Prop Resonance?

[Pmerems]

Gents,

I have been experiencing a "vibration/noise" that I believe is a prop resonance issue. I was hoping someone can help diagnose this.

I have a Hartzell Blended Airfoil 72" prop purchased from Vans mounted to a Superior IO-360 with standard ignition. I have about 50 hours on the RV-7A

A few flights ago as I was on final I reduced power and started hearing a noise which sounded to me like the muffled sound of jet engine. Similar to taxing (with your headset on) parallel to the runway when a passenger jet was taking off. Lasted for a few seconds. Thought it might have been some wind noise since I messed around with a seal on my tip-up prior to the flight.

Today I was on a cross country from Tucson to Wickenburg. On the way up my RPM's were running around 2450 and ~21-22" no noise heard. On the way back I varied the setting a bit and tried 2400-2450 RPM don't recall exactly. As I reduced the MP to 21" or so I started hearing a similar sound but not as loud as the "jet engine" sound as mentioned in my earlier flight. This time I also felt a vibration.

I increased the MP and RPM and the noise/vibration went away. I have flown at these reduced setting before without any issue. However I usually fly when it is cool out but on my return trip is was hotter then normal. I have found that the engine runs the smoothest at around 2450 RPM so I don't think this noise/vibration is just engine vibration.

I haven't had my prop balanced yet. I haven't found any information about any restrictions on the prop, but I might have missed something.

Any ideas out there?

Thanks,

[Bob Axsom]

It does make a difference. The newer blades I believe are F7497 I have the thinner F7496 which are more efficient at higher speeds. I don't have any answers because I never operate the engine at that low of an RPM in cruise but someone else may need that information. I believe Les Dowd of Hartzell's engineering department sometimes monitors this site and he may send you a private message - don't know about that of course.

Better yet, if you go to the Hartzell website/phone and describe your symptoms I have found them more than willing to help. Building on honor you know - I toured their factory in Ohio and got a little feeling of that.

Bob Axsom

[Dmadd]

This might be totally out of left field, but I had something "similar" for a short period of time. Thought maybe the alt belt was slipping or maybe something worse. Turned out it was only the canopy seal. (whew!) Made a weird noise at certain "airspeeds" . I was chasing what I thought was an RPM related phenomenon, when it wasn't that at all. Resealed everything, and all went back to normal...
Just a thought... Good luck.

Dennis

[Bruce]

Paul,

I would suggest Prop Balance when you get
a chance.

Try closing all vents and see if it is the canopy seal.

I had vibrations and it was the nose gear fairing on the
upper side. Will vibrate and you can hear it.

Boomer

[DanH]

Originally Posted by Pmerems

I have been experiencing a "vibration/noise" that I believe is a prop resonance issue.....I have a Hartzell Blended Airfoil 72" prop purchased from Vans mounted to a Superior IO-360 with standard ignition.

Resonant blade vibration severe enough to hear? Assuming the hub is in good condition and the blades are properly installed, I would consider it very unlikely. One very good reason to choose Hartzell is their propeller vibration survey work.

[Neal@F14]

Originally Posted by Dmadd

This might be totally out of left field, but I had something "similar" for a short period of time. Thought maybe the alt belt was slipping or maybe something worse. Turned out it was only the canopy seal.

We had this phenomenon on the RV8 too. Not only was there a bunch of strange air leak noises, but also a rhythmic drumming vibration that sounded like the tail was about to come off the back of the plane. It was mostly the propwash over the sliding canopy causing the rear of the canopy to vibrate against the turtledeck. Adding some rubber weatherstripping to the underside of the canopy skirt to both seal it better, plus add some padding/cushioning solved the problem.

[gasman]

Inspect your baffle seals form the inlets, might have flipped one.

[Walt]

Originally Posted by DanH

Resonant blade vibration severe enough to hear? Assuming the hub is in good condition and the blades are properly installed, I would consider it very unlikely. One very good reason to choose Hartzell is their propeller vibration survey work.

I agree with Dan, very very unlikley you're experiencing prop resonance.
Walt

[lesdoud]

Paul,

Like others here have said, I don't believe you are experiencing a propeller blade resonance issue, especially with the 7497 blade. The only blade vibratory mode that is sometimes felt/heard is the first bending on some manufacturer?s aluminum blades. It can be excited on 4-cyl engines with the throttle closed/idle. However, the frequency of that mode is still typically high and people normally describe it as a noise rather than a vibration. Besides, you would not experience this mode with the 7497 within the normal operating RPM range. All other propeller blade modes are higher frequency and I don't believe you would feel those.

I suggest you get a dynamic balance performed by someone familiar with the spectrum analyzer functions of their equipment. If you can obtain a spectrum plot in-flight with the sensor attached to a longeron or some other piece of structure, then we can determine the frequency of the vibration. That would tell you the likely source of the vibration.

If you have any questions, don't hesitate to contact me directly.

Les Doud
Hartzell Propeller
937-778-4262
ldoud@hartzellprop.com