300 Series Piper Super Cub Kit Bash Into L-4

[kittyfritters]

I'm doing a quick kit bash of a Piper Super Cub into an L-4 Grasshopper for the Blacksheep Squadron "Grasshopper Challenge" on the 17th. The CD has told me that any plane that had an "L" designation would qualify and my Air Force Beaver (L-20) would be OK, but I think this will fly better.

I found reliable drawings of the L-4, traced the critical sections, the nose and greenhouse cabin, enlarged them to the proper scale, printed them on tracing paper, and placed them over the Guillow's plans to see if the kit bash was possible. The nose ended up with exactly the same length (moment) so I went ahead with my project.



The next thing was to re-draw the cabin side, part "H", so that I would have the L-4 greenhouse cabin.



Then I built it.



Since I am building "scratch" , from the plans, I need to cut out some parts. Using my balsa stripper and miter box I cut out 24 rib blanks. Since I am lightening the structure I cut only the root ribs and strut attachment ribs of 1/16" sheet. All the rest are 1/32" sheet



I use the stacked rib method for cutting ribs on constant chord wings. The rib patterns were cut out from scans of the plans, glued to heavy card stock, cut out, and glued to the rib blanks that would be on the ends of the rib stack.



The rib blanks were stacked evenly and stuck together with some new, sharp, pins. If I had needed more than the 20 identically shaped ribs I would have glued the stack together with Testor's "green glue". That way, when you are done shaping and sanding you can take them apart with acetone.



Using my razor saw and a "Cleveland notcher" I cut the bottom spar notch and inserted a piece of spar stock to help keep the stack in alignment.



I turned the stack over and using a fresh blade in my Xacto knife I cut the front curve of the stack down to the pattern. If I had a miniture belt sander this would have gone a lot quicker.



Cut down the rear of the stack to the pattern.



Sand the stack right to the pattern, cut the spar notches, pull the pins and you have a stack of 20 identical ribs. Then I cut the four smaller tip ribs. The whole process to make 24 ribs took less than 1/2 hour.



More as it progresses.

[kittyfritters]

To keep it light I wanted to do laminations of the wing tips and tail surfaces. Printing scans of other parts of the plans I glued them to foam core board and cut them out to make molds for outline laminations.



All you have to do is soak the wood strips in water, apply the glue (Original Gorilla Glue in this case), bend them around the mold, tape them down all around, and put them aside overnight to dry. You gotta love the Scotch Pre-Cut tape dispenser. My wife introduced me to that for gift wrapping and it come in real handy for this.



Even the stabilizer outline is no problem.



Again, printing from scans of the plans, I laid out the formers for cutting, There are some modifications, however. The formers from B1 to B5 are on 1/20" sheet and the formers from B6 aft are 1/32" sheet to keep it light. The hole for the rubber cavity has been enlarged since flying my other Piper 95 showed the need for the additional rubber room. The stringer positions from former B5 aft have been adjusted so that the stringers will be in scale positions for the L-4, and the keel notches, top, bottom, and sides have been adjusted for narrower keels, again for lightness.



To be continued...

[kittyfritters]

Again, trying to keep it light, I am using much thinner keel outlines. I may be unintentionally building myself and indoor model.



The wing spars are also all 1/16" square. The trailing edge is 1/8" X 1/16". The leading edge is standard size, but much lighter wood.



At least it's light. The wings have not been trimmed or sanded here, but are still sub two grams per panel. I'm beginning to worry about just how much rubber it will take. If this works, (That means I can cover it and put it together without breaking it.) it will be sub 20 grams, finished, with 80 square inches of wing area. The glide should be a bit floaty.

[kittyfritters]

At last, the fuselage finally has enough structure that I can handle it without breaking something. This has been one of my most maddening builds. I'm beginning to wonder if I no longer have the manual dexterity to build with "contest grade" (i.e. 4 to 6 pound/cu ft) wood. If I were designing this from scratch for this light wood I would have assembled it a bit differently. The fuselage is still sub two grams here.



The landing gear wire is going in "Beaver style" with an end-to-end wire sandwiched between two formers rather than the individual axle halves on the ends of wooden struts as shown in the original plan. I am still going to beef up the bottom corner of the fuselage with the "L" parts where the landing gear attaches.

[kittyfritters]

Now to make a proper J3 style nose for this thing.

First assemble the flat pieces.



Then get all the block parts together.



Test fit and mark the block parts for outline.



Hollow the top block for prop shaft clearance.



Smooth out the curve with a Gizmo Gri-Tube.



Glue the blocks on, carve and rough sand. Then attach the nose block with a couple of drops of "green glue" for carving and sanding.



Nose sanded with standard Guillow's thrust button. The bottom contour of this cowl, going from a shallow, almost flat, "V" to rounded is tougher than it looks.



Test fit to fuselage.



I decided not to rush to finish this for the contest since the weather predicted will be a little rough for so light a model. The Liaison Plane Challenge will have to be flown by the Beaver (L-20) and I will fly the Porter in the High Wing Cabin Sport contest. Let you know how I do. The L-4 will still progress despite other projects.

[granpa]

Your talent is amazing!

[kittyfritters]

Now to deal with the exposed cylinder heads on the L-4. I decided to make them the easy way.

Foam cylinder heads.



Flex-Straw finned cylinders.



