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Jan 19, 2020, 01:44 PM
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Tim Cullip's Avatar
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Build Log

Peanut P-38


I just finished (okay, maybe 95% done, but at least successfully maidened) my Peanut F4U Corsair:
http://www.rcgroups.com/forums/showt...ut-F4U-Corsair

So now I've moved on to the next project (I'm a week into it already): a Peanut P-38 Lightning.
If your first thought is that I'm insane, just wait till you see the direction I'm going in on this one - it will confirm your assumptions.

It is challenging enough given:
A P-38 has a high aspect ratio wing, so shrink it to a 13" wingspan and you aren't left with much wing area.
Throw in that it is a twin with a small central pod and two fairly narrow booms - where to put all that RC gear?
Now to go from challenging to insane:
I want this one to look real good and am willing to compromise flyability to some extent - so I'm sheeting almost all of it.

Will I succeed? I kind of doubt it, but I sure hope it looks good if/when it crashes on its maiden.

Attached are the "plans" I'm basing this on (not plans for a model, but plans based on the real thing).
Years ago I actually scratch built a couple of bigger P-38's, but nothing approaching this small scale.


Wish me luck (for whatever good that will do),
Tim
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Jan 19, 2020, 01:54 PM
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As I mentioned in the first post, I wanted this one nearly fully sheeted.
To accomplish that and have any hope of coming in at a near reasonable final weight I am spending a lot of time sanding cut out sections of 1/32nd inch balsa sheets down to 1/64th.

Each wing panel sheet (four of them: top and bottom, left and right) started off at 0.9gms each and ended up at 0.5gms after sanding.
Everything in the second photo adds up to about 3.2gms (including the aileron torque rods).

If I can keep the whole thing down to a 28 gm AUW I'll be happy. Doesn't mean it will fly worth a bean, but I'll at least hit one of my goals.

Tim
Jan 19, 2020, 02:03 PM
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Here are a few photos of the bottom half of the central pod.

Rather than keels and stringers with filled in planking, I am just planking the whole thing from the get go.
One advantage to this approach is that none of the formers need notches.
Again, most everything you see is 1/32nd pre-sanded down not quite to 1/64th. And then again sanded down even more to get the smooth finished result.


Tim
Last edited by Tim Cullip; Jan 19, 2020 at 02:22 PM.
Jan 19, 2020, 02:17 PM
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As I think I mentioned in the first post, I was already a week into this project.
This post now brings me up to where I am currently.

The horizontal stab and elevator has the top sheeted, the bottom is an open skeleton structure.
I decided not to sheet the rudders since I would have had to sheet both sides for looks, so they are going to be the only non-sheeted part.

I temporarily glued on the central pod just for the picture, it will come off again before I cover everything with Esaki tissue.
The eventual nose will be carved/sanded/hollowed blue foam.

What you see in the photo adds up to 6.8gms (including the 1.9gm servo, advertised as 1.7gms but we know how that goes).



Tim
Jan 19, 2020, 02:31 PM
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You might have noticed in the previous photo that the aileron servo is upside down and at an odd angle.
I had to do that because the back of the pod where the aileron torque rod bend up to the servo attachment points couldn't be oriented fully vertically (up or down) due to the narrow height of the center pod back there. So things are oriented to have the servo horn below the wing, the pushrods going through the wing, and the torque rods bend to be approximately 45 degrees forward (close to 90 degrees to the rising pushrods). It took me a full day of thinking to figure out how to make that arrangement all fit in that narrow back of the central pod.


Tim
Jan 19, 2020, 05:21 PM
Go small or go home
ruzam's Avatar
You are insane! I love it! You've gone (pea)nuts with two of my favorite WW2 fighters. If I ever have time to build in balsa again, these are the threads I'll be reading over and over again. Thanks in advance for the excellent build progress.

Bold move with the sheeting. Given that the skin has now become 'structural' have you considered reducing the size and weight of internal structure (ribs, spars, bulkheads, etc)?

