Piper Cub J-3 (oz5021)


Piper Cub J-3 (oz5021) from Sterling 1960 - plan thumbnail

About this Plan

Piper J-3 Cub. Radio control scale model. Scale is 1/8. Sterling kit # FS-6.

Quote: "Plansheet for Sterling's kit... Piper Cub J-3. 54in span for RC, sport free flight, or control line; designed for RC single channel. Doesn't specify an engine range, but shows a Fox .15 for power on the plan. Suspect it'd fly fine on .09-19."

Quote (from ad): "Kit FS-6, wing span 54in. For .09-.35 engines. Especially designed for radio control, also turns in remarkable performance as free flight or control line model. Parts are beautifully die-cut and shaped, internotched for fast,accurate construction. Easy to fly. Complete flying instructions included."

Direct submission to Outerzone.

Update 23/09/2021: Added 'How to make a Twin Bee' article, thanks to MikeFoster.

Quote: "How to make a 'TWIN BEE' for under $15. What at first glance may appear to be a very complicated bit of machinery is really the end product of some fairly simple work. The two engined powerplant has been extensively flight tested by NJ experimenters. Give it a try!

Designed by Walter A Watkins. Drawn by JL Dietrich. Plans inked by RF Harris.

Being somewhat of a scale buff I have long been interested in an opposed twin cylinder engine. But the availability of such twins since the days of the OK Twin has been nil. I have given much thought to constructing one using as many stock components as possible. The making of the crankcase and shaft have always been the stumbling block.

Holding a pair of Cox .049's in my hand and still dreaming of a twin, the idea hit of gearing the two to one shaft in the form of a twin. This turned out to be so easy and ran so well that I think there are others who can do as well or better than I and share in the fun.

The Cox Golden Bee's lend themselves to this project almost as if they were made with this in mind because they run in either direction. The only re-work of the Cox's are turning the tanks 90 degrees, removing the prop drivers from the shaft and grinding a half-round keyway in each shaft. Of course, the Cox Babe Bee's also can be used for this project. The only difference being that the fuel tanks are smaller.

The tanks are turned by removing four screws in the rear of each one. Make sure fuel pickup is placed correctly before replacing screws. The prop driver presses off the shaft if you use a little care. The keyways are ground in the shafts using a Dremel type hand grinder and smallest diameter wheel. While this may not be a perfect fit with a spring wire key, keep it as close as you can.

We list gears from Boston Gear Catalog. G168 is an 18-tooth gear with a .562 pitch diameter. H3222 is a 22-tooth gear with a .688 pitch diameter. Other gears will do if you keep in mind (1) the center line distance, (2) prop diameter and (3) rpm at which you want prop to run. The center gear should be larger than the engine gears to keep engine rpm up. I/8 inch face gears will be okay if these gears are hardened; 3/16 inch face if the gears are soft steel or brass. Use a 6 x 32 set screw in the center gear hub.

The front support is made from aluminum stock. Hacksaw and file to shape. A miller would help here, but it is not necessary.

The prop shaft is made of drill rod or cold rolled steel. A 1/4 inch aircraft bolt with the head cut off will do if you don't have a small lathe. However, the prop driver and the thrust washer is a lathe job, so let's assume you have made friends with someone who has a lathe by now anyhow. The prop driver is made to fit tight and the thrust washer is made to fit loose on the shaft.

The front support center hole is drilled and reamed so that the prop shaft is a good slip fit with the front support. With center gear on the shaft install in the support. Make a small spotting punch to match hole in engine gears. Lay engine gears in mesh with prop gear and center punch support for location of motor support holes. While the detailed drawing shows a 1-1/4 inch dimension between centers for these motor support holes, it's recommended that you follow the procedure described above to get a more positive gear mesh.

Drill and ream the two engine support holes. Countersink rear of engine support holes to clear taper on front of crankcase. If this came out okay, you have proper gear mesh and the hard part is over.

Cut rear support and mounting plate, one and the same, from .050 to .062 sheet aluminum. Set the two engines, with front support in place, on rear support to locate eight 2 x 56 tapped holes to mount engines.

Make sure the spacer rod holes in the mounting plate and the front support are lined up, and then follow the procedure described below to locate the engines accurately.

