majortom-RCU

My Feedback: (40)

Setting up a four-stroke in a new model today, I noticed my throttle stick was way out of whack, idle at bottom, then very rapid climb up the rpm curve from there, with stick at one-quarter I was over 8000 rpm, and over 10000 with stick less than half-way up. This on an engine topping out at 10800 static. I remembered from somewhere the tip of putting negative expo (Futaba TX) on throttle. I gave it a try, set the expo all the way to negative 100%... now have the best control I've ever had on throttle stick across the full range. What did we ever do before computerized transmitters?

DarZeelon

Norman,

The throttle should hardly be linear from an RPM standpoint.

In most cases it will not be linear from an output standpoint either.
But it is much closer to that. At 10,800 RPM your engine is making 26% more power than at 10,000.

If you own a computer radio and your question about exponential movement suggests that you do, you have end-point, or ATV adjustments, which are independent on the upper half and on the lower half of the stick movement.

You can tailor the throttle servo to move less in the lower half and more in the upper half and make push-rod, or sub-trim adjustments, making the throttle stick more linear.
In addition, you will have very accurate adjustment of the idling speed, with the trim lever.

majortom-RCU

My Feedback: (40)

Right you are, Dar. Actually my post was more a comment than a question, but I have seen a few intermediate flyers struggle with throttle set-up, so I thought I'd put up a thread to see what further comments this topic might draw--prompted by my satisfaction with the great results I got yesterday by putting expo on my throttle for the first time.

Yes, I moved up to a computerized Futaba 8U a few years back, and have enjoyed learning to use its potential. I routinely use ATV (end-point adjustment) to get throttle servo travel to coincide with engine throttle mechanical limits. Just pick a combination of throttle arm attachment point and throttle servo arm attachment point that is just short of giving full throttle movement with the default settings, then use ATV to extend throttle servo travel out to the mechanical limits at top & bottom ends. I do this with the throttle trim button set all the way down, which facilitates setting the Throttle Kill switch, which I also routinely do.

I also notice that my throttle plate will move to fully open, then rotate a few degrees past that point, so as to drop the rpm's down a bit. I adjust the ATV on the top end so that throttle stick full up moves the mechanical arm to its maximum rpm setting, which is a little below its top limit.

Next I do the Throttle Kill Switch setting, which takes the throttle to its fully closed position. This allows killing the engine when needed to bring down a light plane with no headwind. I've had set-ups where the lowest idle speed would still pull the plane along indefinitely in ground effect, so killing the throttle completely was the only way to get the plane back on the ground.

Next I return the Throttle Trim button to its center position, warm up the engine, then adjust Throttle Trim button to the point where the engine is just short of pulling the plane through the grass. I use that as my flying idle speed. From there I set up my Idle Down adjustment, which switches in a lowered idle trim setting, and take that down to the slowest reliable idle. I use that to switch down to a lower idle for landing.

In previous set-ups, that's all I've had to do. However yesterday setting up a new model I found I had basically two speeds--idle, and high speed, with very little in between. Dialing in the exponential fixed that very nicely, so now I can ease the stick forward and have a little more power with every click up, all the way to the top.

I have yet to resort to another measure addressed in the TX manual, that of mixing throttle to throttle, to permit an even more precise shaping of the power response curve. This technique permits the adjusting of % throttle servo output at five waypoints of % throttle stick input. That's still a little fancy for my needs, but maybe I'll give it a try someday.

This morning I look out at a thermometer reading back down to freezing. I'll wait for the sun to warm the air, then head back to the field for another bit of fun. Dar, thanks as always for your comments.

Boomstriker

The new computer radios are great, but they shouldn't be used to correct or compensate for a bad mechanical set-up.
Exponential or non linear travel can be inadvertently brought in to the movement of the throttle by poor set-up up of the servo horns, throttle arms and the links. When computer radios weren't available, we could purposely set it up for expo, if needed, with the geometry of the mechanics.
If you just pick a hole and hook the servo to the throttle, then play with the radio settings to get it to move full tilt, you're going to get non linear movement. The same goes with the flight surfaces too.

