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Cutting and Prepping Lightweight Foam Cores

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Old 10-03-2013, 02:30 PM
  #26
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I not sure if 1/64 birch plywood would weigh more than .023" birch veneer, i suspect the glue used to laminate the plywood would add considerable weight. *Prices based on a 4' x 8' sheet are as follows. *I paid 69.99 plus 9.99 shipping for the birch 79.98 total. *A 4' x 8' sheet of 1/16" balsa (16 sheets 6" x 48") plus 15.95 shipping (BalsaUSA) would be 97.07.

Woodcraft sells this product from a lumber mill in either North or South Carolina (I think) and since you're from SC,,,,ROADTRIP!!!,,LOL!!

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Old 10-03-2013, 05:02 PM
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Quote:
Originally Posted by jp1961 View Post
An*excellent*book on the topic of making foam core wings is "Radio Control Foam Modelling" by David Thomas. *Shows all sorts of tips and*techniques, paperback. I don't think foam core wings will ever be as light as built up wings, with the possible exception of vacuum bagging carbon fiber cloth over foam, but that would be an expensive proposition. *Each method of building has it's advantages and*disadvantages. *
Jeff
Actually foam wings can be just as light maybe lighter then built up, just depends on how far one is willing to go with them.

Bob
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Old 10-03-2013, 05:13 PM
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Jeff,

Im really interested in the birch veneer idea. I found several sources for the 4x8 stuff. I did the exact calculation of cost of veneer vs balsa as you did. I am really interested in cheap. In high school and college, decades ago, I used poster board, contact paper, wall paneling, corrugated cardboard, etc to be cheap. All kind of heavy. Now that I can buy the stuff I still like cheaper, especially for the large airframes where maybe a few extra ounces will not be a major factor.

Did you ever compare the weight of the birch veneer to balsa? I wonder what it would be.

The .023 veneer is less thick than 1/64 ply which is the thinnest I have used. How is it for hardness, ding resistance, and strength compared to balsa? The wing I am planning is a thick airfoil like yours also. 42" root 24" tip x 50" span per panel. I like the idea of a one piece skin with no edge gluing and sanding. I would cover with .75 oz glass and resin before painting.

Gary
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Old 10-03-2013, 05:36 PM
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MattK,

Some questions from your post if you can answer.

Bow. Is the entire bow made from the fiberglass rods? It would be very light compared from the wood cross beam and music wire springs I made. What do you use for cutting wire?

Gravity pull strings. Are you using a swing arm to pull the strings for different size chord templates?

Templates. I want to make my templates from cheap ply also. Where do you get your teflon tape and what is it used for normally?

Fiberglass skin. I have been thinking of making my own skins from glass as you described. What weight of glass, layers, bagging technique are you using. How much more heavier is it. Could it be cheaper in overall cost of finishing?

Thanks

Gary
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Old 10-03-2013, 06:21 PM
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MaverickJetJockey,

How do you control the depth of cut with the 12AWG wire forms. Do you have any photos of the forms?

Gary
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Old 10-04-2013, 08:37 AM
  #31
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Quote:
Originally Posted by Viper1GJ View Post
Jeff,

Im really interested in the birch veneer idea. I found several sources for the 4x8 stuff. I did the exact calculation of cost of veneer vs balsa as you did. I am really interested in cheap. In high school and college, decades ago, I used poster board, contact paper, wall paneling, corrugated cardboard, etc to be cheap. All kind of heavy. Now that I can buy the stuff I still like cheaper, especially for the large airframes where maybe a few extra ounces will not be a major factor.

Did you ever compare the weight of the birch veneer to balsa? I wonder what it would be.

The .023 veneer is less thick than 1/64 ply which is the thinnest I have used. How is it for hardness, ding resistance, and strength compared to balsa? The wing I am planning is a thick airfoil like yours also. 42" root 24" tip x 50" span per panel. I like the idea of a one piece skin with no edge gluing and sanding. I would cover with .75 oz glass and resin before painting.

Gary
1/64" birch ply is about .015". Birch veneer is 0.023", thicker and possibly heavier than 1/64" ply. These materials weigh around 50 lb per cubic foot

Both materials are heavier than 1/16" medium balsa (8-10 lb stock). Contest balsa is around 4-6 lb stock or around 2/3 the weight of medium balsa and 35-40% the weigh of birch materials (equal volumes)

Both birches are far more ding resistant (but not ding proof) than either balsa even if balsa was covered with 3/4 ounce cloth. There is no need to cover the 1/64 birch ply. Seal it, fill it and paint it directly with no covering of any kind required. It will finish a bit heavier than either balsa even if glassed but it sounds that you may not be as weight conscious as others. But the veneer may need some form of covering not so much for hiding grain but to hold the grain intact and keep the veneer from splitting
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Old 10-04-2013, 08:58 AM
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Quote:
Originally Posted by Viper1GJ View Post
MattK,

Some questions from your post if you can answer.

