A reason to avoid dynamic propeller balancing?

[Pasturepilot]

14 hours ago, JimB said:

Is your 0-360 counter weighted? I thought the only engines with counter weights were the ones with a 6 in the suffix like the M20J engines. (IO-360-A1B6D or IO-360-A3B6D or IO-360-A3B6)

Crap. No, it’s not counterweighted. 
 

Cant believe I let the guy send me down that rabbit hole! 
 

thanks for pointing that out. 

[Pasturepilot]

41 minutes ago, Cody Stallings said:

Anything that Spins needs to be balanced. Human body can’t feel certain vibrations, but your Crankshaft an everything aft can.

3 issues with Dynamic Balance.

1) Old Timers don’t wanna hear About it, it wasn’t available back in their day an everything worked fine. 
2) Viewed as a Gimmick, sorta like MMO Conversation. People think if they don’t feel a very pronounced difference in Vibration, then they were Had!!

3) An in my opinion only. A balance Machine in the hands of an Individual that isn’t properly trained, an equipment isn’t kept Calibrated is Dangerous.

A balance gone wrong can be detrimental to a Crankshaft, Dampeners, Avionics an the overall well being of the Rotating assembly. 
Not trying to throw stones at anyone at all

If you have the Ability to have your propeller balanced by a tech that is trained an uses equipment that’s not covered with dust, do it.

You won’t be disappointed.

Thanks, Cody. Great information here. 
 

Gonna be getting lined up for a balance tomorrow. Between having had the prop off for a new alternator belt and crankshaft seal, new isolator mounts, and shimming the thrust line up, it’s time. 
 

Not to mention that I have a wedge of foam in my compass mount to keep it from dancing. 

[EricJ]

21 hours ago, PT20J said:

Dynamic balancing corrects the prop/spinner combination for a condition where the center of mass is not in line with with the center of rotation. Balancing may or may not produce a discernible improvement in perceived vibration, but it’s still a good idea as it may increase the life of accessories and other components.

An extreme example of a dynamically unbalanced propeller is ground resonance in a helicopter. An airplane propeller would not become this unbalanced (unless it lost part of a blade), but it is illustrative of how the propeller vibration transmits to the airframe.

 

Skip

FWIW, ground resonance doesn't have to do with imbalance in the rotor system, just energy resonance (the usual mechanical resonance phenomenon) between the rotors, the ground, and the springiness/damping of the landing gear or skid system.   It's a well-studied and interesting phenomenon.

My understanding is that the vid below was an intentional introduction of landing gear conditions that induced ground resonance (very successfully).   The rotor system was not imbalanced.   I used to use this vid (along with several others, like the Tacoma Narrows bridge collapse) as illustrations of resonance when I taught engineering courses.   This is still relevant to the current discussion, though, as the yellow or red arcs on tachs are typically due to resonant regions where torsional vibration is an issue.    Once resonance starts happening, it can do a lot of damage, as seen in the above vid and this one.
 

 

[PT20J]

18 minutes ago, EricJ said:

FWIW, ground resonance doesn't have to do with imbalance in the rotor system, just energy resonance (the usual mechanical resonance phenomenon) between the rotors, the ground, and the springiness/damping of the landing gear or skid system.   It's a well-studied and interesting phenomenon.

My understanding is that the vid below was an intentional introduction of landing gear conditions that induced ground resonance (very successfully).   The rotor system was not imbalanced.   I used to use this vid (along with several others, like the Tacoma Narrows bridge collapse) as illustrations of resonance when I taught engineering courses.   This is still relevant to the current discussion, though, as the yellow or red arcs on tachs are typically due to resonant regions where torsional vibration is an issue.    Once resonance starts happening, it can do a lot of damage, as seen in the above vid and this one.
 

 

Agree — mostly. Ground resonance is a hazard with articulated rotor systems having three or more blades. It can occur during start up or with a somewhat hard touchdown which upsets the lead/lag balance of one or more blades causing the CG of the rotor system  to go off center and setting up the oscillation. Normally, the dampers in the landing gear will damp out the coupled motion to the airframe and the rotor system rebalances and the oscillation quickly subsides. If one or more of the landing gear dampers is defective, the oscillation can build exponentially with positive feedback between the airframe and the rotor system unless the helicopter is immediately lifted clear of ground contact whereupon the rotor system will rebalance.

Skip

[GeeBee]

I can tell you for a fact, my Ovation came new from the factory balanced. In fact it is on page 1 of the logbook it so states that fact.

 

 

[Pasturepilot]

Got my balance done today. Went from .51IPS to .05. 

 

I can read my compass now. 

