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A Main Hobbies . Boca Bearings . Modefo's RC Helicopters

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Gasser Model RC Helicopters > Melted rudder servo disk!
 
 
cranester
Key Veteran
Location: Bogota, Colombia

I was hovering my new Bergen EB at about 10 feet tuning the motor,(its its third flight) suddenly I lost tail control, the helicopter started spinning like crazy I managed to stay calm hit throttle cut and autoed safely. I proceeded to inspect the heli and found the rudder servo disc had broken and the pushrod had sliped out of the disk. I grabbed the pushrod and touched the screw and bolt (which were still attached to the rod) and too my surprise they were burning hot!!
upon carefully inspecting the servo disk I noticed it had not actually broken it had melted!

The only thing I can think of is vibration heated the screw and bolts so muche they melted the servo disk.

I never thought this could happen how do you guys concet your pushrod to the tail servo?



Juan Crane
04-18-2008 12:05 AM
HOMEPAGE  
 
 
copperclad
Elite Veteran
Location: ..

hi cranester
i have not seen a servo wheel melt like that before , but i am wondering , does Bergen still use velcro to suport the control rod , or what kind of control rod suport were you using , dana
04-18-2008 12:55 AM
 
 
cranester
Key Veteran
Location: Bogota, Colombia

There's nothing supporting the rod between the servo and tail pitch assembly.
Maybe thats causing harmonic vibes?

Juan Crane
04-18-2008 01:08 AM
HOMEPAGE  
 
 
turbogti
Veteran
Location: St. Andrew, Jamaica

Strange, was the rod in close proximity to any heat source such as the muffler or engine?

****Predator Gasser SE - She's Ready to rumble ***
04-18-2008 01:31 AM
HOMEPAGE  
 
 
GREYEAGLE
Veteran
Location: Sioux City IA

Seen it before on RC aircraft - it's vibration telegraphing

Seen two different cases : one actually started a fire !

1st time was on a 60 size old school 60 piped pattern bird
" Banshee" in which we had a contact line fit between the lower plywood saddle located bulkhead F2, and the contacting surface of the heavey glassed center section of the wing .

Landed the bird, could smell something burning like wood. Remmoved the wing and the complete saddle and plywood was charred black and smoldering. Once we gave it a bit of air it really took off ! Needless to say we poured a can of pop on it.

2nd time was on a set of the earley style plastic grey type pinned hinges on a vertical fin in a Byron F-16. We could see scale like smudges traveling horizontally from the pin across the rudder.

We thought it was a nice scale effect till we noticed that the metal pin in the knuckel of the hinge had actually melted thru 6 of hinge's!

I would try to kill the resonance of the carbon rod "de tune" by soaping up a couple sections of silicon fuel tube and slide them over the rod.
Won't take much to kill the harmonic's

GREYEAGLE
04-18-2008 01:31 AM
 
 
cranester
Key Veteran
Location: Bogota, Colombia

WOW... I'Il try that greyeagle, amazing, never thought that could happen, the bolt and screw were really very hot, I actually could hold them for more than three seconds because they were so hot.

Juan Crane
04-18-2008 01:43 AM
HOMEPAGE  
 
 
FCM
Elite Veteran
Location: Back in Blighty!

That's a new one on me too. Must be localised heating from the look of the damage. I would be very careful about using that servo again as the feedback pot must have gone through hell if it was resonance that caused this.

Good save by the way

Paul.
04-18-2008 01:47 AM
 
 
cranester
Key Veteran
Location: Bogota, Colombia

Quote 
I would be very careful about using that servo again as the feedback pot must have gone through hell if it was resonance that caused this.

WHAT? its a brand new servo!, only three flights on it

Juan Crane
04-18-2008 01:50 AM
HOMEPAGE  
 
 
haggy38
Senior Heliman
Location: Bogotá - Colombia

Hi Cranester, sorry hear that! First time I've seen this thing happen. But how is possible transfer this amount of heat between the push rod "plastic Ball link" and the servo STEEL BALL!! Have you check the if your servo is over heating or maybe the servo nut and bolt get loose?

Hope you can find the problem.