Ditch the screws and bottom cover off the servo. It's easy weight to lose and won't affect operation. You might even get some size reduction and better placement. Also shorten and solder the servo wires instead of messing with heavy plugs. You're going to need every fraction of a gram you can get.

I'm curious at this scale if it's even worth it to frame up rudders for tissue? Wouldn't it be just as light (and easier to work with ) if they were simply cut from solid sheet?

I hope you're going to hide the elevator control rod in the booms and not cheat with an exposed push rod
Jan 20, 2020, 02:50 PM
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Its funny how I had the exactly same thought, when I saw the servo...

Outstanding work, as always.

Do not worry about flyability. You have two power plants. So it will fly, just maybe not slow...
Jan 20, 2020, 03:40 PM
Balsa, depron addict.
Hi Tim,

Like usually I will follow your building.
You are prolific and you build in an easier way that I would like to have.

Stef.
Jan 20, 2020, 09:37 PM
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Quote:
Originally Posted by ruzam

Given that the skin has now become 'structural' have you considered reducing the size and weight of internal structure (ribs, spars, bulkheads, etc)?

I'm curious at this scale if it's even worth it to frame up rudders for tissue? Wouldn't it be just as light (and easier to work with ) if they were simply cut from solid sheet?

I hope you're going to hide the elevator control rod in the booms and not cheat with an exposed push rod
The internal structure is pretty minimal, just a few ribs and spar in the wing. Also picked the minimum number of fuselage formers (two-ply cross-grain sanded back down to less than 1/32 total thickness).

I plan to tissue everything, otherwise I'd have to apply several coats of sanding sealer (or something equivalent) in order to hide the wood grain. I'm hoping that tissue and light coats of paint will be lighter than enough coats of sealer/paint to hide the wood grain (maybe wishful thinking on my part).

The elevator push rod will be hidden in one of the booms and only the very end of it and the control horn will be external (but mostly hidden on the underside of the horizontal stab/elevator assembly and on the inside of the rudder it is near).


Tim
Jan 20, 2020, 09:59 PM
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Today's progress is that I got all the formers for the two booms cut and sanded to shape (each being two-ply cross-grain, sanded back down to be slightly less than 1/32 thickness [except for the formers that the motors will be attached to]). And I have planked one half of one of the booms, and sanded the surface smooth. The front of the booms will probably be blue foam, still thinking about that. What you see in the last picture is currently 0.7gms so the full two booms should be less than 4 gms.

In hindsight I probably should have done similar to the F4U and done top/bottom halves rather than right/left halves because now I will have a chore cutting out the slots for the wing [one of the disadvantages of not planning the whole thing out rather than make it up as I go].


Tim
Jan 21, 2020, 06:43 AM
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Quote:
In hindsight I probably should have done similar to the F4U and done top/bottom halves rather than right/left halves because now I will have a chore cutting out the slots for the wing [one of the disadvantages of not planning the whole thing out rather than make it up as I go
That's ok. I can't be the only one, that learns how to do it better, while building.

I see a problem with your planned elevator assembly. The control-surface won't work symmetrically under load. Had the same problem with a bigger P38, for which I even reinforced the elevator 'heavily'. So you will induce a roll when, yawing? Nicking? -you do the up-down motion...

Stupid thing is that I don't have a clever solution. I just flew the P38 with very gentle elevator, so I could catch the roll tendencies early. But since yours is smaller... do you have cat reflexes?
I am going to use an actuator to be able to place it symmetrically in the stab in my Pea 38.

Three not so clever solutions.
-2 servos...
-1 microflyer servo, or something as tiny, directly on the middle of the horizontal stabilizer.
-Push-pull guided cables.

