Suddenly I'm breaking elevator servos like crazy

[Jessek1486]

I've suddenly encountered an issue where I've gone through 4 elevator servos in the past week, which is more servos than I've gone through this whole year for all parts of the plane.

It'll often happen after a minor crash, or hard landing. The elevator servo will just be stripped to the point there's zero resistance at all.

Now, I know someone will probably say stop crashing or land softer, sure. However, I've never had this problem until this past week, and nothing else breaks on these impacts, not even minor foam smashing...

I also don't think it's the control surface sizing, as I've also been using this exact sizing and technique for months with only 1 failure in that time outside of the recents.

I do not notice any issues in flight, I'll just land and boom, no more servo. Or a minor crash. Something like a stall a few feet above the ground, again, nothing extreme. This is often the ONLY thing that breaks, the elevator servo...

 

[Tench745]

I've suddenly encountered an issue where I've gone through 4 elevator servos in the past week, which is more servos than I've gone through this whole year for all parts of the plane.

It'll often happen after a minor crash, or hard landing. The elevator servo will just be stripped to the point there's zero resistance at all.

Now, I know someone will probably say stop crashing or land softer, sure. However, I've never had this problem until this past week, and nothing else breaks on these impacts, not even minor foam smashing...

I also don't think it's the control surface sizing, as I've also been using this exact sizing and technique for months with only 1 failure in that time outside of the recents.

I do not notice any issues in flight, I'll just land and boom, no more servo. Or a minor crash. Something like a stall a few feet above the ground, again, nothing extreme. This is often the ONLY thing that breaks, the elevator servo...

I don't know what to tell you, except that I had the same problem with those blue 9g servos in certain planes. If the control surface gets hit hard enough, especially if it's near full deflection, it strips the teeth off the gears inside. I use the Turnigy HXT900 servos and they do sell replacement gears for them. But I recently started spending the money on metal-gear servos when I can.

 

[Foamforce]

I’ve also had quite a few blue 9g servos go bad, or start bad. I bought a ten pack of no name blue ones from Amazon a couple years ago and every one of them eventually went out. I’ve had pretty good luck with TowerPro 9g though, and also Hextronic. More recently I‘ve bad luck with TowerPro 5g servos.

If it’s not that, are you actually striking the elevator on the ground when you land? If you are, the linkage you have there seems pretty rigid, and if the linkage doesn’t have any flex, then a direct ground strike with the elevator could strip out the servo. If you make the linkage a bit longer, or thinner gauge wire, or add a V bend in it, then that can help protect the servo.

 

[Jessek1486]

Some very good feedback here, and perhaps it's just that I'm flying harder due to being better at it.

That plus having cheap servos, which apparently break suddenly. I did buy some metal gear ones, so I'm hoping that will help.

Was mostly looking to see if there's anything I was overlooking

In hindsight, it may have started when I wanted more up elevator deflection. I'm at about 35-45°.

Sounds like it's just normal then. It was odd that it began to happen very suddenly though.

No direct strikes to the elevator that I saw, the last one was a normal landing actually 😂

 

[quorneng]

jessek1486
It certainly sounds like it is a servo problem rather than anything more complex.. I suspect in the past the capabilities of the 9g servos have been just enough. A better or bigger servo is the likely answer.

Just a point but you may have noticed that with some servos the arm can be easily moved by hand whereas with others it requires a jiggle to get it to move at all. Guess which is likely to strip gears when a landing impact on a elevator creates a sudden load on the servo arm.

 

[Jessek1486]

jessek1486
It certainly sounds like it is a servo problem rather than anything more complex.. I suspect in the past the capabilities of the 9g servos have been just enough. A better or bigger servo is the likely answer.

Just a point but you may have noticed that with some servos the arm can be easily moved by hand whereas with others it requires a jiggle to get it to move at all. Guess which is likely to strip gears when a landing impact on a elevator creates a sudden load on the servo arm.

That first point makes sense. I was on the upper limit of what the servo can handle, with aerodynamic loads and the earth bumping into it. So any more stress and it shits the bed.

Do you know if the load on the servo is linear with increasing angular deflection?

Hmm, I've noticed that the same servo can sometimes be both. I figured it had to do with the magnets in the motor and whether or not by chance they're locking the stator.

Unless I grossly misunderstood you.

 

[AIRFORGE]

You mentioned it may have started after wanting more up elevator, so...
Looking closely at your elevator it appears there are 3 things that may be the cause of your servos being stripped.

1. Elevator hitting the bottom of the rudder.
2. Elevator binding at the hinge bevel.
3. Elevator horn hitting the horizontal stabilizer. Also part of the horn is in the bevel/hinge area.

 

[Jessek1486]

You mentioned it may have started after wanting more up elevator, so...
Looking closely at your elevator it appears there are 3 things that may be the cause of your servos being stripped.

1. Elevator hitting the bottom of the rudder.
2. Elevator binding at the hinge bevel.
3. Elevator horn hitting the horizontal stabilizer. Also part of the horn is in the bevel/hinge area.

Hmm, so currently it's impossible for the elevator to ever contact the rudder.
I do not feel any binding when moving it without a servo attached, it does require slightly more force at angles >35 degrees but not that much.
There is no damage there nor does it look like it contacts even at max deflection.

Additionally when it breaks, it's sudden, meaning working 100% fine to not at all, after an impact or hard landing.

I am leaning towards you being correct, though, and I just cannot get more than 25 degrees up elevator with my current setup.

