Category: Building

Another modification I recently accomplished on the Cub is to reshape the rudder.  Again, I have decided to just look at the general outline of the aircraft.  I realize that when covered, some of the “ribs” will not be in the right place and anyone looking closely will be able to tell this but in the air I doubt anyone will be able to tell!  They will see the outline, which I hope will at least be suggestive of the real Super Cub!

So here is what I did.  After careful comparison of the shape of each I decided that just a bit of a change to the trailing edge of the rudder would take care of the primary difference between the J3 and the Super.  Besides… I’m always happy to have a little more control surface!  In order to accomplish this I decided I would just laminate a new trailing edge.

Here’s the process.  To start I used my balsa stripper to create some strips of the proper width from a sheet of 1/16″ balsa.  After a quick sand to make a slight flat spot at my “start” and “stop” areas I pinned down the rudder and used some thin CA to tack down one end of the first strip.

I then bent the stick to shape and tack glued down the other end and then trimmed of the remainder with my Xacto knife.

I then added two more layers of stripping.  On each one I used some wood glue between the layers and then tacked it in place with pins and some thin CA to insure it didn’t move before the wood glue set.

Once finished with the third strip, I added some additional support at the point where the existing “ribs” would have contacted the trailing edge.  With just a bit of sanding to round things off a bit and to taper the new material at the ends and mate it up with the existing structure, the final shape was complete.

The structure is quite strong and once covered I think most folks will find it to be a fair match to the outline of the real Super Cub.

There is plenty more to do in order to get this bird finished so… more to come!

Another modification I have decided to make on my Cub is to create functional flaps.  Below you can see the inboard section of the port side wing.

After extending lines from the trailing edge of the outboard section and then accounting for the thickness of the trailing edge I made some cuts.  Here is the result.

I then created a gauge from plywood with the proper angle to match the ailerons and marked and cut the flaps to match.  Here you can see it in use.

Once all the ribs were cut to match this angle and length I attached a plate to the back edge in order to finish out the flap structure and allow for hinge attachment.

I then used a plane to match the size and angle I needed on the top and bottom edge of the flap surface.

I will probably need to add some blocks at the hinge attachments point inside the flaps before they are covered in order to insure a strong attachment point.  When finished, the flaps look very similar to the ailerons.  here you can see the leading edge of the flap (left) and aileron (right)

Aside from the hinge blocks, I will need to work out servo placement in the wing to actuate the flaps before I can call these finished but the major work is done now and overall I am pleased.  The only drawback I see is that the angle of the leading edge of the flap surfaces (and the ailerons) are going to limit the throw to approximately 30 degrees of down travel.  I feel like that is likely to be enough for my purposes but I’ll do a little research to see what I can determine.

One thought that occurred to me after finishing these up is that I may just plane the corner off of the bottom edge where it contacts the wing surface.  I calculated that I could possibly gain another 10 degrees or so of travel this way.  I’d be happy with that since the real aircraft maxes out at about 50 degrees… I’ll call that close enough.

While I’m thinking that over, I may finish up reshaping the tail feathers…

I’ve spent a fair amount of time just sitting and staring at… by which I of course mean meticulously planning modifications for… the Balsa USA Cub.  I have been comparing the J-3 Cub and the PA-18 Super Cub from a visual appearance perspective and one of the first things I realized is that the Super has a slightly different shape to both vertical and horizontal components of the tail surfaces.  Looking at the horizontal feathers first, there is a bit of a difference in shape, especially in that the PA-18 has what I refer to as aerodynamic load balancing.  In the case of the Super there is a section of the elevator that is in front of the hinge line located at the most outer section of the elevator on each side.  This design not only give you increased surface but also decreased load on the servo as the air flow on that part of the elevator will aid in moving the surface in the desired direction.  I.E. When you pull elevator the rearmost part of the surface goes up relative to the hinge line and the airflow is fighting this motion which your servo must overcome.  The part forward of the hinge line however is moving downward and the air pushing on it is helping to rotate the surface and therefore is assisting the desired motion!  This is not only a very noticeable difference visually in the control surface shape and design but it is one that will help to keep the servo(s) on the elevator from having to work so hard!  I like that so I immediately decided to incorporate this change.  Another nice part of this modification is that it is simple since there is a rib at approximately the correct spot where surgery will need to be performed to make this change!

Of course the horizontal surface is also a bit different shape on the Super versus the standard cub.  I haven’t decided if I will do anything to correct that as it would be purely cosmetic… adding very little surface area to the elevator… We shall see.  So here is what I did for this first control surface change.

Here is the starboard horizontal surface.

That first rib looks to be in approximately the correct spot so below you can see the first cut has been made right against that rib.  Plenty of TLAR involved in this project!

After making this “incision” I then needed to add a rib to the tip of the fixed surface.  If I just capped them with a piece of balsa I’d end up with a fixed surface that would be to wide for my elevator to wrap around so I then had to cut off about 5/16ths worth of balsa in order allow for insertion of a new 1/4″ rib (1/4″ x 3/8″ was used as the thickness of the trailing edge is more than 1/4″).

I then installed my new end cap as seen below

And after a bit of sanding to size I ended up with this

Rinse and repeat for the other end and I am ready to glue the removed part to the moving elevator surface.  The builder has done very little shaping of the leading edge surface of the elevator so I think I can glue the removed portion of the fixed surface to the front edge of the moving portion and with just a bit of filling and sanding I should be all set.

I will probably leave the tail surfaces for a bit and work on the wings next as I am in the mood to tackle that project.  I’ve been getting nervous about cutting into the wing surfaces for a bit and I think I have my courage screwed up now since this task came off fairly painlessly.  I’ll get back to the tail surfaces again and see what can be done to shape up the rudder a bit later.

