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Old May 07, 2010, 06:46 PM
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United States, WI, Fond du Lac
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Build Log
Ki-46 Dinah in heat molded foam.

The Mitsubishi “Dinah” was designed to fill Japan’s need for long range high speed reconnaissance. The version I choose to model is the Ki-46 III with it’s streamlined, glassed in nose. In my opinion, it’s one of the most beautiful twin engine aircraft produced during WWII.

I had built the prototype pictured below last year and while it flew well, it was underpowered and a bit overweight (like it’s builder!) It also suffered from structural problems in the fuselage. I had used construction methods typical of WOWPLANES with an EPS fuselage covered in glass cloth. But on the Dinah there’s not much fuselage between the wing and the cockpit openings and I snapped the nose off the prototype several times. I usually don’t paint a model until after initial flight testing and the Dinah was so beat up by then that I never did paint it. I eventually removed the landing gear and flew it as a belly lander. It never had enough power to hand launch so I used a catapult. Even underpowered it was fun to fly and looked great in the air. I always knew I would build another and now is the time.

WS 52”
Length 37”
Power (2) Turnigy 2209 28turn 1050kv
Battery 3S 2200mAh
AUW goal 40 oz.
Wing loading 14 oz./ft^2
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Old May 08, 2010, 02:41 PM
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United States, WI, Fond du Lac
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Fuselage

I’ve been intrigued by Harpye’s method of molding depron and since I still had the nacelle molds for the prototype, I tried it and really liked the result so I decided to try it on the fuselage. I had the plans I drew up for the prototype and it was a simple matter to modify them for the new mold.

First I cut out blanks from pink foam (using the prototype's templates) and glued them to a ½” plywood backing cut to the plan side view. I left a gap between the blocks of foam so I could slide in cardboard temples giving the cross sections. Then I sanded the foam to the cross sections. My experiments with the nacelles showed that it’s better to make the mold over long and cut the shells back to the desired length than to try to mold them to the finished length.

The foam (FFF in this case) was covered in PVC packing tape and cold formed to a curved shape. The foam was then taped tightly around the form. The whole thing is then baked in my foil faced foam oven for 15 minutes at 200-210F. After cooling, the tape can be cut and the formed shell removed.
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Old May 08, 2010, 03:12 PM
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The oven is 16” x 16” x 48”. There’s a sheet metal shelf running down the center about 42” long. The gaps on the ends allow air to circulate. The heating element with fan is scavenged from a small space heater and sits under the shelf, mold sits on top. The temperature is regulated by watching the temperature gauge and manually plugging and unplugging the heater. Once it’s warmed up, it runs about 20 to 30% of the time to maintain the temperature.

I found while experimenting with the nacelles that if I covered the inside and outside surfaces of the shell with glass cloth that the result was very stiff and strong. But trying to coat the inside with one piece of cloth was difficult. The solution I came up with was to cover the mold with heat shrinkable window film that I got from the hardware store. That way I could smooth out the glass cloth on the mold (1/2 oz in this case), coat the inside of the shell with epoxy, and replace the shell onto the mold while the epoxy cured and have a nice smooth continuous layer of glass on the inside.

Once the inside epoxy cured, the shells were trimmed to size. Some minimal internal structure was added including two 4mm carbon tubes that will hopefully strengthen the forward fuselage. The shells (still with the PVC tape on the outside) were then glued together.

After the glue (white Gorilla glue) set I peeled off the tape, glued on a block of foam for the nose and sanded the outside smooth. I covered the outside with a layer of 1 oz. glass cloth.

