Servos jittering/unable to center
11-22-2006, 02:25 AM
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Servos jittering/unable to center
I've got a couple of DS811 servos that are acting weird. They have been fine until today. I went to start my plane and I noticed that when the right aileron servo centered it "vibrated" for a second. Imagine bending a thin, flexible piece of metal back then letting it go. It vibrates around the center until it stops moving. Well, that is exactly what the servo is doing. I then noticed one of my elevator servos was doing it too. This one just does it constantly, then stops for a bit, then does it more. These servos have maybe 30 flights on them total. The DS821 on the rudder seems fine, as well as the other DS811s on the plane. All the servos move normally and at the proper speed. It's only when you let the sticks center that they do this.
Any ideas? I'm using a JR9303 radio with a R790 receiver. DS821 servo on rudder, 4x DS811s on elevators and ailerons and 1x DS285 on the throttle. MPI regulated switch at 6v.
Any ideas? I'm using a JR9303 radio with a R790 receiver. DS821 servo on rudder, 4x DS811s on elevators and ailerons and 1x DS285 on the throttle. MPI regulated switch at 6v.
11-22-2006, 09:14 AM
#3
RE: Servos jittering/unable to center
The problem is in the servos - and on your setup
these servos have tight centering and any looseness in the gear train PLUS running at higher voltage - can cause a self sustaining dither - It is not a situation peculiar to the 811- which is an excellent low cost servo .
The same thing will happen to ANY tight centering servo with a bit of play in the gear train.
As the servo snaps to center -- the momentum of the load it is attached to , will thru cause a mild overshoot - then that will cause the servo to try to move the opposite direction-in order to fully center .
The load - and the amount of overshoot - typically being quite small - will setup this oscillation. Touching the surface will typically stop the movement.
Typically changing the inexpensive gear set - will stop the problem - but there is the chance that wear and weight of the control linkages and surface attached is "just right" to repeat the problem
these servos have tight centering and any looseness in the gear train PLUS running at higher voltage - can cause a self sustaining dither - It is not a situation peculiar to the 811- which is an excellent low cost servo .
The same thing will happen to ANY tight centering servo with a bit of play in the gear train.
As the servo snaps to center -- the momentum of the load it is attached to , will thru cause a mild overshoot - then that will cause the servo to try to move the opposite direction-in order to fully center .
The load - and the amount of overshoot - typically being quite small - will setup this oscillation. Touching the surface will typically stop the movement.
Typically changing the inexpensive gear set - will stop the problem - but there is the chance that wear and weight of the control linkages and surface attached is "just right" to repeat the problem
11-22-2006, 09:40 AM
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RE: Servos jittering/unable to center
Dick is correct.
The best thing to do about it is forget it and go fly.
Until it gets so bad that it is running down your batteries. Then pull the servos out and send them in under warranty coverage. They are guaranteed for three years.
The best thing to do about it is forget it and go fly.
Until it gets so bad that it is running down your batteries. Then pull the servos out and send them in under warranty coverage. They are guaranteed for three years.
11-22-2006, 11:22 AM
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RE: Servos jittering/unable to center
My linkages are all tight. I have virtually 0 slop in all my surfaces. I can't detect any in the geartrain either, but I'll have another look at it.
I am running the servos at 6v (one tester reads the voltage from the regulator at 5.9v, the other at 6.11v - would the 6.11v be harmful?), though the surfaces are not heavy and as I said I can't find any slop. Could vibration be the cause?
The slightest touch of the surfaces does stop the movement.
So I should be ok flying like this?
Thanks for the explanation!
I am running the servos at 6v (one tester reads the voltage from the regulator at 5.9v, the other at 6.11v - would the 6.11v be harmful?), though the surfaces are not heavy and as I said I can't find any slop. Could vibration be the cause?
The slightest touch of the surfaces does stop the movement.
So I should be ok flying like this?
Thanks for the explanation!
ORIGINAL: dick Hanson
The problem is in the servos - and on your setup
these servos have tight centering and any looseness in the gear train PLUS running at higher voltage - can cause a self sustaining dither - It is not a situation peculiar to the 811- which is an excellent low cost servo .
The same thing will happen to ANY tight centering servo with a bit of play in the gear train.
As the servo snaps to center -- the momentum of the load it is attached to , will thru cause a mild overshoot - then that will cause the servo to try to move the opposite direction-in order to fully center .
The load - and the amount of overshoot - typically being quite small - will setup this oscillation. Touching the surface will typically stop the movement.
Typically changing the inexpensive gear set - will stop the problem - but there is the chance that wear and weight of the control linkages and surface attached is "just right" to repeat the problem
The problem is in the servos - and on your setup
these servos have tight centering and any looseness in the gear train PLUS running at higher voltage - can cause a self sustaining dither - It is not a situation peculiar to the 811- which is an excellent low cost servo .
