Just in case someone is wondering, this is a definition that eluded me for a LONG time: what, exactly, is "mixture". Mixture is this: the ratio, by volume (!) of the liquid fuel and gasseous air being drawn into the cylinders. That's why it has to be adjusted with altitude -- the liquid densities don't change with pressure, but the air density does. Pretty much nothing defines this quantity -- more flight instruction sources just say "it controls the amount of fuel" -- but nothing about it makes sense unless you grok that it's a volume ratio.
Curt Olson wrote: > What is the procedure for adjusting mixture with a constant speed prop > such as the 310 has? It gets complicated and engenders long arguments. :) There's a really, really good article somewhere on http://www.avweb.com about leaning procedures. The basic idea is that you always (insert obligatory exception here, like high altitude airports) start out at max rich. As you lean the engine, it is still burning all of its oxygen, but evaporating less fuel, so the exhast gas temperature goes up. Eventually, you get to the point where the engine is no longer providing enough fuel to burn all the oxygen, and the EGTs start going down again. This is the setting that will provide the maximum power output for a given fuel flow. If you could, you would just stop there. But for most engines under common conditions, this is too hot. So typical procedures are to lean go XXX degrees rich of peak. That is, lean it until you find the peak EGT, then add a little extra fuel. It's possible (and this is where the flame wars come in) to do exactly the same thing by going _lean_ of peak, too. The danger is detonation -- leaner mixtures are more likely to explode rather than burn, and damage the cylinders. The AvWeb article talks about this too, and which parts of the reasoning are just old wives' tales. > In the c172 I had been tweaking for max rpm and then backing off a > touch to the rich side. The cherokee I've been in didn't even have an EGT gauge, and that's exactly how it was leaned. You pull back the lever until the engine sounds rough and RPM drops off, and then back off a bit. The rough running is apparently due to the poor mixture distribution in carburated engines; I'm told that fuel injected engines don't rumble the same way at lean mixtures > When I adjust mixture on the c310 engines, the only guage that moves > is the temp. Shouldn't the manifold pressure also be changing if the > engine is generating more or less power as the mixture changes? That's correct (although fuel flow should also moving). The manifold pressure just measures gas pressure. Fuel in the manifold is still in the liquid state, and doesn't contribute much at all to the pressure. You could have a cut fuel line, be drawing pure air into the cylinders and producing no power, but still be at max MP if the throttle was open. Oh yeah, and another fact that never gets said: what the throttle does, literally, is choke off the airflow into the engine. It sits upstream of the carburator or fuel injector, and just turns a little plug that obstructs the air intake. Andy -- Andrew J. Ross NextBus Information Systems Senior Software Engineer Emeryville, CA [EMAIL PROTECTED] http://www.nextbus.com "Men go crazy in conflagrations. They only get better one by one." - Sting (misquoted) _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