Regards,
Gustavo
04-18-2008 02:22 AM
 
 
cranester
Key Veteran
Location: Bogota, Colombia

Hi haggy the pushrod is conected to a ball, the ball in term is connected to a screw that is tightened against the servo disk, apparently it was the friction between the metal screw and the servo disk that generated so much heat. I couldnt't understand why it was so hot! everything else was cool to the touch, servo, rod, boom etc.

Suerte!!
Cuando volamos? necesito meterle unos galones a este bicho aver si se suaviza el motor!

Juan Crane
04-18-2008 02:31 AM
HOMEPAGE  
 
 
FCM
Elite Veteran
Location: Back in Blighty!

I don't think this is a friction thing is it? It's to do with a harmonic causing the ball/ball link to heat up. My understanding of it (and I may well be wrong here) is it's like a crude microwave oven effect? Maybe somebody more learned can fully enlighten us as I can see that although rare, this could happen to just about any machine we fly.

My concern about the sero is real. This is the 9254 and they are known to be a bit fragile. If the rod has been vibrating enough to melt the plastic, a lot of those vibs must have passed through into the servo itself?

Paul.
04-18-2008 02:41 AM
 
 
GREYEAGLE
Veteran
Location: Sioux City IA

Kinda a Zig from the Zag :

Although your problem was caused from resonance - I had a time when I was concerned with a busy rudder Fut 9253 getting warm to hot on my SE which is a boom mounted directley above my Zimmer rear air can.

I just didn't like a hot digital even though it was hanging out their in the breeze. Drove me nut's !

I elected to make a radiant sheild from a piece of 28 ga alu , painted it flat white and hang it from the servo using a couple of nylon rudder horns. It immediatley dropped the temp considerably.

It's a little tough to see in the photo but you can make it out.

" along with 5.24V"



GREYEAGLE
04-18-2008 02:45 AM
 
 
GREYEAGLE
Veteran
Location: Sioux City IA

If you want too peek at the servo ?

It's a hassel but a gear set is under $5.00.

If you open it, most likley you might void the warrenty if it still has one. Look out for a hair line size O- ring that runn's the perimeter.{ See Below } !!!!

Remove the servo, remove the sevo arm , place it on a flat surface and operate it whilest slightley pushing down on the output shaft and see if it feels notchey at all.


To open it, remove the back case screw's, do not pull it open yet !

Place it on a flat surface, hold the output shaft down with one finger and lift up and off the top case half, which will leave the gear train intact . Some times one of the gear locating pin's may leave with the top case half. Once you see em they go back pretty easy, its just that snake bit O- Ring you gota fight with !

Only time I've open them up is when I feel the train, shouls be smooth as silk.

GREYEAGLE
04-18-2008 02:58 AM
 
 
FCM
Elite Veteran
Location: Back in Blighty!

I remember your heat shield greyeagle! It was of course, a cool idea. I would be worried about the feedback pot and motor inside that 9254 not only the gears.

Paul.
04-18-2008 03:12 AM
 
 
copperclad
Elite Veteran
Location: ..

hi cranester

Bergen used to use a velcro strap to support the control rod in the center , this would dampen any resonance with the control rod , or i think you could use aftermarket plastic supports , i think supporting the control rod would help this problem , HTH , dana
04-18-2008 12:08 PM
 
 
lperagallo
Senior Heliman
Location: Westfield, Indiana, USA

Cranster,

Copperclad is correct. You need to add a Velcro strap around the boom and lightly on the tail control rod. Look in my gallery and you will see one. OK, it's hard to see but look at teh blue Gasser by the horizontal fine. That's where it goes. This will also help things if you ever have a another ball link failure as you won't have a long rod hanging down when you auto.

Lou

Bergen Intrepid EB, G-26, 720mm; Bergen Turbine, Wren MW54; Raptor 50V2, OS50-Hyper
04-18-2008 04:40 PM
 
 
AceBird
Elite Veteran
Location: Utica, NY USA

Quote 
The only thing I can think of is vibration heated the screw and bolts so muche they melted the servo disk.

I never thought this could happen

Ultrasonic welding!

