Servo Problem
05-18-2005, 10:53 PM
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Servo Problem
Had a problem today which has me puzzled. Started to fly my Edge 540 which uses a JR955S Rx, DS8231 servos on each elevator half and each aileron (4 total), an 8411 on the rudder and a 531 on the throttle, a 5cell NiMH 1700mah battery and a Jaccio 5.7v regulator.The Tx is an old, but tried and true, JR10S with relatively new batteries. The aircraft has had only about 5 or 6 flights previously.
Did the usual preflight check of control surface movements; all seemed ok. The OS160 started and ran great as usual. Taxied out and took off. Added a click of left aileron on the downwind trim pass, seemed to be flying great. Pulled up to a 45 to initiate a turnaround and experienced a strong right roll but correctable with aileron. Did a level 180 turn to an upwind pass and tried the pull-up again - same result. I surmised one elevator half was not functioning, altho I was sure I saw both move on the preflight check. Made another 180 to set up for landing and the ailerons became erratic but the throttle seemed to be responding. By the time I started to turn base, I had lost all flight control but I think the throttle was still responding. I cut the throttle and watched it disappear behind the trees.
Some luck was with me as it crashed into juniper bush and never touched the ground. It was basically intact, except for a broken right wing. Certainly repairable. On the post-crash inspection, everything appeared to work properly except the servo which drives the left elevator - it's gears were clearly stripped as the elevator flopped freely and the servo shaft turns with no resistance. Its quite possible, in fact I think very likely, that the gears were stripped before take-off. That would explain the right roll on up elevator. Probably some hangar rash incident stripped or fractured the gear teeth and the flight load finished the job.
That's a bummer, but no one to blame but myself - a casual preflight and you pay the price. What I don't understand is why all the flight controls went berserk and I lost control. I've tried to eliminate the obvious - the Rx battery read over 6.3v with a load and later I discharged it on the cycler and it ran nearly 2 hours before hitting th 5.7v cutoff. The Tx has realtively new batteries and was reading 10.4v. In fact, I subsequently used it to fly two flights on another plane, so I'm confident thats not the problem.
I've never had a digital servo fail like that and I'm wondering if it could draw enough current to reduce the voltage out of the regulator to affect the other sevos or the receiver. Are the voltage regulators current-limted to that extent? Would I have been better off without a voltage regulator in the system?
Any thoughts?
Jack
Did the usual preflight check of control surface movements; all seemed ok. The OS160 started and ran great as usual. Taxied out and took off. Added a click of left aileron on the downwind trim pass, seemed to be flying great. Pulled up to a 45 to initiate a turnaround and experienced a strong right roll but correctable with aileron. Did a level 180 turn to an upwind pass and tried the pull-up again - same result. I surmised one elevator half was not functioning, altho I was sure I saw both move on the preflight check. Made another 180 to set up for landing and the ailerons became erratic but the throttle seemed to be responding. By the time I started to turn base, I had lost all flight control but I think the throttle was still responding. I cut the throttle and watched it disappear behind the trees.
Some luck was with me as it crashed into juniper bush and never touched the ground. It was basically intact, except for a broken right wing. Certainly repairable. On the post-crash inspection, everything appeared to work properly except the servo which drives the left elevator - it's gears were clearly stripped as the elevator flopped freely and the servo shaft turns with no resistance. Its quite possible, in fact I think very likely, that the gears were stripped before take-off. That would explain the right roll on up elevator. Probably some hangar rash incident stripped or fractured the gear teeth and the flight load finished the job.
That's a bummer, but no one to blame but myself - a casual preflight and you pay the price. What I don't understand is why all the flight controls went berserk and I lost control. I've tried to eliminate the obvious - the Rx battery read over 6.3v with a load and later I discharged it on the cycler and it ran nearly 2 hours before hitting th 5.7v cutoff. The Tx has realtively new batteries and was reading 10.4v. In fact, I subsequently used it to fly two flights on another plane, so I'm confident thats not the problem.
I've never had a digital servo fail like that and I'm wondering if it could draw enough current to reduce the voltage out of the regulator to affect the other sevos or the receiver. Are the voltage regulators current-limted to that extent? Would I have been better off without a voltage regulator in the system?
Any thoughts?
Jack
05-19-2005, 11:35 AM
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RE: Servo Problem
I'd suspect the voltage regulator. I use 5-cell packs on all my planes with no regulators. Can you test the regulator under load. Say 2 amp load just to make sure? Your symptoms sound like low voltage to the servos and/or receiver. I have flown two different planes with only one elevator half functioning and the other one flopping loose. In my experience, the plane is still quite controllable and surprisingly does not exhibit strong roll tendencies even though only one half of the elevator is working.
