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MEASURING ENGINE TEMPERATURE

I read Dave Gierke's column in the November 2000 issue about 4-cycling and found it most informative. More articles like this would be welcome. You mentioned head temp measurement. I am a beginner and have had a lot of trouble adjusting 2-stroke-engine needle valves. There was an article in another magazine about using a non-contact temp gauge to measure head temp and using this measurement to set the needle valve for correct engine operation. The temperature was around 210 to 230 degrees F. I find head temp to be anywhere from 190 to 255 degrees F, depending on where along the head the measurement is taken. The rear just above the crankcase seems to be the hottest. I concluded that using the head temp to set needle valves is touchy, at best. Any ideas or information you can add would be greatly appreciated. Thank you for your time. [email]

P. HARVEY

I also read that business of using an infrared gun to measure the head temperature and set the needle valve. I also have one of these but find it inaccurate. The manufacturer states that it shouldn't be used on shiny surfaces such as a cylinder head. For my dynamometer work, I use a calibrated thermocouple. I use this only as a reference for when to take torque and rpm readings, as performance will vary greatly if you take data when the engine is cold or too hot. I use an EGT (exhaust gas temperature) thermocouple to set the needle when I'm doing economy tests to determine specific fuel consumption (sfc). This relates to the mass of fuel consumed for the amount of bhp being produced over a period of time.

For all practical purposes, the needle can be set very nicely by using a tachometer. Simply needle the engine to its approximate maximum rpm; then give the fuel-feed line a momentary pinch. If rpm increase, lean the needle a bit more. Keep doing this until no further increase is noted on the tach. This is the engine's max rpm for the propeller load you are using. Next, the needle should be backed off several hundred rpm. Now the engine is ready for flight. Listen to the exhaust note, since you don't want it to run lean in the air. By modifying the needle a couple of clicks at a time, you can arrive at an acceptable setting. Good luck with your engines!

DAVE GIERKE

GAS OR GLOW?

I've been a modeler for several years and am now thinking about buying a giant-scale model. I want to build the new Dynaflite DHC-1 Chipmunk but am having difficulty deciding on the type of engine to power the model-gasoline or glow power. A friend who swears by glow power suggested that I use a Super Tigre 3000, while another friend says I should get a Saito 1.80 4-stroke. The guy at the hobby shop suggested a U.S. Engines 25cc gas engine. There are so many combinations; what is your suggestion?

JOHN SULLIVAN

Groton, CT

John, I do agree that at times, there seem to be more solutions to any given problem than are needed, but when it comes to powering a model, especially a giant size, you should be comfortable with the choices. First, you should base your decision on your experience with model engines. If you've had a good time with 4-strokes and are comfortable adjusting them, then that's what you should use. If the simplicity of a big glow engine feels safe, go for it. I enjoy running gasoline engines, so that's what I'd choose.

You also need to evaluate the model itself. When thinking about suitable powerplants for a model, I look at the type of model (high wing or low wing), and I figure out the wing area/loading. This helps me determine how heavy the model can be and still fly well. The new Dyna flite Chipmunk is advertised as weighing 14 to 18 pounds, and it has a wing area of 1,189 square inches. That's a low wing loading of 27.15 ounces per square foot and a high of 34.90 ounces per square foot. If we shot for the average weight and wing loading, then we could say this is a 30ounce-per-square-foot model.

Gas engines are relatively heavy when compared with glow engines, so in this case, even though I love the gas burners, I'd go with a 1.20 or 1.50 4-stroke engine for a better power-to-weight ratio.

With a little more area or a little less weight, a 30- to 40cc gas engine would be my next choice. Remember that if you choose a large-displacement glow engine, you'll also have to carry a larger than normal fuel tank to feed it. I hope this helps you decide what's right for you.

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