Hacker Piper J-3C Cub - RC Groups
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Oct 15, 2012, 05:12 PM
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Build Log

Hacker Piper J-3C Cub


This'll be the second thread for me on rcgroups, and the second plane I build since taking up this hobby again for a 15 year long break. The first plane can be found here (a SIG 29'er with slight modifications to make it look more like a Fairchild F22):

I've always liked the J-3 Cub, as well as the Tiger Moth which gave me a bit of a problem when deciding which kit to buy because Hacker make a 59cm wingspan Tiger Moth as well as this 71cm wingspan Cub (and a few others). I decided I'd get a little more experience under the belt before tackling a biplane so I chose the Cub. I've done a bit of searching and haven't been able to find anyone on rcgroups who's built this model. Happily there's a clip on youtube showing an example flying nicely, with skis even. The manufacturers homepage is here:
If you're shopping in Sweden this is a good place to get it:
This model on skis (not mine!):
mini RC Piper J-3C CUB (1 min 4 sec)

My last kit consisted of pre-crushed balsa sheets of dubious quality (but very dry, having been in storage for 20+ years), the stuff in this kit looks like it will fit together better, nothing is laser cut, but it is cnc'd rather nicely. A very nice touch is that the tail feathers come pre-laminated in the kit.

Recommended propulsion is either CO2 engine or rubber band, the stuff needed for rubber flight is included. The plans actually have two servos sketched for 2 channel r/c (+CO2 motor), but no instructions cover r/c installation as far as I can work out. I'm going to fly it using 4 channel cheap micro stuff from hobbyking which worked really well for me in my 29'er.

If weight allows, I'll use a smaller motor than the 10g 18-11 Outrunner to get a slower plane which might be usable indoors. All the stuff in the box weighs in at around 80g at the moment, ideally I'd like to end up somewhere around 100g with electronics and so-lite covering, so I may lighten it here and there. Considering the wing is quite a bit larger than what I have on my 29'er which flies fine at 80-90g, anything around 100g should fly very well, and I'm sure it'd be flyable with 150g+ weight too, just not as slow as I'd like it to.

Progress so far is I've opened the box, ironed the plans to stay flat and started poking at the parts, I'll be spending parts of evenings and weekends putting this together so progress might be slow, especially since I'm quite meticulous.
Last edited by XeCutor; Oct 22, 2012 at 01:05 PM. Reason: spelling
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Oct 16, 2012, 12:41 AM
Blew out my flip flop
David Hogue's Avatar
Got one in the box....watching closely
Oct 17, 2012, 06:20 AM
Registered User
I'm quite far off from starting with the wings (I like to think ahead), but I've already decided that I'm going to skip the suggested 5 degrees of dihedral suggested in the kit and make a straight wing for scale looks, with ailerons it should work fine I'm sure, at least it did on my 29'er.

There's no washout built into the wings in the kit. Would it be wise to add it? Input from experienced Cub-pilots would be appreciated. I'm thinking of just making the wing with a completely flat underside as suggested by the kit, and then make the last ribs a little fatter on the bottom creating slightly different airfoil with a stubbier "nose" and a slight downward angle. I don't think that explanation was very good, hope anyone can figure out what I mean.
Oct 17, 2012, 08:35 AM
Übung macht den Meister..
Deuce's Avatar
Originally Posted by XeCutor
I'm quite far off from starting with the wings (I like to think ahead), but I've already decided that I'm going to skip the suggested 5 degrees of dihedral suggested in the kit and make a straight wing for scale looks, with ailerons it should work fine I'm sure, at least it did on my 29'er.

There's no washout built into the wings in the kit. Would it be wise to add it? Input from experienced Cub-pilots would be appreciated. I'm thinking of just making the wing with a completely flat underside as suggested by the kit, and then make the last ribs a little fatter on the bottom creating slightly different airfoil with a stubbier "nose" and a slight downward angle. I don't think that explanation was very good, hope anyone can figure out what I mean.
A Cub does have a slight bit of dihedral, but can't recall right now how much. Leaving a bit there will also prevent the "droopy" look.

