Model airplane fever

Model aircraft engines have always appealed to me, and when Flob McConaghy demonstrated the world’s first model aircraft stirling in radio controlled flight, I knew I had to try my hand at a model aircraft stirling.

Rob had used (and still favors) pressurization, but I decided to stay atmospheric, for simplicity. The key would be to make each part as absolutely lightweight as possible.

A first effort was a one inch bore alpha engine based on a wobble plate drive mechanism. The engine had an inner sleeve and a foil regenerator, Ftulon cup seals, and water cooling. Although it ran adequately, it was grossly overweight, and its water cool­ing made it impractical.

A second effort, conceived and completed in three weeks time, was much more promising. It was a simple beta engine, with Ftulon cup seals on the piston and displac­er shaft, an annular regenerator, a cantilevered built-up crank, and plastic miter gears making the prop shaft concentric to the long cylinder. Less prop and burner, the engine weighed 80 grams, and it produced 90 grams of static thrust. Great pains were taken in keeping wall thicknesses low, and as I recall the wall thickness of the regenerative por­tion of the hot cap is at most 0.008 inch (0.2 mm).

The prop used is a 12-6, made of maple for use on much more powerful gas en­gines. Obviously a lightweight custom prop of balsa would be in order before flight test­ing.

One remaining challenge is to devise a suitable flight burner that is not going to set farmer Smith’s barn on fire in the event of a crash. A butane burner with an inertial gas shut-off valve is one possibility. Another is to use no burner at all, but merely a heat

Model airplane fever

The lightweight model airplane engine is a beta design.

Model airplane fever

Storage canister, filled with aluminum made molten with a ground-based propane torch, then covered with insulation prior to flight.

An "improved" version of this engine was subsequently made, incorporating higher compression (piston to displacer strokes = 1 to 1, rather than 1 to 1.25), a shorter, lighter hot cap (mistake #2), and a simpler, stronger crankshaft frame (the only true improve­ment). This engine weighed 10 grams less than its predecessor, but its performance was substantially lower.

These projects were diversions from my primary aim of making a practical fraction­al horsepower stirling, but they were fun and great morale boosters. There is something transfixing about watching a little stirling, its hot cap glowing red with heat, steadily and quietly turning the big prop to a blur, pushing a stout breeze.

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