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Thread: In air fuel mixture adjustments! 20cc gasoline!

  1. #1
    Pilot Hockeystud87's Avatar
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    In air fuel mixture adjustments! 20cc gasoline!

    Alright guys. I have a problem that I would like you boys to help me solve.

    Task

    I would like to fly a gasoline powered aircraft to high altitudes and have the ability to adjust the high speed carburetor mixture. I have seen pixturethis and his ability to fly high with these motors and have tried contacting him.

    Question

    So i'm just wondering what concepts creative solutions and ideas do you folks have for this?

    Anything helps and all ideas are appreciated for trying to help in the solution.

    Assumptions

    Size of motor is 20cc
    Will be gasoline
    Assume 10,000' MSL or higher
    Assume you will have RPM, cylinder temps provided
    Will the in flight sound of the aircraft be useful?

    Thanks agian guys for all the help.
    sum ting wong
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    Sounds like a typical RcLab1 maiden flight to me -ssassen

    Whatever. This forum sucks. -SENTRY

  2. #2
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    In flight needle adjustments have been done for decades in LoS RC. It's not commonplace anymore as advanced carburetors with 2 or 3 needles were developed for low-mid-high adjustments.

    For a modern go at it I simply put a continuos 360deg Sail Servo either inline with the needle or geared to the needle.

    Obviously their will be much testing to do and I can only imagine how long it will take to land dead stick from 10,000 feet!

    This is automatic but not fine tuned.
    http://www.rcmodels.org/csm/carbsmart.html

    Old school way of doing it.
    http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCN51
    Last edited by 1320Fastback; 15th September 2014 at 08:19 PM.
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  3. #3
    Engineer for Jesus Christ IBCrazy's Avatar
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    Why not just re-jet it for high altitudes? Sure it will run rich initially, but if you climb out fast I'm sure it should clear itself out.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

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  4. #4
    I see you... Derrick's Avatar
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    Quote Originally Posted by Hockeystud87 View Post
    Alright guys. I have a problem that I would like you boys to help me solve.
    There isn't an easy answer here, this is something that you are going to have to path find lots of trial and error with a lot of testing to get it right.

    In general you monitor the temperature... if it gets to hot you richen the mixture, if it gets to cold you lean it out. The rpm is used as a gauge of that is about right, by this I mean that you know about what the RPM should be for a given throttle setting.
    When nothing else out there will suit your needs... design and build it yourself.

  5. #5
    Navigator jrwperformance's Avatar
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    It probably would be better to moniter EGTs than just the head temp. EGTs won't vary as much with outside temperature. That's how the two stroke snow mobile guys tune their rigs.

    Also, nitro engines are tuned on cylinder head temps because alcohol fueled engines get a lot of their cooling from the fuel itself. Gasoline engines...not so much.
    Last edited by jrwperformance; 16th September 2014 at 04:38 AM.
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  6. #6
    GO HAWKS Hucker's Avatar
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    \warning : this might have no bearing on small engines and could be a complete waste of your time:


    HockeyStud: If I know you this is an endurance thing. Not sure if it is 2 stroke or 4 but I can tell you that the path to maximum endurance in full scale aircraft is to understand EGT intimately and learn how to fly lean of peak EGT. The key being that you can run lean or rich and be at safe operating EGT's ... or you can be in the dangerous spot where the EGT gets too hot and bad things happen to your motor. THere are many old wives tales about the "danger" of running LOP but if you watch EGT it is actually very easy to get right unless there are fuel delivery issues on a per cylinder basis (you need independent EGT/CHT to get it right )...but with a 1 cyclinder engine this problem doesn't exist.

    Do note the running LOP is a maximum efficiency concept not a maximum power concept. Intuitively running lean 'should' be best because there is an implied complete burn of the fuel (since every fuel molecule can find air molecules to explode with) whereas for best power one might trade putting more fuel in to ensure burning the most fuel. This is where the concept of "the fuel cools the engine" comes from. The fuel doesn't really cool things in the sense of heat transfer, it 'cools' things by making the engine run at a point where it might put out a little less energy since every fuel molecule isn't burned...racers learn to run right at stoichiometric (perfect air/fuel ratio) and play lots of games to REALLY cool the engine with creative cooling ideas that keep the cylinders transferring as much heat to the airstream as possible.

    Another thing to consider (again depending on your goals) is to run a variable pitch prop. A fixed pitch prop is a compromise (that we can almost always live with BTW). It is basically the art of picking the best "gear". If you want to climb fast you'll pick something that is high thrust low speed and if you want to cruise fast you will do the opposite...then add in the variable altitude you are going to fly at and you will find you need even a bigger bite. Variable pitch will allow you to extract maximum power from the engine at all operating conditions (roughly speaking). Full scale does this with a constant speed prop that automatically maintains RPM thus letting the prop and engine spin at their most efficient speeds. For you, you will either just watch RPM and adjust pitch to get the RPM right ... or if you are clever, trick a NAZE32 and close the loop with an RPM sensor and pitch control (tail rotor assembly from rc heli) to make a poor man's constant speed prop. (I have modified NAZE code for a project so I suspect this might not be too hard and might even be possible with no modification to the code, bonus points for closing the loop around egt or O2 and mixture) VPP might also be a waste if you have a long runway and spend most of your time in the cruise condition (e.g. you might put up with crappy climb performance to have a lighter, simpler plane).

    With a VPP you will be able to land REALLY short by reversing the pitch on landing...and you will be able to taxi backwards.

    THere are double digit percent efficiency gains to be had by tracking this stuff down.
    Last edited by Hucker; 16th September 2014 at 12:47 PM.
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