***FunBuild 3*** Guillow's P-39 Airacobra Conversion - RC Groups
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Feb 23, 2010, 10:08 AM
Registerd Beaver
Smokin' Beaver's Avatar

***FunBuild 3*** Guillow's P-39 Airacobra Conversion

Hi all,
Thought I’d put up another funbuild thread.
This one another Guillow’s challenge – the old P-39 kit which is now out of production.
In a way I'm not taking this challenge on alone as milesperpound - Mark is about to embark on the same journey.
So I'm looking forward to this parallel build very much.
I'm not sure what Mark has selected for his model but I'm going with a setup which I have in stock.
anyhow, so to it...

First things first was to make plaster plugs of the plastic bits for future replacement.
Just the usual – fill it up with drywall patching plaster (doesn’t shrink & goes off in 15 mins).
Not that I’ve ever wrecked a canopy but I made one of these too just in case.

All the airframe design review is still to be finalised but I’ll go with my favourite setup for Guillow’s planes of this size & that’s;

This motor from HC here

This 800ma/hr 7.4V 2S battery from HC here . I do use a 1200 ma/hr pack in the A6M2 for ballast though.

Servo’s will be a good old 5 gram HXT ones here which I’ve had an excellent run with for the elevator & the 9 gram for ailerons here

Prop will be the APC 7x4 SF here this has a nice look about it too, sculptured tips make it look good.

For those who are interested, this power plant is not a powerhouse ‘prop-hanger’ setup.
It requires a firm hand launch at ~40 degrees and it’s away quite well.
My P-40 & A6M2 both use this motor/prop setup and fly extremely well, surprisingly quick when you open it up, very economical too but most of all the motor will run happily without much ventilation which is good for these models. Staying well away from its upper limits, it seems to dissipate its heat very well.
I also choose this one because of its long shaft that is handy for getting thru the cowlings. For the radial, just chop it down to suit.
It’s rated for 9 amps & this setup draws around 6.5 amps = 50 watts.
For a model of 10 oz it equates to about 75 watts per pound which I find acceptable.
Prop adapters are the simple 3mm collett type to suit the APC hub. Most spinners will fit without mods.

Next step will be to nut out the airframe & here I'll bounce off Mark to see what ideas he's got. I believe you've already looked into this.

Tomorrow it's time to clear the bench & get going.
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Feb 23, 2010, 10:20 AM
In Unusual Attitude Recovery
MPP's Avatar
Well it looks like your box is a little more beaten than mine. Sounds like a great plan. Are you going to use the kit wood and such? I am going to cut all new parts and keep the kit in my historic storage. looking forward to working along side and see how you tackle this one. I'm tuned in.


Feb 23, 2010, 12:53 PM
Or current resident
glewis's Avatar
I'm tuned in too.
I made a set of plugs from Mark's kit plastics and will start on one after a couple of pending projects get done.

I like your motor choice; it’s very similar to my CDROM conversions I use in models this size. The KV is a bit higher than mine though. I too like the motor's ability to not overheat without ventilation as long as it's loaded within reason.

What is your plan for the landing gear?

Feb 23, 2010, 06:27 PM
It flew once before...
jofrost's Avatar
Welcome aboard Phil ...again

Great job completing the first one , and this one should be a treat too

Feb 23, 2010, 07:26 PM
Big gov never Works
St. Martin's Avatar
You guy's should really shake up the troops. Enlarge it 200% and mount the mtr behind the cockpit. And of course retracts..am I a trouble maker..or what?

Feb 24, 2010, 08:15 AM
AMA 1124741
Parkk's Avatar
Originally Posted by St. Martin
You guy's should really shake up the troops. Enlarge it 200% and mount the mtr behind the cockpit. And of course retracts..am I a trouble maker..or what?

That's what I would do if I was building a P-39 !
Feb 24, 2010, 04:25 PM
Registered User
Rekitter's Avatar
Hey Phil: Godspeed on the new build. Interesting and unique subject. I have no doubt that you will have a winner on your hands with your gameplan. I do think a 200% enlarged P-39 would be a nice way to go .... now that you have done all of that beautiful plastic work! Look forward to following your progress.

Feb 26, 2010, 02:19 AM
Registered User
Jugjock's Avatar
Dialed in on the beam, Smoke! Lokking forward to another neat Guillows build.
Feb 26, 2010, 05:18 AM
Registerd Beaver
Smokin' Beaver's Avatar
Hi gang
Just swingin' back for a couple of moments to say thanks for joining in.
This is going to be a lot of fun.

I'll get back to some techo stuff later but let's get way ahead of ourselves just for a moment.

Mark, what paint scheme are you going to go?
Either of the two supplied decal versions?

