p51tom

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I am breaking in two RCGF 20cc gas engines in my Great Planes Revolver for a Kemp 95" B25. I just changed engines after 52 flights on my first engine. The second engine runs fine as did the first. The only difference I find is the RPM does not change much until I am below 1/4 throttle. I believe the only difference in the set up of the two engines may be the engine throttle arms. I have the throttle linkage on the outer hole of the servo arm with the end point adjustments down to 25% in each direction. Should I move the the linkage to the inner hole of the servo arm and increase the servo end points? I have not had this problem with the other 5 gas engines I have used. Any suggestion on how to make the throole more lineor would be appeciated. Thanks P51TOM

Tired Old Man

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The linkage should always be set as close to the servo output shaft as possible, and as far out on the carb lever as possible. Your set up is 100% of your throttle resolution problem. If you have been doing your other gassers differently you might want to take some time to correct them.

The vast majority of throttle resoultion will come from the linkage geometry and set up, not through the transmitter trims and end points. Typically the servo travels should end up as close to 100% as possible, and in the low to mid 90% range is the norm. If your end points are a lot less than that you are giving up a lot of finer control in your throttle control.

apalsson

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Jody posted an excellent diagram that shows the throttle / servo geometry, just the other day
It explains the relationship very well plus the angles to make the throttle as linear as possible, using the linkage

jedijody

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You mean this one?

apalsson

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That's the one !

av8tor1977

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Yeah, thanks for posting that again... I keep forgetting that fourth decimal digit on the throttle arm length and it messes my throttle geometry ALL up!



AV8TOR

jedijody

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ameyam

I ususally set up my throttle with zero trim and 100% ATVs so as to get both the throttle and servo arm at exactly 90 deg to the carb / servo and with the baffle and throttle set at 50%. I have a E-Z connector (sliding connector with screw that screws on the pushrod) to allow me to do this on the servo horn. Subsequently, I adjust the movement on both sides equally in the ATVs (Futaba calls it EPA). It usually get the setup fairly accurate atleast on my glow planes. Am awaiting my gas engines, most likely will do the same there. Anything I am doing wrong?

Ameyam

jedijody

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You'll find that a glow engine and gas engine have little in common in throttle geometry set up. For the gas engine, at 50%throttle valve opening it will be providing about 75% of it's power output, at 75% throttle valve opening it will be pumping out 95% of it's available power. Setting the throttle up as you described you do with your glow engine is why people need toprogram heavy throttle curve mixes in their radios. Set the geometry up right for the engine type and it's power profile from thebeginning and you'll have a much more linear feeling throttle without throttle curve mixing.

ameyam

So what would be the correct method of setting up in a simplistic sort of way? And do I need to get rid of the baffle spring as some suggested?

Ameyam

jedijody

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The drawing I posted is the geometry you want base your set up on, you can deviate a little but that's as simple as it gets, numbers and everything.

Leave the spring alone, if you remove the spring the throttle valve definitely and possibly the carburetor body would be ruined in a very short time.

Tired Old Man

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Here's something about throttle set up I wrote a long time back:

Here's a few throttle linkage set up tips.

1) Always determine the desired linkage length with both the throttle servo control arm and the carb throttle level at the half travel points. That distance is very close to the perfect linkage length. So the carb is half open and the throttle servo is midway between end points. Servo neutral to be accurate. Start with 100% travel on the throttle servo. You can adjust travels either direction from there later if needed.

2) Typically, using the innermost hole on a stock short nylon servo arm is the best linkage attachment point on the servo end. You can make a new hole inside of that one and get even better. On the carb, the outermost hole or attachment point works out the best initially. That combination of inner and outer provides the best mechanical resolution. When possible, replace the stock and quite small throttle lever tab on the carb for one longer. Up to an inch in length on the carb side works out nicely most of the time. There are various methods and products available to do this.

#) Servo choice: If you have a 48 ounce inch servo or greater you are good to go on engines up to about 250cc or so. The throttle return spring is of no load consequence to the servo or flight battery. Once the servo achieves it's commanded position the current draw is next to nothing unless you have binding linkage. Do not remove the throttle return spring from the carb. It doubles as a safety device while maintaining the correct positioning of the throttle plate in the venturi. Don't use servos designed for small electric planes and foamies for throttle servos. The gears are much to thin and strip out under combustion pulses.

Back to the throttle linkage. With the linkage connected, check the end points of the servo throw for any binding while visually confirming the throttle plate is opening and closing completely. Be sure the throttle trim lever is fully down, or closed, when you do this. Check the linkage is not a little long or short before messing with radio trims or travel adjusts. Play with the end points through the transmitter as needed after you have verified linkage length. Most cases will at worst have a slight reduction from 100% at both ends to provide the correct travels. Some will require less than 100% at one end and a bit over 100% at the other. if you have multiple attach points on the carb side, work with them a little before changing the travel end points. Often in or out one hole on the carb lever will handle a slight mismatch. Travels above 100% improve resolution. Below 100% detracts from resolution.

After the linkage length and travels have been established, use the throttle trim tab/lever, with the throttle stick fully closed, to verify you have adequate throttle trim travels to fully close the throttle plate and enough to open it up about 1/8" of movement at the edge of the butterfly without using the throttle stick. That established the needed opening for starting when using the choke and sets the stage for a good engine cut when the throttle plate closes using the trim tab.

Now run the engine and see how linear the engine response is to throttle stick movement. You'll be pretty good for the first 1/2 to 3/4 stick travel but the nature of a throttle plate carb will prolly make you want to set up a throttle curve for the top side of the stick. Curves are best done with the engine running and someone holding the tail.

There are a few fiine points I missed in this guide but you have enough to get a good start on a throttle set up. The few small points I've forgotten will become evident as the process progresses. You math majors out there (see Jody will quickly figure out there's a means to detemine the rod lengths by degreeing the arc radii of the servo and throttle but I'm not a math major....

DaleD

Hi jody,

What is the unit of measurement use for the distance given for the servo shaft to hole and throttle shaft to hole on the diagram? I am thinking that is doesn't matter as long as the ratio between the two is maintained. Am Icorrect?

Thanks

DaleD