Pulsating Harmonic Engine/Propeller Sound

[Vantastic12]

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.

[DaleB]

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.

[Walt]

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."

[Piper J3]

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…

[DanH]

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/cla...%20is%202%20Hz.

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.

[Piper J3]

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.

[DHeal]

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.

[AeroDog]

Originally Posted by DHeal

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

[DHeal]

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.

[DaleB]

Originally Posted by DanH

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.