Piper Cub J3 -DESIGN AND BUILD THREAD.

[GremlinRC]

For a long time now, I've wanted to build a cub. More recently I've looking for a bird that I can use to complete my model flying certifications. I need a high wing trainer type plane that will be big enough to handle a little wind but not too big to make it difficult to transport. Soooo, I've settled on a 1500mm wingspan Piper Cub. I've always loved cubs and thanks to some of the great folks here on flitetest I've learned loads of design and construction techniques which I plan to use to the full to design and build this lovely old airplane.

The primary design will be foamboard, but I will introduce some fiberglassing and some 3d printing into this build too. I'll be trying to make this one as much to scale as possible but I don't plan to go overboard. I've made a decent start on the fuse and have enough done to get a thread going. I've been documenting and writing up plans as I go and of course they'll be here at the end. I also want to share my way of taking a conceptual idea for a model and turning it into plans and a reality. Hopefully that might encourage others that designing and building from scratch doesn't have to be a pipe dream.

To start the design I found some balsa plans on the web and traced out the major dimensions and parts in illustrator.

Like so;




Next up is to take these plans into a 2D CAD package and turn them into something I can cut on my CNC machine (replete with the now famous Needle Cutter) and also use dxf format so that they can be turned out as pdfs at a later time.

My own favourite 2D CAD program is CAMBAM. Sure there are more functional programs out there but this one hasn't let me down so far.

Here's a screenie of a foamboard sheet laid out in CAMBAM ready for cutting;



Some of the parts so far have evolved through a suck it and see approach where I'll make a part (e.g. a former) cut it in situ till it's right and then scan it on the scanner so I can repeat the above process and add it to the plans.

So that's basically how the design starts out (in my world). I'm sure there's no right or wrong way to do this. I'm just showing 'my way'

Next up I'll start the actual build and hopefully by the end of this thread we'll have a video of a flying cub and some plans to boot.

I'd really love if some flitetest fans jump in and tell me how to do it better

Cheers

Dave.

 

[97tj-neil]

Looks good so far.

If it helps at all, you are welcome to steal as much as you want from my Aeronca Champ plans, as it is a very similar airframe to the Cub. Mine is about 1100 mm wingspan. The build thread is linked in my signature.

I will post up the latest rev plans for you tonight. They are very much in "beta" form still.

 

[GremlinRC]

Build Started.

First up is to lay out the fuse and add some extension pieces. To build a 1500mm wingspan cub, its possible to get each wing onto a sheet of flitetest wp foamboard, but the fuse is too long to build in one piece. What I've done is cut some pieces off the tail end of the fuse, laid them out on another free part of the sheet and then attached them using popsicle sticks as reinforcment.




This is just the lower half of the fuse the upper half will be build using sideplates, formers etc as you'll see next.

I wanted proper glass windows on this model so the sideplates are cut and some sills added by removing some of the foam around the inside of the window. Cutting a piece of 1mm lexan and gluing in place. The first time around I stupidly cut the sill on the wrong side of one of the sideplates. That's the beauty of having it all in dxf. I can just cut another without too much fuss.



Next up the formers and sideplates are fitted and glued.



Tomorrows job is to mount the tail plane. Tail servo's and possibly the rear turtledeck if progress is good..

 

[GremlinRC]

Hey Neil. Thanks for that I may well indeed dip into those plans. I was looking at your MM Texan. That's a cracking build. I can see myself building that one once this cub is in the air!!

 

[97tj-neil]

Thanks. Be sure to post up some pics if you do build the mini Texan.

 

[GremlinRC]

Been caught up with "Family stuff" so progress has been slow.

So here's where it stands. I've assembled and installed the tail section and landing gear struts. Covered the underside and mounted the rear turtledeck. I've also installed ply reinforcement to act as wing support.

Here's where the fuse is at as I start on the wing assembly.



Here's some of the build pics which got it this far.


added some ply uprights to support a piece of pine which will act as support for the landing wheel struts


Here's the pine support glued in.


Here's the landing strut complete and the underside sheeted over. Not expecting any lack of strength in this arrangement.


For a big(ish) bird like this, I find that unsupported foamboard eventualy warps and fails, so a strategically placed popsicle stick can work wonders and definitely adds to the longevity of foamboard planes IMO.


