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do continuous RPM restrictions still apply after dynamic propeller balancing?

KayS

Well Known Member
Hi All,

on my RV7 i installed a standard Hartzell 72" aluminum BA CS prop. the engine is an IO-375 low compression from aerosport which is kinda exotic. because of that i had an hard time to figure out the RPM restrictions for continuous cruise power with this engine/propeller combo. aerosport told me that the io375 has basically the same restrictions as the 360 but they can't say for sure.

i like more sedate power settings during cruise so the local A/P did the dynamic prop balancing at 2200 RPM. i don't recall the reached value in inches/second but he said that he never managed to get it that low before and the vibrations seem to me low at 2200.

info i have is that for a 360/hartzell continuous operation between 2000 and 2300 should be avoided. does that still apply after dynamic prop balancing in that RPM range and you're happy with vibration?

Kay
 
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The rpm limits recommended by Hartzell are not affected by dynamic balance on the safe assumption the original tested prop was within reasonable balance. The restrictions come from different physical phenomenon (angular resonance vs power impulse frequency and amplitude for instance)
 
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I’ve never seen this prop rpm restriction for the standard Hartzell 72” CS aluminum propeller. I too like to cruise “comfortably” and usually set my flight rpm at 2350. Please give a reference for this restriction so I can read about it.
 
The restriction is listed on the propeller type certificate and in the Hartzell Propeller manual, which you should have. It is not really just advice. If you stay inside of the RPM range, the prop stresses due to power pulses and blade resonance shorten the fatigue life of the blades drastically, potentially resulting in blade loss. The RPM limitation is 2000 to 2200 RPMs for the affected propellers.
For the affected propellers there is also a limit on the time you can spend between 2500 and 2700 RPMs.
 
I have the same engine as the OP with a non counter weighted crank and Hartzell CS prop. Hartzell said to use “Continuous Operation Is Prohibited Above 24” Manifold Pressure Between 2350 and 2550 RPM”. Hartzell have since recommended to not use an aluminum prop on this engine because of the crank. There are other threads discussing this. I have 240 hrs on it now with no sign of any problem but I’m picking up a new MT tomorrow.

I have not dynamically balanced the Hartzell and people have commented on how smooth they felt it was. I will dynamically balance the MT once I get it flying.
 
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info i have is that for a 360/hartzell continuous operation between 2000 and 2300 should be avoided. does that still apply after dynamic prop balancing in that RPM range and you're happy with vibration?

As others have noted, dynamic balance and propeller operating restrictions have nothing to do with each other. Operating restrictions deal with blade vibration, not balance.

I've attached two gifs (courtesy of Chrystal Instruments and Ziyuan Jiao, www.crystalinstruments.com) illustrating 1st and 2nd mode blade vibration. Running within the restricted range excites some mode (1st, 2nd, 3rd, etc) and pushes blade and/or hub stress to levels the manufacturer finds problematical. The simulation exaggerates the motion, but yes, this is really happening out there in front of you.

2nd mode on left, 1st mode on right. Blade twist and mass distribution complicates the mode shape, but basically the 1st mode is a bow, while the second mode forms an S-curve. A third mode would add another opposing curve.
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Bill: this is what i also had in mind. that the propeller balancing does not sufficiently affect these strange harmonics that lead to some RPM ranges to be avoided.

Jim and others: i have no idea where the actual RPM restrictions are. if you use the search function to see what is recommended for an io-360 with a hartzell you'll find all kinds of limitations. Dennis here just came up with a new one i was not aware of. and when you look at some Cessna, Piper and Mooney forums they come up with even more restrictions. at the end we have no RPM at all where we can run the engine between takeoff and landing. :D

if there would be some reliable advice i would be happy to follow that recommendations. is there any way that we find out for ourself with our specific setup? i feel that (besides displacement, compression ration, ignition timing etc. etc.) these resonant frequencies are affected by the phase of the moon.

btw: i called Hartzell once adressing the issue and they basically said "you're on your own with that engine."
 
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Dan: thanks for the great pictures!!!

would it be safe to say that below a specific manifold pressure you can run any RPM for extended period of time? let's say 24"...?

the one thing i am much more concerned of than an engine stoppage is when a propeller blade says goodbye.
 
is there any way that we find out for ourself with our specific setup? i feel that (besides displacement, compression ration, ignition timing etc. etc.) these resonant frequencies are affected by the phase of the moon.

btw: i called Hartzell once adressing the issue and they basically said "you're on your own with that engine."

Do you have one of the 375's without crank pendulums? If so, your best bet is to send it back and exchange the crank.

(Warning, soapbox mode ON)

EAB has become way, way too cavalier about propeller vibration. Knocking Hartzell for saying you're on your own is, well, not good. They, with MT and a few others, actually conduct the necessary testing with instrumented props, and if they have not tested your particular combination, their truthful answer is "We don't know". Other vendors simply guess, parroting Hartzell et al based on what they perceive as similarity.

None of this is new. Note the date on the blade mode illustration. Mr. Lurenbaum traveled here from Germany to present his research. Good thing he did.

BTW, you'll hear claims about wood, composite, and wood/composite props being immune to blade vibration. Fact is, everything vibrates. The real issue is amplitude. The maple prop below resonated and failed when excited at a particular RPM in a dyno cell, with poor glue lines being a contributing factor. A blade with different mass distribution and stiffness would behave differently.
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would it be safe to say that below a specific manifold pressure you can run any RPM for extended period of time? let's say 24"...?

