I'm having problems getting my throttle linkage set properly on my Alpha 60 with an Evolution 61. I'm using a Futaba 7C 2.47Ghz TX.

The throttle position looks good at both full open and full closed stick throw, with trim smack in the middle. What I can't do is get it to close off using the trim to shut the engine down after landing. One of the guys at the field fiddle with the throttle shut off switch, end points and sub trims (not even sure what sub-trim is) and it's just not wanting to close down anymore from idle position.

What do I need to do here? I'm assuming that the first thing to do is re-set the endpoints to 100%, and zero out the sub-trim and start over. Then what?

............and if I'm not mistaken, with the throttle cut off switch activated in the "on" position, the endpoint adjustment for that channel goes inactive.

Bottom line is that I'm very new to this, know enough about my radio to make me dangerous, and don't know how to fix this.

Thanks in advance,
Bob

You need to set your computer radio to zero-zero for the throttle settings. No trim, center of throws at the center and 100% for the throws.

Now, look at the arm on the servo. If it does not make a 90degree angle to the pushrod, then take the hold down screw out and move the arm to as close to 90degrees as you can. NOW use the radio's centerpoint adjustment to get that arm and that pushrod to make a 90degree angle.

OK.... now..........

Now go to the throttle arm on the carb. Unhook it and move the arm until the opening in the intake is half opened. Will the throttle arm and the pushrod now make a 90 degree angle? If it does, adjust the length of the pushrod to connect to the arm where they will have that 90degree angle.

If it does not, then loosen the screw holding the throttle arm and move that arm to the 90degree position. Tighten the arm and adjust the pushrod.

You now have the rigging setup so the servo will rotate the throttle equally in either direction.

Now........

I have also found that it helps if you take the servo arm off, and hold it up to the carb throttle arm. Make sure you are using a hole in the servo arm that corresponds to the hole on the carb throttle arm. That way you get a 1:1 movement of servo to throttle, and you should have to adjust endpoints all that much. This makes a much smoother transition. Whenever you use endpoints to adjust throttle travel, you often times get a point where the throttle moves too much at once, or not enough at once because you are messing up the linear relationship you would expect between the radio throttle movement and the actual engine throttle movement.

Hope this made sense.

Until you have those relationships set, you're working with a servo that will move the throttle more one way than the other.

Having a 90degree angle between the servo arm and the pushrod, and a 90degree angle between the throttle arm and the pushrod is basic. Don't have it and you're wasting time.

Now it's time to see if the servo moves the throttle drum too much or too little. If it moves it too much, move the pushrod connection in one hole on the servo arm or one hole out on the throttle arm. If it doesn't move it enough, move the connection out on the servo arm or in on the throttle arm. When it's close, THEN use the endpoint adjustments on the radio.

It's not a radio problem. It's a basic "building a model airplane" problem.

Every center of rudder/elevator/aileron/throttle is based on the basic truth, "the pushrod should make a 90degree angle with the arm when centered". Only a few special situations require otherwise. And most everything else actually requires the 90degrees.

I think your problem here is your throttle stop screw. Readjust it a bit so your carb will close completely.

If the carb is closed you probably have an air leak.

Wow, talk about fast responses! [X(] Thanks.

OK, with the throttle stick at middle position, the servo arm is at center, the servo arm and pushrod are at 90 degrees, and the pushrod and throttle arm are at 90 degrees. The throttle barrel is half open. That's been the case all along. At full open throttle the barrel is fully open. At full closed throttle stick the barrel is open about 1/16tb inch. Just what the book calls for. The trim switch will NOT close the trottle and kill the engine. That's when folks started fiddling with the cut off switch, end points and sub trim. (still don't know what sub-trim really is).

Edit

Initially the throttle push rod was binding on the engine mount so I slightly adjusted the offset bends in the pushrod to give clearance. Everything is now free and clear.

I can loosen the set screw holding the pushrod in position at the servo arm and easily move the throttle barrel to fully closed with the pushrod, so the stop on the throttle arm doesn't seem to be the problem either. The servo is not binding at either full open or full closed.

So, I'm still at square one.

Did you look at what GunfighterII suggested?

OK, then you need more total movement from your servo to the drum.

You say your radio has end point adjustment? And it's got an "over maximum" ability that comes into play when the kill button is pushed. If you're sure the kill button actually moves the servo a bit more, then give that end of the movement a head start. Adjust your linkage from the kill position.

Move the throttle to closed. Now loosen the lock screw. Now push the pushrod to where the throttle barrel is closed. While holding the pushrod, press the kill button. Tighten the lock screw and let go of the pushrod. Let go of the kill button. Your radio should certainly now be able to bring the carb to idle and the trim and kill should have the movement and freedom to kill the engine. But now.......

Now your computer radio should have enough adjustments in it to do whatever else is needed. You might need to increase the end point for max throttle. But there's nothing inhibiting that movement. If the radio still can't do it, then move the pushrod out on the servo arm one hole or in on the throttle one hole. If that happens you'll need to do all the above all over again. But you will have proven there is freedom of movement at both ends of the throw, and now you'll absolutely have enough movement to do the job.

Almost every model airplane carb is designed with throttle arm movement that matches up with the average servo arm throw they expect to find paired with the engine.

Sub-trim is what Futaba call's their servo centering function. It's a way to make up for the problem servo centering has with the teeth in the servo arm matching the teeth on the servo drive shaft. When you try to get the servo arm to make a perfect 90degree angle with the pushrod (the angle the arm makes with the servo is absolutely of no consequence) the arm usually won't match up with the drive shaft to give perfectly 90degrees. Also, some people like to have their trim levers at zero trim (a pretty good idea) and think the radio should give them a crutch to do that (a less than good idea). And the sub-trim can do that too.

You really want to make sure your sub-trim setting is centered for this exercise.

GunfighterII suggested the throttle stop screw was the problem.

Thanks to you and to the others who replied. We figured it out at the field this afternoon.

The linkages and geometry were all just fine. The problem was that I had reversed the throttle servo in the wrong manner. (The location of the servo required it be reversed). I set the reverse using the "thrtl rev" menu setting rather than just reversing the channel 3 servo in what apparently is the "normal" manner. There is a separate menu tree to "thrtlrev" and the spot where you can reverse all of the servos as needed.

The way I had done it apparently locked out both the trim and the throttle cut functions.

As soon as we undid my reversal and set it up normally everything was fine.

The plane started, idled and cut off just fine.

Five minutes into the "re-maiden" flight, though, disaster struck. With the instructor flying the plane, straight and level, the firewall separated from the remainder of the fuselage. I'm posting a full account in another thread "Second Flight Disaster". At least it's a manufacturing defect. I would expect to have the plane replaced.

Thanks again.

Bob