The same thing will happen to ANY tight centering servo with a bit of play in the gear train.
As the servo snaps to center -- the momentum of the load it is attached to , will thru cause a mild overshoot - then that will cause the servo to try to move the opposite direction-in order to fully center .
The load - and the amount of overshoot - typically being quite small - will setup this oscillation. Touching the surface will typically stop the movement.
Typically changing the inexpensive gear set - will stop the problem - but there is the chance that wear and weight of the control linkages and surface attached is "just right" to repeat the problem
11-22-2006, 06:16 PM
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RE: Servos jittering/unable to center
I just checked my servos and they have developed some slop in the geartrain. How did that happen? They were fine a week ago, and all of a sudden they are worn out. The rudder servo seems fine though, but it's an 821 with the composite geartrain. Is the 811 geartrain not very strong?
11-22-2006, 07:57 PM
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RE: Servos jittering/unable to center
Well, there is a good bit of play in the two aileron servos and elevator servos. I don't want to fly because of the possibility of flutter. I guess I need to upgrade the servos a lot sooner than I thought.
I've got a Fliton Extra 330 with a Saito .82. What would be a good choice? JR DS3421s? Hitec 5245s? I obviously need a servo that will hold up better.
I still wonder how the heck the gears have developed so much play so quickly. *sigh*
I've got a Fliton Extra 330 with a Saito .82. What would be a good choice? JR DS3421s? Hitec 5245s? I obviously need a servo that will hold up better.
I still wonder how the heck the gears have developed so much play so quickly. *sigh*
11-23-2006, 02:00 AM
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RE: Servos jittering/unable to center
I have a Hitec 425 doing the exact same thing. It started when I installed a 6 volt pack. I checked and there isnt any slop in it. For only 15 bucks I am going to replace the servo with a new one.
11-23-2006, 10:22 AM
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RE: Servos jittering/unable to center
ORIGINAL: RVM
Well, there is a good bit of play in the two aileron servos and elevator servos. I don't want to fly because of the possibility of flutter. I guess I need to upgrade the servos a lot sooner than I thought.
I've got a Fliton Extra 330 with a Saito .82. What would be a good choice? JR DS3421s? Hitec 5245s? I obviously need a servo that will hold up better.
I still wonder how the heck the gears have developed so much play so quickly. *sigh*
Well, there is a good bit of play in the two aileron servos and elevator servos. I don't want to fly because of the possibility of flutter. I guess I need to upgrade the servos a lot sooner than I thought.
I've got a Fliton Extra 330 with a Saito .82. What would be a good choice? JR DS3421s? Hitec 5245s? I obviously need a servo that will hold up better.
I still wonder how the heck the gears have developed so much play so quickly. *sigh*
I would static balance the control surfaces to eliminate the flutter.
11-23-2006, 12:39 PM
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RE: Servos jittering/unable to center
I checked for flutter with some low passes at full throttle. I didn't hear or see any, but I guess that doesn't mean it wasn't there. I'll seal my hinges today. There wasn't much of a gap so I never got around to doing it, but I guess I need to now.
ORIGINAL: dirtybird
If you rapidly develop gear slop there is a good possibility you already have flutter.
I would static balance the control surfaces to eliminate the flutter.
ORIGINAL: RVM
Well, there is a good bit of play in the two aileron servos and elevator servos. I don't want to fly because of the possibility of flutter. I guess I need to upgrade the servos a lot sooner than I thought.
I've got a Fliton Extra 330 with a Saito .82. What would be a good choice? JR DS3421s? Hitec 5245s? I obviously need a servo that will hold up better.
I still wonder how the heck the gears have developed so much play so quickly. *sigh*
Well, there is a good bit of play in the two aileron servos and elevator servos. I don't want to fly because of the possibility of flutter. I guess I need to upgrade the servos a lot sooner than I thought.
I've got a Fliton Extra 330 with a Saito .82. What would be a good choice? JR DS3421s? Hitec 5245s? I obviously need a servo that will hold up better.
I still wonder how the heck the gears have developed so much play so quickly. *sigh*
I would static balance the control surfaces to eliminate the flutter.
11-23-2006, 02:37 PM
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RE: Servos jittering/unable to center
If you have enough flutter that you can see or hear you won't have an aircraft very long.
Something must be causing the rapid gear wear.
Sealing the hinges might help but the only sure cure is static balance of tha controls. All full scale aircraft static balance the controls.
Something must be causing the rapid gear wear.
Sealing the hinges might help but the only sure cure is static balance of tha controls. All full scale aircraft static balance the controls.
11-23-2006, 02:43 PM
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RE: Servos jittering/unable to center
How do I do this? It is an ARF, and I've never heard of anyone else with this particular plane having any problems with their servos wearing out quickly. That said, I'll give it a try.
