Bosta (oz14154)

About this Plan

Bosta. Control line combat model, for Super Tigre G21 .35 engine.

Quote: "Many people wonder why I went to the trouble of designing an elliptical Combat plane. And these same people also wonder why I went to the trouble of building it. The answer to both, questions is the same. I'm looking for the best possible combination of speed, strength and maneuverability, and the Bosta is it!

The idea of an elliptical Combat plane was first advanced, as far as I know, by Carl Berryman in 1963. Berryman's plane was called the Super Twister, but it was not a truly elliptical design.

I was Howard Rush's roommate at Purdue University, and as you might guess, we had many bull sessions into the wee hours of the morning about Combat and Combat planes. During one of these sessions, I suggested that an elliptical design would be the ultimate for Combat. Although Howard was convinced that it would have some advantages, he also thought it would be too hard to build. Accepting his opinion as a challenge, I immediately undertook the task of designing and building a prototype.

Several important design parameters were already set up for me. Howard had worked hard on his Nationals winning design, the Nemesis II (August 1972 AAM), so I used his airfoil, area and control system. Then I used Purdue's Control Data Systems 6500 computer to work out some of the details.

The prototype Bosta was, as Howard had predicted, heavy. But it flew better than any Combat plane I'd flown up to that time, so I resolved to improve the structure for more strength with less weight.

Some advantages that an elliptical planform has over a rectangular, or straight taper wing are: lower induced drag, improved mass distribution (it is heavy in the middle and light at the tips), concentrated structure where needed, high aspect ratio in the middle (5.35 to 1), and curved LE (so the tips don't hit the ground to break wings off when you hit straight in!).

I'm not going to make any incredible claims, but I will say that I'm seldom outrun, and never out-turned. The Bosta, due to its superior planform and structure, will survive crashes that would re-kit a lesser plane. The theory here is that, while it takes longer to build a Bosta, it takes longer to destroy one, and since it is not handy to have a wing fold in the middle of a match, you are ahead of the game with the Bosta!

Construction begins with the making of a sign to be hung above your workbench: Lightness is a virtue. Select all of your balsa with this in mind. If you don't know what the different densities and grains of balsa feel and look like, ask someone who does. Or check the first couple of pages in the Sig catalog.

First, cut the horizontal spar out of warp-free 1/8 balsa. With epoxy (white glue tends to warp this structure) glue on the 1/4 x 1/4 in triangular leading edge. Notice that from rib 3 outboard, the LE tapers to 1/8 in at the tip. This is the only tricky part of building this plane. Let the 1/4 x 1/4 in triangular leading edge stick out in front of the spar, and sand the taper in after the epoxy sets. Attach both the top and bottom LE at the same time, and sight down the TE of the horizontal spar to check for warps. These warps are easily twisted out while the epoxy is still wet, but impossible to remove when dry.

While this assembly is drying, cut out the motor mounts and glue them with epoxy or Titebond to the 1/2 in center block. Sand both sides of this assembly flat to prevent damage to an engine being bolted to an uneven surface. Saw the slots into the 1/2 in center block for the bellcrank and horizontal spar. Drill the mounting holes in the motor mounts. I use an old crankcase as a drill guide.

Cut and sand out the center ribs. Remember while making ribs that an airplane is only as good as its airfoil. Only the inboard center rib needs a slot for the bellcrank mount. Glue the center ribs to the motor mount assembly, but be careful not to build in any up or downthrust..."

Direct submission to Outerzone.

Update 26/10/2022: Added article, thanks to RFJ.

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