The finished weight is 115 g (36g foam, 45g internal structure, 34g fiberglass and glue). This is 60 grams lighter than the prototype at this stage and MUCH stiffer.
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Old May 11, 2010, 10:19 AM
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United States, WI, Fond du Lac
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Tail

I tried something new, for me anyway, on the tail surfaces. The rudder and stabilizer are made from two layers of foam glued to a piece of .020" walnut veneer. The advantages are that the veneer strengthens and stiffens the tail surfaces. It also provides a visual centerline reference while shaping them. The hard veneer provides some dent resistance to the edges. And lastly, because the veneer is the same thickness as the Dubro hinges I used, I was able to precut spaces for the hinges, leaving perfectly centered hinge slots once the movable surfaces where cut away.

I started by cutting out the foam and veneer pieces and laminating them with 3M 77 spray adhesive. After shaping them, I cut away the movable sections and covered them with 3/4 oz. glass cloth and WBPU. Once they where finish sanded, I glued in the hinges.
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Old May 11, 2010, 03:50 PM
Build it again, Sam!
Harpye's Avatar
Germany
Joined Oct 2004
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WOW --- Really beauty!!!! thought about this plane several times but every time I was unsure and finally decided for another bird...... I should have given her a trial!!!

Congrats on this results ... ( curiously waiting for progress.....)

Hansjoerg
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Old May 12, 2010, 10:21 AM
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Quote:
Originally Posted by Harpye View Post
WOW --- Really beauty!!!! thought about this plane several times but every time I was unsure and finally decided for another bird...... I should have given her a trial!!!

Congrats on this results ... ( curiously waiting for progress.....)

Hansjoerg
Thanks Hansjoerg,

I am so impressed with how well your technique works.
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Old May 12, 2010, 10:37 AM
My plans are in my blog
Rusty-Gunn's Avatar
Kotzebue, Alaska
Joined May 2006
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I like your laminating idea. Your build looks very nice.
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Old May 12, 2010, 10:39 AM
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Seattle
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Another WOW, great job on the fuselage and tail surfaces.
With fiberglass inside and out it should be really strong.

A question, you indicated you use FFF, is that the regular approximately 1/4 inch thick stuff, mine is blue in color, yours looks pink.
I like your oven, I built one similar to yours last year to shape flat wing panels into cambered ones, again a great job, keep up the good work!

Ivor
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Old May 12, 2010, 11:28 AM
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Quote:
Originally Posted by fuelsguy View Post
Another WOW, great job on the fuselage and tail surfaces.
With fiberglass inside and out it should be really strong.

A question, you indicated you use FFF, is that the regular approximately 1/4 inch thick stuff, mine is blue in color, yours looks pink.
I like your oven, I built one similar to yours last year to shape flat wing panels into cambered ones, again a great job, keep up the good work!

Ivor
The foam is 1/4" Corning FFF. It is pink. I had some left over from residing the house and it works fine. It's a little easier to cold form than 6mm depron and a little less dense. You have to sand out the ripples and the tape marks after forming. I like the way it sands but it's soft and dents easily. Immediately after sanding I give it a coat of WBPU to harden the surface and reduce hanger rash. A final light sanding before the fiberglass is applied is all that's need to give a nice smooth finish.
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Old May 12, 2010, 11:51 AM
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Seattle
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Hey, thanks for the quick reply.
I know what you mean about hanger rash, all my depron airplanes end up with it. I'll have to give this technique a go sometime soon.

Ivor
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Old May 12, 2010, 11:59 AM
Build it again, Sam!
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Germany
Joined Oct 2004
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Fuelsguy - need a plan designed for this technique???

The FW190D ( pic attached) in the foreground is prepared for share and several guys here in forum have tried it ... so there is knowledgebase and experience available - and as they are not able to answere your questions... I'll try...

I KNOW what you mean regarding the hangar ... this is just one wall in my flat...

