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Mar 03, 2018, 09:32 PM
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Build Log

Mini p-40 23 inch

Well this is my first build log so go easy. I've done lots of scratch-built foamies , so many I lost count. But the ones that fly best are usually warbirds. Going back to my youth here - my first plane was a Cox control line Warhawk, those were the days.

Anyways I'm a little crazy calling this a build-log since every plane I make is different, and I run into different setbacks and successes each time. I am especially fond of the mini models at 2S - wingspan near 20 inches, that's the sweet spot that I can fly anywhere. My Nakajima Ki43 Oscar was built pretty much to scale at 23.5 inch wingspan and flew great, really stable and light, so I am doing a similar build with this p-40.

Quick summary of that build and this one. No plans, starting from 3-view, set tile print to proper wingspan, crop the areas of 3-view I need, print - cut - tape them together and glue to poster paper before cutting out as formers. Fuselage : X-method I call it as you see, with as few as possible cross section (egg-shape) formers, then wrapped with 1mm depron (that's the difficult bit). My Oscar has some 2mm and 3mm wrapped up front to give it more strenght, probably not needed so I will try with 100% depron wrapping and see if a paper model helps me in any way.

Wings for Oscar were dollar tree with paper left on, as was the X-fuselage. Wings then had a strip of foam about 3mm high and a 3mm CF rod, and 1mm depron over the top to form a basic airfoil. I've had great luck with KF wings on wings so I will go this route this time, and covered dollar tree foam peels too easily so externals will be paper removed with a bit of epoxy as a very thin layer in some cases.

I am out of 2000 kv motors at around 25 grams, so I will use a 1700Kv one and either go to 3s if it handles the weight, or go to a bigger 7-8 inch prop.

Enough talk, here's a few pics.
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Mar 03, 2018, 09:42 PM
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First setbacks

Well this hobby, like everything else, has gotten crazy expensive just in the past couple years, so it's a good thing I enjoy scratch-building. And there are few warbirds available in this size, and what's out there is not in ARF form.

My first setbacks today, I always think the NEXT one will go smoothly - Turns out tile print with my new printer is wacky, the wing tiles once ttaped and glued gave 1.4 cm difference at the tips in the forward-back direction, so I went with a cut half wing as template - better anyways since this one has dihedral.

Secondly, right after gluing (of course) in the servos I noticed the p-40 has a much longer nose to leading edge distance than the Oscar did, so I had to move the servos back in order to compensate for the nose-heaviness.

Probably one reason why the japanese planes turned better, more pics tomorrow I hope
Mar 04, 2018, 02:42 AM
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Looking forward to seeing how this one goes.
Mar 04, 2018, 12:03 PM
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cross sections

Oh yeah, cross sections. These were printed from a different 3-view after sizing the wingspan to 23.5 inches on tile print. It turns out, as usual, that the sizes of the quarter-pies didn't quite match my X-form fuselage, after marking the 4mm width (dollar tree foam) crosses, So I had to do it the slow way, carefully measure distances and follow the outlines as best I could to remark them before cutting. At least I have formers for a 2nd build if needed. I suppose I could scan these into the computer on 8x10 colored cardstock background to make a plan if anyone is interested.

So far I have used sections B (front and back of battery - bluecor), and D and G ( dollar tree with paper to support long bluecor pieces - for wing support and gluing). The wing piece bottoms I cut angled to match the 6-degree dihedral.

pics today hopefully.

As an aside, I have tried printing tiles with both tile-print and adobe. I think adobe isn't capable of resizing, so it's use may be limited. Both of them gave not so great results on my printer. So I am suspecting it's a printer fault or something to do with the paper size/feeding. Even within a page, I often get 2 areas which are impossible to line up perfectly to the adjacent page. I wonder if anyone knows of a better prog or knows the answer?
Mar 04, 2018, 01:26 PM
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Bit of progress

Few more pics. I got quite a bit done yesterday, and ran into some considerations. Think I mentioned the p40 nose to wing is quite long relative to my Oscar, so I needed to move servos back from that similar position, to end up close to the right CG at the end of build. I have moved only one of them, but I'm thinking I may need to move both, since I may be using a heavier 3S battery due to lower KV motor.

Hopefully this will be all the cross sections I use, to keep things light. I put a thin layer of 20min epoxy on top of dollar-tree section of the wing with foam brush. I didn't cover the complete area, for weight considerations and maybe make it look a bit "war torn" after painting.

KF section on wing top is 3mm depron epoxied on with 3mm square CF tube behind.

I ended up lowering the wing incidence angle a bit from the one on my poster board template. I had already lowered the angle from 2.5 degrees (the only published angle I found for p40) to 1.7, and after looking at it - still seems to high. My monobloc Bf109 is at 1 degree incidence and flies like it's on rails. If you look at photo I went down about 8mm in front, so I suspect I am near 1.2 degrees wing incidence relative to stab.

My last plane from a plan was at a darn high incidence, something like 2.5 degrees and it wants to climb so much that I have to trim the tail down - creating drag.

