Originally Posted by
Crash one
If these engines are running richer than necessary because of the engine/carb design, I'm thinking that if the throttle is set to max static revs on the ground with mixture fully rich, then leaned to best revs, this would not be " fully rich" but would depend on elevation, pressure, density, temp etc? and should give best take off power on that particular day/place?
I'm not trying to rock the boat suggesting oddball ideas and I doubt if this would save anything worthwhile in fuel but, if the theory is correct then it would help me get my head completely round understanding this.
Crash One, the engines are not just running rich because of engine/carb design. The most important reason for running overly rich (at full mixture forward) is due to heating. The engines we're talking about are air cooled, but they do not have sufficient cooling capability in their cooling fins to keep the engine cool when it's producing 100% power. So we run the mixture extra rich at that power setting. The fuel that's not burned helps cool the engine through evaporation.
What you're suggesting is entirely correct (for a fixed pitch prop - for a CS prop you'd use a slightly different method as you can't use RPMs as your indicator). You can run up the engine to full throttle, then lean for best power (max RPM) to get the best performance out of your engine. However, in that situation the engine is developing 100% power so it'll heat up very quickly to beyond the maximum allowable values (CHT mostly) which may lead to detonation and other sorts of engine damage. So don't do this when you care about your engine.
Having said that, that procedure is exactly what you would do in a high density altitude (approximately > 5000' DA) situation, such as when you're taking off from an airfield in the mountains. In that case, the density altitude is such that the aircraft cannot produce more than about 70% of rated power anyway, and you need all the power you can get from the engine in order to fly before you run out of runway.
(And to help your understanding even further: I was flying as a passenger in a kitplane with very sophisticated engine controls. We took off from a sealevel airport and even during the early stages of the departure the pilot started leaning. However, he did not lean for peak EGT, max RPM or anything like that. He only leaned just a bit and used the CHT temperature as his most important indicator. For every twist of the vernier mixture control his CHTs would rise a few degrees, and he leaned so that his CHTs were just below the yellow bar. Through this, he achieved a few percent more power, and a bit of fuel saving, without the risk of engine damage. So yes, you can even lean when the engine is operating at 100% power, but you really have to be careful and know what you're looking for. And it would be foolish to attempt this in a carbureted engine with just one CHT probe.)