Musings from the Combat Pits

A fleet of 2010 Dominators (left) and Warlords, built by the author using the jig described in this article. All photos by Buzz Wilson.

Fabricate a construction jig for a Diesel Combat plane

By Buzz Wilson

When I decided to build Dominators for the 2010 Combat season, I decided I would first build a jig. I have used a jig for years to build Warlords. Here you see the 15-year-old Warlord jig.

One reason that I write about and document these projects is that it forces me to rethink how I do things and improve on them. Secondly, it becomes a process sheet to use when I build.

A jig is supposed to save you time and give you a repeatable piece of work. A good jig will do that. Once you have materials, it will take about eight hours to put together the jig shown in the photos.

The first step is to find a straight, level piece of plywood. Once you have this in hand, cut the plans to size and use 3M-77 to attach the plans to the board. Spray a light coat onto the board and carefully place the plans down. Start with one edge and lay down and smooth out as you go.

Next take a piece of FasCal and place over the plans. This will protect the plans from glue. You will however have to cut the FasCal when you place the jig pieces. Cut small slots and let CA run under the wood.

In making a new jig, I find it helpful to have the pieces for the first plane cut out. This helps visualize problems. Secondly, I use tracing paper and lay it over the plans to see where jig pieces will be placed. I want enough support and definition, but not overkill that will not allow me to adjust for alternate materials. I use a Sharpie and rough sketch the positions.

I like to work using metric dimensions; unfortunately, balsa wood uses the English system. I made a conversion chart beginning with 1/64" and increasing by 1/64". The following text will mix both systems.

When you make a jig, you need to establish a reference line. Since our planes are symmetrical and the wings are on the thrust lines the natural reference is the thrust line. This will vary with the engine you use. For the Dominator, the reference centerline height with a Sharma is 23.75 mm. The motor mount resting on the board sets the jig centerline height reference.

With the thrust line defined, next establish the trailing edge. If you use different thickness woods for the trailing edge set the height for the thickest and shim for thinner. For the Dominator, I will use 1/4" balsa for the first two. Since I have some good 3/16", I will use it for the next two. The trailing edge jig will be set for the 1/4" and will be shimmed when I use 3/16". A quarter of an inch trailing edge has 1/8" above the centerline and 1/8" below. A 3/16" has 3/32" above and 3/32" below. This means when you use the 3/16" you have to calculate a shim dimension (1/8" - 3/32" = 1/32").

The Warlord jig had a continuous piece for supporting the trailing edge that had to be cut for the ribs.

The Dominator jig breaks at each rib.

The front and rear of the trialing edge are supported. Weights or pins can be used to hold the trailing edge in place.

The motor mount is the lowest point on the plane and will rest on the board. The leading edge is fitted through the mount.

The thickest part of the rib is 40 mm or 20 mm to the centerline, therefore the ribs will need to be raised 3.75 mm (23.75 mm reference centerline height with Sharma - 20 mm to the rib centerline).

The Dominator leading edge offers some unique challenges. First it tapers at the tips. Secondly it sweeps back. Taper is a matter of shimming to get the support since the taper is about the centerline of the leading edge as it runs span wise.

Leading edge sweep is a different issue. Here you will need some adjustability. The reason for this is you will not get the sweep exact every time. What you want is to be symmetrical on either side of the centerline. To get the adjustability use a 1/8 inch router bit and route slots in the leading edge hold-downs. Set the back of the router fence to where you want the slots and then place wood stops on either side of the bit to set where you want the cut to end. Cut one end and then the other. Raise the bit and repeat until you complete the slot.

The leading edges are round in front and flat on the backside. What you want is to apply force on the front side to hold the rear tight to the backside jig. To hold the leading edge, the front will be adjustable to the sweep.

The rear of the leading edge will fit to a hard position. I find this type of system to work better than trying to rubber band to a fixed support. By doing this, any variance in the sweep between one leading edge and the next, will be compensated. When building the jig you should use a leading edge to help establish where stops need to be positioned. You will also angle blocks. A test fitting of the leading edge positioning and holding part of the fixture holds the leading edge tight enough that the taper will not need to be shimmed. Use small pieces of wax paper to protect the jig where glue seepage is likely to occur when you are assembling a plane.

Because of the leading edge sweep, I thought the front of the ribs would need to be sanded to match the angle to get the correct fit. This was not the case. Because the LE is routed, small gussets are installed for additional support. A small template is made to cut these the same size.

Tanks vary in size; the rib out board of the tank is not jigged. The rib will be put in after the plane comes out of the jig and the tank is installed. This is done for both the Warlord and the Dominator jig

I do not use hard hold-downs, but weights on the trailing edge. Because the trailing edge is slotted, a tight fit holds the ribs. If additional holding is needed, use angle blocks to hold the ribs prior to gluing.

The center motor mount crutch is held to the jig with a clamp. Wax paper is under the mount to protect the jig from glue seepage.

Here is the completed jig ...

... and here is plane that has been framed using the jig ...

... and finally a completed plane.

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This page was upated Feb. 9, 2010