Building the Piper Super Cub 95 (Kit 602)

[Mfezi]

I built the "Build-by-Numbers series" Piper Super Cub 95 kit more than 30 years ago as my first balsa model - so I thought it would be interesting to re-live that experience. I was also hoping my skills had improved a little since building that model which, by the way, turned out to be a key factor in my eventual career choice. At the time I was only about 7 or 8 years old if I remember correctly. My motivation then came from my dad who also built (and still builds) free-flight, control line, plastic and radio control models. I have a very vague memory of him building an all-blue Sopwith Pup at the kitchen table when I was probably only about 4 years old. That model sure triggered an interest that never went away. So, when I got to around age 7/8, which I figured was old enough, I convinced him to buy the Piper Cub kit for me. I built it over several weeks, but never flew it. I remember covering it in yellow and red tissue, but cannot recall if that tissue came with the kit or from the local stationary store. It hung from my ceiling for many years until I must have eventually played with it to destruction.

So, let's get to it. Here is what you get in the box today:


I honestly cannot remember whether the box contents today is any different from those that came with the kit all those years ago. You'll notice small stickers on the three die-cut balsa sheets: I always weigh the balsa in my kits and calculate the density, which I write on those stickers as a reminder. I do the same with every sheet of balsa that I buy. Here is what I found in this kit:

Sheet A (1/20" balsa): 120 kg/m^3 (7.5 lb/ft^3)
Sheet B1 (1/20" balsa): 238 kg/m^3 (14.9 lb/ft^3)
Sheet B2 (1/20" balsa): 244 kg/m^3 (15.2 lb/ft^3)

The die-cutting itself was actually not too bad, but those two B-sheets were really way too heavy for me. Regardless, I wasn't really planning to use the die-cut sheets as I don't like the 1/20" balsa. I recently built the Typhoon and found once you sand the balsa to release the pieces and get all the fuzz off, it gets so thin that tail surfaces, in particular, would start warping over time. On the Typhoon I ended up re-building the tail parts from normal 1/16" balsa and they came out much stiffer and still at the same weight. My plan was therefore to re-make all the die-cut parts from my own 1/16" balsa. I could have used the die-cut parts from the A-sheet as that balsa was OK, but eventually decided to re-make all of it. The strip-wood was also pretty heavy and hard, but I prefer it that way for these kind of models. The model will be flown but only for fun, so I thought the strip-wood in the kit was a good compromise for the added strength and stiffness.

I have a couple of other criticisms: The tissue colours are really not very inspiring. I will use my own Esaki for this build. I can understand why Guillows doesn't use Esaki as it would make the kit too expensive, but some brighter coloured domestic tissue would really have been nice. They included a bright red domestic tissue in their new Pilatus Porter kit that turned out to work pretty well, so I think it would be nice if they could include something similar in this kit instead of the white and grey tissue.

The plans are very nicely drawn and the instructions are very clear. As I said, I built the same kit when I was about 7 years old with only minimal help from my dad, which gives you some idea of how clearly everything is explained. A minor point that I discovered when I started cutting my own parts was that not everything is perfectly symmetrical - that is obviously because the plans were originally hand drawn and not CAD drawn. That really wasn't too much of an issue though.

Everything else in the box looks like it can be used. Better rubber for the motor will obviously be a good idea. My 4-year old son, Kirill, is going to help me with this model. I'll try to post an update about once a week. Unfortunately I am not the fastest builder out there...

[Wildpig]

looking forward to your build thread.

So what was your career choice?

[Mfezi]

Since I decided to use my own balsa instead of the die-cut balsa that came with the kit, I had to do a bit of extra work before building. I thought I'll show the process that I follow here - it is not the most efficient, but I have been doing it this way for many years and actually enjoy doing it. I have made entire scale radio-control "kits" this way. Of course, there are many other ways that also work well.

The first thing that I do is to trace all the parts that I will make from the plans. You could just photo-copy the appropriate parts of the plan, but I like to do it this way as it puts everything together and gives me some time to study the plans properly. I use either a high quality tracing paper or draughting vellum, both of which tend to be quite stable dimensionally. I use a sharp pencil for tracing, as you do need to make the occasional correction.



Once I have traced everything, I make two copies of my parts sheet. I actually only needed one, as I use the first part as a template for the other one when doubles are needed. However, the extra copy is useful for the odd mistake which you are bound to make. Next step is to cut the paper templates from the copy (I save the original tracing for future use, or additional copies):



I then glue the parts on to the balsa with a glue stick and start cutting. I use a #11 scalpel for most of the cutting out and a Great Planes Easy-Touch sander for final shaping. I think the following pictures are self explanatory:



The picture below also show some of my home-made sanders. The sandpaper is glued on with glue sticks. The ice-cream sticks with balsa strips and sandpaper are for different sized notches. I have a very large collection of home-made sanders, sanding sticks, sanding sponges, etc.



