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HomeAircraftHelicopterMain Discussion › Setting head speed and tuning engine
04-30-2004 12:39 AM  14 years agoPost 1


Centreville, VA

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I've been reading over as many posts and articles about tuning an engine and setting head speeds as I can and I've only managed to confuse myself more. Let me try to give an example: discusses tuning in this way:
Next you set the high speed needle. You have to set it first because the low speed needle is dependent on the high speed needle. The low speed is only a ratio of the high speed. When you richen the high speed, that also richens the low speed. Bill calls the high speed needle the gate keeper because all fuel flow into the engine is determined by how much you have the gate keeper open. Start the helicopter and do a little mild flying/hovering to let the engine warm up. All tuning will be done in normal mode (not in flight mode) with the rpm around 1650. Come to a hover, then punch full throttle and watch your RPM. If the rpm increases, then the high speed needle is too lean. If the rpm stays the same then your high speed needle is set right. If the rpm decreases, then either the high speed needle is too rich or the blade pitch is too much for that engine. To determine if it's too much pitch, do a climb out with the throttle about 80%. If the rpms remain constant, then decrease the top end pitch. With an engine in good condition, you should be able to run +9 on the top end.
A couple of things bother me with this.
1) If the engine is very powerful, you could still be very rich and maintain a 1650 RPM at full power, so you haven't really "gotten there" yet.
2) What if you then want to have an idle up speed of ~1800? Everything is kind of tuned for 1650 now, isn't it? In fact, wouldn't your top end be limited to 1650?

I'm trying to get my TT39 setup right and haven't really hit on the best method of doing it. Most methods seem to agree on setting the high speed needle first since it's the master needle, and then the low. Trying to combine all I've been reading, here's my plan of action:
1) Get engine warmed up, start with a +9 top pitch, linear throttle curve 0-100%
2) From hover, smoothly go full throttle and tach head.
3) Lean needles slightly and repeat 2). If head speed goes up more than before, continue to repeat this cycle. If head speed gets too high (more than you think you'd ever want to run, I guess), increase top end pitch. When head speed decreases from leaning, you've gone too far.
4) Richen the needle a few clicks - done with high.
5) If smoke and temperatures feel good after a few minutes of hovering, done with low so long as engine doesn't die in idle. If it does, lean some until it doesn't. If engine is too hot, richen low end, unless it won't stay on in idle, then richen high end some.

What do you think?


04-30-2004 02:35 AM  14 years agoPost 2


Jacksonville FL

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I think your confusing head RPM with engine RPM

04-30-2004 02:38 AM  14 years agoPost 3


Centreville, VA

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No, I don't think so I am. I'm trying to tune the engine for best performance. Setting the head speed will be part of this process. What part do you believe I'm confusing?


04-30-2004 05:39 AM  14 years agoPost 4
Arthur Dent



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I'm also little bit confused of this matter.
Is there any one who could explain it clearly.
What and How to find best performance.


04-30-2004 08:25 AM  14 years agoPost 5
the collective

rrKey Veteran

Bayside, NY, U.S.A

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I've always set the high end needle first, start with a rich setting and go to full throttle, then land and lean it a few clicks and go up and hit full throttle again. Land again and lean a few clicks. Each time you lean it a few clicks you should see an increase in the full throttle RPM. Once you get to the point where leaning a few clicks produces no increase in RPM at full throttle, back off a few clicks and give up some RPM. You want to be slightly rich of maximum RPM.

Once I've got the high end set a bit rich of max RPM, I set everything else with the idle/mid needles. On a two-needle carb, I set the low needle for hovering and descents, and live with whatever idle results I get. I'd rather have a lumpy idle than wah wahs or a lumpy hover with the associated tail jitters.

At all times make sure there's plenty of smoke and check the backplate temperature constantly. IT should never be hot enough to feel painful to touch. Uncomfortably warm is okay, but if it's hurtin hot then the engine is not happy.

Oh... I usually make the adjsutments with around a quarter tank of fuel. If you set the high end with a full tank it may go too lean as the fuel level drops.

Tweak headspeed with pitch and throttle curves, not mixture.

This may not be the absolute best way to do it, but it's worked pretty well for me over the years.


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