7955 - Centering issue

[djmisterpe]

Guys I have just installed new 7955s on my 50cc YAk.I have very recently noticed an issue with the 7955 istalled on the rudder.And thank God this was detected with my prechecks on the ground.When I give left rudder and let it come back to the neutral position, the rudder doesnt center fully...I have to sligtly push the stick to the opposite side and take it back to the neutral position to let it center fully.I cant figure out wht this actually could be...servos totally new.Im just praying the servo's not damaged.or cud this be an issue with my pull pull set up.I know my rudder isnt warped at all.

[bodywerks]

Try JR, futaba, or airtronics servos...

[jmoesdad]

had that happen also...make sure the subtrim is at 0 and center the arm as best you can before trimming.

[djmisterpe]

I'll try the options.here is the picture of how it looks when the rudder and servo is fully centered.I know there is a slight tilt in the arm to the left but dunno if this could be a problem as both servo and rudder are center in this position of the arm.Its like the servo doesnt fully return to the neutral position but once given a slight thumb in the opposite direction and the it stays neutral.guys im a bit new to setting up giants.correct me if u observe any mistake.

[jamesrxx951]

i would swap that servo out for elevator or aileron servo and leave everything the same and see if that problem follows the servo. if it does it must have a bad pot or something. hitec will fix it if there is a issue. i have had excellent service from hitec. including repairing a servo that was damaged in a crash and fixed it no charge when i told them i would pay for it.

[djmisterpe]

I have also noticed the rudder servo buzzez alot even zero movement and sounds as if its under alot of load.

[bodywerks]

I would mount those ball joints on the bottom of the servo arm. You are putting massive torsional load on the output shaft bearing and that can be affecting things, especially if your cables are too tight. If you can play a note on them(like a guitar string) you are WAY too tight and this will affect centering.
Also, that looks like a pretty huge arm (4"?) for a 50cc plane. What is the distance from the hingeline to where the cable connects at the rudder control horn? It should be AT LEAST 2 inches. Any less and you are asking for trouble and you lose resolution and centering ability.

[djmisterpe]

	Quote: Originally Posted by bodywerks
	I would mount those ball joints on the bottom of the servo arm. You are putting massive torsional load on the output shaft bearing and that can be affecting things, especially if your cables are too tight. If you can play a note on them(like a guitar string) you are WAY too tight and this will affect centering. Also, that looks like a pretty huge arm (4"?) for a 50cc plane. What is the distance from the hingeline to where the cable connects at the rudder control horn? It should be AT LEAST 2 inches. Any less and you are asking for trouble and you lose resolution and centering ability.

Bodywerks,

I have included the picture and marked the points from where I measured the distance.It gets to 1'' .Not sure if the distance between points I measured from are exactly what you suggested.and could this all sum upto damaging the servo in anyway.I've had two flights on it.a few knife edges and a flat spin.

[bgold]

You need to get the pivot point of the connection in line with the hinge line. From your picture it looks like your pivot point is 3/8 to 1/2 inch behind the hinge line. With this setup you must have one line tight and the other loose when the rudder is activated and neither is the same as tension as when the rudder is neutral. The dimension bodywerks was talking about was from the hinge line out to the pivot points. The distance from between the pivot points (width) should be the same as the distance on your final servo output arm i.e. if the rudder is 3-1/2" then you should have a 3-1/2" servo arm.

I don't think it's the servo, it's your setup.

[wfield0455]

	Quote: Originally Posted by djmisterpe
	I have also noticed the rudder servo buzzez alot even zero movement and sounds as if its under alot of load.

The servo buzzing indicates it is trying to find a better center. I would like to second some of the suggestions other have already made and make a couple of others as well.

Try to reduce the torsional load on the servos output shaft bearings by moving the ball links to the bottom of the servo arm. This assume you can do that without having them hit anything.

Use a shorter arm. If you want to avoid problems with uneven cable tensions at full rudder deflection I would recommend making the distance between the ball links on the rudder control horn be the exact same spacing a the ball links on the rudder servo outupt arm...

Disconnect your pull pull cables and get the rudder servo perfectly centered. Start by centering the trim and zeroing out the rudder subtrim. If the servo arm isn't perfectly centered, remove it and try rotating it 180 degrees. Because of the geometry of the splines on the servo output shaft
the arms usually fit better one way then the other... Once you find the best fit use the subtrim
to get the arm perfectly centered if it isn't already. Or, if you have a servo programmer, use it to tweak the centering of the servo... If it had been tweaked before you might want to start from a factory default setting...

With the cables still disconnected, try rotating the servo arm. No need to really crank on it, just a firm twist. It shouldn't move.... If it does, determine if the servo output shaft is actually rotating because of poor centering or if the arm is moving because of a sloppy fit of the splines in the arms to the spline on the output shaft.. I prefer the arms that are slotted and have a locking screw to tighten them to the output shaft as they are easy to get on and off and they lock tight to the output shaft once tighted down. If the arm is slipping on the shaft, get rid of it. If the servo rotates under reasonable pressure, make sure there are no loose electrical connections that could be causing a voltage drop to the servo. If that all looks good and it still rotates under reasonable pressure send it back for repair...