Oct 17, 2012, 10:08 AM
Registered User
It does? I browsed through this thread, and none of the scalier (a word?) have any visible dihedral:

I haven't been able to see any on pictures of real Cubs either, so if it has some (which would kind of make sense for the type of plane it is), it's got to be very little?
Oct 17, 2012, 10:49 AM
Registered User
I'm kind of answering one of my own questions here, but couldn't stop googling on dihedral until I found out something that seemed trustworthy.

It's not actually info for the J3, but as someone says in the thread, it ought to be the same:

It seems there's is some dihedral, but it's so little it's almost not noticeable, and on a model my scale, you probably couldn't hit that angle precisely anyway.
Oct 17, 2012, 05:27 PM
Übung macht den Meister..
Deuce's Avatar
No, there's not a lot of dihedral, but if you put in just a bit, it'll prevent the "droopy" wing look. I'm at work right now, so I don't have documentation at hand.

A quick search turns up this: http://www.j3-cub.com/forum/f84/dihedral-19427/ Though it discusses dihedral of a full-size Cub, it appears not to reach any other conclusion than "some" dihedral!

Oct 22, 2012, 01:22 PM
Registered User
During the last week, I assembled the two fuselage sides and I've started gluing these two together. The balsa is 3x3 mm, which seems like a bit of overkill in many places, but I don't feel I would save enough weight to motivate changing out the wood from the stock kit to something different. The quality and weight of the sticks is quite varied, so choose carefully which bit goes where.

I try to use the lighter bits in the tail as much as possible, and put the stronger and straighter ones as fuselage longerons. If they're naturally bent in one direction, I try to place it so the bend follows the shape of the fuselage once it's being mounted together and bent into shape.

I'm not super impressed by the suggestions in the build instructions, saying you should crack the pieces in some places in order to create a distinct angle in the longerons. For example, the sheeted front-end of the side fuselages is meant to broken in four places so it can be bent inwards in the nose (you can probably make this out in the picture). This'll create very obvious weak points in the structure and I think it'd be a good idea to add a few milligrams of weight in these spots to strengthen the joints.

If you build this kit, it might be good to have a decently accurate scale at hand. The weight of just the leading and trailing edge of the wings, plus the ribs could, if you take all the heavier ones on one side and all the lighter ones on the other, lead to one wing being around 3-4g heavier than the other.
Nov 01, 2012, 02:42 PM
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I've had very poor progress during this last week. The only evening I had some real time to spare, I modified by Spektrum DX6i radio to get a nice backlight, it took some time but I consider it time well spent.

I've assembled parts of the tail, the elevator is 3mm thick unless you modify it/sand it down which I will, especially on the trailing edge.

I've started looking at the wing, there are a lot of ribs, but it's going to be hard to redistribute the remaining ones nicely while keeping the ailerons in the correct location so I'll probably not bother. So far my weight savings are pretty much non-existing.

The ribs of the wing are not as nicely cnc'd as I'd have liked them to be. They all match each other, but they also all deviate from what they ought to look like according to the plans by a 0.5-1mm here and there. Since the trailing edge of the ribs feels too thin/pointy, and the front of them aren't as thick/high as it should be to match the pre-made leading edge of the wing, it feels like they cut off a bit too much on the underside, and the only way to do anything about that is to either glue a wafer-thin strip on the bottom of them or make the whole wing slightly less wide.
Nov 03, 2012, 11:56 AM
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I've decided I'm annoyed by the wing-ribs enough to modify all of them slightly (see picture). There's twenty of these thin ones, plus the 4 thicker ones for the wing roots. I'm very happy I've got a balsa stripper which at least makes the job of creating equally thick strips of balsa very easy.

I'm sure there are better ones out there, but this the one I've got and it works fine:
Last edited by XeCutor; Nov 03, 2012 at 12:16 PM. Reason: spelling
Dec 23, 2012, 03:43 PM
Registered User
I've been slow building this plane and even slower at posting my progress, but today I'll write a few updates.

I made the wing ribs a slight bit thicker than they were (which made them match plans and leading+trailing edge balsa pieces. This added some weight of course, so I though I'd make a few strategically placed holes to loose weight again. Picture of that is attached, took me a couple of hours at least, and weight saved was around 2g so not really time well spent.

Building the wings was pretty straight forward, although to be honest I'm not completely done yet have some details left.