If the screw was loose it would have vibrated and showed signs of wear. You can weld or melt plastic by subjecting it to very high frequency load changes (ultrasonic’s). It is not a vibration in the sense that the parts are not moving but a force is applied and then taken away very quickly. This heats the molecules in the plastic to the melting point. Two parts will be held together and subject to this characteristic for a short period of time. When it is stopped the plastic cools and you have a perfect weld between two parts (you hope).

If you have a very high frequency servo driven by too much gain on a gyro and then those frequencies were added to some natural frequencies from some other source (like an engine) you might hit the ultrasonic range (would be my guess). At the very least I would back off the gain of your gyro before I flew again.

Of course you could change the composition of the servo disc or even go to metal which will prevent the melting from occurring but that disc is telling you something and it ain’t good.

BTW you can ultrasonically weld metals together but the energy required is much greater. It was the plastic that heated up not the screws. The screws acted like an energy director. They stayed cool.

Ace
What could be more fun?
04-18-2008 05:41 PM
 
 
Brunobl
Veteran
Location: Pomerode, SC, Brazil - 26 40S 49 11W

Quote 
If you have a very high frequency servo driven by too much gain on a gyro and then those frequencies were added to some natural frequencies from some other source (like an engine) you might hit the ultrasonic range (would be my guess).

The fastest thing rotating in a gasser is turning at between 11 to 12K RPM. At one pulse per revolution that is below 220Hz. Digital servos run at a couple of hundred Hz (analog servos are even slower at 50Hz). Those are off by a very wide margin from ultrasound.

One other possible source of high-frequency vibration might be the fan. Air pumped by individual blades through a fixed opening might generate, I suppose, a tune at a frequency of the engine's RPM times the number of blades. However, a 10-blade fan at 12KRPM would generate something around 2KHz. Even its third overtone would be just 6Khz, still way below ultrasound.

No ultrasonic welding there IMO. Probably just good old vibration generating heat and/or mechanical stress on the plastic part.

-------------------
Best regards,
Bruno.
04-18-2008 06:39 PM
 
 
AceBird
Elite Veteran
Location: Utica, NY USA

Quote 
The fastest thing rotating in a gasser is turning at between 11 to 12K RPM.

What about gears and bearings or any other source of impulses. If two things are a source of impulses they can fill in to each other and make the frequency greater than either one. I have no idea if the frequency hit 20 khz., one of the common frequencies for sonic welding but the truth is it didn’t have to hit the melting point. All it had to do is absorb enough energy to soften and I would guess it occurred over a long time. Sonic welding occurs in less than a second.

Quote 
No ultrasonic welding there IMO. Probably just good old vibration generating heat and/or mechanical stress on the plastic part.

Vibration implies motion. So when you say vibration are you saying the ball link loosened up and the friction between the metal part and the plastic part generated heat? There would have been evidence of chafing and there is none that I can see.

A control rod flapping in the breeze can certainly ad to other frequencies present in a helicopter. Did the plastic link on the end of the control rod melt? The metal ball did not get hot I am sure of it.

Ace
What could be more fun?
04-18-2008 09:14 PM
 
 
Brunobl
Veteran
Location: Pomerode, SC, Brazil - 26 40S 49 11W

Quote 
What about gears and bearings or any other source of impulses.

They are all at or below engine RPM.

Quote 
If two things are a source of impulses they can fill in to each other and make the frequency greater than either one.

Yes, that is called heterodyning, but the resulting frequencies are the sum and difference of the input frequencies. So the mixing of pulses around a couple of hundred Hz each, will result in other frequencies that will not begin to approach ultrasound.

If you mix 200Hz with 300Hz, for instance, there will be 4 different resulting frequencies: the original 200Hz and 300Hz, and additionally 300-200=100Hz and 300+200=500Hz.

Quote 

are you saying the ball link loosened up and the friction between the metal part and the plastic part generated heat? There would have been evidence of chafing and there is none that I can see.

No, the part that melted was the servo output arm, not the ball link, so I would guess that the loose part might have been the bolt that held the ball head to the servo output arm. If that were the vibrating joint, it could explain the servo output arm's failure.

But the above paragraph is just speculation. My point here is that there is no source of ultrasound in an RC heli.

-------------------
Best regards,
Bruno.
04-18-2008 10:24 PM
 
 
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Gasser Model RC Helicopters > Melted rudder servo disk!
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