Crazy Radio Issue, Help Needed.

[JPI33600]

I hope I'm not suggesting something silly, but would it be possible to perform a test with engine running at 2200 RPM with propeller removed? Depending on the noise still being present or not, that could provide some useful clues.

On the other hand, I suppose removing a propeller is not a trivial operation, at least regarding security aspects. And running an engine without its propeller affixed may just prove impractical...

Mechanics ? What do you say ?

[MurphyWasRight]

I hope I'm not suggesting something silly, but would it be possible to perform a test with engine running at 2200 RPM with propeller removed? Depending on the noise still being present or not, that could provide some useful clues

A related test has been done, From HF's original post:

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.

Since removing the prop and running engine is not feasible anothe test would be to borrow a "seriouse sound system" from a car nut, put it in the cabin and sweep 20 to 80 hz at aboout 120Db spl.

BTW: that level that is basically just above idling for the class of sound system I am thinking of.
I have never seen one in person but the competions for loudest system are intense, blowing out a windshield is a classic beginners mistake I heard. Needless to say nobody is actually -in- the car during the tests.

[mike-wsm]

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.

That blows the prop vibration theories.

[MurphyWasRight]

Quote:

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.

That blows the prop vibration theories.

Not sure if it does or not, unless HF stepped on the clutch the engine would still be rotating , pistons up and down etc.
There could still be a resonating effect that might result in increased vibrations in direction of spin.

This however is where my lack of (any!) practical knowledge of the behavior of prop vibration caused by interaction with engine rotating mass is evident.

[mike-wsm]

Yes, still reciprocating masses, but the engine wouldn't be going bang-bang-bangy-bang so no torque impulses to make the prop vibrate.

To be annoyingly pedantic, Lycoming pistons, like VWs and Porsches, go left and right, not up and down.

[mm43]

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.

Going back to very early on in the thread, I did mention that a zener diode could, if reverse biased, generate the "white" noise. It seems that condition could exist in the alternator regulator when current to the base of the pass transistor is cut off. A capacitor associated with the zener will need to be open circuit for this to happen, certainly a possibility, and something that a standard regulator test would not identify.

[MurphyWasRight]

MM43 wondrs:

Going back to very early on in the thread, I did mention that a zener diode could, if reverse biased, generate the "white" noise. It seems that condition could exist in the alternator regulator when current to the base of the pass transistor is cut off. A capacitor associated with the zener will need to be open circuit for this to happen, certainly a possibility, and something that a standard regulator test would not identify.

Zener or other electical white noise source fails to explain why the interfenrence when pressent is at the same relative level over a 40Db or more Rx signal level range.
(HF did stat 10+ miles range so my 200M to 20Km range is valid.)

From HF original post, a couple more reasons why it is very unlikely to be an external (electically generated) RFI problem:

Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.

Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change

When looking for caues of a bizzare issue such as this it is important to keep all the clues in mind, even then it can get pretty confusing.

MikeW: Yes I knew that the pistons go left right just let fingers get ahead of brain.
Even was part owner a VW bus once until one of its pistons ended up goind in/out and then allover the place...

[ampclamp]

G'day Mike no I dont think the antenna was physically removed.

Paddybee , I suspect you are on the money.A defect or condition that is evident on the comms but not actually a radio defect as such.

Someone asked what are they using to do bonding checks.I do hope it is a proper boding meter not just an avo or fluke.

[mm43]

Originally Posted by MurphyWasRight;

Zener or other electrical white noise source fails to explain why the interference when present is at the same relative level over a 40dB or more Rx signal level range.

My reasoning is that the noise had become equalized in a mixer, or some other non linear device, but that in itself doesn't explain why Rx signals from forward of the antenna are affected markedly more than from other directions.

It has clearly been demonstrated that the noise cuts in at a particular rev range, but also noted that it lacks variation with the revs. That's why, rightly or wrongly I looked at the alternator regulator. You have killed off that idea, so what's the alternative?

The Faraday shorted turn proposal currently doesn't rest well with me.

HF has said that the null effect is only forward, and it would need to be the same when going away from the Rx source.

[FullOppositeRudder]

"What if this is caused by presence or absence of front seat pax, or caused by another factor in the flight profile that is concomittant to the RPM and not actually the RPM at all? AOA or trim at that weight/speed condition?" It appears that all the logical steps have been taken (for a radio problem) already. Good luck if this starts a new chain of thought.

Thanks for looking in and thinking outside of the box for a while. We need that, we certainly do. One of the problem here - if I read HF's notes correctly is that the problem - when chooses to be evident - can be reproduced on the ground (post #92 applies).

The positive outcome from relocating one antenna to the bottom of the future is a ray of hope, but the over-riding frustrations remain.

