Making Bowed Outlines
by; Pat Tritle
The one question I hear more then any other regarding model building from kits
or plans is, “How do you make the bowed outlines around the tail and wingtips”?
The technique we’re speaking of is the laminated balsa outlines formed to the
shape of the control surfaces and wingtips. The advantage to these pre-formed
outlines is two-fold.
First, the outline is very light, a real plus on a model with a short nose or long tail. And second, they’re strong. With a segmented balsa outline there are several parts with many glue joints, and the grain direction can’t always be optimal for the location of the part. With a bowed outline, the grain is always running in line along the perimeter of the bow.
There are no special tools required to make the bows. All you’ll need is a roll of masking tape, a paper measuring tape, and a pan to soak the wood in. There are several ways to soak the wood. A cheap alternative is to use a disposable wallpaper wetting pan, or, a section of PVC plastic pipe works well too by plugging one end and loading through the other. I do a lot of bowing, so I made a dedicated soaker tube from a 24 1/2-inch long piece of 3/4-inch I. D. Lexan tubing.
For gluing the wood strips together, medium Cya works well. If you prefer “white glue”, Elmer’s or Titebond wood workers glue works well also. Avoid using craft glues like Liquid Stitch, Tacky Glue or Pacer Canopy Glue as they don’t dry hard and will not sand well. No matter what glue you use, apply it sparingly or it will leak onto the form and make removing the bow difficult if not impossible.
Making the Bowing Patterns
My favorite material for making up the bowing pattern is 3/16-inch thick Artists Foam Board available from arts and craft stores. On some plans the pattern outlines are provided, on others you’ll need to take the shape from the framing plan. I use a band saw (or scroll saw) to cut the shape, but the foam board also cuts easily and cleanly with a common hobby knife. Once the form is cut to shape, rub bar soap around the edge to help prevent the glue from sticking to the form. It isn’t necessary make up more then one form for stabilizer halves or wingtips as the patterns can be used several times.
Preparing the Wood
Select medium firm sticks with nice straight even grain. To get the required length of the sticks, measure the perimeter of the form using a paper tape measure, and add 2-inches to that length. Next, glue the strips together for about 1/2-inch at one end to form the proper cross section. Then the final preparation is to soak the sticks in water for 20 – 30 minutes. For harder wood, the soak needs to be a bit longer, softer wood will require a shorter soak. With practice you’ll get a feel for how long to soak the wood.
Forming the Bow
To form the bow, tape the glued end of the balsa sticks to one end of the form, oriented so that one stick will lie atop the other. Then carefully pull the wood around the form and tape the inner stick to the form. Next, run a bead of glue around the inner stick, then pull the outer stick around and tape it in place to hold it against the form. The bow can be air dried overnight, but for those of us who like to get with the program, the wood can also be dried in the Microwave oven on the High setting for 12 – 14 seconds while still on the form. If the foam board gets too hot it will puff up, so keep the time short, and repeat a second time if necessary.
When dry, remove the bow from the form, then repeat the process as needed to make up the second stabilizer half and additional wingtips.
As you can see, making bowed outlines is not difficult, but the technique does require a bit of practice to perfect. The most important part of making the bow is proper wood selection. It the wood is too hard it will snap when forming it around tight radiuses. If it’s too soft, the wood will kink. Knowing how long to soak the wood is important too, but with just a little practice you’ll find that making up the bows is quick, easy, and will actually speed up the building process.
The items needed to make bowed outlines is simple; Artists Foam Board, a glue stick to attach the pattern to the foam board, a paper measuring tape, masking tape, and a pan or tube for soaking the wood sticks.
The bowing patterns are cut out using a band saw, scroll saw, or a hobby knife. The perimeter is measured using a paper measuring tape and the wood sticks cut to length.
The sticks are glued together for about 1/2-inch to form the required cross section then soaked in water for 20 – 30 minutes to soften them up.
The sticks are taped to the form on the previously glued end, then the inner layer is pulled around the form and taped at the opposite end.
A bead of glue is applied to the inner layer, then the outer layer is pulled around the form and taped in place.
The bow, while still taped to the form is placed in the Microwave oven for 12 – 14 seconds to dry the wood. If the bow isn’t completely dry dry, let it cool and put it back in for an additional 10 seconds.
The completed Bows are removed from the patterns and the second elevator bow made up in the same way as the first. At this point, the finished bows are ready to use.
The finished rudder assembly was built using the bowed outline technique. Notice that the bow can also be formed with both inside and outside curves.