Honest engine answers

Model Airplane News, Jan 2001 by Chianelli, Chris

INVERTED OK?

Dear Chris:

Please advise me of the disadvantages or possible complications of installing an O.S. 70 inverted. My O.S. instruction manual doesn't list a contact for technical help.

RUSS

Hey, Russ:

I've run O.S., Saito and Magnum 4-stroke engines inverted with no problems at all. Four-stroke engines don't seem to suffer from inverted operation as 2-stroke engines sometimes do. The 4-stroke's higher combustion-- chamber temperatures and more efficient fuel consumption characteristics probably have something to do with this. On some brands of 4-strokes, the glow plug has been moved closer to the exhaust valve-a hot area that helps keep the plug's filament glowing. Such a move also takes the plug farther away from the intake valve and its incoming cool fuel/air mixture. The only thing you must be aware of is not to flood the engine while it's inverted. If a 4-stroke engine becomes seriously flooded to the point of hydra-lock, damage to the engine's internal parts can result if a strong electric starter is applied under such condition. No matter which brand of 4-stroke you are running, make sure it is equipped with an O.S. "F" glow plug. It is simply the best.

-Chris

MARKETING BABBLE

Dear Chris:

Something puzzles me. Why do engine manufacturers publish horsepower ratings for their engines? Why don't they specify a thrust at a certain rpm with an appropriate prop, using a stated percentage of nitro fuel?

I ask because when it comes time to talk about props and rotors, our primary concern is thrust. It is thrust that keeps our birds flying. Props and rotors convert torque into thrust, no? Horsepower just doesn't seem to be very meaningful data. Am I missing something?

RANALD HAY

Dear Ronald:

You are missing nothing! Manufacturers use peak horsepower ratings because it is effective marketing to do so; these ratings have very little to do with how an engine is going to fly our models. Those factory-- claimed peak hp ratings are often attained at very high rpm levels-levels that aren't usable for 99 percent of the sport, aerobatic and scale models we fly.

Thrust claims are a problem, too, because we're talking static thrust, and static thrust is also misleading. It doesn't tell us much, especially if measured with higher-pitch props that can actually stall when run in the static mode, thus giving a false low-thrust reading. For example: it is theoretically possible to take readings on two props of the same diameter (different pitches) and get a higher thrust reading on the finer pitched one than on the one that's more coarsely pitched. Very coarsely pitched props stall-- cavitate, if you will-in the static mode, and this gives a lower thrust reading. Formula One racing props stall badly during takeoff and don't supply optimum thrust on takeoff. Once that plane is moving, however, dynamic thrust is in force, and those racing models really start moving at very high speeds, where the highly pitched props can start to do their thing.

We can take static rpm readings with finely to moderately pitched props of differing diameters, and by slightly over- and under-propping an engine-and, of course, by taking readings on all props of the correct sizes in between-we can get a relative idea of where (at what rpm range) an engine produces the most power. Thrust, however, becomes a truly meaningful assessment only in the dynamic mode when moving through the air. Something that is meaningful and that should be stated for practical purposes is torque-a term you also mentioned. It's the peak torque/horsepower that twists our props for needed thrust. Manufacturers never mention torque in their specs, but I, too, wish they would start doing so.

-Chris

WATCH OUT FOR THOSE "EXPERTS"

Dear Mr. Chianelli:

I read your recent Model Airplane News article about engines and oil content in glow fuels. Since I have just bought my first four-banger, an O.S. Surpass .91 FS engine, I am a little concerned about the manufacturer's recommendations: "At least 18% oil content' in the fuel to be used. I said I am concerned because we at the field can only get 16 percent oil with 10 to 15 percent nitro Byron fuel, which is very good for our 2-cycle engines, right ?

My friend has a 4-cycle 1.20; he bought castor oil from a lab and added a measured quantity to a brand-new gallon of 16-percent-oil-content fuel to increase the oil/fuel ratio. Is this practice adequate, or should I get the correct fuel?

The "experts" at the field have always recommended using the same 2cycle 16-percent-oil-content fuel, but they say opening the needle valve so the engine runs cooler is more than enough protection. Is this a valid practice? Should I "break in" the engine first with 18-percent-plus-oil-content fuel and then switch to a glow fuel with a lower oil content?

GABRIEL ZAMORA Mexico City, Mexico

Dear Gabriel:

If simply opening up the needle is an acceptable offset to too little oil, what happens if your engine inadvertently goes lean in the air? (And sooner or later, it will.) Obviously, this is not an adequate safeguard. I'm sorry, but I do not think 16 percent oil is enough for a 4-stroke-or a 2-stroke-period! And I'm not alone in this belief.