Cylinders assembled to heads with Uhu styrofoam blue. It's a contact rubber cement. Apply to both sides, let dry 10 minutes and press together. The conical objects in the background are new exhaust stacks for one of my Pilatus Porter models which was on the bench at the same time.



"Crankcases" of 1/32" sheet to attach the cylinders to nose. Tried to make the cylinder baffle air scoops out of straw material, but the plastic has too good a memory to be bent right. Ended up using paper and printed patterns from Fiddler's Greed J-3 Cub.



Test fit to nose. Exhaust stacks will be foam, pushrod tubes pins. Slightly enlarged patterns from Fiddler's Green J-3 are right size and shape, but will be stiffer light card stock.




By the way, didn't do well at all at the contest the other day, Had one of those days where I couldn't get anything to fly. Beaver and Porter are flying fine now. (Of course!)

[kittyfritters]

Starting the painting with the engine cylinders.



My wife took a picture of my "Research Assistant", Schrodinger, reading over my shoulder the other day. He's not allowed in the garage where I am building now, so he hasn't been taking an active role in building lately. Avoids "cat-tastrophies".

[Bill Gaylord]

I noticed that you also hand cut parts from prints. I would have almost thought that the kit is now a laser cut kit, without seeing the prints. With a steady hand, it's not difficult to have better parts than the die-cut parts, and you parts appear to be cleaner than the kit parts. Very clean work. Love the "flexi-straws" for the Continental engine also.

This is a kit that I keep thinking about building from scratch again myself. I built the original as one of my first kit builds, and came out with the bright idea that I could make it more robust, using plastic 1/16" stringers on the fuse, of all things. Standard weight iron-on covering was also used. Robust yes, but practical no, and the covering job is not something that I would show to the public. Some years later as an experiment, I did manage to prove that with roughly 50W of BL Feigao power, the plane was flyable with RET control at 9oz.

Per "cat-tastrophies", I lost a canopy to my first 400 series FW190, shortly after I got my Pomeranian. Couldn't find it anywhere, and then I pretty much figured it out. You can't find it, because it was dropped on the floor and moved by someone else. Went upstairs, and there it was, all chewed up. I recommend learning basic canopy molding by pulling heated sheet plastic over hand formed balsa molds, to all modelers with pets.

[kittyfritters]

The reason that I hand cut the parts from prints on this one is that I was trying to build it as light as possible. Lighter than even the wood now supplied with the laser cut kits. All the sheet wood is 1/20" and 1/32", 6 pound stock. The blocks in the nose are from some old 4 pound stock that I happened to have. The strip wood is 1/16", 3/32" and 1/8", 4 to 8 pound stock. With the difficulties that I have had assembling this with my aging hands I should have gone for the 8 to 10 pound strip wood currently in the laser cut kits. So far, as shown, the all up weight of the components is 10 grams. If I were to do this again I think that I would start with a current laser cut kit, but it is what it is.

Schrodinger's problem is that he is genuinely trying to be helpful.

He supervises design sessions...



contributes to development ideas...



and inspects parts for quality.



Cats! You've gotta love 'em.



Howard

[ADW 123]

Kittyfritters, I am curious... what CAD program you use?

[kittyfritters]

It's not a CAD program, per se, but an illustration program called Inkscape that I run under Linux. The package works in scalable vector graphics files (.svg) so that it's easy to resize anything and can save drawings in just about any format. My Linux box is about four times as fast as my Windows box with the same processor and memory and the software came, free, with the install. (I was formerly an accounting programmer for systems on Unix and Linux servers.) I have considered going to a 3D CAD program, but this works well, so far.

[kittyfritters]

I was probably getting a little carried away with the exposed cylinder heads on the dummy engine. I added push rod tubes to the bottom and top and bottom ignition wires. The intake tracts and exhaust pipes, of painted foam, will be added and the cooling baffles of paper. Not absolute scale but close enough for a model intended as a sport flier.



Then, as I was sanding to get ready for covering I had an OH $%*#! moment. I felt one stringer start to give in my grip and before I could let go of it the results were as shown in the photo.



I just tired to build it too light. I have started on another fuselage with the same weight formers, but 8 to 10 pound stringers. A few years back I probably could have gotten away with the super light wood, but I just don't quite have the feeling in the finger tips that I had. As my father says, "Getting old ain't for sissies! It may still come out under 20 grams, but I will be happy if it is under 30. The Piper 95 I built out-of-the-box for last year's "G" Challenge weighs 78 grams with the rubber in it and it still flies, so 30 grams should fly well.

[kittyfritters]

OK, here we go again. After the "crushing failure" of my first attempt at a super light, 300 series fuselage, I went searching for a specific balsa sheet to use for strip wood. Back in the day, when a plan called for a component to be made from "rock hard balsa" this is the stuff they were talking about. I used it for the keels and the stringers at the edges of the fuselage where the stingers failed the first time.



The formers are still contest grade stuff. 1/32" aft of the C.G., 1/20" forward, and 1/16" for the front former.



The rest of the stringers are eight to ten pound balsa like the material in the new laser cut kits.



I find that miniature Scunci hair clips make good, low crush, clamps for delicate work.



The project continues...

[Bill Gaylord]

I admire you for starting an entire new build. I end up just patching in the broken stringers with new ones, which I end up breaking even when using the ironwood stringers.