When duplicates are needed, I just use the same glue stick to glue the first set of finished parts back onto the balsa as templates. However, it is important to use the absolute minimum amount of glue, otherwise you won't be able to separate them again:



To remove the paper from the balsa, I just wipe the paper with a wet cloth, wait a few seconds for it to soak through to the glue to soften it and then peel it off. I then wipe the parts with a slightly damp paper towel to remove any glue residue. When making duplicate parts you should try to separate the two halves before the glue completely dries. I separate them very carefully by wedging a single-sided razor blade inbetween the two halves. After a few evenings, I ended up with a complete set of parts:



You'll notice I did end up using a couple of landing gear parts (L1) from the die-cut balsa. One of the nice things about making your own parts is that you can select the appropriate wood for each part. I used 5lb, 8lb and 10lb balsa for the various parts. The tail parts and wing ribs (except the root rib) were all made from 5lb balsa. The medium balsa was used for the majority of the fuselage formers and the heavy stuff only for the root ribs, the most forward fuselage former and the former to which the landing gear attaches. You could probably make everything from 5 or 6 lb balsa, but since I don't fly competition I think the extra strength is a useful compromise.

[Mfezi]

So what was your career choice?

I'm an aeronautical engineer - ended up with a Ph.D. in applied aerodynamics and later specialised further in flight dynamics, flying qualities and aerodynamic systems identification. These days I work mostly in the military flight test world. I never lost interest in model aircraft though

[Phugoid]

This kit can be made to fly really nicely even with heavy-ish wood. I look forward to seeing yours fly.

Andrew (also an aeronautical engineer by education, but not as quite as qualified as you!)

[Mfezi]

Thanks, Andrew. I really enjoyed your thread on the Cessna 180! I hope this one looks as good and flies as well as your Cessna did. When you balanced your model I saw that Gas Turbine Theory book you used as part of your balancing rig and realised aeroplanes were probably also more than just a hobby for you...

[Phugoid]

Ah yes, Cohen, Rogers and Saravanamutoo. Standard reading for gas turbine theory.....

I built the super cub too a while before the Cessna, and unlike the Cessna, it's still in one piece. There is a thread on here somewhere not quite as detailed as the Cessna but it might be of use to you.....

Andrew

[WIDDOG]

Great Build, very interesting. I recently started using my own wood and cutting out the parts.

[Mfezi]

OK, some progress pictures (I did this about a week ago). I simply followed the build order recommended on the plan as it is the same as I would have done anyway. This means tail and wing struts first:



To make sure the left and right wing struts were identical, I waited for the first one to dry, sanded it off and then placed it back on the plan with a littlebit of wax paper on top of it. I then built the other one directly on top of that. This method does ensure they are identical, but it is probably not really necessary to get them that accurately identical. It is a useful technique for some parts though. It is a little difficult to see the first part through the wax paper, but I think you can vaguely make it out:



Some sanding and the first few parts are essentially ready for covering:



By the way, when I do the cross pieces in the tail, I use a very traditional technique: I mark the length of each part with a very soft pencil (7B), then cut it about .5mm longer than it needs to be. For the cutting I use a single-edged razor blade. I then use one of my sanding blocks or sanding sticks to trim it to the exact length. It is very quick and with some practice you get an exact fit every time: It should basically stay in place without the need for pins, yet not require any force that might result in residual stresses in the structure.

[Mfezi]

I built the wings over the weekend. They were also very straightforward:



Notice what I use for applying glue: It is a syringe with a very thick needle. I ground the tip of the needle off with a Dremel tool so it just ends in an even blunt tip. The little bent wire that sticks out of the needle is just to prevent the glue in the tip from drying out. I have had this particular syringe and needle for about 8 years now. Each time when I refill it, I first clean it out thoroughly with water, let it dry and then refill - good to go for another model or two. Once or twice the needle became blocked and I just worked a thin piece of wire through it a couple of times to clean it out. The glue is a locally made (South African) aliphatic resin, similar to the aliphatic glue you get from Great Planes or Elmers in the USA. All aliphatics are pretty similar. I prefer them over standard white glue, as they seem to sand better once dry. White glue has other uses again. I know everyone has their favourite glue and applicators, but these have worked for me for a long time. I even use it for the majority of my R/C models.

Once removed from the building board, the wings were sanded smooth. The little tabs at the trailing edges of the ribs were blended with a small sanding stick. A long sanding stick was used to get the whole wing nice and straight and all the ribs perfectly equal; a smaller one was used on the curved wing tips. I know some people hate sanding, but it is one of my favourite parts of the build. I particularly like doing leading edges - I feel the wings transform from a bunch of glued-together sticks to a real wing structure during the sanding process. I have a small razor plane that I first use on bigger areas, such as leading edges of R/C models and bigger free-flight models, but here it was never needed.