Bow. Is the entire bow made from the fiberglass rods? It would be very light compared from the wood cross beam and music wire springs I made. What do you use for cutting wire?

Gravity pull strings. Are you using a swing arm to pull the strings for different size chord templates?

Templates. I want to make my templates from cheap ply also. Where do you get your teflon tape and what is it used for normally?

Fiberglass skin. I have been thinking of making my own skins from glass as you described. What weight of glass, layers, bagging technique are you using. How much more heavier is it. Could it be cheaper in overall cost of finishing?

Thanks

Gary
Enire bow is made from the glass rods. Inexpensive, flexible but stiff enough.

Yes the gravity pull stings are pulled by a swing arm. Simple ratios are determined as to where to place the pull cord hooks, based on the root to tip ratios

Adhesive backed teflon tape is standard issue from McMasterCarr. You can get the high temp adhesive (silicone) or the low temp (acrylic). Either works fine. The great thing about the tape is that it is stretchable so it can be made to conform perfectly on the airfoil of the template. Sand the template smooth with 320-400, clean well to remove all dust and work the tape from one end, stretch it, and watch it lock down smoothly. There is some technique involved but it's not difficult.

Glass skins I've played with start with 4oz cloth cut on bias. A second orthogonal layer of 2 oz is added from root to about 1/2 span. That's for wing panels of around 550 sq inches. BUT it isn't the final answer. Still playing with that technique. It's heavier than contest balsa, carbon tissue and epoxy on the inside and Esaki tissue, nitrate dope on the outside.

Another technique I have just started to play with is to fully form a balsa/carbon/glass composite skin directly on a foam core. The foam core is made from heavy and very strong foam like 4 lb density stock. it's is used only as a mold not as a wing. The wing foam is from 1 lb density or less. Basocally, I use sheet teflon as my release on the foam core and go from there. This also is not fully fleshed out so I don't want to go into detail right now. It promises to be a poor man's way to composite wings....
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Old 10-04-2013, 11:43 AM
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Here are a couple of shapes I use to cut out servo holes and retract mounting plate holes. I control the depth of the hole by the shape of the wire form I use and the template I cut out. I use heavy card stock such as the cardboard off the back of a pad of paper for the template. I cut the exact size of the hole I want to make. I tape this template onto the wing core with the cutout in the exact location it needs to go. When I am ready to make the cut, I pull the trigger on the gun, push the wire form down into the foam until I bottom out against the template. I pull the wire form to the other end of the template and pull it out of the wing. I remove the cutout piece and have a cleanly cut hole. To get the hole to the exact size, I do not let the wire form get too hot or it will melt excess material and over cut the hole. I control this by moving the wire form through the foam as fast as it will go. A little practice on a sample piece of foam will give you the feel for this.

Lloyd

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MaverickJetJockey,

How do you control the depth of cut with the 12AWG wire forms. Do you have any photos of the forms?

Gary
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Old 10-04-2013, 01:31 PM
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I picked up two cans of 3M Polystyrene spray adhesive (#78) from Menards for 10.99 each. *One can was barely enough to attach the Birch veneer to my wings. *Each wing is 31" long (62 total), 17" wide at the fuselage (minus ailerons) and 14 1/2" at the tip (minus ailerons). *If you plan on making wings bigger than this buy two cans. I wouldn't fiberglass over the Birch, not needed. *My wings look like they were molded from solid fiberglass with cheap Towerkote iron on film. For some reason the Towerkote wrinkled on the fuselage but the fuselage has 2" holes to lighten it. *The Towerkote held up great over the birch on the wings, go figure?

One word of caution, my profile fuselage is two pieces of 4mm marine ply epoxied to 1" thick Extruded Poly Styrene (pink or green foam), even low temperature film will erode (melt) foam with no wood covering.*

Other veneer options would be pine or western red cedar (if your as weight obsessed as say a super model,,,LOL) Woodcraft sells clear pine but it's way more than Birch. *You may be able to find western red cedar via Google. Wow, sounds like your building a monster!! Any bigger and you'll be able to sit in it!!