Here's the basics....

1) Start by going to default settings- centering the trims, setting the travels to 100% and getting rid of any expo, sub trim, offset or other mixing.

2) All servo movement should be based on 60 degrees of total travel. This is the amount of rotation the servo sees with full deflection of the stick and 100% of travel set in the radio as above. I'm not going to get into the geometry of it, but anything more than this results in more nonlinear movement of the rod at the ends of travel because the servo is turning, but the rod is going more side-to-side, not back-and-forth anymore.

3) All servos arms should be at 90 degrees to the rod when the stick and servo are at center. If the arm is is not 'centered in it's travel', it will move the rod farther in one direction than the other. If one arm doesn't line-up at 90, rotate the horn on the splines until you find one that is, not by using the subtrims or offsets.

4) Measure the distance from the center of the throttle arm to the farthest hole out in the arm. The hole used in the servo arm will be nearly this distance from the center of the servo horn because most carbs rotate around 60 degrees also.

5) Make the rod with good quality adjustable ball-links or clevises and drill the holes so that no slop or binding exists in the arms or set-up when installed between the arms.

6) Just as in step 3, the throttle arm must also be at 90 degrees to the rod when the servo is centered and IT'S arm is at 90. At this centered point the carb should be half open, if not, re-adjust the carb barrel to the throttle arm.

7) Move the stick up and down, checking for full range of movement and not over-travel of the carb and binding. If the carb doesn't open all the way AND doesn't close to an idle slot, move to a further out hole in the servo arm so the carb will see more travel. Likewise, if the carb is over traveled on both ends, move to the next inner hole. Don't move the end points from 100% unless it can't be achieved with the servo horn.
If the the carb over-travels at full, but doesn't close enough, or vise versa, be sure that the arms are at 90 degrees to the rod when centered, (step 6) and adjust the carb barrel to the throttle arm, or the length of the rod for better centering.

Once you have the travel and the centering set-up mechanically, the radio can be used to fine tune it.

The radio controls the servo movement, but if the mechanics are screwed-up, the radio will just make it worse trying to compensate.
The mechanics must be set-up properly first, then worry about the radio.

Kirk

JoeAirPort

My Feedback: (41)

This is good timing. I have to set up my throttle on my FuntanaS/YS63 engine (again) and I want to do it right. Reading this thread really helps. One thing I've found is that if I set my throttle up to go from full throttle to closed at 100% endpoints, I usually have to reduce my travel on the close side from 100% down to around 70% or I can't get the idle high enough. So I end up not using the full travel. Then I usually can't use my engine kill switch feature (Futaba 6EXA computer radio) since the trim is subtracts isn't enough to kill the engine. Maybe I'm doing something wrong too, but IMO throttle setup is NOT as easy as I thought it would be

Thanks to all for your tips. I'll try it out this week and see how it goes.

majortom-RCU

My Feedback: (40)

Joe, I remember having the same problem you describe; in fact that was also on a YS 63. I found that if I set the low end ATV with the trim beeped all the way down to its lowest setting, then later when I would return the throttle trim to its mid setting I would have no problem with a steady idle. Then when I would throw the Throttle Cut switch, it would take the throttle right down to rock bottom. All this would be done with the engine cold, constantly eyeballing the throttle plate and listening for the buzz of servo stalling. Then once the engine is started and warmed up I would adjust the throttle trim button for a smooth and reliable idle. I suppose it matters where you have your Throttle Trim Rate setting. I always leave that set at Trim Rate = 10, and have never felt a need to change that setting.

Boomstriker

Hey Joe, I'm glad it helped.

If the throttle goes open fully with 100% but closes the barrel at low stick with 100%, then the total travel is too much so you would need to move in one hole on the servo horn. You will need to lengthen the rod to get full open now but it should be closer.

Kirk

Hobbsy

My Feedback: (102)

I always set my throttle on my JR 10SXII by starting with the carb barrel at what I judge to be 50% open, not 50% power, with the throttle stick set at the center of it's travel, then I set the end points. This seems to get about the same rpm change per click.