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Sep 19, 2020, 08:35 AM
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All foam Martin Mars


Build thread for Ivan Pettigrew's Martin Mars 120" wing span. Plans bought here https://store.flying-models.com/cata...oducts_id=1888

Will be built from white and pink foam and fuselage will split between F7 and F8. Outboard wing panels will be detachable between R8 and R9. This will make this large plane transportable in a small car.

Not sure about power yet, will need to get a better idea about the weight of the model. Looking for 80 watts per #. 300 watts to 500 watts per motor 3 or 4 cell battery pack.

Will be a 2020 and 2021 project, should be finished in spring of 2021.

If you have never worked with foam would recommend this book "Building with Foam" https://parkflyerplastics.com/cart/i...roducts_id=644

This is not my 1st foam plane build, have built several. Three all foam examples pictured, all with split fuselages.
1. Al Master's 81" (now 94") DO335, 2500 watts total, 26#
2. Sig Bravo 2200 watts, 25#
3. Phillip Noel 88" Connie 80 watts per pound, 16.5#

Lets get started
Tony
Last edited by cornj; Sep 20, 2020 at 09:48 AM.
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Sep 19, 2020, 09:06 AM
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Making templets, first step is to trace all ribs and formers etc. on poster board. Using a glue stick I glue the poster board traces to 1/8" MDF. The MDF is then cut out and will be used to cut the foam.

The fuselage and wing will be made from several sections, it's much easier and more accurate to use shorter sections between templets. The hot wire must start at the right and left templet at the same time and must finish at the right and left templet at the same time. Usually there's a large and small templet used to cut out a section so the hot wire needs to run faster on the large templet to keep up with the small templet. This is very important for accurate sections.

The wing will be made using 4" x 24" x 48" white polystyrene https://www.foambymail.com/polystyrene-foam.html
The wing will be made up from 5 sections, the blue tape shows the outboard wing split. The wing sections will also be hollowed out to reduce weight and create conduit for motor and servo wiring.

Hollowing out the wing, I use a 7/32 sharpened rod heated with a Wagner adjustable heat gun set to 1300 degrees F to get a hole for running the hot wire. You can see I use a book spine to accurately track the rod.

4th pic shows red line for 1/4” washout at TE of wing tip


Also use paving brick for weight (cheap)

Tony
Last edited by cornj; Sep 20, 2020 at 11:40 AM.
Sep 20, 2020, 10:15 AM
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Gluing wing beds together with white PU glue.

Glues used
PU - polyurethane glue (Gorilla)
PU white
PU brown (mainly used for sheeting)
Wood glue
Foam safe CA+
CA thin and + (not used on foam only wood)

Outboard wing panel wing tube - using Estes BT50 rocket tube and 24MM carbon fiber (CF) tube, each wing tube will be 19.5" long. Will be using one of the following to act as a stop for the CF wing tube, Estes tube coupler, centering ring or engine mount. Wing panel ends will be capped with 1/8" light ply, will have a 1/4" dowel for anti-rotation and will be held in place with rear earth magnets (will also have a backup staple shaped wire held in place with rear earth magnet, mounted top of wing, when painted should not be noticeable)

Due to the cost of shipping in the future will use wing tube sets from TNT Landing Gear https://www.tntlandinggear.com/ or Gator RC https://gator-rc.com/

Also pic of my foam squaring board.

Tony
Sep 20, 2020, 10:31 AM
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Fuselage sections will be cut from stacks of 2" pink foam. I use Camie 373 https://www.foambymail.com/C-/camie-...-adhesive.html or Loctite 200. If using the 200 spray you must keep the can 16 to 18 inches from foam, any closer will melt the foam.

I build a carboard box 24" high to fit the 2" foam blocks for spraying the glue, this stops the overspray and gives the 16" to 18" depth if using 200 spray. I spray both pieces to be glued together.

The above sprays will not impede the hot wire
The 4' ends of the pink or blue foam should be cut back about 6" maybe more, this part of the foam sheet is not usually flat, you need to check for flatness.

Tony
Sep 20, 2020, 11:16 AM
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Covering the wings and horz/vert stabs, these are cut from white foam and will be fiberglassed so they will need to be covered with something, white foam does not sand out well without a lot of prep
1. cover with 1/8" sheets of pink or blue foam then fiberglass, sands well but does not stand up to hanger rash (very cheap)
2. cover with brown medium weight paper (used to protect counter tops and floors during construction) dose not sand out well for smooth finish (very very cheap) The DO335 and Bravo above are covered with this paper.
3. 1/8" or 3/32" balsa works well but is expensive and hard to get lite weight balsa.
4. Red Oak paper backed veneer, what I will use for the Martin Mars, not to expensive, easy to get, Home Depot $25 for 24" x 96" sheet. Sheet thickness is about .022 inches includes paper backing . I was using Okoume veneer but can no longer easily buy. Big question is weight, see pic, left to right, medium weight balsa 6g, Okoume veneer 5g and red oak 6g. The red oak veneer should work. I've used Okoume paper backed veneer on a stick plane see pic worked great, gave a very hard surface, hard to damage.
Will be glued with brown PU glue

Tony
Sep 20, 2020, 11:38 AM
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The fuselage will split between F7 and F8 for easy transport BUT that will make a big problem IT WILL LEAK WATER might as well drill a hole in the hull. The pic shows the 2 ply F7 and F8 pieces that will be used to bolt the fuse together. What I plan on doing is installing a 1/8" O-ring cord on F8. This cord will be glued (CA or CA+) into a 1/8" channel that I will make with my Dremel tool. I hope this works. 1/8" cord from McMaster Carr https://www.mcmaster.com/o-ring-cord...-cord-stock-7/ $4 for 10 feet.