Right now, at the end of 1.25" servo arms, I get about 1/8" of movement.
Right now, at the end of 1.25" servo arms, I get about 1/8" of movement.
ORIGINAL: dirtybird
Sealing the hinges might help but the only sure cure is static balance of tha controls. All full scale aircraft static balance the controls.
Sealing the hinges might help but the only sure cure is static balance of tha controls. All full scale aircraft static balance the controls.
11-23-2006, 02:52 PM
#13
RE: Servos jittering/unable to center
sealing the gap will NOT prevent flutter -Old Wives Tale
what it does do is act as a movement damper. proper balance really helps eliminate vibration induced servo "manglement"
Here is the WHY?
the servo tries to hold the surface in position. the engine bam bam tries to move the fuselage and wings and stab
So it becomes a fight - the inertial resistance of the movable surface is transfered thru the linkage to the servo
By balancing all surfaces close to the hingeline -- these forces are reduced
Example: If you shorten the CG of the aileron by 50% -the force is reduced --50%
The friggen engines we use -- often have a sympathetic resonance with the plane - and this is a hit and miss thing.
some setups are no problem others will shake the balls offn a pooltable.
what it does do is act as a movement damper. proper balance really helps eliminate vibration induced servo "manglement"
Here is the WHY?
the servo tries to hold the surface in position. the engine bam bam tries to move the fuselage and wings and stab
So it becomes a fight - the inertial resistance of the movable surface is transfered thru the linkage to the servo
By balancing all surfaces close to the hingeline -- these forces are reduced
Example: If you shorten the CG of the aileron by 50% -the force is reduced --50%
The friggen engines we use -- often have a sympathetic resonance with the plane - and this is a hit and miss thing.
some setups are no problem others will shake the balls offn a pooltable.
11-23-2006, 03:08 PM
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RE: Servos jittering/unable to center
Hmm yeah I know about all this. I figured vibration had something to do with it. I'm just surprised it happened so quickly. The surfaces are already really light. Not sure I could move the balance on them forward any.
I've leaned out my engine quite a bit further than it was when I first put it on, so vibrations are reduced a great deal. I'll get more gears for the servos and see how they hold up. If they give out again I'll have to either go with a servo with stronger gears or figure out a way to eliminate vibration from a 4-stroke.
I've leaned out my engine quite a bit further than it was when I first put it on, so vibrations are reduced a great deal. I'll get more gears for the servos and see how they hold up. If they give out again I'll have to either go with a servo with stronger gears or figure out a way to eliminate vibration from a 4-stroke.
ORIGINAL: dick Hanson
sealing the gap will NOT prevent flutter -Old Wives Tale
what it does do is act as a movement damper. proper balance really helps eliminate vibration induced servo "manglement"
Here is the WHY?
the servo tries to hold the surface in position. the engine bam bam tries to move the fuselage and wings and stab
So it becomes a fight - the inertial resistance of the movable surface is transfered thru the linkage to the servo
By balancing all surfaces close to the hingeline -- these forces are reduced
Example: If you shorten the CG of the aileron by 50% -the force is reduced --50%
The friggen engines we use -- often have a sympathetic resonance with the plane - and this is a hit and miss thing.
some setups are no problem others will shake the balls offn a pooltable.
sealing the gap will NOT prevent flutter -Old Wives Tale
what it does do is act as a movement damper. proper balance really helps eliminate vibration induced servo "manglement"
Here is the WHY?
the servo tries to hold the surface in position. the engine bam bam tries to move the fuselage and wings and stab
So it becomes a fight - the inertial resistance of the movable surface is transfered thru the linkage to the servo
By balancing all surfaces close to the hingeline -- these forces are reduced
Example: If you shorten the CG of the aileron by 50% -the force is reduced --50%
The friggen engines we use -- often have a sympathetic resonance with the plane - and this is a hit and miss thing.
some setups are no problem others will shake the balls offn a pooltable.
11-23-2006, 09:44 PM
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RE: Servos jittering/unable to center
As precisely as my DuBro balancer allows. Hub and blades all balanced out.
Do you think the plane is flyable with the slop in it? I'd hate to be out of commission until I got replacement gears in, and it is my only plane that is capable of flight at the moment.
Do you think the plane is flyable with the slop in it? I'd hate to be out of commission until I got replacement gears in, and it is my only plane that is capable of flight at the moment.
ORIGINAL: andyt
balanced prop?
balanced prop?
11-24-2006, 08:50 AM
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RE: Servos jittering/unable to center
I have JR 8231s on a Yak-P profile and the aileron and rudder do this when the engine is idling on the ground. I'm still flying beacuse almost most air time the engine is above low idling and the control surfaces are moving.