Here's the tail assemble complete with support popsicles. Also the popsicle sticks on the elevators will give me a solid base to attach the control horn to.



Next up is to make the wings. I'm planning a fairly standard flitetest style airfoil but with some carbon fibre rods running through the wing struts.


Couple of decisions which I'm still mulling over. Steerable tail wheel or fixed? I'm worried that adding a steerable tail wheel will add too much weight. SInce most of my flying is done off a grass strip, I'm tending to lean to a fixed wheel or skid

The other decision is do I chop the nose and make a 3d printed cowl or do I just run with the standard foamboard nose. I'm inclined to think the 3d printed cowl will look better.

Hopefully the coming week will allow for more progress.

 

[Hai-Lee]

A simple steerable tail wheel can be made using a little wire, a washer, half a paddle pop stick, and some glue.

Glue the paddle pop stick half to the underside of the fuselage so that in protrudes beyond the rudder hinge line.
Drill a small hole in the stick, in line with the hinge line, to allow the passage of the tail wheel wire axle etc.
Bend the wire so that you have a small wheel axle and clearance for the wheel to rotate and then measure the height or clearance you require for the wheel when fitted and mark the wire.
Solder the washer at the mark so that the washer takes the impact of landing rather then the rudder.
Fit the wire back through the stick hole and bend the wire so that it runs along the bottom of the rudder and bend the end of the wire to be inserted in the base of the rudder.

Glue the wire into the rudder and along its bottom edge and when the glue is set/dry the wire can be adjusted for alignment as required and you have a simple steerable tail wheel. To mount the wheel itself I also use washers, soldered and glued.

The simplest source of tiny wheels is the cheap $1 plastic cars sold in variety discount stores.

Have fun!

 

[GremlinRC]

Beem slow progress here mostly because I encountered a serious issue and had to figure out how to deal with it as best as possible. Specifically warped and twisted tail feathers I suspect the problem was partially leaving it in the cold shed for a few days and the other part has me not installing enough reinforcement.



Waaaay to bad to just ignore. Problem was it was well and truly glued in. So it took quite a bit of surgery to remove it. However I managed without too much damage. I had to sacrifice part of the rear turtle deck but thats an easy replacement.



Before I move to the replacement tail I just want to say that I am having a LOT of issues with warping and the new FT waterproof foamboard. Its usually not a problem when its folded in A and B folds etc as these put it under tension, but large unsupported surfaces like these tail feathers really show it.

Anyway I found the flattest piece I could find and cut the tail feathers from the flattest part of that board. Plenty of reinfocement including a 4mm CF tube spar and some strategically placed popsicle sticks. Which reminds me. Not all popsicle sticks are straight which *might* have been part of my original problem.



If this one warps, I give up!

Anyway tail was reinstalled and its all now looking pretty good.



Next up is the power plant and motor mount. I settled on this 900kv 3548 motor. Mostly because I had it and it should provide plenty of power with a 4S and a nice 12" 3 blade prob which I also have.



Here's the power pod installed. Enough hot glue in there to ensure I wont have too much problem with tail heaviness!




I also added the upper crossmember to provide support for the front of the wing.



Not one to ignore HaiLee's good advice I added the tail wheel wire which was bent from a wire hangar. I did remember to add the support washer before gluing it in



Next up was the wings. Build photos are on my other camera so Ill post them later. But for reference here they are sitting in place with Missie the dog taking full credit!!



So in short its now back on track and progress should be fairly rapid from now. Still mulling over whether to do a 3d printed nose cowl. I think it would definitely add to it, the question is whether my 3D modelling skills are up to it.

 

[FPVology]

the cub is looking very sweet. I have always loved cubs as well. the BJ-J3 cub was my first design and its still my favorite to park fly

 

[agentkbl]

I don't mean to be rude as I've never built a cub before, but it seems rather overbuilt. I am no expert though, and I'm sure it will make for a very strong airplane. Good luck!

 

[GremlinRC]

I don't mean to be rude as I've never built a cub before, but it seems rather overbuilt. I am no expert though, and I'm sure it will make for a very strong airplane. Good luck!

Thanks for the feedback. The wingspan on this is 1500mm, so it does need all that extra reinforcement. I've learned from hard experience that once you go to that sort of size with foamboard, if you don't do the reinforcement, they don't tend to last very long.
Cheers,
Dave.