So what do we know ?
1) The problem suddenly appeared without any apparent reason.
2) Even so, it is intermittent in nature without any identified reason(s) thus far for that factor.
3) Both radios are affected even though they are (I think)of different types.
4) Substitution of one of the radios with a known "good" one still produced the problem as previously.
5) Changing the loads on the DC circuit /bus seems to have no effect on the behaviour of the problem.
6) Improving the engine's running (sorry - I'm sure there's a better way of saying that) by changing spark plugs etc changed the onset of the problem to a point some 300 erpm higher than previously.
7) A different alternator (and presumably) regulator was tried - no change.
8) The problem is directional and appears within the predicted rev range over a wide range of engine operating parameters - but always at the known RPM.
9) Problem cannot be reproduced on a handheld in the cockpit and independent of the aircraft power or antenna circuits.
10) Problem is evident over a very wide range of distances (and therefore signal strengths) - could hardly be a RF signal then could it ...?

I'm still without explanation or answer. It the defies every understanding of radio behaviour that I have ever read about or encountered. The only consolotion is that I'm not alone.

So is the radio system essentially OK - something else altogether is at fault?

Can we establish that this has been observed with a sample of other transmission sources - ie just not the tower or base - other aircraft?

I know nothing about ADF operation. Is there something which "cuts in" within the ADF system within the observed heading range? (but why only at that RPM range also ??)

This one really defies logical analysis. I hope I'm still around when it's finally solved once and for all. It's a real worry.

FOR

[mike-wsm]

FOR

Excellent summary of what we have been told.

Local information:
Au CASA Visual Pilot Guide for Jandakot:

[GarageYears]

Getting back on-topic...

Thanks to Machaca for posting the pics a couple of pages back, having looked at them for a few moments and understanding the fact that the new antenna attached to the underside of the aircraft works perfectly (still true hangflyer?) really does bring into question the interaction of the ADF antenna and the comm antennas.

So a flight with the ADF antenna removed or even just ground tests would be revealing. Somewhat obviously, the fact the ADF antenna is inline with the orientation of the aircraft, which also corresponds to the directional confluence when the problem is heard would seem compelling.

I'm still not sure I can understand how/what the prop RPM setting can have over the behavior though - if the ADF ant. is causing some kind of RF shadow to fall over the VHF upper body antenna, how is RPM (above/below 2250RPM) factored into the equation? I presume some vibration effect?

A real puzzle indeed!

- GY

P.S. DERG - yes, VA is a nice place to live, but I'm not sure I'm totally lined up with the tanker decision - though in the end I suspect the Air Force would have been able to make either option work. Living as close to Wash.D.C as I do, there are far more politics in the decision than fit-for-purpose - my perception is Boeing has far stronger lobbyists than EADS could muster, ergo Boeing wins. In the first go-around Boeing simply didn't think it could lose... until it did, hence the re-compete.

[MurphyWasRight]

GarageYears

I'm still not sure I can understand how/what the prop RPM setting can have over the behavior though - if the ADF ant. is causing some kind of RF shadow to fall over the VHF upper body antenna, how is RPM (above/below 2250RPM) factored into the equation? I presume some vibration effect?

It most likely has to be some vibration that cause whatever bit of metal that is the culprit to intermitantly connect/disconnect above a certain prop speed.

This mostly fits with the observation that the "onset" rpm rose slightly (but repeatabley) when the ignition was optimized which (probably) resulted in a smoother running engine.

Either that or the work somehow affected the culprit, access panel cycling or whatever.

Too bad we don't know if the engine off windmilling "onset" speed changed at the same time.
That would remove a smoother engined (and yet again the spark plugs ) from the suspect list

[mike-wsm]

Here's a generalised schematic of the electrics of a light aircraft. If anyone has a more appropriate one please send me a pm and we can arrange to post it instead.

from Welcome to the free online private pilot ground school

[Machaca]

This is the stack in VH-FJI :



The map light is partially obscuring the intercom at top.

[boguing]

Mike-WSM.

That diagram implies that there is an alternator on this engine. I stupidly discounted that, assuming that the mags would also handle DC charging/supply.

So apart from my droning on about the mag AD et al, what rpm does this alternator start charging at? My Series III Land Rover is a bit dodgy and it needs about 4,000rpm to kick in. And because it's not A1 condition, I'd bet that its' output is more than a little noisy.

[Machaca]

Hard to discount the contribution corrosion may have in affecting components, switches, cables, etc.



I imagine hangflyer has cleaned things up substantially since acquiring the aircraft, but there's a lot of electrical componentry that will not be so easy to put eyeballs on.

[MurphyWasRight]

Please check the original post, a lot was already covered.

HF's original post contains an impressive list of debugging efforts before he even posed the question here.

As to alternators:

Quote:

Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.

Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change

[boguing]

MWR.

You clearly have better mid term memory than I. You're quite right.

[forget]

Machaca, I don't understand the photographs you've posted. Is that aircraft actually airworthy? I've dealt with flying wrecks in the past but that is a bloody disgrace. Tell me it's not the one we're discussing here.