In the following picture it looks like the main spar is everything but straight. The zoom on the camera and the angle exaggerates the effect - in reality it is not too bad. I wanted to show here one of the advantages of making your own parts - note how perfectly all the notches fit when you make them yourself with the notching tools that I showed before. This never happens with die-cut parts, but laser cut parts are another story and usually just as good or better than what I got here.



The wings are now essentially also ready for covering. Note one small but important modification from what is shown on the plan: I added two small triangular gussets between the root rib and leading edge and root rib and trailing edge. These small additions helps to prevent the rib from deforming, thereby causing wrinkles in the covering. That is also the reason why I used relatively hard balsa for the root rib:



One final thing that I did before setting the wings aside was to make sure that the slots in rib F3, where the wing struts meet the wing, are sized perfectly for the struts. It is very difficult and frustrating to try and do this after the model has been covered while trying not to damage the covering.

[Mfezi]

Over to the fuselage. Now is probably the best time to make modifications to the nose block if you plan to do your own thing there. Because the cowl on this model is so shallow, and it is supposed to be a simple model, I decided to build it more-or-less like on the plan. The only modification I made was to add a small piece of 1/8" (3mm) ply behind the most forward former that was drilled out to the exact size of the nose button. The idea is that it helps to prevent the hole for the button loosening up over time. It is obviously much heavier than balsa, but it is small and you will normally need some weight in the nose anyway.

The vertical strengtheners behind the former had to be moved outwards a bit, requiring the slots to now run through them. The balsa was so hard, that I just used my dremel to cut the slots in the strengtheners. It is not as neat as the slotting tool, but it worked.



I also test-fit the cowl at this stage: There is nothing as annoying as discovering the front former is the wrong shape after you have already assembled the whole fuselage:



I also clamped the landing gear wire between the two formers at this stage, but forgot to take a photograph. I used epoxy to glue the wire to the formers and the two halves together. Epoxy glue is heavy, so I only use it for very specific applications. In this case, gluing steel wire to balsa was such a place. Medium or thick superglue would probably also have worked, but I prefer epoxy. In order to recess the wire into the balsa formers prior to gluing, I first pressed the wire down to leave a mark and then used a small embossing tool to make a deeper slot. An old ballpoint pen would probably have worked if you didn't have an embossing tool. Since the wire is thin, there was no need to carve a slot. Sorry about the missing picture, but I think the description should be quite clear if you had the plan next to you.

The frames for the fuselage were built over the plan as recommended. This is one of those places where you could have built the two sides on top of each other like I did with the struts. That would have ensured they were absolutely identical. The disadvantage with that method is time: You have to build one, wait for it to dry, remove it, sand it, put it back, add some wax paper and only then start the other one. I just built the two simultaneously over the plan.



The plans call for some strip balsa "doublers" on the frame in a few places. This would have made it difficult to sand the assembled fuselage sides. I therefore removed the frames from the plan at this stage, and sanded the joints and everything else nice and smooth on both sides:



After sanding the two fuselage frames smooth (and making sure they were identical), I pinned them back on the plan and then glued on the doublers and strengtheners.

[Mfezi]

It took me about four evenings to build the fuselage, but that was because I only had a little time each evening, and because I sometimes allowed the glue to dry fully before going on to the next step. I just followed the instructions on the plan and really didn't deviate much. Here and there I wasn't happy with the lengths of the cross-pieces as shown on the plan and sanded them down a little to give me the correct curve to the fuselage. I didn't take pictures of all the steps, but here are the ones that I thought might be useful or interesting:





The next picture shows a tricky part on the bottom of the fuselage where a cut-out will be made to help you load the rubber for the motor:



Then on to the sanding. My principal sanders are my two Great Planes Easy Touch sanders, one loaded with a medium-coarse sandpaper and the other with a medium-fine sandpaper. I also have a big variety of other sanders that I use as required. The sanding block in the picture below came with an X-Acto knife and carving set and I have been using it for more than a decade. Its rounded ends are particularly useful. The thin sander, on the other hand, is a strip of 1/32" plywood with a strip of sandpaper glued to it using a glue stick. It is very useful for getting into small spaces.







At this point I suddenly realized I was done with the basic structure! Nothing left to do before covering, which I will start to do this weekend.

[Mfezi]

Of course, one always gets a bit nervous when your local FAA inspector arrives. Mine was particularly critical this time

"Ok, let me see..."


"Ooohh, this does not look right..."


"Hmmm, this part looks a bit better..."


"Right, I'm happy. You may continue with the covering now."

[Steve Blanchard]

I was always nervous when my FAA inspector showed up too. It seemed if there was something he didn't like about the ship or thought was unsafe he'd just smash it or break off the offending part. Luckily he's much older now and has moved on to much more pressing issues such as driving and high school football. Let's hope he's over his smashing phase (for driving of course, I'm sure he needs to maintain the smashing for football). Anyway, Looks great so far. Best of luck!

Steve

[Scott]

Thanks for posting your build pics.