11-24-2006, 08:59 AM
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RE: Servos jittering/unable to center
ORIGINAL: dirtybird
I would static balance the control surfaces to eliminate the flutter.
I would static balance the control surfaces to eliminate the flutter.
11-24-2006, 10:59 AM
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RE: Servos jittering/unable to center
ORIGINAL: Josey Wales
Can you explain how this is done ?
ORIGINAL: dirtybird
I would static balance the control surfaces to eliminate the flutter.
I would static balance the control surfaces to eliminate the flutter.
Basically all you have to do is hang a weight ahead of the hinge line to make the surface balance. Some elevators with aerodynamic balance make it easy. Just load the part ahead of the hinge with lead until the surface balances.
On surfaces with flat tips just add a plate on the end of the control surface and attach a rod to it with a weight on the end of the rod that extends ahead of the hinge line. If you don't have flat tips get an old picture of a Lockheed Lightning and take a look at the horizontal stabilizer to see how they did it. That's not too hard to duplicate.
This works. I had a 1/3 scale Extra where I added the balance. Before my flight the airplane had been struck on the aileron unknown to me and the gears wiped out on its servo. When I took off I noticed the ailerons lacked sensitivity. I landed to investigate and found no control over the left aileron. There was no sign of flutter
11-24-2006, 02:03 PM
#20
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RE: Servos jittering/unable to center
My experience is that 6v batteries and even a small resistance (1/2 ohm) in the connector or multiple connectors will aggravate and sometimes cause oscillation about servo center. The resistance seems to be more critical when using 6v batteries. You might temporarily plug in a 4-cell battery pack and check for oscillation as a test.
Linkages that are exactly neutral (no bias) are sometimes hard on the pivot points and gear train. In spite of what we think most control surfaces push slightly against the servo while flying. The slight push removes the slop or free play.
Bill
Linkages that are exactly neutral (no bias) are sometimes hard on the pivot points and gear train. In spite of what we think most control surfaces push slightly against the servo while flying. The slight push removes the slop or free play.
Bill
11-24-2006, 04:51 PM
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RE: Servos jittering/unable to center
I'll try it out with my 4.8v pack.
I flew 4 times today and there is no more slop in it now than it was when I took off. It is still irritating though, since I had 0 slop to start with.
I won't need to worry about getting the servos straightened out for awhile - I came in a little bit hard and broke the engine mount. lol
I flew 4 times today and there is no more slop in it now than it was when I took off. It is still irritating though, since I had 0 slop to start with.
I won't need to worry about getting the servos straightened out for awhile - I came in a little bit hard and broke the engine mount. lol
ORIGINAL: BillS
My experience is that 6v batteries and even a small resistance (1/2 ohm) in the connector or multiple connectors will aggravate and sometimes cause oscillation about servo center. The resistance seems to be more critical when using 6v batteries. You might temporarily plug in a 4-cell battery pack and check for oscillation as a test.
Linkages that are exactly neutral (no bias) are sometimes hard on the pivot points and gear train. In spite of what we think most control surfaces push slightly against the servo while flying. The slight push removes the slop or free play.
Bill
My experience is that 6v batteries and even a small resistance (1/2 ohm) in the connector or multiple connectors will aggravate and sometimes cause oscillation about servo center. The resistance seems to be more critical when using 6v batteries. You might temporarily plug in a 4-cell battery pack and check for oscillation as a test.
Linkages that are exactly neutral (no bias) are sometimes hard on the pivot points and gear train. In spite of what we think most control surfaces push slightly against the servo while flying. The slight push removes the slop or free play.
Bill
01-13-2007, 12:07 PM
#22
RE: Servos jittering/unable to center
I had this problem recently with an 8411, sent it in to Horizon and they replaced the gears and pot under warranty. I think Dick is exactly right in his description of what is happening where the servo is over driving back and forth around center. I first noticed it when flipping the prop over to start the plane. I have a fellow flier that both his ailerons do this and he continues to fly with apparently no side effects. I flew that day and then came home and took the servo out and sent it in. JR/Horizon has a GREAT service dept. it would be worth it to send them in. I think they just develop slop over time and eventually it gets to the point where this will happen. For me the prop is balanced, both blades and hub, all linkages are under probably 6" and I'm using ball links so the only slop is from the servo gear train to begin with.
01-14-2007, 10:55 AM
#23
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RE: Servos jittering/unable to center
Dick Hansen gave you excellent advice above. There is one more thing that can cause flutter, a flexible surface. Ideally you want that moving surface to be a rigid as possible. If it can be easily deformed or twisted, it is prone to flutter, even if statically balanced.
01-14-2007, 01:01 PM
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RE: Servos jittering/unable to center
I was able to get everything rebuilt and whatnot quite easily with the advice given. The only difference is I used some silicone grease I had instead of ordering some ceramic grease. I'll get some on my next order though.
