shaping , cutting, or sanding a piece as an experiment.
Molded planes can be magnificent... so is the skill and time required to make the plug and molds.
A first plane should be a simple one that is relatively easy to actually complete in a satisfying way.. there must be thousands of 1/2 finished models rotting in basements.

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Old May 19, 2001, 01:43 PM
EDF Head
Haldor's Avatar
Stavanger, Norway
Joined Feb 2000
7,999 Posts
You can take a look at my L39 project at www.rcwizard.com/haldor to see how I did my first scale EDF. I used green foam to sculpt the fuselage, hotwired then sanded.

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Old May 19, 2001, 02:06 PM
2014 EDF JET JAM We be Jamming
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
6,801 Posts
The Boeing JSF has been done and it is a simple subject to model, as long as you aren't gonna enter it in the Master Scale class. Keep it simple.

Here is my page on it: http://hometown.aol.com//kcox854272/x32.html

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Old May 19, 2001, 04:54 PM
Registered User
memphis, tn
Joined May 2001
24 Posts
Nice looking plane, Kevin.

Do you have any more info on how it was constructed and designed, like how you knew what wing thickness to use, and how to shape the curved leading edge?

What did you make the scoop on the bottom out of, and where did you use the balsa? What is the thickness of the wall on the bottom scoop? What about the thickness and material of the two rudders (i am sorry that i don't know what they are called, but i mean the two things sticking up on the back, stabilizers maybe?)?

How fast does this thing go?

Finally could you please explain the error you made, I don't really understand it.

sorry again about all of the questions

Thanks,
Austin Carpenter
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Old May 20, 2001, 12:49 AM
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memphis, tn
Joined May 2001
24 Posts
Thanks for your reply,

After doing some more reserach, I found out that the sr-71 is a really tough plane to do with the hard edges, orientation, and all. Maybe a boeing JSF or B2 would be a better choice. The JSF would be pretty hard to shape though.

What about shaping EPS foam? And, I have some (a little) experience making fiberglass plugs and molds, but I am not sure what you put into the mold once you have made it. Can one purchase foam which comes as a liquid that would still be light and relatively durable? Or, would the plane be made out of some other material such as fiberglass or kevlar?

Yep, this wasn't going to be a first plane. I am getting a zagi pretty soon for that, but since this may take a while to build I am thinking I could get started in not too long.

Thanks,
Austin Carpenter
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Old May 20, 2001, 09:46 PM
2014 EDF JET JAM We be Jamming
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
6,801 Posts
The wing thickness and airfoil, in my opinion, is a function of the planned flight envelope. So it depends on what you are trying to do. The X-32 on the page had a 6% symmetrical airfoil at the root and 8% at the tip if I remember right. Most of the LE of my models are fairly sharp, but it is because I use balsa triangle stock. This makes it easier for me to consistently shape the LE.

The X-32 in its final setup was probably about 80mph.

The error that I made was on the inlet opening. I made it about 70% of the fan swept area instead of the planned 90-100%. It still worked well.
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Old May 20, 2001, 11:49 PM
Registered User
memphis, tn
Joined May 2001
24 Posts
Thanks Kevin,

I am planning on building a small albatross, then once I get that working, a larger one.

To construct my wing, I was going to build a a foam wing cutter, make the shape out of possibly plaster, then cut the core with the foam cutter. Then, put a balsa support sheets on part of the foam wing, leaving most of it uncovered ([until painting], like what you seem to have done (if I am looking at the picture right). What balsa thickness did you use for those cut-off triangle like shaped supports?

It's now time for me to learn about airfoils and make a wing patter.

You all have been a great help.

Thanks,
Austin Carpenter



[This message has been edited by austin9000 (edited 05-21-2001).]
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Old May 21, 2001, 09:48 AM
Dude, where's My Plane?
JasonJ's Avatar
Green Lane PA USA
Joined Mar 2001
2,167 Posts
Austin,
I am a fellow EDF newbe.
I have used blue construction foam on sevral jets, is is very durable. It is also about 6 bucks a sheet and I am on my 4th air frame off one sheet. I cut it out with a razor knife, rough shape the edges with the razor knife, glue the layers together when I need more thickness, and sand it with 80 grit paper. When I am satified with the curvs, I use very fine sand paper to smooth it out. This stuff sands very fast but makes a mess. I have been using less and less blue foam and more white to keep the weight down. The white foam is harder to work because the beads are so coarse. I frame the wings with basla like Kevin, and basicly try to read every word he has ever written. I use a balsa spar or plate in the nose of the jets because this is where they take the most stress in the Jason style landings . This is also needed for a good anchor for the bungee launch hook. There is nothing like sending the nose of your plane and radio gear down the field with the rest of the plane still on the deck . The blue foam is the way to go for easy fast and very curvy shapes. You can crash it every time like me and jsut take the 5 min epoxy to the field in force . Oh, if you do use foam and dont already know, regular CA glue is corrosive to the foam, and so are regular spray paints , so make sure all your glue and spray paints are foam safe like Testor. Kevin suggested to me to make the duct work out of construction paper and to keep it very straight forward to start. Make sure you use a good round intake lip too. Fuel tube works well. EDF is funny because just trying to throw something together for testing is often the big mistake. You need to make the best of every advantage you can just to get airborne. Kevin was offering his F-18 plans on this board, I jumped on the chance to see the guts of one of his jets to learn more, I suggest doing the same . Good luck, keep us posted and dont give up! I love building these jets so much I dont even mind if they dont fly because it just makes me more determined and satisfied when they finally do! Sorry about rambling, too much coffee .
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Old May 25, 2001, 09:57 AM
Master of Desaster ;)
Christian Abeln's Avatar
Dreieich, Hessen, germany
Joined Jun 1999
118 Posts
Hmmmm think the B-2 plans over...
Were building one and trust me:
If you arent into mathematics you will have problems flying this baby without vertical stabilisers...
We calculated the hell out of our brains and it seems like we finally found a solution

just to look how complex such a thingy can be have a look at: www.jet-connection.de
click on projects then on B2...
sorry I did not manage to make an english version, but Im working on it

Chris
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Old May 25, 2001, 10:00 AM
Master of Desaster ;)
Christian Abeln's Avatar
Dreieich, Hessen, germany
Joined Jun 1999
118 Posts
I also saw a picture of someone who had built a scale B-2, and it was really cool. What construction technique did they use?

-> Maybe that was me
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Old May 25, 2001, 02:00 PM
Registered User
Palm City, FL
Joined Dec 2000
628 Posts
Chris Gould has B-2 plans available at Electric Flight International magazine (EFI) from Traplett. Had a couple of really good construction articles on it.

Anyone getting into EDFs should start reading his articles; he's a former jet jock and has a wide range of plans there from Speed400s to 14' B-52s.
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