The Engine Shop
BY THIS TIME-half a year after my first "Engine Shop" column appeared in MA - I've received nearly 100 letters from readers. Most had questions, and I wrote a personal reply to every one. (Please enclose an SASE if you send me a query that needs an answer!) More than a dozen of those readers asked me about model fuels, lubrication, rust protection, and after-run oils. Because misunderstandings seem common in this area of model engine operation, I'll explain some of the "wet" aspects of model airplane engines as thoroughly as I can. The oil in model fuel does a lot more inside our engines than just keeping their moving parts slippery! It forms a "barrier film" over everything, to prevent corrosion and wear. With proper lubrication, the metal parts will never touch.
The Engine Shop
PERFORMANCE of any internal combustion engine depends on how much oxygen it's able to inhale to burn fuel. It needs plenty! Air contains about 21% oxygen, and for every cubic inch of fuel/oil vapor that enters a glow model engine's inlet, roughly seven cubic inches of air is required for complete combustion. (Diesels and gasoline-fueled engines need about 12 cubic inches of air.) Those numbers are for "normal conditions," but conditions vary considerably much of the time when model airplanes are being flown. Engines lose power in hot weather (the air is less dense) and with increasing altitude. Humidity also causes power loss; whenever water vapor is absorbed into the atmosphere, it takes the place of about 1/5 of its volume of oxygen. (Moist air's density is low, too, which is what makes it rise and form clouds high above the earth.) And low barometric pressure (from an impending weather change) lowers the air's density.
The Engine Shop
MISCELLANY MONTH! There's a new address at the head of this column. My recent move to Alabama will allow me to do considerably more model flying from now on than western Pennsylvania's weather permitted in the last few years. I'll also be able to experiment more with model engines. (I have some truly unusual engine developments to complete and test, and to write about in "The Engine Shop.") Several readers have written to discuss topics I mentioned in previous columns. One of those was my two-stroke/four-stroke comparison, where I stated that the effective displacement of two-stroke engines has always been wrongly specified. That's because the mathematical formula for "geometric piston displacement" neglects the inactive part of the stroke, between Bottom Dead Center (BDC) and the point where the piston crown passes the top edge of the exhaust ports.
The Engine Shop
SEVERAL READERS have written to me about the engine intake extensions I mentioned in this column a few months ago. Some wanted more specific information: "How long?" "What's the best length?" "Where can you buy them?" and similar questions. Here's more on the subject of intake extensions: Length: Make them as long as possible, within reason. Extending the length four to five times the inlet's inside diameter makes a noticeable improvement on most two-stroke model engines. More than that might further improve performance, but the added gain won't be as great.
The Engine Shop
INTERNAL-COMBUSTION ENGINES aren't the only piston-type power plants that are useful for model aircraft flying. Years before the first "gas engines" came on the market, compressed air motors powered many a model airplane. (They're called motors rather than engines because, like electric motors, compressed-gas power plants obtain their motive power from external "stored energy" sources.) In fact, between Lindbergh's transatlantic flight and the onset of the Great Depression, more US companies made compressed-air motors for model airplanes than there are manufacturers of glow engines in America today. One of the best pre-1930 motors was the Hoosier Whirlwind, a three-cylinder design by Bert Pond, one of AMA's founders. Except for its steel crankshaft and aluminum connecting rods, the 1929 Whirlwind was made from brass. A bore and stroke of .375 gave it a total displacement of .12 cubic inches. With rubber-power-type high-pitch propellers from nine to 14 inches in diameter, it flew Free Flight models as large as six feet in span, and weighing as much as 20 ounces.
