No metal to metal contact - a furphy?

I am advised by the cluey and experienced guys at my club that metal to metal contact in glow planes is a no no - ie the metal throttle control rod mustn't attach to the metal throttle lever on my OS61, as it will cause glitches etc.
However, on thinking a bit about it I don;t see how this can happen as there is no potential difference between the motor/throttle lever and the control rod - in my current case the other end of the rod just attaches to a plastic/nylon/whatever lever on the throttle servo.
I am not sure that electricity can be generated just by rubbing two bits of metal together when there is no potential difference between them
I note that in other situations - eg mobile phones in cars, car radios etc there is LOTS of metal to metal contact surounding these devices, and is not a source of interference (ignoring the common problems of earth loops etc which are s different matter)
So, I am thinking that maybe the problem doesn't really exist - in the situation as I have outlined above - but I have been wrong before, so....
If you believe it is a problem I would appreciate a short explanation of how the interference is generated, and whether your view is based on personal experience or information gleaned elsewhere
Cheers and thanks in advance
David

Can't explain why. On range checking my newly built Citabria Pro, at about 50 feet the throttle and ailerons would just go nuts (good thing someone was holding plane). Replaced metal clevis at engine with a nylon one and problem was cured! Just to check I put the metal one back on and had problem again! Have used nylon clevises on engines sense without any glitches.
Happy landings, Skyhawk

Thanks - that is pretty convincing I reckon - bugger, I'll have to refit with nylon
David
Skyhawk wrote:
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Skyhawk wrote:
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I'll guess there is a good chance you have a Hitec radio... Metal to metal has affected every Hitec I have had, not so much my Futaba's. I am not flame-ing Hitec, but be aware of metal to metal with any radio and be safe.
Ray

funny, I have no trouble with Hitec but Futabas go crazy. Any chance your Futaba is on PCM? PCM will mask the problem pretty well.
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Did you have any Tx mixing.
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at about
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someone was
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yep, golden rule of r/c. NEVER NEVER NEVER have any metal to metal contact! If the throttle arm on your carb is plastic then metal clevis is OK. but on metal throttle arm you have to use a plastic clevis.
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VERY true. I have actually SEEN the electric sparks generated by metal to metal contact when running up an engine after sundown.
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Just to add something we discovered at the field on the weekend.....
One of our expert pilots (I think he's 15) and his father (also an expert pilot) brought there usual trailer load of planes to the field on Sunday. One newish aircraft wouldn't pass a range check at 30ft. After lots of inspection and testing I noticed a rather large field charger topping up one of their other planes. I moved it away from the suspect plane and things got better, disconnected the battery and the range check was perfect, reconnected the charger and it went bad again.
Moral to the story is, always be aware of other potential sources of interference when doing a range check. Even a humble charger 10ft away can cause problems.

| I am advised by the cluey and experienced guys at my club that metal to | metal contact in glow planes is a no no - ie the metal throttle control | rod mustn't attach to the metal throttle lever on my OS61, as it will | cause glitches etc.
Metal on metal contact is OK as long as it's a good solid contact. Pieces of metal soldered together for example work fine. But anything that's loose or intermittent is bad.
If you have two pushrods that have to touch each other, but can't be soldered together, you can get a small wire and solder each end to each pushrod and that should take care of it.
The longer the pieces of metal, the worse the effect.
| However, on thinking a bit about it I don;t see how this can happen as | there is no potential difference between the motor/throttle lever and | the control rod
You're thinking DC. In a DC world, that would be correct. But this is AC -- radio frequency AC. The rules are very different.
| - in my current case the other end of the rod just | attaches to a plastic/nylon/whatever lever on the throttle servo.
I know, I know, but the effect is real.
| If you believe it is a problem I would appreciate a short explanation of | how the interference is generated, and whether your view is based on | personal experience or information gleaned elsewhere
I'm not precisely sure exactly how it works -- I'm guessing that the two pieces of metal are working as antennas somehow and picking up the signal and re-radiating it (well, that part is certain) and each time they touch and stop touching it makes RF noise somehow.
Rather than convincing yourself that it can't happen, show yourself that it does. Turn your transmitter on, leave the antenna down and put it 30 feet away. Turn your plane on, then take two pushrods or pieces of piano wire and rub them together right next to the plane. As long as you don't have a PCM or `smart' receiver with a DSP, the servos will twitch and possibly jump.
I suspect that the RF noise emitted by the two pieces of metal is extremely weak, but it's only 2" away from your receiver and antenna (and very well may be parallel to the antenna, which will pick up the signal very well) and your transmitter is much further away, especially if you're flying.