All wood parts will be sealed with WBP

Tony
Sep 20, 2020, 02:51 PM
Arrowhead
The Mars is an awesome bird!

Can’t help but think... if she’s going to ship water anyway, what about a functional water drop?
Sep 21, 2020, 12:03 AM
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tspeer's Avatar
Quote:
Originally Posted by cornj
Covering the wings and horz/vert stabs, these are cut from white foam and will be fiberglassed so they will need to be covered with something, white foam does not sand out well without a lot of prep ...
4. Red Oak paper backed veneer, what I will use for the Martin Mars, not to expensive, easy to get, Home Depot $25 for 24" x 96" sheet. Sheet thickness is about .022 inches includes paper backing . I was using Okoume veneer but can no longer easily buy. Big question is weight, see pic, left to right, medium weight balsa 6g, Okoume veneer 5g and red oak 6g. The red oak veneer should work. I've used Okoume paper backed veneer on a stick plane see pic worked great, gave a very hard surface, hard to damage.
Will be glued with brown PU glue...
The veneer sounds very interesting. If using a built-up foam structure instead of a solid foam core, I wonder how thick the sheet foam skins would need to be, since the veneer would provide the strength?

How do you spread PU glue over a broad area and achieve a thin even coat before it starts to foam?
Sep 21, 2020, 07:43 AM
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CF105
Yes I've been looking at 12v water pumps, maybe used with 16oz glow tank. Mix red food coloring in the water for effect.

TSPEER
See pics
I use tile spreader 1/16" square notch and credit card room keys depending on what I'm using to cover, paper covering seems to work better with 1/16" square notch. Why I use brown PU glue, setup time is quite long. If doing a small area white PU glue would work, setup time is 1/2 the time of brown.
The fuse foam thickness on the stick plane is 1/4", my guess is could be reduced to 1/8" if looking for very lite weight.

Decided on using Estes BT50 centering rings as stops for wing tubes.

Foam cores being glued back together using white PU glue.

Tony
Sep 21, 2020, 03:11 PM
Official Boat Bum
Eddie P's Avatar
Cool! Lots of interesting things going on here.
Sep 21, 2020, 03:34 PM
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ErcoupeEd's Avatar
I find your building with foam interesting!
I have and have read Keith Sparks "Building With Foam" book quite interesting
I am considering one or two airplanes from foam.

A 100 inch wingspan T-28 Trojan which I have plans for and also a Wendell Hostetler Sirius with a 102 inch wingspan, which I aalso have the plans for.
Sep 22, 2020, 12:24 AM
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I'm signed on for the watch Tony
Sep 22, 2020, 08:25 AM
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ErcoupeEd

Last year got a built wood T-28 from a friend, see link. Converted to electric power but did not fly well. Gave the airframe away https://www.rcscalebuilder.com/forum...188&PN=1&TPN=1

If anyone is thinking of doing a foam build and has not used foam before than I would recommend building a simple ugly stik. The building with foam book indicates that you need to practice cutting scrap foam, my opinion just build a practice RC plane. This is what I used for 1st practice RC foam plane, the green camo twin started out as the "Classic Ugly Stik", I never thought it would fly (fly's good) and made lots of building mistakes. The plane weight is 14.5# very heavy, but still fly's.. Classic stik plans https://outerzone.co.uk/plan_details.asp?ID=7108 had them printed at Staples. For 1st foam build constant cord wing with flat sided fuse (fuse has no compound curves) is highly recommended.
2nd foam practice plane, white camo, about the same size plane but got the weight down, 9.5# This plane uses low wing twin wing from Outerzone and Telemaster fuse, vert and horz stab.

Gone flying

Tony
Last edited by cornj; Sep 23, 2020 at 07:29 AM.
Sep 23, 2020, 01:11 PM
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ErcoupeEd's Avatar
cornj,
I have a lot of experience at foam cutting.
I have made many foam wings for sailplanes.
Also built a complete Glasfugel Alpina sailplane ( 4 meter) from foam.
Tried several different wing airfoils.
Eppler 205 flew great!
I added flaps and enlarged ailerons slightly
Literally 1/2 the AUW of the original kit!
Kit prices on the Alpina go for $400 to $800, I built the foam version for about $80.00 ( less hardware and electronics)

Ed
Sep 23, 2020, 06:15 PM
Registered User
Tony I was thin king about your fuse split/water issue. Here's a thought for you to consider. Where you have the formers F7 and F8 set up for being able to screw the 2 fuse halves together you could put another sub former that is solid (as in no holes and completely sealed) behind these 2 formers that completely seal off the 2 fuse halves. That way you would only have a minimal area that could possible get water in rather than the whole fuselage. You could still run the rubber seal to prevent water getting in to that small cavity but having the other formers completely seal off the fuse should work.

Obviously you'll have servos down in the tail that the wires will need to get forward so in that case you could use something like a multiplex plug so that when the fuse is joined the 2 connectors get connected together.

I hope all of that makes sense haha.


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