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.

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. :rolleyes:

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.

MM43 wondrs:
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:

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...

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.

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.:hmm:

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.

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 :uhoh:??)

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

FOR

Excellent summary of what we have been told. :ok:

Local information:
Au CASA Visual Pilot Guide for Jandakot:

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 :hmm:

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.

GarageYears 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:rolleyes: ) from the suspect list

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.

This is the stack in VH-FJI :

http://i337.photobucket.com/albums/n...F-FA-200_5.jpg

The map light is partially obscuring the intercom at top.

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.

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

http://i337.photobucket.com/albums/n...F-FA-200_6.jpg

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.

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:

MWR.

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

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.

Year of manufacture 1972

(regosearch, refreshed from casa 22 feb 2011)

I wish I'd spotted this thread from the get-go. Fascinating.

Is it still an ongoing mystery, or is the fitting of the lower aerial considered a final fix/workaround?

I used to commute from the east coast to Luton, often at night in a 4 seat metal aircraft. It was fairly new, but the radio used to get worse as I passed near to Stansted, then improve with distance. I did exactly what I'd want to do with this problem, and measure the AGC voltages. However, in those days, I was able to open the box without breaking hidden seals.

In this case, after a few pages I have to not spend any more time today, but after so much testing and swapping - I was getting a gut feeling (after a C&G and being a telecom tekki) in my yoof, that something exotic was happening. Thinking laterally, I wondered if the basic design of the comms boxes radiated a spurious signal, which was being reflected by by a prop disc that only qualified as a dish at a certain RPM. I'd want to know what happened to that AGC line as the signal was swamped/diminished.

I know it was easy in my day. AGC feedback was a simple voltage that could be measured with an AVO. Now I suppose it's a digital 'statement.' :hmm:

Yes - I agree. I'm afraid I see almost infinite potential there for poor electrical connections and intermittent operation of equipment - and noise. Somehow it doesn't quite fit in with the sweet looking machine we've seen elsewhere in this this thread.

Another puzzle (hopefully resolved soon) (sigh......)

FOR
PS Please, no-one start posting photos of the bl**dy spark plugs :(

Was this aircraft resurrected from a serious accident in 1984 or was the reg number later assigned to another FA-200-180?

Registration details for hangflyer's aircraft are:
Model FA-200-180
Serial FA-200-203
First Registered Date: 07 November 2006

The accident report says the aircraft involved in the 1984 accident was destroyed.

FA-200 wiring diagram:

http://i337.photobucket.com/albums/n...F-FA-200_8.jpg

Hi-res version here.

Thanks for the continued stupidity DERG, you seem to have adopted Charlie Sheen's approach to life ---> "Nutter". :mad:

I presume you are going to explain that these particular cockroaches have some kind of Adonis DNA and can sense when the radio is in-tune with a station and start generating 'white noise'. :D

Since not much useful has been added in 24 hours ('cept the electrical schematics perhaps) I will continue to remind all the folks that can't be bothered to read the ENTIRE thread, that much fault-finding has already been done and the single MOST IMPORTANT point discovered to date are these:

- No white noise with the engine RPM below about 2350 (was originally 2250, but went up a little following replacement of plugs and 'tune-up' bits in the engine
- ONLY NOISE when the tuned station is within +/-20 degrees of the nose, goes away otherwise
- ONLY NOISE when a station IS TUNED, opening the squelch on an open frequency does NOT produce the problem effect
- Noise is NOT heard on a hand-held in the cockpit, or the Nav receivers
- Noise is the SAME whether the station is the tower 200m away or many km distant, the receive volume of the noise is constant
- Noise is heard on BOTH VHF receivers (2 fitted), originally 2 antennas on topside rear of craft
- New antenna installed on the underside of the aircraft does NOT suffer noise

(There might be a couple of other pointers I'm forgetting but I think these are the key ones)

My dis-assembly of all this is:
a) Noise is not RF signal (otherwise hand-held and nav rx would 'hear' it too)
b) Noise is not RF signal (otherwise an open frequency would carry it too)
c) Directionality - prop is in front and effect is linked to RPM.... vibration effect???
d) ADF antenna is directly above topside location of original antennas... interaction?

What am I missing (DREG - again, if you mention cockroaches I have suggestions where to insert them....)

- GY :hmm:

First registered as VH-FJI in 2006 but manufactured in 1972.

Click on: RegoSearch/CASA.

DREG is so much more appropriate than DERG !

From a web-site. VH-FJI is maintained in the Charter Category and is probably the finest example of a Fuji in WA.

I'm still baffled by Machaca's photograhs. They must be from a different aircraft - so why post them? :confused:

As I said earlier - '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'.

hangflyer

I would like to suggest a test, please. One of the explanations suggests that there is no external noise, and that the white noise you hear is just the normal internal radio mixer noise heard when the received signal gets very weak and the agc (automatic gain control) turns up the radio gain.

Would it be possible to check reception on a distant station and see if the noise level comes up and whether it sounds the same? To one side and using each receiver in turn.

Many thanks, Mike.

The images I've posted are indeed from photos of VH-FJI -- taken by hangflyer himself last June.

Bloody hell! And he's worried about the performance of the radios........?!!!:mad:

I'd be more worried about the control column coming off in my hands. :eek:

I really don't get it - the external condition doesn't reflect any of the mess that appears to be inside. :ooh:

To be honest I'm surprised much of anything works. Ever heard the term ground-up rebuild??

- GY

Thanks Machaca. There's something very fishy here. No one in their right mind would charter that mess - and surely the aircraft has an independent annual. Who would sign that off? Very very fishy.

I'm out.

I would expect since acquiring the aircraft hangflyer has sorted out most mechanical ills and surely followed the technical bulletin and replaced the control column tubes.

forget -- a lot of time passed since hangflyer acquired the aircraft and he subsequently offered aerobatic flights for hire. Much work could have been accomplished since last June.

Alternator Regulator
 

http://i54.tinypic.com/2z71wzr.jpg

It would appear that the alternator is a belt driven Prestolite brand, and the above schematic represents the associated regulator.

Potential noise culprits are the Zener diode Z1 (with open circuit C1) and L1 - a neon. MWR has dismissed the regulator, based on reports from HF, but as nothing has conclusively been determined as the cause, it is still open to conjecture.

mm43, MurphyWR posted #201 a reminder that Hangflyer had run the engine with the field disconnected, to no avail. pm also sent.

You mentioned it only occurs with the radio tuned to a station. Could this be some sort of coupling between the two radios? Does it only do it when both radios are on?

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

cwatters wonders
From HF original post:

We appear to have some remaining tech posts interspersed with a range of silly posts. I have had several complaints that the latter are spoiling the game for those more interested in the tech aspects.

I will monitor the thread more closely and, if the standard of some posts doesn't rapidly rise, either move the thread elsewhere or remove some posters as appropriate.

regards,