This is important

I’ve gone to scores of funflys over the years, and it seems like at every single one there’s at least one flyer that complains about the way his model performs. Most of them think their mixture is off or maybe their engine is worn out.

I don’t know how many times it’s been written, but evidently the word isn’t getting out or maybe these guys just didn’t believe it’s that important. Or, maybe they just weren’t willing to put in the time to get it right. Here’s the deal. Even if it takes a week of Sundays, GET YOUR THROTTLE SETUP PERFECT.

What’s perfect? Perfect is simple to define, harder to achieve. Perfect is when your ATV / Travel Adjust is 100% on both sides of the throttle and at 50% servo output the throttle barrel is exactly half way between full open and closed to the point the engine quits.

Notice I didn’t say half stick. 50% output is half stick only when the throttle curve is set so half stick is set to 50%. For this procedure, it’s best to set your throttle curve to it’s default settings. That being 0% at low stick, 50% at half stick and 100% at high stick. Other points should be deleted or set to default. Turn off any throttle mixes.

Now imagine a rectangle, which has two equal sides and four 90 degree angles. That’s what we’re striving for. Draw an imaginary line between the center of the throttle barrel and the center of the servo output spline. That’s one side. The side opposite this is the throttle control rod. It will be the same length as the other side, if things go well. Start out by adjusting the rod to this length. The other two sides are the servo arm and the throttle arm. Ideally, they should be the same length also.

Diagram

Find a servo arm that will be 90 degrees to the rod. This is assuming the throttle and servo arms go either up or down. In cases where one is up and one is down, things might need a little tweaking. Gary Wright makes a nifty tool which lets you use a wheel and drill the hole exactly where it should be.

Test the setup and adjust the throttle arm length and position on the throttle arm, if necessary. You might have to make some very small adjustments to the length of the throttle rod or some small adjustments to the ATV (try to keep them as close as possible to the same on both sides). Take your time an do it right.

You’ll see that, when you’re finished, at the center of the servo’s throw the throttle arm moves quickly compared to the way it moves at the extremes. That’s why we went through all this. The response of the engine is quicker and more linear where it needs to be. Take a look at the drawing at the bottom of this page.

Now readjust the throttle curve to your liking. Normally you will be hovering at about 60 - 64%. Don’t be surprised if your model seems a lot more responsive and just runs better overall.

Why... some details

When your throttle barrel rotates, the air flow looks kind of like the drawing below. Actually, when you throw in the differential the servo provides, it's even worse. That's why it's so important to have the area of the best throttle response coincide with the most linear portion of the servo movement.

Imagine if the linear portion of the servo movement was way up on the curve. There wouldn't be much throttle barrel movement in the bottom half. When a descent is begun, the throttle wouldn't drop and the engine would likely over speed. Obviously, guys with a dozen points in their throttle curve could adjust this out by making their throttle curve look like a pretzel, but people that have only 5 points must depend on a proper setup.

If you were to have the linear portion of the servo movement toward the bottom, you wouldn't have very good throttle response in a climb out. Mike Lehr, from Oklahoma City came to me at a funfly and wanted to buy a new engine because he thought his was worn out. He said it never had made good power. I took a look at his throttle curve and ATVs. They were pretty hosed up, so we started over and set it up right. Mike couldn't believe it was the same engine. It climbed out twice as fast, and had much better transition in all directions. I guess I could have sold him a new engine, but that's not the way we operate.

By the way, you guys with 91s might think this isn't important, since this engine has so much torque and power that it climbs great no matter what....wrong. If you'll take the time to do it right, you'll realize the full potential of the engine.

Diagram of throttle movement

You'll notice that the curve is shifted to the right about 13%. That's the amount of rotation between engine off and idle.

With this setup you can set your points as follows in your 3d curve.

Rest assured you'll be really close to your final settings. Of course you'll have to make some small adjustments depending on how lean you run your engine, blade size  etc, but you will be close.

Try it, you'll like it.

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