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Metal to metal interference

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Old 10-26-2006, 12:55 PM
  #1  
kairos
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Default Metal to metal interference

Hi,
Can somone explain to me the old chestnut of metal to metal vibration causing interference to your receiver. I am not saying it could not happen but no one has explained to me exactly how it occurs?

To me you need inductance ,capacitance and a source of the frequency being transmitted
i.e. Crystal ( or the metal to metal vibration?). or a spark could cause an electromagnetic field if you could engender enough volts to create the spark!.

I think the original problems were with the early RC receivers but it is still sited today as to be avoided at all costs.


Many thanks

In ignorance

Richard.


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Old 10-26-2006, 01:19 PM
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Default RE: Metal to metal interference

LOL i cant explain it but i can show it to you if want to come and see my bipe. you touch the engine with a metal wrench with the receiver on and things will jump and twitch. don't know all the details but basically if you stimulate the monocules of metal weather it be by vibration or or put it in the microwave you can create a electrical Field this can cause interference epically if it is carried near the receiver by a metal control rod or cable.
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Old 10-26-2006, 01:46 PM
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Default RE: Metal to metal interference

It does happen and doesn't have to be metal to metal. If a metal push rod rubs against balsa or ply in the fuselage, there's a minute screeching sound that will create RF interference thru the antenna to the RX. On my gas (not glow) engines, can't have a metal rod directly from the throttle servo to the carb control arm. Learned that on my first big gas and with a running pre flight check, all the surfaces went crazy. This past summer had the same thing happen during a pre check only to have a small crack in the spark plug wire on a G62.
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Old 10-26-2006, 02:30 PM
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Default RE: Metal to metal interference

An electrical spark "broadcasts" on a huge spectrum of frequencies. Lord knows what the metal-to-metal could be generating. There are harmonics and nodes throughout a radio spectrum that may cause "glitch" transmissions. If something vibrates at exactly 1/1024th of a wavelength but is 1024 times closer than the transmitter? Who knows.

Vibration causes material fatigue and that is enough reason for me to abolish it as much as reasonably posible.
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Old 10-27-2006, 09:26 AM
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Default RE: Metal to metal interference

I have had a similar problem on one of my planes where my muffler was a little loose and when I started the engine the servos twitched a lot. When I turned the engine off the servos stopped twitching. After tightening the screws and adding a pipe clamp, it has not twitched on me yet.
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Old 10-27-2006, 02:01 PM
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kairos
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Default RE: Metal to metal interference

Thanks for all the replies .I think it is quite a fascinating subject. It looks as though it is a phenomenon that we all know exists but no one has carried out any research to find the answer ,which is surprising really considering the state of the art of RC and the planes getting bigger and more sophisticated ( read expensive!).Of course there is also safety to be considered.

I think Randy and Charlie are on the right lines , may be it is static build up , if you think of the wing area for instance friction through the air could induce static build up.Metal to metal contact could be the make and break to set the charge off.
With regard to static I was thinking of the moving parts of the engine causing static and then thought of the plastic/nylon (What ever)
prop, again causing friction with the air.

It would be a good research thesis for a student.
As I said earlier thanks for all your comments if you have anything further to add please feel free.

Richard
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Old 10-27-2006, 03:59 PM
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Default RE: Metal to metal interference

I've got a bipe that shows it in spades.

I've had metal to metal on many planes but never any intereference problems. Then I built a Balsa USA EAA Bipe and added flying wires. I've got metal connections soldered onto the wire struts that the metal quick-links on the flying wires attach to. This plane glitched all over the place. Since it's a modern PCM radio, the glitches translate into frequent periods of no control. Remove the flying wires and the plane is 100% solid. Every flight with wires mounted has many glitches. Every flight without is solid.

The thing that I find interesting is that while I've had metal to metal before, I've never had it in a situation that was as likely to build up a static charge as a flying wire is.

Not an answer, not proof, but interesting.

Dave
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Old 10-27-2006, 06:45 PM
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Default RE: Metal to metal interference

A static discharge will certainly produce RF interference. Any sudden change of current flow, even a very small current, produces frequency components throughout the RF spectrum. A metal push rod will act as a sort of antenna, receiving signals from the environment. The received signals set up currents running along the pushrod. If that antenna is in intermittant contact with another metal object (say the engine throttle) then RF noise is producted as the contact makes and breaks.

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