There is no such blanket assurance.

the one thing i am much more concerned of than an engine stoppage is when a propeller blade says goodbye.

As you should.
 
KayS - when I contacted Hartzell before first flight in Jan 2017. They said “The only recommendation we have would be to follow the most restrictive placard” and quoted me the one I indicated in Post 5.
 
Dan: i don't remember what Hartzell told me exactly but it was for sure not "you're on your own" and then they hung up the phone. i exaggerated a bit. i recall that they mentioned that there's no test data available with that engine.

i do have the non counterweighted crankshaft and understood it was not necessary due to the lower compression of 1 : 7.8 for that engine version.


Do you have one of the 375's without crank pendulums? If so, your best bet is to send it back and exchange the crank.

(Warning, soapbox mode ON)

EAB has become way, way too cavalier about propeller vibration. Knocking Hartzell for saying you're on your own is, well, not good. They, with MT and a few others, actually conduct the necessary testing with instrumented props, and if they have not tested your particular combination, their truthful answer is "We don't know". Other vendors simply guess, parroting Hartzell et al based on what they perceive as similarity.

None of this is new. Note the date on the blade mode illustration. Mr. Lurenbaum traveled here from Germany to present his research. Good thing he did.

BTW, you'll hear claims about wood, composite, and wood/composite props being immune to blade vibration. Fact is, everything vibrates. The real issue is amplitude. The maple prop below resonated and failed when excited at a particular RPM in a dyno cell, with poor glue lines being a contributing factor. A blade with different mass distribution and stiffness would behave differently.
-
 
KayS - when I contacted Hartzell before first flight in Jan 2017. They said “The only recommendation we have would be to follow the most restrictive placard” and quoted me the one I indicated in Post 5.

that would be great. usually at altitude the MAP is lower than 24" anyway.

i have to admit that after some "internet" research the entire topic seemed very confsuing to me, so in someway i decided to ignore it at all for the last year. which isn't very wise.
 
Right - the MP/RPM restriction is not a problem unless your screaming along in a low level strafing run.
 
thanks all. my original question was answered... dynamic propeller balancing does not (unfortunately) affect RPM restrictions.

what comes to mind... if the problem is caused by the pulses from the engine's combustion process "hitting" the resonance frequency of the propeller blades, then lowering the manifold pressure should help.

do we further decrease the magnitude of the pulses from the engine if we run LOP as this decreases the internal combustion pressure?
 
It is not all rotational acceleration . . . .

The cylinder pressure acting on the connecting rod tends to spread the crank axially between the main bearings. That provides an axial displacement to the prop hub. Clearly the frequency will change with RPM, and the force/displacement will change with MAP and timing. While there is some damping of the blade resonance, excitation energy clearly drives actual blade forced displacement and resulting stress.
 
The current Hartzell docs only have these 2 limitations. Perhaps if we have an engine/prop combo that is similar it would be a good idea to avoid the ranges of RPM and MP that they list for other engines.

"Do not operate above 24" MP below 2450 RPM"

and

"Avoid continuous operation between 2000 and 2250 RPM. Operation above 2600 RPM is limited to takeoff. As soon as practical after takeoff, the RPM should be reduced to 2600 RPM or less."

Hartzell RPM Limitations.png

My personal method is 2700 at takeoff, drop to about 2500 after obstacles cleared for noise abatement, and then once I get to my target altitude, I drop it to 2350. Full throttle all the time until ready to descend. I'm not super consistent but perhaps avoiding dropping into these restricted ranges should be a consideration.
 
As others have noted, dynamic balance and propeller operating restrictions have nothing to do with each other. Operating restrictions deal with blade vibration, not balance.

I've attached two gifs (courtesy of Chrystal Instruments and Ziyuan Jiao, www.crystalinstruments.com) illustrating 1st and 2nd mode blade vibration. Running within the restricted range excites some mode (1st, 2nd, 3rd, etc) and pushes blade and/or hub stress to levels the manufacturer finds problematical. The simulation exaggerates the motion, but yes, this is really happening out there in front of you.

2nd mode on left, 1st mode on right. Blade twist and mass distribution complicates the mode shape, but basically the 1st mode is a bow, while the second mode forms an S-curve. A third mode would add another opposing curve.
-

Tangentially, you'd be amazed at how short the hop from here it is to talking about electomagnetic acoustic system tuning.
 
The restriction is listed on the propeller type certificate and in the Hartzell Propeller manual, which you should have. It is not really just advice. If you stay inside of the RPM range, the prop stresses due to power pulses and blade resonance shorten the fatigue life of the blades drastically, potentially resulting in blade loss. The RPM limitation is 2000 to 2200 RPMs for the affected propellers.
For the affected propellers there is also a limit on the time you can spend between 2500 and 2700 RPMs.

This whole propeller thread got me thinking and I read again my Hartzell Propeller Manual and Logbook. Upon re-examining it I still couldn’t find anything other than a statement relating to limitations being “engine specific and refer to your POH.”

Since we write our own POH’s I went to the Hartzell website and ask the question about my own specific engine and propeller.

They’re response was very specific to my engine and propeller saying as long as I had my current engine/propeller combination with magneto ignition I had no operating restrictions.

I’ll post their email in my propeller logbook and make an annotation in my POH. I appreciate this VAF thread and the thought provoking issue it identified. Hartzell was extremely quick to respond to my questions.
 
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