I have yet to decide if I’m going to do anything else on the elevator before I call that part good and move on but for now I am pleased with my progress.  There is at least one big “mistake” that the original builder made that will prohibit me from making this plane truly scale looking… at least it’s a big enough discrepancy that I don’t think I will tackle it.  Once I decided that it made me start considering what the limits are going to be in terms of how much effort I am willing to put into this thing.  I feel his pain as I have problems when building sometimes telling my left from my right as well but I’m not going to point it out yet.  Maybe you can try to spot it someday when the project is finished and I get it ready to fly and take a few photos to post here.  It’s nothing that will effect the flight performance of the airplane but there’s no way this thing will ever be in a scale contest!  As a result, there is a fuzzy line in the sand in regards to how far I’ll go to make it scale.  I want it to be recognizable as a super cub and even as a particular airplane but I’m not going to get to crazy making it perfect.  I’m especially leaning toward anything that not only makes it look more Super and less J-3… especially when that affects it’s flying characteristics as well.  I guess it’s just going to be Goldilocks scale.  Not to much.  Not to little.  Just right!

Recently my club had a batch of RC gear contributed and much of it was sold off to club members over the course of a few weeks with proceeds going to the club.  One of the items that got sold off was a Balsa USA 1/4 scale Cub.  The kit had been almost completely framed up and even some covering done.  The craftsmanship looked good and the price was right so I decided it would be my next large scale electric project.  I have immensely enjoyed flying my 84″ wingspan Carbon Z Cub and I figured 108″ of Cub might mean almost 30% more fun!

It also seems like a great excuse to try something with an even larger electric motor and with it in the 90% framed up state, I figured it was ideal to do some “bashing” and make it into something a bit different than the usual “big yellow cub”.

I searched for a suitable subject and came across several attractive options.  I needed to balance the desire for something a bit more unique with a practical eye toward ease of adaptation and availability of covering options etc…  I didn’t want to go to the level of scale competition but I wanted something recognizable as “not” an available ARF.  I looked at the NE-1, the Grasshopper (complete with Bazookas) and a couple others before I settled on a conversion to a PA-18 Super Cub.

There are a number of color schemes that are close enough to available covering colors to make it unnecessary to resort to painting… and many variations of the Super Cub exist… So many of these aircraft have been modified “in the field” that it would be difficult for anyone to point out a combination of “optional” gear like tire size, window size and shapes, cowl changes to accommodate various motors, etc… etc… that isn’t out there somewhere!  The one’s that attract me most are all bush planes with the big tundra style tires!  Those things not only add a nice scale touch but they are practical in that they absorb a bit of the abuse that the air frame would normally have to absorb.

Super Cub cowls are available for this kit so that part of the profile is easy to achieve.  Bush wheels are also available as are light kits, interiors, articulated landing gears, etc… etc…  So a bush variation PA-18 is what I will shoot for.  I picked a picture off the internet of an Alaska based aircraft that I thought would be “within reach” to model and started planning, plotting and modifying.

First on the agenda was gathering info on equipment I would have to acquire.  Bush tires and a nice shock absorbing landing gear are a necessity and I found PR bush wheels…  These guys are out of Alaska and they created scale bush tires and rims for several sized and types of Cubs because no one else really has another really good option that I could find!!  You really need to check out their video of a customer doing wheels skims across a pond!  They are pricey as one would expect for such custom items but one of my flying buddies stepped up and ordered me a set in return for some RC shop work I had done for him recently.  I wasn’t looking for that kind of payback but it is appreciated!  Thanks Kelly!

Of course this forced me to order the appropriate Robart 1/4 Scale Cub landing gear!  Can’t have those fancy wheels on some old, ugly wire gear can I??  So now I have pretty much all I will need to have an awesome landing gear setup for the PA-18.

Here’s the Robart gear with the wheels attached…  These are going to look great!

I also went ahead and order a new cowl from Fiberglass Specialties Inc…  They have a stock PA-18 cowl that fits the Balsa USA kit.  It looks to be their standard quality workmanship (which is high) that has already survived sever drops from my workbench onto the concrete floor with nary a crack.  I really need to quit dropping it!!

The project has since languished a bit as work got busy and I spent some time working on other projects. I also had to spend some time stuffing the piggy bank to purchase a suitable motor, speed controller, etc…  I have finally gotten back to doing a bit of work on the Super Cub and I have found a few more things I felt “needed” modifications.  There are a number of noticeable differences between the venerable J-3 and the PA-18.

I have the cowl taken care of but the tail feathers are noticeably different with aerodynamic counter balances on the elevator and a more rounded and larger rudder as well as some obvious bracing difference in the cockpit.  Also, the vast majority of PA-18s have flaps while the majority of J-3s do NOT.  Those items seemed to be high on the list of what makes a PA-18 look different and they all seem to be modifications that are “in reach” since they will require very little but time and effort to accomplish.

During this busy time I started stripping all of the covering off of the control surfaces and a bit of the tail which was about as much as the gentleman had gotten to.  This was mostly accomplished during short windows in the shop after work was finally done and before bedtime.

I also removed the glow motor mount that I won’t be needing…  The blind nuts I punched out before I realized that the top pair were trapped in an enclosed space and now make a fantastic rattle!  I can see some Dremel work is coming to retrieve those!

This promises to be a fun project so I will have more updates soon as I expect to get in the shop a bit over the next 2 weeks around the holidays.  I’ve already made some progress on the cockpit area, tail surface shapes and flaps so will try to post on some of that soon.