I brought my Tower Trainer back out to investigate again why the elevator servo buzzs.

I built another pushrod, changed the connections from a basic nylon clevis to a L-connection. Still buzzing.

Then I found it . . . Whenever I move the elevator to the down position THE SERVO SHIFTS. It is fine when up but I can see the servo shift every time in down position.
The servo is screwed down all the way to the point where the grommet is impressed lightly and no more, so yes it is secure.

Why is the servo shifting only when the elevator is down and how can it be remedied?

Thank you

If it's moving it is because the servo arm is "torquing" or twisting as it tries to push/pull the pushrod. The pushrod is resisting the "command" from the servo. There is resistance in the control somewhere. A bind. It may not be stuck but stiff.

The other point is How much is it moving? A little movement is normal because of the grommets and stuff.

It sounds to me like your elevator is in a bind. Disconnect the servo and check to see if the elevator moves freely through its entire range of motion without locking up. A servo that is pushing hard against a control surface that won't move will buzz and drain your reciever battery extra fast as well. It's likely that the elevator was not bevelled enough around the hinge.

I would have to agree with Bruce here. How much movement are we talking about?? You will have a little bit of movement in a properly mounted servo because of the rubber grommet, but that's good because the rubber is doing it's job of isolating the servo from vibration. But if you are getting a lot of movement then you have something seriously wrong with your setup. My guess is that you are trying to move the control surface past it's limits. Once you hit the limit of movement something has to give as you continue to push against it. Right now it's only the servo moving, but eventually the control surface will tear lose from the plane. If this is indeed what is happening then you need to change the position of your pushrods in the control horns to adjust the amount of travel of the surface.

If you have flexing or moving of the servo going on it's because you have something wrong with your setup. You can't just look at the pushrods, but you have to look at the entire setup as a unit. The servo, servo arms, control horns, pushrods and control surfaces need to all be looked at to find problems like this.

Ken

I agree, a servo buzz is a cry for help so to speak. You must have a bind somewhere. either in the elevator or the pushrod. disconnected from the evevator and make sure the travel in both directions is even and smooth. If it is okay check your pushrod again, same way, nice and smooth both directions, full travel. if thats okay check to see the control horn is not binding when hooked to the clevis.
good luck

I hope it is not the elevator itself, as I already had to cut the old one off because of a warp. The cut CA hinges from that removal are still enbedded in the stab trailing edge, so if I have to remove the elevator again I will have no new area to cut new hinge slots.

The control horn is at a slight angle as per instructions, but maybe I should have had it straighter anyway. But I don't believe the horn has anything to do with it because the buzz is still there even with the horn end disconnected. The pushrod opening is clear and the rod slides cleanly in and out of there, so it looks like that leaves the rear-end OK. The servo end is not hitting or rubbing anywhere and the arm is moving freely. That leaves inside the fuselage, but looking inside I still don't see anything rubbing or binding. Please don't tell me I have to rip the covering off to investigate further.

Disconnect the pushrod from the servo. Now, while holding the rod end as close as possible to the servo arm, move the rod in/out as if the servo was stroking it. Feel VERY carefully for any snagging or resistance.

Hate to say it, but the elevator itself is binding. I disconnected as you said bruce and moved it by hand. The pushrod itself is not obstructed but I notice with the down elevator there is some tight resistance in the elevator itself, with some kind of sticky noise like the sound of a piece of tape being pulled off covering (no, there isn't any covering coming loose, it only makes that noise) and it is coming from the hinge line. So let me be the first with the bad news [:@], I will have to take the elevator off yet again and rehinge [sm=cry_smile.gif] But as I stated earlier, with me having to remove the elevator because of a warp, I have no place to reslot for CA hinges. Ideas [sm=confused.gif][sm=confused.gif]

Being its a trainer, I would cut off the elevator, find whats binding then just use packing tape or covering to re-attach the elevator.

try moving the elevator up/down until it loosens up a little servo buzz is not as much of a big deal as you would think.
question is are you trying to move the control surface more than 20% over the recomended throws,if so then don't.
it just makes the model more out of control if you do.
move the push rod to the next outer opening of the control horn so you are in the recomended range, then work in the elevator by the up and down method after a few movements past the buzz zone then re-check to see if the buzzing is gone and keep going past the tite spot untill it opens up the gap,it doesn't take much.
re-hinging is a pain, and unless you glued the elevator to the stab it should loosen up with a little work in.
if you don't line up the hinges just right your going to get your buzz right back.
then you will be right back where you started after doing a bunch of work that you thought you needed but didn't.

Make sure there is NO covering in the way anywhere around the elevator but especially near the hinges. You'd be surprised how little it takes. Make sure the previously cut off hinges are not sticking out and rubbing. Then tighten down the servo a little more and go fly it as long as the elevator actually moves through the desired range. Check the battery before EVERY flight, something you should do anyway.

There are just the tiniest bit of covering bits sticking out near the hinges, and a few of the previously cut hinges are sticking out about 1/16" but none of it appears to be rubbing anything. I did notice at one end of the elevator is off-line maybe 1/16" off center - will that effect it alot? The elevator does move evenly across without bending anywhere that I notice. If I end up having to take the elevator off again using covering for hinges like Groundforce-RCU suggests prob wouldn't be enough for a .40 powered trainer? They are more suited for park-flyers and some electrics.

With the servo disconnected, work the elevator by hand throught it's total travel, end-point to end-point. (even though it will be further travel than needed to fly) Do this 100 times, or so. If it's going to loosen up, this should do it. Return the servo to operation and place the pushrod on the nearest inside hole that will give you the recommended throw. This will provide less servo strain without over-controlling the elevator.

how much moovment are you getting before the bind? remeber its a trainer you cant expect to get 3D trows....

Could be as simple a fix as just working the elevator back & forth to get it freed up. Just disconnect the pushrod & flex the elevator up & down until it frees up. I just went thru a similar situation with a small electric. Thought I had a bad servo because it moved the aileron so slowly. So I took it apart & changed out the servo only to realize I had fixed the good wing.[>:] So I took it apart again & replaced the servo on the bad wing this time, only to get the same result. Finally the light bulb came on & I realized that I forgot to snap the hinge line after allowing the CA to dry. So I just spent a good bit of time working the aileron back & forth to break in the hinge line & now it works just fine.