Servo failure in flight - oscillates from one extreme to the other

[dmcquinn]

I've never had a servo fail in this manner. Flying my 35% Yak when suddenly it began pitching up and down. I could see the left elevator going full up / full down from one extreme to the other about one cycle per second. I was able to land (sort of) because I had normal control of the right elevator half. After I "landed", I took this video clip with my cell phone. All the servos are still locked in solid except for the left elevator. After I turned off the receiver and turned it back on, no more problems. I cannot make it happen again. The servo is a JR DS8411 - several years old. I checked the electrical connections and found nothing unusual.

[Tired Old Man]

Time to get a new servo. I lost a plane to one of another brand that did that in flight a few years ago.

[crash-n-burn]

yikes, way to get it down intact that must have been an interesting landing.

[zoomer260]

That is freaky. I had something like that happen with a servo reverser.

[WangoTango]

If you have this servo plugged into an extension, don't trust the extension either. (Especially if it is a non-gold plated "sport" (cheapie) version. I've seen this kind of problem traced to just the extension only!
WT

[dmcquinn]

Thanks for the comments. I flattened the landing gear a little, but did not break the prop. I should be back in business by next weekend.

[smokin]

WT is right!!!!! if you are disposing of the servo....you must dispose of everything between the servo and the rx.. are you using a power system? i had a smartfly PE that had a bad channel for solder reasons.... that would've caused this. now, i still fly all smart fly systems and in no way intended that to be a negative remark about SF in any way. i love their systems and the service! my point is check all things from servo to rx.

[dmcquinn]

Good Point, Smokin. I don't have anything between the servo and the receiver except for about 3 feet of 22 gauge extension cord. I plug the right elevator into channel 8 on the JT PCM receiver and use the dual elevator function of the transmitter.

[Anthony.h]

Dmcquinn, I had the same experience with a JR8411SA servo on the left elevator last Sunday. About 100-150 feet high the model pitched down, I managed to gain some control while the elevator was pulsing down. At about 10 feet I had very little control so I flicked the engine kill switch, landed hard but saved the engine and radio.

After some testing I found the left elevator servo to be faulty, when I tapped the side of the fuselarge with a small hammer the elevator servo went nuts- the right side did not do this. Easy to test with the transmitter in servo moniter mode.

I put it down to vibration, maybe somebody could answer this, how long do these servos last? and are the JR8411's critical to vibration?

model was a Comp-arf 2.12m extra
DA50
JR8611 ,ailerons and rudder
JR8411SA elevators
un-regulated 5cell 1950mAh NiMH battery- 6.3V under 1.5A load after crash

[Anthony.h]

I should mention the servos are all new except the elevator servos, that came out of and older model that I sold, but had limited use and an easy life until being subject to the vibration of the DA50. Again the Comp-arf 2.12m had a limited life. Receiver is a Futaba R608

[dmcquinn]

I sent my 8411 to horizon hobby. They repaired it for free (even though it was past the warranty date). I have been flying it ever since - probably another 50 flights since it was repaired.

[JackL]

I just boxed up a new JR 8711 (12 flights) to send back to horizon for a similar problem. It was on the left elevator and when I would turn the power on it would go to about 80% down deflection. You could make it travel about 1-2" up and down, but it was like the center of travel had moved to about 80% down. After a while it would return to normal neutral until you turned the power off/on again. Was using a Power expander with a Fasst reciever. Tried direct to reciever with the same results.

[STLAV8R]

	Quote: Originally Posted by dmcquinn
	I sent my 8411 to horizon hobby. They repaired it for free (even though it was past the warranty date). I have been flying it ever since - probably another 50 flights since it was repaired.

What did Horizon say was wrong with your servo?

[dmcquinn]

I think they replaced the potentiometer.