Can precipitation static affect RPM sensor?

[charrois]

Hi there. I have a stock IO-540 with Slick mags, and an RPM sensor installed in the non-impulse magneto.

Normally, my RPM readings are rock solid. On initial climb my prop is turning at 2700 RPM, then I bring it down to 2500 RPM for continued climb.

Last week, I did a fair bit of flying in IMC, and on two occasions (both climbing at 2500 RPM, in cloud and in the rain), I noticed a surge in the indicated RPM, with the readings climbing up beyond 2800 RPM, even reaching 3100 for a few seconds once. But I think this is a sensor problem, not a governor one, since I didn't hear the propeller surge or sound in any way out of the normal (I would think that a real RPM surge from 2500 to 3100 would sound quite noticeable, and the engine sound didn't change noticeably). Still, it was quite disconcerting.

In talking to a magneto manufacturer at Oshkosh, he suggested the events might have been due to precipitation static. That makes some sense to me, I think, since any spurious noise on the RPM sensor line might be interpreted as additional pulses on the sensor. But I'm curious what the rest of you think.

Does this make sense to you? If so, it's an easy "fix" to just ignore intermittent high RPM readings when climbing through precipitation. Or perhaps look into shielding the wires to the sensor better. If not, does this indicate to anyone that I should look into replacing the RPM sensor entirely? I surely hope that it's a sensor problem and not a "real" RPM surge, because I'd definitely rather not consider replacing the governor.

Thanks for any feedback!

Dan

 

[Carl Froehlich]

What EMS system are you using to read and display RPM?

Carl

 

[lr172]

Hi there. I have a stock IO-540 with Slick mags, and an RPM sensor installed in the non-impulse magneto.

Normally, my RPM readings are rock solid. On initial climb my prop is turning at 2700 RPM, then I bring it down to 2500 RPM for continued climb.

Last week, I did a fair bit of flying in IMC, and on two occasions (both climbing at 2500 RPM, in cloud and in the rain), I noticed a surge in the indicated RPM, with the readings climbing up beyond 2800 RPM, even reaching 3100 for a few seconds once. But I think this is a sensor problem, not a governor one, since I didn't hear the propeller surge or sound in any way out of the normal (I would think that a real RPM surge from 2500 to 3100 would sound quite noticeable, and the engine sound didn't change noticeably). Still, it was quite disconcerting.

In talking to a magneto manufacturer at Oshkosh, he suggested the events might have been due to precipitation static. That makes some sense to me, I think, since any spurious noise on the RPM sensor line might be interpreted as additional pulses on the sensor. But I'm curious what the rest of you think.
Does this make sense to you? If so, it's an easy "fix" to just ignore intermittent high RPM readings when climbing through precipitation. Or perhaps look into shielding the wires to the sensor better. If not, does this indicate to anyone that I should look into replacing the RPM sensor entirely? I surely hope that it's a sensor problem and not a "real" RPM surge, because I'd definitely rather not consider replacing the governor.

Thanks for any feedback!

Dan

Yes, this seems to be a logical conclusion. That RPM sensor is a VR (variable reluctance) type, I believe, and they are notorious for being sensitive to electromagnetic noise, usually entering via the cable. I used a VR sensor when I put an aftermarket EFI system on an old porsche engine and had a lot of RPM bouncing. Replaced the cable with twisted pair, shielded cable and the problems disappeared. Many setups that use VR have trimmers for tuning due to the noise issues and the odd sinusuidal shapes they create, but doubt that is the case for your EMS.

IMHO if your RPM rapidly went up 300 RPM and then down again, you couldn't really miss the sound and sensation associated with it. I can sense the minor surging on T/O as the governor takes over a few seconds after going WOT and that is well less than 100 RPM.

Larry

 

[jcarne]

IMHO if your RPM rapidly went up 300 RPM and then down again, you couldn't really miss the sound and sensation associated with it. I can sense the minor surging on T/O as the governor takes over a few seconds after going WOT and that is well less than 100 RPM.

Larry

Agree with Larry, if your RPM really did shoot up that much not only would you hear it but you would without question feel it.

 

[charrois]

Thanks for the comments, everyone! Glad to hear that it seems to be a general consensus that if the RPM really did surge I would have heard and felt it, and that noise on the RPM sensor line was very likely the culprit. I've never seen it do that before - just twice while climbing through cloud in precipitation, so there does seem to be a smoking gun.

My EFIS is an Advanced Flight Systems 5600, and the engine monitoring is done through that.

Though seeing the surge in indicated RPM on the EFIS certainly woke me up (and will continue to do so if it happens again), it's good to hear that it probably isn't something of great concern. If it continues, I might try to see about shielding the sensor wire better than it may already be.

 

[Bill.Peyton]

This is why I installed static wicks. I had both my radios totally fail with static one day in the clouds.

Static Wick Placement for the RV-10