I would not go for glass as the fabric structure takes much resine to get filled ( and would coat only the outher side - the coating is able to take the surfaces tension and prevent it from breaking - the inner surface is protected by the opposite half of the fuse and so you just would have to install some spacer ( inner structure) to keep the stability and distance of both halves) -Glass + resine without bagging could become HEAVY ... Iprefere tissue from the laundry ... and some thinned waterbased PU Laque ... the surface is smoother and easier to repair at comparable hardness to 20 gr glass!!! and lots cheaper!!!
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Last edited by Harpye; May 12, 2010 at 12:04 PM.
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Old May 12, 2010, 12:14 PM
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Seattle
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Harpye,
Thanks for the offer on the FW 190. I design most of my own planes so I'll have to think about the 190, although I will confess to buying a few foamies lately. I like your thoughts on WBPU and tissue, I have tried WBPU and 3/4 oz fiberglass for belly protection.

Ivor
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Old May 12, 2010, 12:29 PM
Build it again, Sam!
Harpye's Avatar
Germany
Joined Oct 2004
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Double or triple layer of tissue is better

Currently I try myself in EPP again ... - I usually design my planes on my own but mostly refer to original warplanes of WWI and WWII
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Old May 13, 2010, 11:48 AM
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Laser Method

I had cut an extra set of foam wings when I made the prototype so I finished those with a 6mm carbon tube spar, covered with brown paper and WBPU. They came out fine but since that time I've learned of the so called "laser method" of wing construction and I thought I'd give it a try. It may look like the most complicated way to build a wing conceived by the mind of man but it really wasn't that bad. See the photos below.

1.Start by laying out the wing on a flat board. I used some melamine shelving.
2.Attach the rib blanks to the board with double sided tape.
3.Trim them to the wing outlines. I also made a horizontal cut to get them all the same height.
4.Attach the upper board to the ribs with double sided tape.
5.Fix templates and hot wire cut the upper wing surface.
6.Cut the slot for the spar. Mine will be 2 layers of 1/16 balsa to make a slot for the 1/8" ply dihedral brace. Outboard of the nacelles the slot has 1/8" balsa glued in place.
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Old May 13, 2010, 12:47 PM
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Laser method continued

7. Glue on the top skin. I used 1/16" balsa and white Gorilla glue. you apply the glue and the skin and then replace the cut off top and pile weights on it. This applies clamping force only where the ribs are. Very slick........
8. Apply templates and cut the lower surface. Cut holes for wiring and install spar, LG mount, and block of foam for aileron servo. I cut the wiring holes free hand with the HWC. The LG mounts are 1/16" and 1/8" plywood. This was a somewhat tedious cut and try affair but I traced the parts so the other wing should be easy.
9. Sand the bottom of the wing including the trailing edge balsa skin. Glue the bottom skin on. Like before, use cut off board to apply clamping pressure.

I really should have left the wing attached to the board to sand the LE and attach the 1/4" balsa LE but curiosity got the better of me and I had to cut the wing loose to weight it and see how stiff it was.

The method promises laser like accuracy and it delivered that. The wing was absolutely straight and I've never had a more perfect trailing edge on a wing. The wing was very stiff, even without the LE attached. I could flex and twist the foam wing a bit (which is not a necessarily a bad thing).

The laser wing, as expected was a bit heavier. 92g vs 81g per half wing. The foam wing is almost ready to paint and the balsa wing will have to be filled and/or covered in tissue. Both need the tips added and the foam wing has yet to have all of the LG mounting parts installed. I estimated the laser wing will end up about 230-250g and the foam wing 200-220g.

The laser wing is straighter and I know I'll be able to get a smoother finish than with paper over foam. However, under flat paint I won't notice the difference.

Was is worth it on a wing this size? It's a toss up. But on a larger wing, the weight benefit of not having a solid foam core begins to give the laser method a weight advantage in addition to it's superior accuracy. Part of the reason to try this (as well as forming the fuselage) is that I have a giant scale DH-91 on the drawing board that has long narrow wings. I need accuracy, low weight, stiffness and a surface good enough to paint silver. I hoped this method would fit the bill and so far it looks good.

I wasn't able to find out who invented this method, anybody know?
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