Metal to metal interference

[Myron Beaumont]

Phil

Not to mention the chattering 

[Former Member]

[This posting has been removed]

[Phil Claridge]

i bought a viva from our local radio mechanic back in the cb days, it had earth leads all over it, doors to body, boot and bonnet to bodyand hub caps(remember them) to the hubs. one day it would not start, it sounded like a flat battery, as it was dark i tried the lights and they were quite bright so i turned the key and she started easily the flipping earth lead between the engine and chassis was loose but putting the lights on gave it some sort of earth return, i thought it was a fluke so i tried it again and it would only start with the lights on

[Evan Pimm]

This sort of 'metal to metal' noise is no stranger to model flying, been with us since the start. 2.4  is no more immune than any other transmission system. Often it will only affect one control, and be seen as servo 'chattering' and strangely only when the engine is running...Sometimes attaching and removing the plug igniter will induce a little shiver through the servos. You see, it does not need to get in through the receiver aerial, any length of wire will do and a servo lead is near perfect. As someone has already said, this sort of RF interference splashes across the whole spectrum so really no system is any better than another. The same precautions as ever need to be taken to avoid the malaise, and the same troubleshooting when you have it. It seems that this 2.4 stuff is seen as the answer to all ills...some will be sadly disappointed. So far, at the local clubs, at least one model per day has been lost on 2.4, but mostly, after the investigation, down to the usual culprits, batteries, switches, failure to do the takeoff checks. It has ever been so...

Evan.

[Tim Mackey]

EP said...

"It seems that this 2.4 stuff is seen as the answer to all ills...some will be sadly disappointed. So far, at the local clubs, at least one model per day has been lost on 2.4, but mostly, after the investigation, down to the usual culprits, batteries, switches, failure to do the takeoff checks. It has ever been so...

 

WHAT ! - at least one per day lost on 2.4 G ? - glad I aint in those clubs

[Evan Pimm]

Yep Timbo, I fly at at least 3 different club fields, and the average is one model per flying day lost on a 2.4 system. The kicker, of course, is the next sentence, the usual culprits. I guess I was trying to point out that this sort of interference does not care what transmission system you are using, and that 2.4 will not save you from the usual stupidity. I blame the modern 'ARF plug and play' thing, the operators are not getting the basic grounding ('grounding', good that, what!) in proper installation and checking that we used to have to (and still) do. The shattered remains of bits of foam and printed plastic provide mute testimony to the fact.

Evan.

[Phil May]

Hi guys

   decided to change push rod and fit a plastic clevis, just in case

   thanks for the replies.

                                     B.P        

[Erfolg]

Will there not be some metal to metal noise from IC engines, if it is mere contact between metal to metal parts?

 

I am particularly thinking of the thrust washer on the crankshaft to prop driver. Bearing to crankshaft, as lubrication will not be 100% effective, particularly rolling element type, particularly given that the oil film sees pulsating loads. Also there is the piston/ring to bore.

 

Most modellers have heard of metal to metal noise. Without quantifying the characteristics of the noise, the arguments revolve around antidote events and there relevance. The bodies in the ditch will continue to rise with this discussion, as each side defends and counter attacks. Particularly as the evidence, for what it is, is of the type, I sneezed and the cat  became ill, thus proving that you should not sneezing near the cat. Or I sneezed and the cat is fine.

 

By the way I do not believe to have ever suffered this issue. I was particularly thinking of my open gliders. In these I had (still have) a metal Bell crank, with metal clevises, an arrow shaft (aluminium) as push rod (very stiff and light) to the servo, a simple metal push rod in some instances to the All Flying tailplane, which also had a metal block as a clevis , which was then connected as a bearing to the metal tailplane dowels.

 

What should have made it worse, is that the aerial was/is run parallel to the push rod in a plastic internal tube to the rear.

 

As an alternative I have in the past used a metal Bowden cable inner as a push and pull on the elevator.

 

You may wonder why I used so many metal parts. It came about as a consequence of plastic parts failing.

 

I also was copying other competition  modellers practices about 15-20 years back. 

 

It seems that I am incredibly lucky. Or the model for producing RF frequencies, was not achieved.

 

As an aside i now have a current  model which I fly quite a lot, with metal horn, clevis and push rod to my wing aileron servos. To date no issues of the type described, plenty of others though. Electric model.

 

Of course my experiences do not prove that metal to metal is not capable of producing RF frequencies, or that frequencies achieved can affect 35. I think it perhaps shows that it is not necessarily always an issue.

 

Erfolg

[Evan Pimm]

As you say Erflog, not always an issue, and not ever likely to be with a glider, you need some energy input to the system, via the vibration. If your engine is producing metal to metal noise, then the oil film has failed, and the engine will soon to, or the muffler is loose, and that has caused servo jitters. As an example, I use nylon bolts on a UPF to anchor the flying wires, these are removed for transport. If I forget, and fit even one metal bolt, the throttle will have occasional random operation. Replacing the metal bolt with a nylon one, and everything is solid again. Now, I'm not going to say that it is 'metal to metal' noise, but it only happens when the engine is running, the radio system is on, and a metal bolt has been inserted where a nylon one should be. Pretty solid grounds for the suspicion, but. And you actually need two bits of metal to be in contact, with just enough space between the to allow some relative motion between the two bits.