Ding resistance is outstanding,,,I usually damage my planes more getting them in and out of my walkout basement than flying them. *

This plane sat in the bed of my pickup with no tonneau cover, exposed to sun, washboard dirt roads, dew, *when I was at work, so I'd say I gave this construction technique a thorough durability test.

Jeff

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Old 10-04-2013, 03:58 PM
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pic of the engine
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Old 10-05-2013, 05:50 PM
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Jeff,

Did your .023 birch veneer have a paper backing? I see lots of sources for veneer with paper 10mil backing. Did you glue it on with the backing?

I did some calculations and it seems that using 1/64 ply skins is only slightly heavier than using 1/16" balsa but is more expensive depending on price of balsa sheets. However it is easier because it is in larger sheets. The birch veneer is really heavier than both ply and balsa plus the weight of the paper if it is used.

I really would like to avoid edge gluing balsa sheets. However the largest 1/64" sheet I can find is only 4' x 4' and would have to be edge joined also.

The price of the birch veneer is really good but I'm concerned about the overall weight.

Gary
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Old 10-05-2013, 07:52 PM
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If you tape the balsa sheets together, use laminating resin and a vacuum bag, there is no reason to edge glue anything and there is also a weight reduction when spline sanding the wing for trueness.

Bob
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Old 10-05-2013, 08:21 PM
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Gary,

Balsa is the lightest wood you can use for skinning a foam wing. Obeche is used in europe, because either they can't get balsa or it's too expensive. *From a weight perspective here is a list of woods from lightest to heaviest, balsa, cedars, pine/spruce, obeche, mahogany then birch. *Yes, the birch veneer I used was paper backed. *You can get veneers minus the paper, but I haven't tried those. Another option would be vacuum bagging carbon fiber cloth over the foam, this would be as light as balsa, with much better ding resistance. SIG sells rhino paper for skinning. *So you have lots of skinning options, each having advantages and disadvantages. * Vacuum bagging removes alot of the epoxy resin from the cloth (either fiberglass, carbon fiber, kevlar, etc), if you decide on that route.*

Jeff
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Old 10-06-2013, 07:30 AM
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This is the process that I use for applying wood wing skins to foam cores. I made the videos quite a few years ago so the quality isn't spectacular. I really like the durability of 1/64 ply skins. There are lighter alternative though.

https://vimeo.com/2357551

This one might help for doing composite skins with the mylar transfer method.

https://vimeo.com/2368007



Last edited by wyowindworks; 10-06-2013 at 07:33 AM.
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Old 10-06-2013, 01:48 PM
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I ran a spreadsheet exercise today just for fun and was surprised at the results.

The project is a scaled up turbine powered tailless design called Lancer by Mike Oser from Texas. Photo of glow powered Lancer and wing plan posted below for reference.









My project will be called Sabre XLT for "Extra Large Turbine" Approximate dimensions are on the spread sheet. I estimate the empty weight to be 30-35 lbs. With 22.9 sq ft wing area that gives about 21-24 oz/sq ft wing loading empty and 28-31 with full fuel and smoke oil. I found values for wood density/cubic ft, prices, and estimated shipping on the internet.



Hope the chart is readable. It posted very small.

I had just assumed that balsa was the most expensive method of sheeting but the spreadsheet shows it may be the least expensive. I like Bob's method of joining the pieces with no edge gluing needed prior to bagging

I would vacuum bag the skins using epoxy so I assumed the glue weight and cost would be a wash. Also the birch veneer has a paper backing which I do not know the weight, but it would make the veneer heavier than calculated. Since my expected wing loading is going to be light for a turbine plane I have not been too worried about the extra weight so much. I plan to finish the wood with .75 oz glass and paint.

However, it seems balsa would be the best option from a weight, cost, delivery, small size of pieces for less waste etc. I like the 1/64 ply for ease and stiffness, but it is costly to ship or I would have to drive 5 hours and back to get it.

What do you guys think of this analysis?

Gary
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Old 10-06-2013, 03:41 PM
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Bob,

You said this on the other thread.

"Now for the wing skins, I use medium density 1/16 balsa 3" X 48" sheets, I true the edges and tape them together, next I cut 3/4 oz. fiberglass cloth strips 1" wide and place them on the opposite side of the tape directly centered over the seems and utilizing EZ POXY 10 epoxy laminate system purchased again from Aircraft Spruce & Specialty I laminate 2 plies and allow to cure. After cure I lightly scuff over the laminated areas with 320 grit abrasive and clean all the dust away. Using the same epoxy system I sparingly roll the epoxy onto the cores and skins place in the shucks, and into a vacuum bag not exceeding 5 hg throughout the cure cycle (overnight)."

