I have this weird feeling, that maybe the main needle would need seasonal adjustment/altitude compensation, but not the bypass valve.
But that is just a gutfeeling, based on "D'uh!" and nothing more. Hence wanting to know how this set-up would hold out on long term flight tests/

I have a feeling that in running this that it will be more like a solution allowing a gas conversion to run like a nitro engine. I'm thinking that the main needle might require a "tweak" once in a while. I don't see a way around that because we lack mixture correction at the top and bottom ends unless we use the bleed at the extremes also. The bleed is quite a "course" adjustment and using it at idle will be difficult as the volume of air that is required for mid range correction is quite large (5mm hole in the case of the 1.80 Saito). Might not be impossible but would require a "logarithmic" type flow curve in my option. I don't think we will get mixture resolution like the solenoid.

When i get a chance i plan to install the 180 on the Yellow testbed plane (was the 160 boxer and now has the 23cc super tiger two stroke) as i like how the 180 runs and sounds on the stand. That will give a better idea of what this system will be like in the air on a four stroke.

have you thought of using a slot with a varying width in the air valve like most manufacturers use in automatic balancing valves? you would get better control at the ends of your air valve doing something like the slot in this:

Hmmmmm.. There we go. I like the look of that valve. The curve i get could be "flattened" by using a valve such as this, and would have the possibility of fine enough control at the bottom end to do the fine adjustment needed for compensation.

I don't know if i can do it with my Graupner, but can a OpenTX system measure temperature and pressure and adjust output to an air bleed valve?

Yes... The TX can do it, the question is whether you can program it.

The solenoid is active all over the range, and regardless of the needle settings, when the solenoid is given a dutycycle "X", the atmospheric correction "Y" will be relative to the value of "X". And this happens to be exactly what the engine needs. All you need to do is get the factors that determine "Y" correct, whch basically is to put the common gas law into formulas.
With the airbleed, that is quite a bit different, because now you are acting on the air side, not the fuel side. And not only is the airbleed valve probably not linear what the airbleed does basically comes down to "1+X", because the airbleed is adding air to the mixture, not fuel. That's an issue for which I cannot think of a simple formula to correct the airbleed with. I get the feeling that would require "mapping" (individual curves for each and every thinkable combination of temperature and barometric pressure...

What if instead of using a servo have a piston (with a return spring inside the cylinder) move the bleed air valve?
Then use the vacuum from the crankcase (trough a one way valve) to actuate/move this piston.
This assumes that the vacuum inside a engine is directly corrolated to the fuel demand (rpm ect) and therefor regulates on the actual fuel demand of the engine instead of the position of the trottle servo.

thoughts ?

Agreed - we are not going to trump the accuracy of the solenoid with adding air.

I think what your describing is a bit like the fuel circuit of a Variable Venturi or "constant-depression"Carb - (SU / others) - I'm not sure the vacuum signal would correlate that well to the (mostly midrange) adjustment needed to use a glow carb on gas.

There would be too much dead time in that control loop. You would be much better doing the compensation in the controller. We would need to come up with a proper compensation equation to do that but adding the code would be fairly easy.

That won't work...
The principle is a bit like that of a "constant velocity" carb, but they work very different than what you describe.

The how and why would probably take me 2 pages to describe, so I won't even try.

Depends... If it is the atmospheric pressure and temperature, the same way we use it for the solenoid, the delay really is no problem. I have several planes still running with a separate pressure/temp sensor feeding info back to the TX over telemetry, and the TX by means of a LUA script correcting the solenoid output.

But if it is intended to compensate for the airbleed, that won't work for reasons I described earlier: impossible to mathematically describe the correlation between that atmospheric data and the airbleed requirement.

I finally got a moment to play with the controller board xanaphyst sent to me. I flashed my firmware to it but it's reading temp and pressure incorrectly. I tried changing the base address of the sensor and reflashing but that gave me the "SENSOR ERROR" warning so I flashed it with the original base address. I see the sensor soldered to the board (at least I think I do), and the firmware is reading from a sensor at that base address.


Is this a different sensor requiring a different library than the bmp280, xanaphyst?