Originally Posted by
xanaphyst
Howdy,
I've had a little more time to read through some of the threads and have a better picture now.
I was wondering your thoughts of a hybrid solution between air bleed and weather condition compensated solenoid.
The short idea is a high speed continuous rotation servo that contains a feedback line would operate a standard remote needle valve to do the air injection handling but
in overview the same idea could be applied to the solenoid solution and perform fuel limiting. This would eliminate the complexities of building a solenoid housing and eliminate
the solenoid driver. The servo would be controlled with the Feather TFT to allow for weather compensation.
That would not work, since the "weather-compensation" with the solenoid, adjusts fuel admission over the entire range exactly to proportion, which is pretty simple using a dutycycle-modulated valve for the fuel only, and a (mechanically constant) single air throttle... Very crudely spoken: assuming a baseline value, the the CHANGE in density of the air is calculated, and fuel is admitted more, or less, in accordance. That is very easy to do because the fuel needle determines a flowrate, and the solenoid LINEARLY increases or decreases fuel flow, regardless of needle setting. Within the range we see it, and liquids not being compressible, volume-flow is directly related to massflow
But to have a fixed fuel supply and TWO air throttles, becomes a nightmare to compensate for the weather. Suddenly you need to have info on the ratio between direct air and bleed air (you need to have throttle position sensors on BOTH throttle and bleedvalve), in order to work out how much the bleedvalve has to be opened to effect the change in airflow in order to keep fuel/air ratio constant, because for air, given the pressure differences over the throttles and gasses being compressible, the relation between volume flow and massflow of the air is, to put it mildly, "a bit flexible".
I would most definitely NOT advise to go that way.
The air bleed system will work, as long as you check the main needle setting before flight, and it MIGHT require a retune of the idle needle setting IF the last previous flight was in very different circumstances.
The solenoid system simply works, period, with the only periodical changes to the needle setting originating from imperfections in the correction factor.
What I have seen, that limits itself to one click richer or leaner between winter and summer (western European North Sea climate).
Occasionally, I have to, what I call, "flush the needle". That is when the engine runs lean despite NO weather change, but as soon as the needle is opened one or two clicks, it immediately becomes rich again and I have to return the needle back to its initial setting within 10 seconds or so. I guess no matter how much fuel is filtered, due to the evaporation going on in the carb, miniscule deposits WILL happen...