How do you true the edges of the balsa sheets?

When you laminate the 1" strips do you bag then or just let them cure in the open. Post #37 sounds like you vacuum bag the strips. If you bag them do you stack the joined skins or bag one at a time. I was thinking if you bag them you could stack them with some plastic between.

Could you more explain your process.

Thanks
Gary
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Old 10-06-2013, 04:01 PM
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Quote:
Originally Posted by Viper1GJ View Post
Bob,

You said this on the other thread.

"Now for the wing skins, I use medium density 1/16 balsa 3" X 48" sheets, I true the edges and tape them together, next I cut 3/4 oz. fiberglass cloth strips 1" wide and place them on the opposite side of the tape directly centered over the seems and utilizing EZ POXY 10 epoxy laminate system purchased again from Aircraft Spruce & Specialty I laminate 2 plies and allow to cure. After cure I lightly scuff over the laminated areas with 320 grit abrasive and clean all the dust away. Using the same epoxy system I sparingly roll the epoxy onto the cores and skins place in the shucks, and into a vacuum bag not exceeding 5 hg throughout the cure cycle (overnight)."

How do you true the edges of the balsa sheets?

When you laminate the 1" strips do you bag then or just let them cure in the open. Post #37 sounds like you vacuum bag the strips. If you bag them do you stack the joined skins or bag one at a time. I was thinking if you bag them you could stack them with some plastic between.

Could you more explain your process.

Thanks
Gary
I true them with a long straight edge and an X-Acto blade, then I touch them up with 80 grit and a 24" flat sanding block. On the 1" strips, I lay them up and and allow them to cure on their own before scuffing, but you could bag them as long as you use a release media on top of the layup.

Bob
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Old 10-07-2013, 02:50 PM
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Gary,

West system epoxy sells a small booklet called vacuum bagging techniques. *It's a pretty good reference on getting started in vacuum bagging. * The initial cost can be high (vacuum pump (although some people make one from a*refrigerator*pump), vacuum bag, epoxy, peel ply, fabric ((being fiberglass, carbon fiber, etc).

Jeff
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Old 10-07-2013, 03:30 PM
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There is also a vast amount of technical knowledge base right here in this composite forum free for the learning, all one needs to do is open their eyes.

Bob
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Old 10-21-2013, 04:43 AM
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Gary, I stumbled on your thread and you ask some good questions. My answers may have been covered by others, I don't know, I didn't read most of it.

I use a bow to cut foam. It is a simple fiberglass reflector post from Home Depot, about 3/8" in diameter and about 45" long, and flexes just right for applying tension on the cutting wire. The posts are about 6 feet long, The extra material is used as the ends of the bow, glued at about 110 degree angle and reinforced with some scrap 1/16 ply

I suspend my bow from a shelf to counterbalance its weight. I use the gravity technique with pull strings for moving the bow through the foam

My power supply is the Burt Rutan Special from Aircraft Spruce. Simple and inexpensive to make....Works fine. Search under Foam Cutting Supplies.

My templates used to be rather elaborate polished phenolic. What a pita to make those......Well there had to be a simpler way. Now I use simple plywood templates. The edges are lined with teflon adhesive backed tape. Nothing is slipperier than teflon so this works better than anything else I've tried regards to templates. Of couse a CNC cutter uses no templates and is likely more accurate but I'm not about to spend a few 000's for a foam CNC cutter.

Regarding glues, for the past 25 years I've used laminating epoxy. Since I vacuum bag all my panels, very little is required. I use epoxy with about 90 min pot life which gives me plenty of working time. Gorilla glue is also used by many, except, you have got to work fast or it kicks off right before your eyes.

On skinning, I've used regular cardboard stock from Staples years ago. It builds rather heavy tho. Many moons ago I used 1/64 birch ply veneer from Aircraft Spruce, but used it only once. Sure wings were bullet proof but they were also far too heavy.

