Sig J-3 1/4 Scale Cub Build Series – Part 12 – Wings and Wing Tube

Since the Sig Quarter Scale cub is an older kit I decided I wanted to “modernize” the wing design by incorporating a bolt on fuselage center section, wing tube, and a two piece removable wing. The original design called for the center section to be permanently attached to one of the wing halves. I did preserve the functional wing struts for this build however.

To begin the wing design I had the plans scanned at my local UPS store so I could work off an image of the plans using a program called DevWing. Many builders in this hobby are familiar with the “Profili” program offered by the same company. https://www.devcad.com/eng/devwing.asp

DevWing is a great program that allows you to completely design a wing with spars, wing tubes, dihedral, servo boxes, and even multi-part wings just to name a few of the features! I started by basing the design off of the scanned sig plans and drew in the positions for the aileron and spar locations. I also used some basic trigonometry to build in 1.75 degrees of dihedral into the wing. (A little dihedral in a cub is a good thing giving stable flight characteristics and also prevents the wing from looking like it’s “drooping” in flight compared to a completely straight wing with no dihedral.)

DevWing also allowed me to specify the wing tube locations and it automatically accounts for the dihedral I had specified. I decided to make the first three ribs incorporate a wing tube from TNT Landing Gear. I used their 5/8″ by 30″ long aluminum tube with a .049 wall thickness and socket.

Another thing I decided to do was build in two degrees of washout into the wings. DevWing allows you to configure this and builds the washout into the building tabs it creates for each rib. This allows you to build the wing on a flat surface without the need for a wing jig! Washout will prevent the wing tips from stalling as quickly as the center of the wing toward the fuselage – helping to reduce the chance for the dreaded tip stall. (NOTE: the test flights with this built into the wing have been extremely stable and without event. Landings and take offs are very graceful and smooth!)

DevWing also allows you to lighten the ribs by automatically generating cutouts into the ribs. This really helps to keep the weight down and doesn’t compromise strength. It also serves to make running servo wires much easier. With the DevWing program you have complete control on how much material the program is allowed to remove. I used 3/16 balsa – which is a little thicker than stock. Using the thicker ribs will make up for an deficit in strength and wider balsa is easier to work with while building the wing on the bench!

Once the wing is fully designed (you can preview a 3D view of the wing from any angle as you build so you can get a good idea of what is looking like) DevWing provides an export feature allowing you to take your parts to the nearest CNC machine for cutting. (A couple of years ago I invested in one of the eBay CNC machines – so far it’s been reliable and has worked really well! You’ll have a lot of fun with all the precise parts you can make!)

Once all the parts were cut out I began the building process on my magnetic work bench. I’ll spare you the detailed description of building the wings since the process is the same for most wings. Just remember that the flatness and straightness of the building surface will determine how straight and precise your wings are.

I normally start by laying down the bottoms spars in place and securing them with magnets to keep them from shifting around. Next I start by placing the ribs in one by one – dry fitting them to ensure a good fit before any glue is added. Once the ribs are in place I’ll glue them with CA making sure to use a square next to each one to make sure it’s perpendicular to the spar and lines up properly on the plans. With the ribs securely in place I’ll put on the leading and trailing edges. Finally I top things off with the top spars. Once that is all dry I then add in the shear webbing which greatly increase the strength of the wing at this point. Once the spar webbing is dry I sheet the top and bottom using the traditional D-tube construction method. Once the sheeting is in place the wing is very stiff and strong – just the way we want it!

Here’s a gallery of photos of the building process!

I will write a separate post showing some screenshots from the DevWing Program.