Evan.

[Erfolg]

Evan

 

I am not sure that rolling element bearings, are not totally supported on an oil film, as the plain bore bearing should be.

Even the babbitt bearings needed some engineering to provide the oil film ramp and pressure to maintain a definite oil film. I worked on a number of test rigs monitoring and recording results for such arrangements (went on for months of accelerated conditions).

 

May be not surprisingly, bronze bearings with steel shafts, did not make use of the ramp effect, but dissimilar metals, which did make intermittent contact. I suspect that the scraper rings on the piston are not dissimilar.

 

It could be all the lubricants suppress any RF noise potential.

 

Some of this does remind me of the problems when testing Large Transformers, where the whole of the very large shop was festooned with thin wire grounded conductors, to create a Faraday cage. Yet spurious signals could be picked up from both instrument wiring and power leads, where shielding, separation were not sufficient or did not cross at (0 degrees. An awful lot of time could be spent eliminating, noise, often from sources not possible. or the supposed source, not being the culprit. Sometimes took a lot of time, other times, no issue. But it was surprising how high passions could get.

 

Erfolg

 

 

 

 

[Evan Pimm]

Well Erflog, I can confirm that the rolling elements most definitely rely on the oil film for survival, part of my job is to provide Root Cause Analysis for gas turbine failures, and you would be surprised how short the life of a bearing is when deprived of oil, how rapidly things get really hot, and the amount of energy stored in a compressor rotor at 10,000 rpm. Even piston scraper and compression rings in IC reciprocating engines have a remarkably short life when the oil film on the cylinder walls disappears. It isn't the running that wears engines, it's the starting, when the oil film is either missing or not completely there that allows the wear to take place.

Evan.

[Peter Beeney]

I think I would have to agree again with Erfolg on this one. I don’t think I’ve ever seen a prolonged case of metal to metal noise. Certainly there are always temporary ‘local’ cases, such as the glow plug clip (another electrical source) when it vibrates or sometimes just by touching the engine with a screwdriver, say, but never a case of persistent glitching that’s been proven to be metal to metal. Many beginners trainers seem to have metal push-rods to the throttle with a z bend to the metal throttle arm but I’ve never seen any that cause a problem. Glitching, yes, but inevitably we are able to attribute it to something else. Crystals maybe, they have been known to cause problems.
As an example, a new member turns up with a trainer and a GWS budget radio, most newcomers are on 2.4, this one is an exception. Flying it showed there was an occasional but persistent twitch; and because the GWS had no buddy facility we used my system. We are a bit tight on flying time at our site so we tend to have our ‘own’ frequency, and I’ve got a couple of old SkySport tx’s which I’ve used for training since the nineties that are now slowly becoming redundant. My own radio is MPX, so one Futaba tx has a MPX rock in it, I use this as the master, on my channel, and I put a MPX rx xtal in the pupil’s rx. Now we can fly all the time. I did this to the GWS and the twitch instantly disappeared! Solid as a rock! I use an assortment of rx’s for my own models and they all have MPX crystals, apart from dual conversion. I must emphasis here this is a personal choice, the recommended procedure is to fit the radios with own-brand crystals; for me, it’s a case of trust in the gear. Having said that, I’ve probably used everyone else's mostly Futaba radio more than my own and it’s always been immaculately behaved. I don’t know what the problems were with the GWS but they certainly disappeared. Reappeared, though, when we went back to the GWS tx.
I’ve also seen glitching related to vibration of the receiver, wrapping in some foam rubber cures this.
Evan, is it possible your metal bolt problem is some sort of ‘close proximity‘ effect? Reflected signals, perhaps? If the bolt is tight it’s difficult to see how that actually generates any spurious interference, although it maybe not of course be impossible. I would be tempted to try another receiver. In any case, a Futaba PCM, a Multiplex IPD or even a Corona DSP receiver will cure any occasional random operations. Full stop!
Regarding the oil-on-metal scene, whilst oil is a insulator it might not make a great deal of difference to the ability of any metal-to-metal contacts to generate interference. Think of the corrosion preventing grease on low voltage car battery terminals through to the contacts immersed in oil in high-power contactors. These do not cause high resistance connections. Some aluminium bus-bar joints have petroleum jelly in between as an oxidisation preventative but the resistance of the joint has to be a least as low as a similar length of bar next to it. No insulation here. Not sure how relevant all this is, but I guess if the engine is going to cause trouble the oil isn’t going to make much difference.
The GWS man eventually went 2.4 and talking of 2.4, the severe interference we witnessed here, (not this model, though!), as related in another thread, was borne on the power leads.

Tomorrow it will be all m-t-m interference!     PB