 

[AIRFORGE]

Hmm, so currently it's impossible for the elevator to ever contact the rudder.
I do not feel any binding when moving it without a servo attached, it does require slightly more force at angles >35 degrees but not that much.
There is no damage there nor does it look like it contacts even at max deflection.

Additionally when it breaks, it's sudden, meaning working 100% fine to not at all, after an impact or hard landing.

I am leaning towards you being correct, though, and I just cannot get more than 25 degrees up elevator with my current setup.

When a problem starts happening after making the only change you can think of, basic troubleshooting says that could be the cause. Try undoing your change, and/or use a more reliable servo.

Note: If the bevel was on the bottom side of the stab with the horn on the top, it would allow more up travel with less load on the servo. But it still won't fix servo reliablity.

 

[Jessek1486]

When a problem starts happening after making the only change you can think of, basic troubleshooting says that could be the cause. Try undoing your change, and/or use a more reliable servo.

Note: If the bevel was on the bottom side of the stab with the horn on the top, it would allow more up travel with less load on the servo. But it still won't fix servo reliablity.

That is my first thought, if stuff starts breaking out of nowhere, consider any changes. I'm not entirely sure the timing of the servos breaking coincides with the change to deflection, though. I have reversed it, as well as put in a metal gear servo.

Interesting, so you're saying just flip the stab upside down, bevel facing down.

 

[Burnhard]

I would recommend to get some metal gear servos. I have broken a lot of plastic gear servos but the metal gear servos seem to be more durable.

 

[Merv]

+1 on the metal gear servo.

I use the cheap blue 9g servos & find them fairly reliable. All of my friends use expensive name brand servos, theirs break at about the same rate as mine. On occasion I have a plane that breaks them, not sure why some planes just break them more often than others. I will then switch to the cheap metal gear servos. I have never "broken" one of them. For me, the metal gear ones wear out very quickly when used with any kind of flight stabilizer. I think is is the constant corrections in more less the same spot that wears them out so fast.

 

[AIRFORGE]

Interesting, so you're saying just flip the stab upside down, bevel facing down.

That is the most common way to do it, but not a must. You could just move your servo to the bottom with the bevel up.

 

[Jessek1486]

+1 on the metal gear servo.

I use the cheap blue 9g servos & find them fairly reliable. All of my friends use expensive name brand servos, theirs break at about the same rate as mine. On occasion I have a plane that breaks them, not sure why some planes just break them more often than others. I will then switch to the cheap metal gear servos. I have never "broken" one of them. For me, the metal gear ones wear out very quickly when used with any kind of flight stabilizer. I think is is the constant corrections in more less the same spot that wears them out so fast.

Ah, okay. I was considering buying better ones, only cus a servo failure means you gotta go home, fix and all that. It's annoying.

What's even more annoying is when conditions outside are perfect, field is empty and you practice inverted flight too close to the ground wrecking literally everything 😂😂

I haven't gotten into the flight stabilization yet, but I've heard similar about interference/chatter causing them to fail.

Also sounds like a certain config eats servos for others, for no reason. Just change stuff until it stops is what it seems others do 😂 glad to hear I ain't the only one

 

[quorneng]

Jessek1486
The aerodynamic load on the control surface increases rapidly with both air speed and its angle of deflection.
It follows that if a servo is working at its limit then increasing the deflection may cause it to fail.
To be honest even plastic geared servos are strong enough to stall the motor if the load is applied gradually. It is a shock load particularly if applied when the servo is already under load that will cause the gears to strip.

I have a couple of bigger heavier models that if dived steeply under power cause the elevator servo to stall as it approaches full deflection as you try to pull out. Shut off the motor, the plane slows down and it pulls out normally.
A bit scary the first time it happened but no problem if you know and incidentally it does in effect limit the g you can pull so avoiding ripping the wings off!

 

[Shurik-1960]

The most correct advice has already been: install a servo in the fuselage and increase thrust. A short pull is a rigid pair and a direct transfer of sharp forces. I installed the blue 9g servos (elevons), crashed the model and moved the servos to the aerobatic plane.

 

[Jessek1486]

Jessek1486
The aerodynamic load on the control surface increases rapidly with both air speed and its angle of deflection.
It follows that if a servo is working at its limit then increasing the deflection may cause it to fail.
To be honest even plastic geared servos are strong enough to stall the motor if the load is applied gradually. It is a shock load particularly if applied when the servo is already under load that will cause the gears to strip.

I have a couple of bigger heavier models that if dived steeply under power cause the elevator servo to stall as it approaches full deflection as you try to pull out. Shut off the motor, the plane slows down and it pulls out normally.
A bit scary the first time it happened but no problem if you know and incidentally it does in effect limit the g you can pull so avoiding ripping the wings off!

Well that'd explain it then. Basically it'd happen upon landing, which usually means flaring(or in the case of trying not to crash, max up elevator) and then a slight bump strips the gears pretty instantaneously.

Hmm, I haven't encountered the second issue you've described. I'll definitely watch out for it tho, since my model keeps gaining weight like she's at an all you can eat dirt buffet...

 

[Jessek1486]

The most correct advice has already been: install a servo in the fuselage and increase thrust. A short pull is a rigid pair and a direct transfer of sharp forces. I installed the blue 9g servos (elevons), crashed the model and moved the servos to the aerobatic plane.

Ah! That makes perfect sense, essentially use a longer pushrod as a dampener, since it'll bend and flex under load. That's likely why it's opnly the elevator servo giving up, it's the shortest and the elevator itself will naturally bend downwards upon the plane landing on its belly. Inertia. pulls it down.

Not sure I follow about the elevon servos?