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Author Topic: Rules of thumb for setting up engines  (Read 937 times)

Offline Chris Gilbert IRL-1638

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Rules of thumb for setting up engines
« on: November 12, 2006, 04:48:33 AM »
Hi Folks,

Just back from a great mornings flying, marred by only 1 thing, between the two of us we only had one model (a streak) which actually ran right for a whole flight.

In my book a flight with a poor engine run is better than no flight, but with so much tweaking going on does anyone have any rules of thumb for setting up an engine to run sort of OK in the air for those first few flights - the streak is the only model we didn't tweak, mind you, it was the only 'old' model there, the rest are in the trimming stages.

We've done all the basic stuff - no sharp bends in the fuel/vent lines, feed positioned approx correctly for the venturi/needle, but a run which sounds great on the ground then leans out far too much in the air.

Any suggestions on how to reduce the number of wasted flights, because of poor engine settings?

Thanks
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Alan Hahn

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Re: Rules of thumb for setting up engines
« Reply #1 on: November 12, 2006, 12:24:35 PM »
At least for me, too lean in the flight usually means I have a wide uniflow on a profile or the fuel is foaming (due to the higher rpm hitting a resonance on the front end of the plane). The fix to the first is to try and get a narrower tank (uniflow closer in to where the spraybar is, and the fix to the latter is to try a little ArmorAll (a couple of drops per gallon) or put a little foam rubber under the tank to isolate the tank from the engine vibration. You could also play with props to raise or lower the flying rpm.
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Offline Jim Thomerson

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Re: Rules of thumb for setting up engines
« Reply #2 on: November 12, 2006, 04:56:22 PM »
If you have a uniflow tank and it is not leaking anywhere, you should get a consistant run from beginning to end of the flight.  I have never experienced the problem Alan mentioned with the uniflow being too far outboard.  The engine thinks it is sucking fuel from the submerged end of the uniflow, and you can easily picture the difference in where it thinks the fuel is coming from when the airplane is sitting on the ground and when it is flying.  It is a matter of adjusting the engine for flight rather than for sitting on the ground. 

If you are running a non uniflow tank, it will lean out as the flight goes on.
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Offline Steve Helmick

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Re: Rules of thumb for setting up engines
« Reply #3 on: November 12, 2006, 05:19:00 PM »
A lot of that nonsense is caused by flying with a new engine, with very little running time.  Bench running is still a PITA, but with most engines, it's the safest way to go. It depends a lot on what engine you're using. The OS LA's work good on the 2nd flight, but each flight will need to be leaned out a touch, as it loosens up. By the time you have 10-20 flights on it, that will be finished. 
:X Steve
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Offline phil c

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Re: Rules of thumb for setting up engines
« Reply #4 on: November 13, 2006, 09:39:28 AM »
Get a couple of starts on it before trying to fly.  See how rich you can set it and still get a smooth run.  See how much adjustment there is until it hits peak rpm.  If the motor won't run steadily at a very rich setting try switching to a hotter plug.  the OS A3 is readily available and usually does a good job running rich.  If you see airbubbles in the fuel line, or the motor runs erratically, try the tank/Armorall fixes suggested before flying.  For the first flight, set the motor in a fast four cycle and only fill the tank half full, just enoug to get in the air.  No need to run a full 5-7 minutes with a bad run is there?  After the first flight gradually lean the motor out for the next flights until it is running where you need it.  Once you've got a decent run on it, check the rpm and also the lap time for reference.
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Alan Hahn

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Re: Rules of thumb for setting up engines
« Reply #5 on: November 13, 2006, 03:15:43 PM »
Jim T.
I guess I wasn't clear in my uniflow comment (it  was clear in my head--honest!  %^ ). What I meant is that with a wide uniflow tank, the ground setting was always richer than the flying setting (when "centrifugal" force gives a little pressure drop to the spraybar). One time it was so rich that I had a hard time geting airborne. Going to a flatter tank helped a lot, but even then I usually set the ground a little richer.

Also a comment---I find short tanking a good thing to do when you are looking at a new setup. Nothing like a 10 minute + lean run. With a uniflow, once you are in the air, the run you get should be pretty constant.
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Offline Jim Thomerson

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Re: Rules of thumb for setting up engines
« Reply #6 on: November 14, 2006, 08:27:10 AM »
Alan, I'm not disagreeing with you. On the ground the side-mounted engine thinks, "I'm sucking fuel from close to level with the spray bar." In the air the engine thinks, "I'm sucking fuel from way below the spraybar." So, as we both say, set the engine on the ground so it is happy in the air.
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Offline Jim Morris

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Re: Rules of thumb for setting up engines
« Reply #7 on: November 14, 2006, 05:16:21 PM »
What kind of engine is it? the reason i ask is I had an engine that did that very same thing, the spray bar was not installed directly inline with the venturi. I had to remove the spray bar, JB weld the holes,then redrill through the center of the venturi.It worked,but the engine was not made to run like the typical stunt motor.
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