Harmonic vibration in composite prop

I noticed a hairline crack on one metal leading edge of my Catto prop recently. The crack extended through the front and rear of the leading edge, and continued through the paint on the front face. There was slight paint chipping around the crack, and also on a similar area of a second blade. The chipping developed since I last waxed the prop at annual 30 hours ago, but the crack was new because I felt it while running my fingers over the leading edge as part of my normal pre-flight routine.

https://www.dropbox.com/sc/x8nm57fkgvg8cgf/AADWeW_g4sO5xe6S8iev7Ktda

Cutting to the chase:
After discussing with staff at Catto Propellers, they offered to repair the prop and upgrade the leading edges for the cost of freight, and even offered to alter the pitch and color if I was interested. My plane is back in the air after only 5 weeks down to repair, re-pitch, and re-paint the propeller to look like new. (shipping took two weeks) Thanks to Nicole, Erik, Craig, and other staff at Catto for standing behind their product, excellent service, and the quick turn-around!

Background:
The prop was mounted on my RV7-A with Aerosport Power IO-375, and had 230 hours TT when I noticed the crack. I had the engine/prop dynamically balanced about 100 hours ago, and installed an Electroair ignition about 30 hours ago (during annual inspection). I don't know if any or all of these issues are related to the crack.

I ordered the original prop with coarse pitch to maximize top speed, and could just barely reach 2700 rpm in level flight with best power setting. With hindsight, I rarely flew at top speed, and spent most of my time around 50% power sipping 7 gph LOP at 2300 rpm and showing 23 mpg on the EFIS. (cheap, I know) I might have been too close to the harmonic vibration range, saving fuel, but damaging everything else.

I asked Catto to pitch the prop a bit finer during the repair to bring my economy cruise setting up 150 rpm to hopefully eliminate any harmonic vibration issues. I made a short flight with the re-pitched prop yesterday, and am very happy with the results. I continue to indicate 138 knots at my economy cruise setting of 7.0 gph LOP, and rpm increased from 2300 to about 2450. I picked up about 200 rpm on the take-off role, causing a big improvement in performance and "grin factor". It was too bumpy to try a high speed run yesterday, but I will run some formal tests when weather cooperates to see how much top end speed was sacrificed. I might even increase my "economy" setting to 7.5 or even 8 gph if it feels better for the engine/airframe as a whole.

Finally, I took Catto up on their offer to change the prop color to match the new paint job on my airframe - it looks great!

Happy flying!
Jay


https://www.dropbox.com/sc/2s2ilovdifo93jf/AABh4EiN6BplSMIgQmCFc0qna

The only thing that outshines a Catto prop is the way Craig takes care of his customers!

Been flying with Catto 3-blade since 2003 and still happy!

i don't know....

I put that senenich GA composite on my RV4 and I am liking it alot.

It is performing very well and I can't believe the increase in acceleration from 0 to 80 knots. I would love to test it at sea level to 1000' msl versus the 7-8,000 DA I usually take off at.

The pitch can be tweaked in 15 mins too.

The only drawback is that its pretty light up front and maybe the cost. vs


Catto.

Cm

I've had the Sensenich composite GA prop on my plane for 100 hours now, and performance, smoothness and even the aesthetics have been fantastic...

However, a few weeks ago I noticed a fresh new crack just appeared in the paint on one of the blades. The crack is chordwise across the face of the blade, about 5/8" in length, starts about 1 7/8" from the leading edge, and is about 12.75" from the tip of the blade.

I sent hi-res digital photos of the crack to Sensenich, and they wanted to inspect the blades so I shipped them and Sensenich has had them for almost two weeks now, but I've not heard anything back yet.

interesting

I sure like mine I hope they don't have an issue. I figured if there was going to be a problem it would be in the hub not the blades...

I guess I will hang on to the catto for a while..

cm

I'm going to phone Charley tomorrow for a status update. They will have had the blades for a complete 2 weeks then. Not hearing anything from them this long is starting to worry me.

Great news from Sensenich today, the crack was only cosmetic... something about a small wrinkle in the surface layup under the paint and normal flexing of the blade made the paint crack only. Nothing structural and no harmonic issues. It's in their paint shop now.

good info

That's great. I REALLY like this prop
cm

I have been under the impression that composite props don't suffer from actual harmonic vibration. So, to the experts on the forum, is that true, or not?

I am not an expert, but like aluminum propeller manufactures, Whirlwind advises against continuous operation between 2050 and 2300 RPM for their composite props. Their manual explains why running low RPM may not be a good idea, even if the composite material can take the stress. The italicized text below was cut from Whirlwind's RV 200 manual.

Jay

Recommended Non-Continuous Operating Ranges
Whirl Wind follows historical industry standards and recommends
that continuous operation between 2,050 - 2,300 RPM and 2,600 -
2,700 RPM be avoided when used on Lycoming and similar four
cylinder aircraft engines. (Take off and climb out is not considered
continuous operation.)
This recommendation is based on the fact that the four cylinder
engines produce torsional vibrations on each power pulse (ignition
firing). These vibrations are transferred from the crankshaft through
the propeller hub to the propeller blades. Extensive research
conducted by propeller manufacturers has demonstrated that these
vibrations at engine speeds between 2,000 - 2,300 RPM and
greater than 2,600 RPM can cause increased vibratory stresses in
the propeller blades and in some cases this torsional vibration can
be close to the blade?s resonant frequency. Blades vibrating at their
resonant frequencies, or more likely higher order overtones, can be
stressed beyond their design limits and could result in limited or
complete failure. This phenomenon is a greater concern with
aluminum propeller blades because of their lower natural resonant
frequency. The natural dampening, high natural resonant frequency
and high fatigue resistance of Whirl Wind composite blades makes
the likelihood of destructive failure less probable, but to ignore the
RPM concerns recognized by the industry would be unwise.
Furthermore, flywheel dampening and / or crankshaft
counterbalancing can mitigate the torsional vibrations but not
eliminate them completely. It is only prudent to avoid operating in
these critical RPM ranges whenever possible.

It should also be considered that the propeller is
not the only component of the aircraft that experiences these
stresses; everything including the prop, engine and airframe are
subject to these stresses simultaneously and it is better for all of the
equipment to avoid these conditions.

Didn't Catto change their layup to carbon fiber or something to address the harmonic issue, if I remember, it was to address the SS leading edge cracking or something......

Harmonics

I have an RV7A with Lycoming 0-360 and the factory recommended Sensenich fixed prop. When not in a hurry and without a real destination, I often pull the throttle back to approx 2000 rpm for a 5.0 gph fuel burn. I'm not aware of any rpm ranges that are off limits for this combo, but the thread that references 'historical industry standards' got my attention.
So, is this an rpm range that should be avoided when going out for an evening flight around town?

Jim Diehl 7A #1976
Lock Haven, Pa.

bret said:

Didn't Catto change their layup to carbon fiber or something to address the harmonic issue, if I remember, it was to address the SS leading edge cracking or something......

Click to expand...

I don't think the change had anything to do with harmonics. The change from fiberglas to carbon fiber created a more sleek, lighter weight, and stronger prop.
A few of the early fiberglas props developed cracks in the nickel leading edges due to too much flexing. I have one of them hanging on my office wall.
The more rigid carbon fiber prop do not exhibit this problem.