The Engine Shop
ANYTIME someone makes a snide remark to you about your "toy airplanes," you might squelch them with the fact that model aircraft are the highest-powered propeller-driven vehicles in existence. And that's not by any narrow margin, either! Take the speediest and fastest-climbing prop-equipped fighter ever built: Grumman's F8F Bearcat. It weighed 9,334 pounds and had 2,800 horsepower. That's 3 1/3 pounds per horsepower for the Navy fighter version. The recent world-record-setting specially-modified Rare Bear Bearcat came out somewhere around two pounds per horsepower. That's a mighty impressive weight-to-power ratio - until you consider that today's competition Free Flight models nearly double it.
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.