Deflect the rudder back and forth through it's full travel and make sure that there is no binding in the hinges or anything like that..

If all that is good, reconnect your pull/pull cables and adjust them for an even tension but not really tight.. If everything above checked out Ok, it really has to work now....

Sorry for the long winded post as you probably already know most of this or have your own ways of getting things done.. Just wanted to list what I do step by step in cases like this just in case you may find something useful...

Hope you get it sorted out..

Wayne

[djmisterpe]

	Quote: Originally Posted by wefcisco
	The servo buzzing indicates it is trying to find a better center. I would like to second some of the suggestions other have already made and make a couple of others as well. Try to reduce the torsional load on the servos output shaft bearings by moving the ball links to the bottom of the servo arm. This assume you can do that without having them hit anything. Use a shorter arm. If you want to avoid problems with uneven cable tensions at full rudder deflection I would recommend making the distance between the ball links on the rudder control horn be the exact same spacing a the ball links on the rudder servo outupt arm... Disconnect your pull pull cables and get the rudder servo perfectly centered. Start by centering the trim and zeroing out the rudder subtrim. If the servo arm isn't perfectly centered, remove it and try rotating it 180 degrees. Because of the geometry of the splines on the servo output shaft the arms usually fit better one way then the other... Once you find the best fit use the subtrim to get the arm perfectly centered if it isn't already. Or, if you have a servo programmer, use it to tweak the centering of the servo... If it had been tweaked before you might want to start from a factory default setting... With the cables still disconnected, try rotating the servo arm. No need to really crank on it, just a firm twist. It shouldn't move.... If it does, determine if the servo output shaft is actually rotating because of poor centering or if the arm is moving because of a sloppy fit of the splines in the arms to the spline on the output shaft.. I prefer the arms that are slotted and have a locking screw to tighten them to the output shaft as they are easy to get on and off and they lock tight to the output shaft once tighted down. If the arm is slipping on the shaft, get rid of it. If the servo rotates under reasonable pressure, make sure there are no loose electrical connections that could be causing a voltage drop to the servo. If that all looks good and it still rotates under reasonable pressure send it back for repair... Deflect the rudder back and forth through it's full travel and make sure that there is no binding in the hinges or anything like that.. If all that is good, reconnect your pull/pull cables and adjust them for an even tension but not really tight.. If everything above checked out Ok, it really has to work now.... Sorry for the long winded post as you probably already know most of this or have your own ways of getting things done.. Just wanted to list what I do step by step in cases like this just in case you may find something useful... Hope you get it sorted out.. Wayne

Great info Wayne! its actually really helpful to be given detailed instructions on how to try out various stuff and eliminate points of error.will def try out the steps you've suggested.

Thanx a bunch guys!

[bodywerks]

	Quote: Originally Posted by djmisterpe
	Bodywerks, I have included the picture and marked the points from where I measured the distance.It gets to 1'' .Not sure if the distance between points I measured from are exactly what you suggested.and could this all sum upto damaging the servo in anyway.I've had two flights on it.a few knife edges and a flat spin.

That is not what I am talking about. I am talking about the measurement from your hinge line out to that nut-and-bolt holding the clevise to the control arm in the pic.This is the effective length of the control arm. This is basic geometry stuff here. Under no circumstances whatsoever should the control arm length be shorter than the servo arm length.
In that pic, although I am looking down in the measurement I am talking about, it looks like you are only about 1-1.5" out - not good.

[djmisterpe]

	Quote: Originally Posted by bodywerks
	That is not what I am talking about. I am talking about the measurement from your hinge line out to that nut-and-bolt holding the clevise to the control arm in the pic.This is the effective length of the control arm. This is basic geometry stuff here. Under no circumstances whatsoever should the control arm length be shorter than the servo arm length. In that pic, although I am looking down in the measurement I am talking about, it looks like you are only about 1-1.5" out - not good.

Ok I think I got it right this time.Each control horn is 1.35'' from the hingeline.makes it 2.7'' wide.The mpi control arm at the servo is 3'' wide.

[bodywerks]

Are you sure it's not 4"? Regardless, your geometry is off. You should be able to get all the throw you will ever need with 3.5-4" on the rudder(point-to-point) and a 3" servo arm.

[djmisterpe]

	Quote: Originally Posted by bodywerks
	Are you sure it's not 4"? Regardless, your geometry is off. You should be able to get all the throw you will ever need with 3.5-4" on the rudder(point-to-point) and a 3" servo arm.

yep... measured the mpi arm again with the scale...its 3'' .will try to extend the rudder arm length to 3'' - 3.5'' aswell.have changed the ball link placement on the arm as u suggested.they are on the bottom side of the arm now.but the cables are now short enough to be very tight.I have to pull the arm setup up with force for it to get to the servo shaft.so i have put that aside for until i increase the length of the cables.dunno how to get an idea how loose or tight should the cable setup be.