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Miniature 4-stroke gas radial

Model Airplane News, Sep 2001 by Gudaitis, Frank

George Luhrs' amazing miniature engines have graced the pages of Model Airplane News before. This micro masterpiece is his latest-- and perhaps greatest-- achievement: a 5-cylinder radial displacing just 0.061 cubic inches. Just 21/2 inches in diameter, it is the world's smallest 4-stroke gas-ignition radial aircraft engine, and it is not a model of a full-size engine. Instead, George designed, machined and assembled it from scratch in his Shoreham, NY, shop. He fabricated every one of the 244 parts, plus laminated 33 parts to form the 5-inch-diameter, 3-blade propeller. Each cylinder bore is just 1/4 inch with an equal-length stroke, which yields a displacement of 0.0122 cubic inches for each of the five cylinders.

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The cylinder heads, crankcase, rocker arms and the fuel-tank cover are all wrought from 7075-T7 aluminum. The cylinders, pistons, connecting rods, crankshaft, cams, followers, wristpins, master rod and pins and all bearings are made of hardened tool steel. The pushrods are 0.018-inch-diameter spring steel. The valves and the prop hub are stainless steel. The valve guides are made of copper bronze, and the valve springs are 0.006-inchdiameter spring steel wire.

The carburetor is made of brass and has a choke and a throttle. Fuel is fed into a manifold on the rear of the crankcase, then brass induction tubes supply the mixture (lantern fuel with 10 percent WD40 lubricant) to the intake ports. The distributor cap is made of Delrin, the rotor is Teflon, and the points are 0.046-inch-diameter tungsten; the unit is adjustable with a timing lever. The spark plugs are stainless steel with Macor insulators. George initially used plugs from one of his previous engine designs, but these have not operated reliably in the radial, so he is fabricating new ones.

At present, George has two workable prototypes of his engine. The primary difference is in the cylinder sleeve material; the second engine uses hardened tool steel for both the piston and the sleeve, whereas the first used it only for the piston. The black sleeves on the engine on the right in the photo reveal it to be the second prototype.

In April of 2001, this extraordinary little engine was awarded first prize at the National Model Engineers Exposition held in Detroit, MI. This is the secand year in a row George has taken that honor; last year, he won for his miniature in-line 4-cylinder engine (September 2000 Model Airplane News). This contest is judged by the public; obviously the folks in the Motor City recognize a work of engineering art when they see it.

Miniature 4-stroke gas radial

Model Airplane News, Sep 2001 by Gudaitis, Frank

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