Also. If you stand by the asymmetrical link, take 2 servos or cables, why not put the elevator link into the rudder? (Like I did with my international link in my latest Ta152) And you could hide it all, since you want to cover the rudders as well.
Last edited by JustAHuman; Jan 21, 2020 at 07:02 AM.
Jan 21, 2020, 08:16 AM
in pursuit of low wing loading
Gordon Johnson's Avatar
Tim,
I wish you the best of luck with your P-38.

Back in 2005 I did a printed tissue profile P-38 with twin 4mm watch-gear drives that Ralph Bradley built for me. I spent a bunch of time drawing up the printed tissue graphics, and built the plane. I had had quite a bit of success with profile Depron actuator planes and was optimistic about the chances for success with the P-38.

https://www.rcgroups.com/forums/show...m-Pager-Motors

That was all dashed on the first flight as the plane crashed into the gym floor. What followed that night and other nights was hacking up the plane in an attempt to get it to fly. It suffered lots of crashes. I put in more dihedral. I enlarged the rudders. Basically I tried everything I or anyone else could think of. In the end I was able to get it to fly more or less in a straight line and sometimes turn. And, it had been hacked up so much that it looked like it had been in several too many combat missions.

In the end I removed the equipment for use in other projects. One of the 4mm watch gear drives (with a new hot wind nano quad motor) recently made its way into my Stick Citabria CF plane.

Your P-38 is looking very very nice.

On servos you could consider replacing the heavy rubber coated wire with Litz wire. Last night I was removing the standard wire from Horizon linear servos by touching a soldering iron to the bottom side of the board and pulling the wire out. I don't know if you are using rotary or linear servos, but getting rid of plugs and rubber coated wire saves weight. And, I find is also easier to route Litz wires in the plane. Not original to me, but pulling Litz wire over red and black Sharpie pens colors the wire and makes it easy to keep track of which wire goes where.

Gordon
Last edited by Gordon Johnson; Jan 21, 2020 at 08:21 AM.
Jan 21, 2020, 08:47 AM
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Quote:
Originally Posted by JustAHuman
I see a problem with your planned elevator assembly. The control-surface won't work symmetrically under load.
I'm hoping it won't be a problem. I have built two larger P-38s with the same setup (elevator horn at one end of the elevator) and they didn't exhibit any roll tendency with elevator deflection. If I were using thin flexible foam for the elevator I could see it being a problem, but with two layers of balsa (one solid sheet on top, the other a stick framework under the sheet [though each is only 1/64th thick]) and two layers of tissue I expect (might be disappointed, but expect) it won't be a problem with this one either - time will tell.


Tim
Jan 21, 2020, 09:11 AM
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Quote:
Originally Posted by Gordon Johnson
Tim,
I wish you the best of luck with your P-38.

Gordon

And I will probably need all the luck I can get with this one!

I liked your thread on your indoor P-38, too bad it turned out to be a bear to get working.

Since I don't have an indoor site for flying I can afford to go higher weight with faster flying. Unfortunately this one will definitely be pushing the envelope in wing loading.

I probably will replace the servo wires with light gauge magnet wire (probably not litz though) and solder things directly to the Rx.
I'm not looking forward to running all the wiring and soldering all the things in place. Once finished I'll never be able to get to any of it again.

By the way, I'm planning on using my two Pesky light weight ESCs in this one along with a couple of rewound Racerstar 08028 motors.


Tim
Jan 21, 2020, 03:22 PM
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I now have both boom's inner half shells done including the wing cutouts for them.

Everything you see is currently at 8.2gms (including the 1.9gm aileron servo). I'm happy with the current weight and might actually hit my 28gm AUW, though it will be tough. BTH, I hoping to reduce that 1.9gm servo down to close to 1.5gms by removing the lower case, replacing the wire with magnet wire, and removing the connector and soldering it directly to the RX. I'm really not looking forward to placing, routing, and soldering all the parts together.

Now I need to build the two outer half shells and start placing the rest of the RC gear because once I glue the half shells together I won't be able to get to much if any of that gear.


Tim


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