gtmattz

Hey guys, I've been wanting to do a scale plane, and have been kicking around the idea of getting a GP Piper Cub 60 kit. I would like to 'do it up' as a L4 however. In doing research on the L4 I am seeing 2 different versions. One appears to be a normal civilian J3 done up in OD green with military markins, this one seems to be more rare. The other, and the one I would prefer to try and build, has a 'greenhouse' type cockpit with windows that wrap around the back and over the top of the fuse in the rear. Has anyone done this, maybe have some tips on how I could accomplish building the greenhouse cockpit and maintain structural strength to hold the wing in place?

khodges

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I've done it, and it's a fun build, but I wouldn't go with the GP to do a kitbash. The Sig 1/5 J-3 kit just begs to be modified, it's the one I used. There are several variants of L-4, which is just a military designation for the J-3 Cub. The L-4C and L-4D, as well as the Navy NE-1 and NE-2, are "standard" Cubs in layout, whereas the L-4A,-B,-H, and -J have the full greenhouse center section. Actually, there are a couple of L-4's that aren't truly J-3's. The J-4 Cub Coupe, had side-by-side seating, an enclosed cowl, longer wingspan, wheel pants and 75 hp engine was designated L-4E, and the J-5 Cruiser was the L-4F and -G, and the Navy used several of these as air ambulances and called them HE-1 (later, AE-1)

In Korea, the Cub was updated and designated L-18.

At any rate, changing the center section is fairly straightforward. I have a ton of photos, both full-size and model, of the greenhouse. There are several other guys here on RCU that have done this, too, and I'm sure they will chime in, the more, the merrier. I'm happy to share whatever i have that will help, here's a few pics to get you started:

khodges

My Feedback: (1)

I missed where you said GP-60 size, I was thinking 40 size. If you want the larger scale, I'd go with the BUSA J-3 kit to modify---they even sell plans for the L-4 based on the J-3 kit, but it isn't a truly accurate conversion. The forward center structure is sheeted, even though the rear greenhouse is there.

The advantages to using the Sig 1/5 or the BUSA 1/4 are that both utilize the center section as part of the fuselage, and the wings attach by the root to the side of the fuse, the bolts are hidden. On the 1/4 Sig, and both the .40 and .60 size GP kits, the wing contains the center section and it bolts down from the top using nylon wing bolts that aren't concealed. There is much less engineering to do if the center section is part of the fuselage to start with. In either case, for the conversion to work, the wing struts must be functional to share the wing loads with the center section. Both the 1/5 Sig and the 1/4 BUSA use functional wing struts already, so the hard points are already part of the design.

Another plus is that the Sig is a more precise 1/5 scale than the .40 GP, and the BUSA 1/4 is a more precise scale than the .60 GP, so if you want scale gear from Robart or Wallace to trim it up and make it purty they will fit better.

A friend of mine is building a Sig, and is using my mods. I did not make pics of mine "in the bones" when I built it, but he made pics of his. Here's a sample:

gtmattz

Wow, all those pictures look awesome. Thanks for the tips! I understand now why the GP kit wont work, and am willing to go with a smaller size, I just want to get a decent scale looking L-4. So it looks like the SIG 1/5 J-3 kit would work well for what I want. Now all I have to do is slip it past the ol' lady ;D

khodges

My Feedback: (1)

Some other tips--the plane will build out heavier than the stock J-3; all the balsa in the center should be replaced by either spruce or basswood for the added shear strength, and lite ply should be replaced with hobby ply (more plies per thickness) in that area also, For good performance, select an engine at the top end of the recommended range. I fly mine with a .65 Saito, which gives me all the performance I need for prototypical flight, turns a scale diameter prop, and it will cruise at 1/4-1/3 throttle. A OS .70 or Saito .72 would also be great choices, and if mounted inverted, will almost completely hide in the cowl. My plane weighs about 10-1/2 pounds, and I have the scale gear from William Wallace, www.piperj3gear.com but Robart also makes a very nice set.

The wing tube system is eliminated in my modification; each wing is attached at the root by two 6-32 hex screws, and by the wing strut to the lower fuse. The Sig kit is nice because the drop-down door and swing-up window is functional, and allows good access to the inside of the cabin for radio, batteries and servos. The Sig comes with a pull-pull rudder linkage, and is all laser-cut, a very enjoyable build.

I covered mine in Koverall / dope, highly recommend this, it's worth all the extra work. The cover has not loosened at all, and there are no wrinkles anywhere, and the seams just disappear. I have had to patch mine a couple of times, and the repairs are easy and also just disappear. If you want a pre-finished material, I'd recommend Solartex.

I have pics of the plans with the airframe mods and a ruler for scale to make the greenhouse mods, let me know if and when you're interested.