I'm not either way yet but I thought of another unique twist to this tandem P-39 build & if Glenn joins in (throws out a bait here ) There's a possibility to do a very unique thing.
To build models with consecutive Bu. numbers

Feb 26, 2010, 07:34 AM
Or current resident
glewis's Avatar
Plop,,,the sound of another lure tossed my way..

Back before I entered the contest I showed my wife pictures of the P-39 and Vega. She said I should do the P-39.
Instead I decided to go ahead with the Vega knowing Mark was going to build the -39.
Before I had firmed up my plans, somebody else took Earharts Vega so I changed to Post's aircraft to avoid a 'me too'.

You guys really want me to join in? A three way build would be fun.

Sequential build numbers would be really cool if we can find documentation on all three aircraft.

Feb 26, 2010, 11:32 AM
In Unusual Attitude Recovery
MPP's Avatar
Glenn you know you should always listen to your wife . Phil I really like your idea. Now to really make it interesting and perhaps more colorful. VVS liveries. I know this is going a bit further away from the golden age and the fun build is, well, fun. However guillows did recognize the significance of the P-39 and the Red Star. Maybe a winter, summer, Fall camo scheme of the same plane. I believe the russians did change Camo to the season. Just thinking out loud.

Either way i am open to it, and is a neat idea.

BTW Phil I am going to preserve the whole kit including the decals. These are not that hard to cut myself, well that is except if we go with Russian lettering . Are you going to use the kit or cut your own wood?

Let me know what you think

Markus Rooski
Feb 26, 2010, 01:17 PM
Or current resident
glewis's Avatar
Kit? What kit?
Thanks to a fellow modeler all I have are copies of the plans and parts.

If this puts this build in the kit catagory I'll have to enlarge the plans and build at a larger size. A plastic cowl isn't necessary and I can carve my own canopy plug.

I'm still leaning toward a 'former on removeable crutch', Ryan style construction and would be redesigning the structure anyway.

I would want to do mine up in a PTO scheme of some sort. I once worked for a guy who flew one in the pacific theater and he loved that airplane. He said firing that canon was something you'll never forget.

Feb 26, 2010, 06:58 PM
Registerd Beaver
Smokin' Beaver's Avatar
I like that idea too Mark, keeps it simpler but still unique.
I intend to increase the value of your kit for you by taking another one out of circulation.

I'll use most of the wood and the decals.
to that end, I'll duplicate the parts I use to one day build the other version.
I don't get a lot of 'new build' time these days which was why I like the dual build last time.

We'll see...

Mar 01, 2010, 10:32 AM
Registerd Beaver
Smokin' Beaver's Avatar
Tonight I finally got to lay out the plans in prep for the build & laid the keel.
Appears I have lucked in with all the wood thinner than 1/8'. It is all of very good quality, the die cutting is particularly sharp. So much so, I'll keep the scrap frames in one piece so as to provide a cutting template for another time.
So what I'll do is use the wood & reduce weight aft of the CG.
A quick calc of a 24% CG location on rib F1A puts it 36mm from the LE so everything aft of that will get looked at for weight.
the wings are swept slightly so it will likely end up 23%-ish. A good starting point.
This model appears to have a long nose moment so I think the setup will be very generic as far as my builds go, I don't see any CG issues.

Now - a target weight would be nice here, I'll do a wing area measurement next to see what the numbers could be.
At any rate, my setup is light & cheap at 4oz & $50 in the air, I'm thinking a 6oz airframe would make quite a sprightly model.

Looking at the wing ribs, I'll modify them more to a Clark Y using an under shim to give me a wider entry LE.
I like what I saw Glenn post somewhere else on the 400 series versus 1000 series airfoils and it looks really easy.

I'll drop the LE of the wing down at the saddle much as I did on the P-40 & A6M2 to improve that.

ailerons will uses torque rods same as the other models.

Once again I plan to fully sheet the entire plane. inserts is good but too slow for me and I've learned ways of feathering the sheet to match cowlings etc.
that extra time is way less than the insert method so it's a fair trade to me.

Tail feathers - light 1/8" sheet, same as my others.
After cartwheeling the A6M2 after a bad launch, this method stood up well, one side got a bit bent, I just pulled it back into position, pricked a few holes thru the Solite & wicked some thin CA in thru the holes.
Problem solved in...oh - about 15 seconds! You cant do that to a framed wing as easily.

Anyway, we're off. A blow has been struck.

Back later with pics (not much to see right now)

Mar 01, 2010, 10:56 AM
Or current resident
glewis's Avatar
I was looking at the plan yesterday and was thinking of cutting ribs with the leading edge higher in the airfoil and modifying the upper and lower curves to match. The leading edge would be blocked up off the board during construction.

About the sheeting, I like to reduce the former outline by the thickness of the sheet and deepen the stringer notches.

I have used 3/32" for tailfeathers and it's just thick enough to slit for floppy disk mylar hinges.

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