RunRyder - Melted rudder servo disk!

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Gasser Model RC Helicopters > Melted rudder servo disk!

cranester
Veteran
Location: Bogota, Colombia

Great.... just what I needed, my helicpter is a flying ultrasonic welding machine.

Juan Crane

04-19-2008 12:54 AM

HOMEPAGE

FCM
Elite Veteran
Location: Back in Blighty!

Sounds likely to me what Ace has described. What about harmonics caused by the apparently unsuported carbon pushrod? We have no idea what frequency this may have be vibrating at. Gears on a gear set can also cause vibrations not from the rotation but from the teeth meshing. This would be a higher frequency would it not?

Paul.

04-19-2008 01:12 AM

Brunobl
Senior Heliman
Location: Pomerode, Santa Catarina - Brazil

Hi Paul,

Quote
What about harmonics caused by the apparently unsuported carbon pushrod?

The fundamental frequencies involved are so low that any harmonics would be heavily attenuated by the time they made it into the ultrasound spectrum.

Quote
Gears on a gear set can also cause vibrations not from the rotation but from the teeth meshing. This would be a higher frequency would it not?

Yes, but still the frequencies involved would be well in the lower audible range, quite distant from anything like ultrasound.

My Spectra has a 93-tooth main gear. Even if I did spin it at 2K RPM, which I don't, still the pinion/main gears would be meshing at just over 3KHz (93*2000/60). It can be shown that other gears will provide similar or lower mesh frequencies.

If this issue were ultrasound-related, we'd be seeing more of this in other helis. I don't believe this event was caused by anything so exotic.

-------------------
Best regards,
Bruno.

04-19-2008 01:44 AM

AceBird
Elite Veteran
Location: Utica, NY USA

Quote
They are all at or below engine RPM.

I don't agree with this one. If there is a 12 tooth gear on the engine shaft the impulses are 12 times the speed of the engine. No?

Quote
My point here is that there is no source of ultrasound in an RC heli.

And I am not saying your wrong. I am more inclined to agree. My point is you can generate heat in plastic by just loading and unloading a plastic member at a fast rate. It most likely is happening because of vibrations and impulses from all kinds of sources. Since it doesn't happen with every helicopter I suggested looking at the gain value of the gyro which has to be one source. There certainly can be others.

Ace
What could be more fun?

04-19-2008 12:50 PM

FCM
Elite Veteran
Location: Back in Blighty!

Well, for once I can't argue as this is all new to me

There have been reported fractures of carbon tail pushrods on Gassers. They can fracture not at the carbon rod itself but the threaded steel rod where it threads into the ball link. What I think happens here is that the threaded portion of the rod is a stress raiser and vibrations passing through cause fatigue at this point.

The 'fix' was to ensure thee was no exposed threaded portion of the steel rod i.e. all the thread is enclosed by the ball link. As far as I am aware, this was a Gasser only problem so this obviously points to the engine as being the culprit.

This heating this may be related to Gasser engine vibs?

Paul.

04-19-2008 01:14 PM

Brunobl
Senior Heliman
Location: Pomerode, Santa Catarina - Brazil

Quote
If there is a 12 tooth gear on the engine shaft the impulses are 12 times the speed of the engine. No?

Of course. When I said the fastest thing rotating was at engine speed, I was referring to RPM. Of course gears may have vibration pulses multiplied by the number of teeth, as I have already addressed in my last post. I even went on to provide an estimate for the specific frequency in my Spectra, as you may recall:

Quote
My Spectra has a 93-tooth main gear. Even if I did spin it at 2K RPM, which I don't, still the pinion/main gears would be meshing at just over 3KHz (93*2000/60). It can be shown that other gears will provide similar or lower mesh frequencies.

The frequencies we are seeing here are at the low end of the audio spectrum. I really feel that it is a bit of a stretch to consider any of this to be ultrasound-related. IMO it is a simple case of vibration inducing failure, possibly in an already loose component. My guess is that the screw fixing the ball to the servo wheel might be a bit loose, had the chance to vibrate and destroyed the servo wheel.

-------------------
Best regards,
Bruno.

04-19-2008 02:44 PM

FCM
Elite Veteran
Location: Back in Blighty!

I have heard of plastic air boxes melting on Zenoah engines. This is the square black filter box I am refering to. Maybe the vibrations that are causing this damage are acoustic and not ultra-sound?

Paul.

04-19-2008 03:10 PM

Brunobl
Senior Heliman
Location: Pomerode, Santa Catarina - Brazil

Quote
Maybe the vibrations that are causing this damage are acoustic and not ultra-sound?

Yes, that's what I think.
There is an awful lot of vibration associated with those gasser engines, and quite possibly anything not securely fastened has the potential to vibrate itself to destruction, or melt at the poor-attachment spots.

Maybe this is a resonance issue. Instead of some exotic frequency multiplication or overtones that would reach far into the ultrasound region, isn't it possible that on some rare occasions a component's natural oscillating frequency just happens to be near one of the many vibration frequencies present in a gasser heli? It would in this case vibrate more than would seem obvious from the vibration levels involved, i.e, four people could overturn a small truck if they push/pull in the right tempo. Change the tempo and they will never do it.

The fix in this case would be simple, just add or remove a few grams to the affected component and thus "de-tune" it from the offending vibration.

This might be an interesting exercise.

-------------------
Best regards,
Bruno.

04-19-2008 03:47 PM

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Gasser Model RC Helicopters > Melted rudder servo disk!

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