The Basics of Stick Framing
In recent months we've seen many "converts" from ARF-ing to Modeling. And with so many new builders cropping up, it looks like the time is right to offer a bit of "How-To" on the subject of Stick Framing. Stick Framing is not difficult, but as with everything we do, there is a learning curve involved. However, once you learn the basic techniques, you can build anything.
The tools required for Stick Framing are minimal and basic. Here's what you'll need to get started:
1- X-Acto Knife
2- Sanding Block - I made one 2 1/4" X 4" from 3/4" thick pine with 100 grit on one side and 220 on the other. The Sand Paper is glued on with Super 77 spray adhesive.
3- 24" Aluminum Straight Edge
4- Small Dress Makers pins - available from Fabric Shops
5- Building Board - I made my 18" X 48" board from 2 layers of Ceiling Tile glued together with Super 77 spray adhesive.
6- 2- Machinists Squares - available from Hardware Stores or Home Centers
7- Waxed Paper to protect the plans
8- 6" Steel Ruler
In addition to the basic tools, a few lead weights will come in handy, as well as a few other items you'll discover along the way. Be creative, you never know what might work for you as your building style develops. And the type of glue used is strictly up to the individual modeler. For basic framing I use medium Cya, but any good quality wood glue will do nicely.
The whole concept of stick framing is not new, and has been common to free flight modeling since modeling began. The idea is to build a strong, yet very light structure that just can't be achieved using large sheet balsa or plywood parts. And though each model you build will be different, the techniques are common to all, so rather then cover specifics to the model used to demonstrate the process, I'll speak general terms that can be applied to all of the models in my Short Kit line.
Wood selection is also a factor in getting the frame built straight and true. For the longerons you'll want to use a medium/firm wood for stiffness, and durability in handling. Match the wood for both sides so that it will bend similarly so's not to build in a pre-load that will induce unwanted warps. Firm wood should be used in the higher stress areas like landing gear mounts and such. Then for the vertical and diagonal bracing, a medium wood is fine, especially at the back end where overly heavy wood will be a penalty in balancing the finished model. So with that, lets get started.
Building the Side Frames:
Study the plans to find any items that might need to be pre-assembled before framing begins. Also note that not all wood thickness is the same, so be careful to make a L. H. and a R. H. assembly where applicable.
The secret to a good strong frame is in the joints. A good fit is absolutely essential, so take your time to get the angle cut properly. I begin by scoring the wood with the knife in place. Then cut the stick and test fit the part. Use the sanding block to true up the final fit through trial and error, and don't glue it in until it fits. You want a nice snug fit; too tight will induce twists and warps -- too loose will make for a week joint.
To start the assembly process, pin down the main former that will set up the frame alignment for assembly. Then pin the top and bottom longerons in place, gluing the joints where contact is made between components. I like to add the 1/8" Sq. balsa main structure first along with all of the cut parts where they mate up with the primary structure.
After the main structure is glue together, then the vertical and diagonal bracing can be added. Fit the bracing carefully to insure a good fit and glue joint. The compound angles of the diagonals can be a bit tricky, so work slowly and carefully to insure an accurate fit before gluing it in place. Then with the first side frame finished, the second side will be done in the same fashion. But be careful, if the sides are dedicated Left and Right, be sure you build in the differences. If necessary, you can coat the plans with either WD-40 or Wesson Cooking Oil to make the plans transparent and build over the back side.
Joining the Side Frames:
Pick a flat spot on the frame and pin the frames over the Top/Bottom view of the plans. Add the main formers on top -- or bottom if the fuselage would be easier to pin upside down -- using the machinists squares to align the frames vertically.
While the fuselage frames are still true and secured on the plans add the remaining formers where the sides are parallel.
With the top formers in place, build up any sub-assemblies involved and start adding the bottom former or cross pieces. Keep a close eye on the alignment at this point, though once the first couple of formers are in place the frame will stiffen up nicely.
Once the first former(s) are in, the remaining bottom formers or cross pieces can be added in the parallel cabin area.
Using a smooth jaw pliers to support the longerons, crack the stick where they will bend at the point where the frame begins to narrow. You don't want to break it off, crack it just enough to make the bend.
Re-align the frame over the plans and add the forward-most former and glue it in place. Align the tail post in the same fashion and glue it in place. A cloths pin works great for clamping the tail post together. (The M&M's are for keeping your strength up during build) At this point, while the frame is still aligned over the plans I'll put a drop of thin Cya on all of the points where the longerons were cracked to reinforce the weekend sticks. You can also add the top formers at this point.
Now you can add all the remaining formers, wing joinery, or other items that might need to go in before the stringers are added which might limit access into the fuselage. Then once all that is done, add the stringers using the plans Side, Top, Bottom, and Cross Sectional views for reference.