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Pulsating Harmonic Engine/Propeller Sound

Vantastic12

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On my RV-12 with a Rotax 912 ULS at full throttle my engine and/or propeller makes a pulsating harmonic sound. It’s particularly noticeable at full throttle while climbing, and less noticeable in cruise. It’s never just a constant level engine/propeller sound. I don’t notice any indication of it in the RPM or in performance.

If you have any ideas, propeller pitch, carb sync or anything else, please advise. The Carbs were sync'd with a Carb Mate and the two-bladed prop pitch is set per the construction drawings.

I fly out of Colorado at 7,600.' Horsepower and take-off/climb performance is obviously diminished, but it will easily make 5,500 RPM at cruise and 120 knots, as advertised. Regrettably, the pulsating harmonic sound is just "plane" annoying. I have about 130 hours on the airplane and it has done this since inception. Otherwise, it flys great!

Thank you for your time and any suggestions.
 
You are not alone...

590 hours here. It’s done it since I bought the plane at about 160 hours. It got a little better with better carb sync, but it was still there. Then it got better when we had the prop blades refinished and static balanced by Sensenich while getting the prop hub addressed. Still detectable, but there’s much less vibration and the rhythmic noise is barely there. Don’t really know what it is. Another RV-12 on field doesn’t do it.
 
Maybe try dynamic balancing, recommended by Rotax, from their manual:

"Dynamic balancing of the propeller as specified by the propeller manufacturer must be carried out."
 
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I have never experienced this problem. I have several hundred hours flying in two different ULS powered RV-12’s. Rotax uses a “hunting” ratio on the gearbox to spread wear pattern evenly and I would imagine to reduce harmonics.

• Any chance you have an Active Noise Reduction headset that is malfunctioning?
• Wear hearing aids that have slow processing amp?
• Does center instrument panel vibrate? Try putting your hand on panel to dampen vibration.
• Check prop pitch - both blades same pitch angle?

Just spit-balling here…
 
Probably a beat frequency, the sum of two distinct vibrations with very slightly different frequencies. Reducing either vibration makes it less irritating, and eliminating either will kill it. I point it out because a mechanic may make himself crazy looking for a single oscillating source which does not exist. The real task is to find one of the underlying vibes.

There are plenty of good tutorials available via Google. Here's a decent one:

https://www.physicsclassroom.com/cl...at frequency refers to,beat frequency is 2 Hz.

I used "may" above, because oscillating sources are possible, if unusual. The 3-cyl Suzuki I flew in a JN-4C replica had no balance shaft, so an unfortunate combination of its block vibratory motion and mount stiffness values could wobble the prop disk in certain RPM ranges, basically the whirl mode vibration which took down early Electras. Bystanders on the ground heard a wa-wa-wa-wa when I flew past, while from the cockpit it was imperceptible.
 
Aeronca 11-AC Chief I learned to fly in had placard for McCauley metal prop that stated no continuous operation between 1860 - 1930 RPM. Prop / engine combination apparently had bad harmonic in that range which could cause bad day.
 
Out of the box here but...could it be one of the tires rotating while in flight? It wouldn't be the first time that a tire rotating in the slipstream produced a pulsating sensation felt in the cockpit.
 
Out of the box here but...could it be one of the tires rotating while in flight? It wouldn't be the first time that a tire rotating in the slipstream produced a pulsating sensation felt in the cockpit.

Along those lines, do you have wheel pants? Partially enclosed tires (wheel pants) are the only ones I've seen rotate continuously in flight. The rotation is slow.

Jerre
 
I have seen wheels without pants rotate inflight. Most of the time it's the nose wheel -- I suspect due to the nose wheel having no brake drag. Also, the wheel tends to rotate in its normal direction which I suspect may be due to the airstream more cleanly hitting the bottom half of the wheel versus the draggier (e.g., fork, strut, etc.) top half?? Any aerodynamicists out there care to comment? Thread drift -- sorry.
 
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Probably a beat frequency,
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.
 
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.
 
Not making light of this... wondering if this is somewhat of a prevalent problem - why are we just hearing about this now?
 
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.
 
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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.
 
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.
 
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... :D
 
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