Drill and tap one hole for each motor. Use short 2 x 56 screws and locate both engines on the mounting plate, then locate the remaining holes and drill and tap same. Grind off any excess screw length flush with back of plate.

Make two spacer rods from .156 diameter brass tubing to be used with a through bolt or .156 diameter drill rod as shown in the detail drawing. These are made .005 inch shorter than distance from front to rear support in order to pull everything together when assembled.

The keyways in engine gears are ground the same way as the crankshafts. Short keys are made from 1/32 inch music wire and are held in place by the washer in front of gear. Allen screws with a 5 x 40 thread, 5/16 inch long are the best to use to hold gear to engine. Use a lock washer along with a brass flat washer here.

Before mounting the last gear, time the engines. This set up will run with motors timed to fire alternate, together, or one behind the other. You can come up with some wild sounds. I find there is less wear and tear if timed to fire together. That is with both pistons at top center together. Also the engine will start easier this way.

The problem of two needle valves is no problem once you get settings after the first run. I hacksawed slots in tops of the needle valves and use a screw driver for setting.

A Sterling J3 Cub Kit was built up and it proved to be a perfect fit and match to our engine width and power. One of the problems with an opposed twin is getting the cylinders out in the air stream. Here, again, gearing the two engines was an advantage because of the greater width.

The two glow plugs can be wired together, or do as I do, use two battery sets. No lubrication of the gears is necessary because of the fuel blow-by which is common to the Cox engines. You can use an 8-4 prop although she'll handle a nine inch diameter prop with ease.

You will notice in the pies that we used a couple of ball bearings. These were Delco New Departure 77R3's (1/2 inch OD, 3/16 inch ID) which can be mounted in the front support. However, this is strictly an optional feature.

As you can see now, it is not much more involved to go to four cylinders opposed by removing the tanks and making a fuel manifold. Let us know how you make out - with two or four."

Supplementary file notes

Article (Twin Bee engine).
Decal sheet, thanks to Tony.


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Piper Cub J-3 (oz5021) from Sterling 1960 - model pic


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Piper Cub J-3 (oz5021) from Sterling 1960 - pic 003.jpg
Piper Cub J-3 (oz5021) from Sterling 1960 - pic 004.jpg
Piper Cub J-3 (oz5021) from Sterling 1960 - pic 005.jpg

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User comments

Greetings, It was nice to see the article on building the twin .049, but it isn’t the best scan. This article was in American Modeler, or American Aircraft Modeler. Can you provide a specific citation for the article? Both those titles can be found online, but I don’t recall the specific issues, and don’t really want to wade through the random online files. Thanks for any help you can provide. Let’s be careful out there.
James Hickman - 23/09/2021
The twin Bee would also look good in a suitable scaled Luton Minor, Aeronca C1 or C3 or any other aircraft with the engine cylinders poking out the side of the cowl.
Mike Foster - 23/09/2021
Doh, my mistake, I had failed to actually upload the pdf. Done now. See supplement file download. The scan quality is fine, James. The article was printed in May/June 1966 AM.
SteveWMD - 23/09/2021
There are no fuselage formers shown on the plan.
cgn - 24/09/2021
Correct. That's why it's tagged 'formers incomplete'.
SteveWMD - 24/09/2021
Fuselage formers can be easily reconstructed, because you get some basic informations even without these being listed separately:
- Top and side view drawings of the engine are give the basic measurements of this area;
- Side view gives the basic measurements of the shallow triangle of the top rear fuselage;
- Wing center gives the width of the cabin area;
- Central triangle of the front part of the stabilizer gives the outside width of this area, the drawing on how to fasten the tail wheel gives the width of this area inside the side sheets.
As the Cub is of very simple construction, the fuselage just tapers from the trailing edge towards the triangular section that supports the elevator.
Should be a breeze to reconstruct these measurements.
BTW: I love the Cub. One of the prettiest aircraft ever built.
Martin - 25/09/2021
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  • Piper Cub J-3 (oz5021)
  • Plan File Filesize: 1514KB Filename: Piper_J-3_Cub_54in_oz5021.pdf
  • Supplement Filesize: 3667KB Filename: Piper_J-3_Cub_54in_oz5021_article_twin_bee.pdf
  • Supplement Filesize: 137KB Filename: Piper_J-3_Cub_54in_oz5021_decals.pdf
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