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

Hi!
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?
Thanks!
- Angel
Reply to
Angel Abusleme
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Given the cost of new servos, it seems false economy to use doubtful ones on your airframe. Glitching can destroy servos but then so will impact! Your range test should be around 50ft with the TX aerial down. Next time in that situation, you may achieve some gain by holding the TX vertical and as high as your arm can reach. Running towards the model (if safe) may help also. You don't mention how the RX aerial was deployed? CM
Reply to
hexhome
On Mon, 12 Nov 2007 20:17:10 -0800, "Angel Abusleme" wrote in :
Range check with the antenna collapsed BEFORE flying.
You should be able to get 100' away without any glitches.
The RX may have had a cracked crystal or other weakness that only became apparent at a distance.
It's possible, maybe.
Or the gears stripped in the crash.
Manually run the servos back and forth to make sure that all the teeth are present and accounted for.
Put them in the system and test them at the extremes that your setup will allow.
Then range check, fly, and see what happens.
You're not putting a huge amount of money at risk with the stuff you've got. See what works and let us know what you discover.
Marty
Reply to
Martin X. Moleski, SJ
Easy way to test a used servo.Hold it over an open garbage can,drop it,.If it bounces back it's a good one,if it doesn't leave it there.Economy servos are so cheap now why buy used ?.
Reply to
Paul
check them in the RX for full travel and tourqe(put pressure on the servo arm) take the case apart and look at the gears. You can buy gears, no need to throw it away. mk
Reply to
MJKolodziej
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.
Reply to
Robert Reynolds
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. CM
Reply to
hexhome
Angel,
Two questions, was the rudder locked in the direction of the bank Would you say that you were trying to fight the plane up with elevato and once in the spin did you try full up elevator
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Reply to
sfsjkid
I agree with that. I like selling servos on ebay because people usually pay too much for them. I wouldn't buy them there.
Sometimes it's fun to buy a junk airplane from the hobby store ceiling and strip all the good stuff out of it. I bought a plane once and sold the engine on ebay to pay my entire cost plus profit, threw the plane away and kept the servos. Most of them were perfectly good.
Reply to
Robert Reynolds
I suppose you're trying to say that I proved your point for you? The engine I sold was a like-new OS 20 4 stroke. Nice little engine, but I wasn't going to keep it when there are so many people paying crazy money for them. The plane was a piece of crap, and I doubt that it ever flew. It was built from door skin and covered with Tyvek. The servos were just a bunch of average Hitec gear. Nothing wrong with them. I think I paid $75 for the whole mess and got $130 for the engine. It was a fun little project.
What was your point anyway?
Paul wrote:
Reply to
Robert Reynolds
I am not sure. I am trying to reconstruct exactly how it was. I think I was in a slight left turn when it violently banked left. I tried to correct and maybe it responded for a fraction of a second, and then kept going left. When I found it, the rudder was locked left and did not respond to my command.
The unexpected turn was so violent that the airplane rolled and the nose aimed down immediately. I tried up elevator in the first few yards during the fall (a reflex when the nose is low). With no effect, I then let the elevator straight to gain some speed and then tried up elevator again when the airplane was about to crash; at that time I also turned the motor off to minimize the damage. I could not see the crash, because it occured down a slope, but I believe that the spin and up elevator right before the crash helped to reduce the crash speed, and that is why the only damage was the propeller. The rudder was in place, and the servo tray and pushrods were intact.
Thanks for your help. Best regards,
Angel
Reply to
Angel Abusleme
Thanks for your help.
I bought the servos mostly as parts for robotic projects when my son grows up. Maybe I can use one or two in steering the nose gear, which is not as critical as elevator.
In this case, for an $8 airframe, I did not matter much.
The Rx antenna was deployed along the fuselage, outside it. There are metal pushrods, but are rather short.
Best regards,
Angel
Reply to
Angel Abusleme
I did that before installing them. I discarded a couple that had a barely noticeable click. I believe that the servos were OK.
I did check the extremes before trimming, but the extremes may have changed a bit after trimming...
Thanks for your help. I don't expect to work on that airplane for a while... no time right now...
Reply to
Angel Abusleme
The structure was intact, only the servo was broken.
It was almost straight, slight left rudder.
I agree! Thanks for the advise!
Angel
Reply to
Angel Abusleme
I have a few United Hobbies servos that work great (HXT500 or something like that) and some others that came DOA (Vigor brand).
Does anyone know how reliable are all these, compared to HS-55 or similar for small airplanes?
Reply to
Angel Abusleme
On Tue, 13 Nov 2007 16:46:27 -0800, "Angel Abusleme" wrote in :
For future reference (because it may have nothing to do with this incident) you might note that adding elevator when a plane is in a banked turn only tightens the spiral.
The only time that up elevator will, in fact, raise the nose of the plane is:
1. When the wings are level.
2. When the airspeed and/or thrust is adequate to counteract momentum and gravity.
Adding more elevator in a spin will only intensify the spin, as a general rule. It won't get you out of the spin. For most beginner's planes, the thing to do with a spin is let the controls return to neutral and wait. When the spin stops, level the wings. Let the plane pick up a little airspeed, then gently pull back on the stick.
With aerobatic planes (less or no dihedral), you should try the neutral method with low power first. If it stays in the spin with no signs of coming out, you might the rudder opposite to the direction of the spin, with or without blipping the throttle (adding power may intensify the spin).
A really nice flat spin may not do too much damage to the plane. Getting into a death spiral usually does a lot more damage.
Here is a story about a full-scale pilot who chose the wrong spin recovery technique and lost a beautiful Ultimate Biplane as a result:
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I've done that several times. I think it is the right thing to do when all is lost.
Marty
Reply to
Martin X. Moleski, SJ

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