Nothing but 1/16" contest grade balsa since. It also allows taper sanding the balsa for further reduction in weight. But if you like to paint stuiff as I do, finishing balsa can be a pita. Time honored nitrate and Esaki tissue over balsa simplifies things and keeps things light, but it's an awful lot of work, work that I do willingly on all my competition models where lightness and weight specs count the most

Lately I've been experimenting with casting fiberglass onto plate glass first to form and cure the stock sheets, vacuum bagging that to the smooth finish of the plate glass. Then epoxying that onto foam directly. G10 sheets are similar, except I control where to place the strength and control its weight. The glass smooth finish is hard to beat and it is done with almost no effort. I haven't worked an optimal glass weight versus strength yet, but it's coming. It isn't lighter than balsa but does offer considerable ding resistance and much less work in the long run.....
Hi Matt,
Perhaps a preformed sandwich is worth considering.
Could be done flat or onto a form with mylar.
When I was molding fuz's in the late 80's I ended up using Rohacell 51 - was the lightest and hardest (good compressive resistance) that we could find at the time.
Anyway I decided to look it up again as I was looking at Herex the other day.
I found this;
http://www.rohacell.com/product/roha...les-range.aspx
Of particular interest is the IG-F.
At 31Kg/m\3 (that's 2lb/ft\3) it's 1/2 the weight of super light contest grade balsa.
I'm sure it's not cheap but it must be worth investigating.
A skin with 0.75oz to 1.5oz glass on the outside and 0.5/6oz on the inside (or maybe 0.3oz carbon veil) c/w 1mm 31IG-F would be good for a 2m pattern wing.
Just a thought !

Brian

Last edited by serious power; 10-21-2013 at 04:49 AM.
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Old 10-21-2013, 09:25 AM
  #46
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Quote:
Originally Posted by serious power View Post
Hi Matt,
Perhaps a preformed sandwich is worth considering.
Could be done flat or onto a form with mylar.
When I was molding fuz's in the late 80's I ended up using Rohacell 51 - was the lightest and hardest (good compressive resistance) that we could find at the time.
Anyway I decided to look it up again as I was looking at Herex the other day.
I found this;
http://www.rohacell.com/product/roha...les-range.aspx
Of particular interest is the IG-F.
At 31Kg/m\3 (that's 2lb/ft\3) it's 1/2 the weight of super light contest grade balsa.
I'm sure it's not cheap but it must be worth investigating.
A skin with 0.75oz to 1.5oz glass on the outside and 0.5/6oz on the inside (or maybe 0.3oz carbon veil) c/w 1mm 31IG-F would be good for a 2m pattern wing.
Just a thought !

Brian
Brian,

One company that has responded with small 10cm square samples was Clarasonic. Distribution from Thailand and California. Thailand,...99/1 Houyai, Ampher Muang, Phetchabun, Thailand-67000; Tel:668 1753-0298.................US: 28 Washburn St, Simi Valley, CA 93065, Tel: 805 526-7568

IGF is one sample they included. They also included several others, as well as cross grained balsa in 1mm thicknesses. I don't know if it would be strong enough as a wing skin or if it would shave any weight. Worth building a panel and testing it. For a bipe wing, the panel should be able to hold about 40 to 50 pounds at about the MAC. The panels may need a spar....I think a simple 1/8" x 3/8-1/4" balsa spar that covered about third to half the span would be plenty if needed. Or 010" carbon strips top and bottom instead of balsa

On preforming, I've been thinking about using the actual foam core, lined with mylar sandwiches top and bottom (mylar, glass, rohacell (or balsa), glass, mylar), and vacuum forming the skins directly on the foam first. Then skinning the foam cores in the usual way. I haven't actually done it but have wanted to for some time. Actually haven't done much lately due to health issues. I think the 1 ounce cloth (out) and 0.3oz carbon (in) combo should be enough, but I think it might be heavier than standard balsa and doped paper.

Rohacell cost (last I saw) was around 70$ (??) (I forget the area but it was large enough for the typical wing panel). That's about 4X the price of 4-6 pound balsa, but if it's strong/light enough, should be worth it.
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Old 10-21-2013, 10:40 AM
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Hi Matt,
This time it will be balsa but at less than 2lb/ft that Rohacell has to have possibilities.
Re the form I was thinking a core allright but one cut from heavy form so as to take more vacuum - less resin !

Brian
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Old 11-01-2013, 10:47 AM
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We built this Sea Fury Reno Racer called Furias about 15 yrs ago. The whole plane weighs 65 lbs. The wing is around 115 in wingspan. We hotwired it in three sections. The centre and two outers. Cutouts for servos etc, were done with the wire in soldering gun method. Assembled and covered with 3/32 balsa, then covered in three layers of carbon fibre. Firs layer went out 1/3 of the wingspan, second layer out 2/3 and the last layer, the full span. It was vacuum bagged with a layer of heavy teflon or mylar, can't remember which, to give a slick smooth finish. We built it super strong to stand 200mph+ speeds. Has a Quadra 200 R 200cc motor of 24 HP. Never did get to the races as they were canceled up here a month before scheduled. It id flown at funflies and such.
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