Re: Gear-stripping glitch? and How to test a used servo?

Robert Reynolds wrote:
Angel Abusleme wrote:

So this is what happened. I bought a used Cox Cub airframe for $8 and equipped it with two used servos I found in a swap meet (rudder and elevator) and a 4-channel, single conversion 72MHz receiver that a friend gave me. I had never used any of these components before. Range check was kind of short compared to dual conversion or DSP receivers, so I decided to fly not too far. Enough for a 30ish-inch wingspan.

First flight. Perfect takeoff. Gaining altitude to trim safely. After 20 seconds or so, the airplane banked left drastically, as if a glitch had happened. As a consequence, the airplane stalled and spinned down, crashing. I had some control during the spin, but nothing to save the airplane from crashing. When this happened, the airplane was about 150ft away, and the transmitter antenna was horizontal (fully extended). I don't remember where it was aimed.

Interestingly, the only broken parts were the propeller (although I was using a prop saver...), and the rudder servo. I opened it, and found that some gears had been stripped.

I guess that a glitch did happen, and it forced the servo to a extreme rotation, stressing the gears and breaking them. Is this possible? Any other explanations?

By the way, I bought a few used (and maybe abused, who knows?) servos in the swap meet. Any advise on how to test them and figure out which servos are about to fail?


- Angel

If the structure that the servo is mounted on comes loose in a crash, the servo can lurch forward and yank on the push rod, stripping the gears. If this didn't happen, then maybe your gears were bad in the first place, and you should have checked them first. Were you flying straight and level before the plane crashed, or did it go crazy when you gave it a bit too much rudder?

The other thing I would check would be the receiver crystal. Sometimes they will work great until you run the engine and get the model about 50 feet away. Try subjecting the receiver to vibration and watch the servos to see what they do.

And never mind the people who tell you to buy everything new. Some people don't enjoy anything unless it still has shrinkwrap on it, but you and I know that it's often more fun to experiment.
Personally, I would agree with this statement. However the cost of new servos now is often below second hand ones. Compare United Hobbies with Ebay! It's great to make do and mend but you must factor in something for experience and safety here.