Probably no more building practice.
Last edited by rotagen; Mar 04, 2018 at 01:33 PM.
Mar 05, 2018, 02:47 PM
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wing on

Bit more done- wing supports and wing on, looks pretty true from frontal view. Dimensions looking pretty scale. I forgot to sand down the leading edge of the wing to round it off, will do it carefully after epoxy sets.

Some specs.
wingspan - 23.5 inches
motor - 25 gm 1700kv - 2000kv would have been better since I will probably fly 2S
prop - 7x5 slow fly - may move to 8 inch
esc-dualsky 12 amp
battery -will most likely be 850 maH 2S but a 500mah 3S would match KV for 7 inch prop.
servos- 5gm HXT - I have smaller ones but I have yet to find a dependable smaller servo.

Rudder only plane (for simplicity and weight) I may regret this but - I am very impressed with my old Cox micro zero warbird, which turns on a dime with only a tiny rudder.

Soon I will have to prepare for wrapping, preparing my mind for a bit of frustration.
Mar 07, 2018, 12:39 PM
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Well I guess I'm sorta talking to myself here but that's OK, I wanted to document this building style for mini warbirds because if I had only one plane to fly, it would be one of these. And the oscar flies so stable, more so than most of my larger planes. Just so versatile and fun to watch while flying in close quarters.

Maybe I should have put it in the scale forum but I'm not that confident in my ability, lol.

I really like the colors here, but I will make them even a little brighter - going with the historical colors would make it too tough to see, I'm thinking a nice greyish light green for undersides.
Mar 07, 2018, 02:20 PM
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some wrapping started

Made the whatchamacallit under-cowling housing shapes and cockpit shapes with styro hot wire table and sanding.

I've decided that paper models are pretty useless as templates for my wrapping, probably just too many differences with my model and I like to wrap in about 4 sections. So I'm doing it the slow way again using a strip of 1mm for measuring.

Saving card stock templates of everything for next time.
Mar 07, 2018, 05:55 PM
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Coming along nicely I see.

Are do doing literally only rudder or elevator and throttle to?
Mar 08, 2018, 04:14 PM
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wrapping - servo move

Yes Jab, I may regret it but my 20 inch wingspan zero (old Cox micro warbird) turns really fast with just a small rudder and no ailerons. Actually the rudder is only about 1 cm on that plane... The interesting thing is it just takes a quick little jab with your thumb and it follows through on the turn for about a semicircle, perfect. Not only that, I would say the movement of that rudder is just a few millimeters - crazy.

Ran into a setback, As usual I should have done more thinking before building, after measuring the nose length and distances to leading edge of battery and motor on my little Nakajima and comparing it to this one, there's a HUGE difference. Multiplying weights by distance and adding up, I knew I had to move my servos back as far as I could on this p-40.

So here I have gone back as far as reasonable, it will show some servo poking through my 1mm wrap but I'm not that crazy about perfection so OK.

By my calcs I will only have to add about 1 gm to the tail now, and I will have to use the smaller 2S 500maH battery that I used with the Oscar (hayabusa).

Wrapping goes slow but getting there.

In retrospect I could have possibly even put the battery area right on the CG, enabling use of other batteries, but still, if it flies half as good as the hayabusa I will be happy.
Mar 08, 2018, 05:39 PM
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Looking good. So you have RET control?
Mar 09, 2018, 11:44 AM
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Yep, that's it, trying to keep things simple and light. Hoping to do a bit more wrapping today.

If you look in the similar threads list below, I found this one that somehow didn't come up in my previous searches. Well the bar is set high and I don't think mine will look so smooth and scale.

He posts no pics of his build but I am assuming he used stryofoam and somehow hollowed out the interior for electronics.

I considered doing mine this way, similar to a large p51 fuselage I made by making a sandwich -hot wire cutting the 2 sides of fuselage leaving the flat factory cut sides on one side for gluing, These were glued to a thinner-foam central piece which included the vertical stab and this kept things nice and straight. But in the end I think this might have given me a heavier plane, and I find the sanding/shaping work is just as labor-intensive as this wrapping method.
Mar 09, 2018, 07:39 PM
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OK - Got some more done today, it's called blast the judas priest grab a beer and go for it.

I'm happy to say my least favorite part is done, wrapped and ready for small amount of lightweight spackle at seams etc to try and smooth things out.
Mar 09, 2018, 08:18 PM
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foam putty

I have some of this stuff so I decided to give it a try. Weight seems almost identical to lightweight spackle - just by weighing the jars.
Supposedly this is made for foam and easier to sand , goes on just like spackle.
Mar 10, 2018, 06:10 PM
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Undersides painted

Well it took me a while to get going, as usual, the little pencil-type airbrush was clogged, even though I always run soapy water through like crazy after painting.

Remind me to never buy a pencil-type airbrush again,

Luckily I had a cheapo trigger style airbrush which I like a lot better, and it throws more paint in a wider area.

Undersides came out great, the yellow/tan upper layer not so much, but it does give it a nice war-torn look. I tried a brighter yellow then didn't like it, so mixed paints to give a brownish yellow but it didn't spray as evenly, pic later after I get the green on.

That foam putty is good stuff, when it dried the little bits are much lighter than lightweight spackly - feather light and easier to sand.

I was gonna go with Australian decals , who knows.

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