The reason I asked is because the plane (ARF) it had a metal throttle rod connected to the metal throttle lever on the OS61 when I built it 2 years ago. It was checked over by experienced club guys and passed all tests, including the range test, without any problems. Also flew well without glitches etc.
The issue came up with a new plane I built recently, and while of course I followed the advice the cluey guys gave me and changed to a non metal pushrod, I like to understand the why and how of things, rather than ONLY follow blindly
David
Doug McLaren wrote:
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| The reason I asked is because the plane (ARF) it had a metal | throttle rod connected to the metal throttle lever on the OS61 when | I built it 2 years ago. It was checked over by experienced club | guys and passed all tests, including the range test, without any | problems. Also flew well without glitches etc.
As long as there is a good electrical connection between the two pieces of metal, there is no problem. And if there is no connection between two pieces of metal, there is no problem.
It's only if there's a poor or intermittent connection that there's a problem. And the longer the pieces of metal are, the bigger the problem, because they're acting as antennas.
A range test doesn't always show the problem, because without the engine running at full throttle, you don't have the vibrations that may cause the problem. Or it could be caused by moving the sticks, or by the plane actually flying.
| The issue came up with a new plane I built recently, and while of | course I followed the advice the cluey guys gave me and changed to a | non metal pushrod, I like to understand the why and how of things, | rather than ONLY follow blindly
Well, in this case there's no need to follow blindly, because it's very easy to reproduce the problem and convince yourself that it's real. I mentioned one way of doing so in my previous post.
| Doug McLaren wrote: | | > Rather than convincing yourself that it can't happen, show yourself | > that it does.
Well, you _did_ seem quite busy explaining why you thought it was an old wives tale :)

From Doug M.:
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Correct! It's the _intermittency_ of contact that causes the problem.
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Correct again. They act as receiving antennas, and they also re-radiate. Picture having one piece of metal (say a pushrod) undergoing a make-break, make-break connection with another piece of metal (say an engine block). Both are acting as RF receptor/re-radiator elements. Now introduce a 3rd element- the receiver antenna. What the receiver is likely to 'hear' is a mishmash of static superimposed on the desired signal. The strength of this 'noise' in relation to the signal (signal-noise ratio) and the receiver's ability to reject the noise, largely determines whether you'll get glitching or not. And there's another potential noise source in the mix if both elements are electrically 'floating', i.e., not connected electrically to anything else: ambient electrical potential of the air itself. Both elements acquire a miniscule static electric charge, a literal voltage, in referance to earth.. AND there can be a voltage difference between the two elements. If they're in intermittent contact, this alone can generate spurious RF hash that can be heard on an adjacent receiver. So the strategy is to simply avoid any metal-metal intermittency, period.
Bill(oc)

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Very good idea. We should do this more often, say in politics........... :) mk

Any metal to metal contact that is not bolted/bonded solidly, may allow those metal pieces to vibrate (rub) together. This rubbing action has been demonstrated many times to cause interference, with servos twitching uncontrollably. In the air, this rubbing may lead to total loss of control, especially with PCM RX which go into failsafe. Vibration prone Heli provide a good example, all connections are ball links or swivel linkages - no metal to metal. Most engines now provide a nylon throttle arm so as to prevent the problems caused with "z" bend ended metal rods supplied in many ARF kits. In the past, some engines with metal arms were supplied with a ball link and nylon cup e.g. O.S. .90 to ensure metal to metal was not used. Du-Bro make a good range of cup & ball links and swivel ball links either of which are best when used with carbs where the barrel slides out as it rotates - less stress on connections. A good read on analysing causes of crashes and how to prevent unexpected "glitches" is chapter 21 "Fault Finding" page 181 of the "Radio Control Guide" by Norman Butcher. This has to be the best book ever written on installation of gear, linkages, differential and general RC modelling - especially for those brought up with computerised TX and do not know how to manually set up a model for zero TX trims, leaving plenty of room for extra control as necessary when really needed. If metal clevis is required to a metal throttle arm or metal horn, solder a piece of desoldering wick (or slotcar braid) to clevis and arm/horn so as to firmly earth the flexible joint. Many good links which expand on the subject under "Radio Systems, Accessories, Alterations and FAQ" on my web page below. regards Alan T. Alan's Hobby, Model & RC Web Links http://homepages.ihug.co.nz/~atong / .................................................................
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You are correct in the statement that no electricity can be created by metal to metal objects being rubbed together with no potential difference. What can occur is ....
The metal to metal contact is a non-linear device. In contrast a resistor is linear. Its value does not change if you increase current or voltage.
The non-linear device (metal to metal) can act as a diode. This can rectify RF energy into unwanted energy.
I have seen this on gas engine planes, where tightening all metal to metal surfaces and removing metal clevis joints eliminates the spurious RF energy.
The question that remains is what RF signal is getting retified into unwanted signal. The RC transmitter itself, a TV broadcast signal, radio stations etc?
The solution is to make all metal to metal contacts solid.
RCS
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David:
You should use similar materials when they are making contact. For example, servo arms and clevises should either be both metal or plastic. The same thing holds when you are connecting control linkages to anything.
A bad example is when the modeler hooks a metal rod (90 degree bend) to a plastic servo arm using a nylon clip to hold it on. Over time, the harder material will wear the softer. You'll see this as the hole in the servo wearing larger. It generally isn't a problem, but it can cause control surface flutter. You'd notice it most in the ailerons.
Anyway, I'd say that something is very odd with your radio! Next time get a Futaba!
Ciao,
Mr Akimoto

On 8/16/05 5:52 PM, in article snipped-for-privacy@g14g2000cwa.googlegroups.com, "Mr Akimoto"
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I hope that was an attempt at humor. There is nothing wrong with his radio, and A Futaba would be affected in the same manner.

Donny boy:
You're totally incorrect! A good radio shouldn't be bothered by any sort of outside intererence and that includes the odd situation described by the original poster. My Futaba 9C never glitches, and the same can be said for my el cheapo Air VG400.
Ciao,
Mr Akimoto

Mr Akimoto Wrote:
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And what Galaxy are you from?????
-- Iflyj
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