Pulsating Harmonic Engine/Propeller Sound

[Piper J3]

L-R EGT's match pretty closely at cruise RPM?

[Sam Buchanan]

Originally Posted by DaleB

That's what I have been thinking also... though the beat between engine and prop RPM would be fairly high, 40-50 Hz. This is more like maybe 2-3 Hz at cruise (5200 engine/2140 prop RPM). Still... pretty sure it's a beat somewhere with the prop, engine, and gearbox. There are a lot of parts involved, moving at different rates. Then there's the natural frequency of the firewall...

For us it's been constant through several years of flying, including new tires, new nose fork, and flying with ans without wheel pants. Pretty sure it's not wheels. And as I said, for us at least it's gotten much less pronounced as we got the prop and carburetors better balanced.

This "beat" is something I heard many times over the past twenty-five years with aircraft that have Rotax engines, both two and four-stroke. I've always assumed it is somehow related to the gearbox/prop harmonics even though I can't explain it and don't know how to address it. Some planes have it and some don't.

[Piper J3]

Not making light of this... wondering if this is somewhat of a prevalent problem - why are we just hearing about this now?

[NinerBikes]

3 RV-12's, maybe a 4th, 2010 build, 2012 build, 2016 build at KWHP here in So CA.

None of them here exhibit that 3 hz noise.


2010 build has prop pitch set for cruise at 7500 to 10500 elev, a bit bigger "bite" of air.

2012 and 2016 are set at Van's factory recommended setting of 71.4 degrees pitch, and both those were set with 0.1 or less pitch variance between the blades, after numerous attempts to get a dead match.

[Vantastic12]

Thank you all for your prompt and interesting takes. I was beginning to wonder if I was all alone on this as I have nothing here to compare it to. But, some of the responses verified what I thought I heard on some RV-12 You Tube videos--some do and some don't.

California Power Systems (CPS) is suggesting re-checking the propeller pitch, but I'll check some of the other suggestions as well. I like the dynamic prop balance idea. Still waiting for a response from Van's A/C and Lockwood Aero.

There is no detectable vibration on my instrument panel. I do have ANR headsets, but the two sets are identical so the harmonic is the same in both of them. I'll borrow a set by a different manufacturer and try it. That did occur to me, as well. And, I do have wheel pants, but I've flown without them and with different tires--no change.

Thanks again.

[Vantastic12]

Thank you Jim.

I'll check the EGTs next flight. I've been dealing with a leaking #2 exhaust pipe at the muffler since changing out my stock non-leaking pipe for the SB called for reinforced pipe. Van's exchanged the first out-of-round reinforced pipe for a better one. It leaked slightly. So, I just gooped it up with lots of anti-seize, put it back together and I'm flying it now. We'll see when I take the cowl off.

First round may have been operator error. I neglected to apply sufficient anti-seize. The harmonic was present long before the leaking exhaust and replacement. Actually, from day one.

[Piper J3]

One more thing to try... unplug the ANR headsets and run them strictly as noise canceling with supplied battery power. In other words - eliminate comm and intercom for testing purposes.

I flew again tonight and have no pulsating at all. The ANR Zulu3 with hearing aids operating with telecoil Vs. microphone make the RV-12 sound like a turbine...