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A whine with a frequency proportional to RPM

My plane has done this for probably years, though you cannot hear it in headsets. You hear it clearly in videos whose sound track has been recorded on a “semi pro” mp3 recorder wired to a spare headset socket. The typical spectrum at 2400rpm is

The first of the three sharp peaks is about 3060Hz.

This is at 2400rpm and thus the peak of the main “lump” is 120Hz.

I can get rid of the 3kHz peak with a 2.5kHz lowpass filter, especially if combined with a notch filter. Normally I use a freebie called Audacity for this. But what is the reason? It is a pretty high harmonic of anything rotating. The voltage regulator is switch-mode but that switches at a fixed frequency (it varies the duty cycle).

Shoreham EGKA, United Kingdom

A stream of air cyclically interrupted by a gearwheel could produce a fairly clean tone – in fact, this is how a mechanical siren works. The first thing that comes to mind is the starter ring gear, but yours has 149 teeth and would thus produce 6 kHz at 2400 rpm. Hmm…

LKBU (near Prague), Czech Republic

The tone is heard in the headset socket, so it must be getting into the intercom circuit. That could happen via one of the headset mikes of course, but if I take off the headset I can’t hear this whine (though that could be due to the high general noise level).

Maybe I need to record the cockpit noise with the mp3 recorder and see if the 3kHz is in that. That would eliminate an electrical source.

Shoreham EGKA, United Kingdom

Noise from the alternator (in proportion to its rotation) may make it past the voltage regulator, and then it gets amplified by the intercom.

Andreas IOM

Yep, there’s a little Filter on the alternator which is exactly for this.

Use a multimeter and measure AC (!) Voltage in your cigarette lighter or somewhere else. If you have a significant high ac voltage that also comes because of that filter not working anymore.

I have a suppressor on the alternator output, described here.

I hugely doubt that sort of filter will do anything in terms of removing an AC component on the main bus, because anything coming out of the alternator is coming out of a low source impedance. But maybe not… rectifiers are known to produce RF when switching. I also can’t account for the frequency seen, relative to 2400rpm.

I could use an oscilloscope to look at the 24V/28V bus.

Shoreham EGKA, United Kingdom

I wonder if it is a harmonic from the alternator which is three phase. There are either six or nine diodes in there – imagine one of them is breaking down just slightly. That could produce an interesting output waveform. No idea how fast the alternator actually spins…

The alternator spins very roughly 5x faster than the crankshaft

There should be 6 diodes, for a 3 phase rectifier.

I was trying to make the numbers add up, to get 3kHz, but could not.

Shoreham EGKA, United Kingdom

A ripple frequency from the 3 phases will be alternator speed times 3 over 60 (2400 * 5 * 3) / 60 or about 600 Hz. 3kHz is near as dammit the 5th harmonic of that (the lower harmonics are likely swamped by other noise). Your intercom probably has enough low pass filtering such that other higher harmonics don’t show up so strongly.

Andreas IOM

These are usually the result of using the airframe ground (whose metal probably bears most of the ground current on your airplane) as part of the intercom circuit.

The solution is to make sure all intercom grounds are insulated from airframe ground.

On my airplane, for insulating headset jacks installed on the metal panel, which is grounded to the airframe, I have used S1029 shoulder washers from behind the panel and S1028 fiber washers on the front. This did away with all my intercom whines.

They are available from Spruce , Mouser and other electronics shops, as well as, surely, Peter’s warehouse

My iPad, iPhone and/or VIRB/GOPRO camera can also be connected to the intercom for playing/recording but I have to make sure it is not grounded…if I connect my panel’s EI USB charger (whose ground is shared with airframe ground) apparently all of these portable devices have their charging ground also connected to the ground on their sound input/output jack which makes the wrong ground circuit and brings the offending whine back. There is no way to avoid it other than disconnecting the charging port, since they are internally connected in the device.

Edited to add that the metal case on these portable devices is also grounded to the sound input/output, so if, even with charger disconnected, the case touches an airframe metallic part it also generates the whine. I have them on insulated RAM mounts.

Last Edited by Antonio at 03 Jul 10:44
LESB, Spain
16 Posts
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