One thing I should have done beforehand was to plan for servo mounting a bit better. I knew I wanted to put them inside the wings, and I knew I wanted them to be as invisible as possible. I had to create some hard-points to mount them so I created some out of 1.5mm plywood. Making holes for servos, adapting them to the wing shape, making the hole in the rib they were supposed to be mounted onto, drill screw holes for servo screws etc. was all very fiddly work, especially since both wings were already assembled, I should have done this beforehand. Ended up pretty tidy in the end though, with little added weight. The previously made holes in all the ribs will make servo-wiring easy at least.
Dec 23, 2012, 03:58 PM
Registered User
The nose section is made out of the included kit-plastic. It's made out of two halves that I've CA'd together, in the front I added some ca-soaked fabric as reinforcement. Generally, it fits together pretty nicely, but I don't like the very visible edge you get between plastic and wood, I wanted it to be a seamless transition.

The kit includes some 1mm thick balsa, the curved part of the cowling is made out of it for example. I bought some more 1mm wood and glued it on the nose to create a snug fit against the plastic. This was then sanded down all around to create smooth transitions. I've got small metal files which I find work well for this kind of work. They hardly flex at all like sandpaper glued onto a piece of wood does. That means it's much easier to sand over a hard section that's been saturated with glue without creating a deep groove in the otherwise super-soft wood.

Out of pure dumb luck, my 10g hobbyking outrunner engine together with my 2mm ebay prop adapters happened to be the perfect length after being mounted on the kit-supplied piece of engine mount plywood. The combination protrudes just right from the plastic nose section which didn't really have any room for adjustment after my efforts to get it smooth with the wood.
Dec 23, 2012, 04:31 PM
Registered User
The next fiddly bit that I probably spent a little bit too much time on was the landing gear. The original kit dictates a single piece of 0.8mm piano wire. I thought it had way too much flex in it, a still spinning propeller would definitely hit the ground on even a slightly hard landing.

With inspiration from the real cub, I did a slightly more elaborate landing gear with suspension crafted out of rubber from bicycle valves. At first I intended to leave the rubber naked but the metal hooks looked ugly so I put some shrink tube on it, which doesn't adversely affect the suspension effect. The gear is now made up of the original 0.8mm wire + a bunch of pieces of 0.5mm wire. All of it is held together with kevlar thread (extremely strong) with an added drop of very thin ca on all joints. I'm pretty confident the metal will be bend beyond it's point of returning to normal shape, way before the kevlar thread breaks.

The wheels are the ones that come with the kit, made out of hard plastic. They look more scale than anything else I've found in this size so I'll stick with them, just hope they hold together. They're made out of two halves of plastic and are supposed to be glued together. They're meant to stay on the axle by gluing a little stop-thingie onto the wire before the outer part of the wheel is glued onto the inner one trapped in place by the stop-thingie.

I've also started on the tail. On my previous (and in modern time only other) build I found control horns etc. in this scale to be very time consuming. Especially since I tend to want everything very precise. Not much has changed, control horns and hinges are still very time consuming.

Also quite fiddly, and not really worth any pictures is wiring up the electronics at this scale. Especially since the servos I use don't appreciate 5V which my ESC supplies and require one or even two voltage drop diodes. I'm using the tiny cables and connectors that come standard with hobbykings smallest servos. The ESC doesn't have those so It need resoldering and I'm putting a diode on it's output so the whole receiver and everything it's supplying gets 4.3V. The aileron servos can't even handle that much, so on the Y-splitter I've yet to make, I need to add another diode to lower the voltage even further.
Last edited by XeCutor; Jan 04, 2013 at 07:14 AM.
Dec 24, 2012, 08:48 AM
AMA 986339 FCC KB5LAM/4
davidterrell80's Avatar
So, you're using zener diodes to subtract out the breakdown voltage of the line? That's cool! I had not thought of that. I like it, too.

I appreciate the work you're doing. Very prettily done!
Latest blog entry: Remembering Bob Hoover
Dec 24, 2012, 07:41 PM
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The voltage drop diode is not a trick I can take credit for. I had the problem of voltage being to high for my servos and found it as a brilliant suggestion proposed by someone here at rcg. Always put the diode on the positive/red lead, otherwise you can expect weird behavior/interference, ask me how I know...

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