[Will.iam]

Bring up this older thread. I got my prop balanced yesterday. It started off at 1.2! At the limit in the balancer manual for needing to do a static balance before doing a dynamic. Since my annual is not until Aug and i just got my governor back from that pesky AD I didn’t relish the idea of downing the plane for the static balance. Upon opening the spinner cone to access the backplate we noticed a balancing weight on the heavy side already. We removed that and it brought the ips to .9 then adding that same weight to the opposite side got us down to .4 and i could really feel the difference in how smooth it was getting. We then added double the amount of washers to the weight and got the ips to .07 idle and .13 at 2700 with .15 at 2400 and .09 at 2200. The manual says anything below .2 is good and at this point the angle to add more weight is shifting 45 degrees from the present location. We called it good enough for now until i get to my annual where I’ll get the prop static balanced and then re dynamic balanced to a tighter tolerance. I can definitely tell a difference and the prop balance guy said it’s the worse out of balance he has seen to correct. My take on this is that I didn’t know how smooth the engine could run until I experienced the engine running after the balance was done. I didn’t ever notice a “buzz in the rudder pedals” and the yoke vibrations were not distracting and seemed normal until they were gone after the rebalancing. Now i know what to expect from a balanced system. Now that it’s gone it’s better to know I’m not rattling my avionics and airframe unnecessarily. 

[Tim Jodice]

On 6/29/2020 at 2:15 AM, N201MKTurbo said:

It is due to torsional vibration. I don’t think many people understand what that is. It is the prop speeding up and slowing down with respect to the crankshaft. This twists the crankshaft and can cause it to fail. It has to do with the moment of the propeller and the power pulses generated by the pistons along with the torsional compliance of the crankshaft. This sets up resonant modes that can be destructive. The red arc on the tach are there to keep you out of these resonant modes. 

BTW these torsional vibrations don’t shake your engine and you really cannot feel them.

An MT propeller removes all RPM restrictions. Is it simply because is is lighter or does it have to do with the material going from aluminum to wood?

[N201MKTurbo]

22 minutes ago, Tim Jodice said:

An MT propeller removes all RPM restrictions. Is it simply because is is lighter or does it have to do with the material going from aluminum to wood?

I don’t know for sure, but I would assume it is because it is lighter. It’s moment must be so low that any torsional vibrations are so small that they don’t matter.

[EricJ]

4 hours ago, Tim Jodice said:

An MT propeller removes all RPM restrictions. Is it simply because is is lighter or does it have to do with the material going from aluminum to wood?

FWIW, the Hartzell top prop removes the restriction as well, and it's heavier than the original prop.

 

[carusoam]

Great pirep Will!

Sounds like the prop came off once, and got put back on 180° out of phase....

Something to consider each time the prop gets removed...

There must be a way to mark a reference.... for knowing years from now...

PP thoughts only, not a mechanic...

Best regards,

-a-

[A64Pilot]

On 6/30/2020 at 12:08 AM, PT20J said:

Agree — mostly. Ground resonance is a hazard with articulated rotor systems having three or more blades. It can occur during start up or with a somewhat hard touchdown which upsets the lead/lag balance of one or more blades causing the CG of the rotor system  to go off center and setting up the oscillation. Normally, the dampers in the landing gear will damp out the coupled motion to the airframe and the rotor system rebalances and the oscillation quickly subsides. If one or more of the landing gear dampers is defective, the oscillation can build exponentially with positive feedback between the airframe and the rotor system unless the helicopter is immediately lifted clear of ground contact whereupon the rotor system will rebalance.

Skip

Pretty much exactly it, The AH-64 is supposedly immune to ground resonance as it’s been “tuned out” by design, in theory it would take both a flat tire, and a collapsed strut to allow an Apache to get into ground resonance.

Just as a comment if a pilot recognizes it in time and is at 100% rotor, ground resonance can be stopped instantly by taking off quickly, once contact with the ground is broken, the resonance stops immediately.

‘Anyway fast forward to when we did Carrier qualifications for the Ah-64, the Navy won’t “break chains” until your at 100%, we were concerned that being chained down to the deck could initiate ground resonance, but it didn’t.

As far as dynamic balancing a prop, I put a three blade Hartzell on my Maule and never could get it smooth, you couldn’t balance it out as it wasn’t a 1 to 1 vibration.

‘I flew once with a Avia Propellor Engineer from the Czech Republic, he said it’s pretty common on aircraft engines that have a lot of time with two blade props, that he believed the crankshaft and bearings wear to the two blade and when you go to a three blade, you’ll never get it smooth.

Maybe this is what the OP was saying about don’t balance an engine prop combo that has a lot of hours on it? 

‘For whatever it’s worth as a longtime helicopter guy, dynamic balancing makes everything better, everything last longer, I mean all bearings, avionics, everything, you can’t be too smooth.

Edited March 26, 2021 by A64Pilot

[A64Pilot]

17 hours ago, N201MKTurbo said:

I don’t know for sure, but I would assume it is because it is lighter. It’s moment must be so low that any torsional vibrations are so small that they don’t matter.

I think wood does an excellent job of dampening vibrations, and while it usually upsets some to hear it, the MT is a wood prop

[N201MKTurbo]

25 minutes ago, A64Pilot said:

I think wood does an excellent job of dampening vibrations, and while it usually upsets some to hear it, the MT is a wood prop

I attended a maintenance seminar about repairing MT props. I learned all about mil-spec lag screws and repairing stripped out holes with mil-spec match sticks....

[A64Pilot]

This is the MT prop I have the most experience with and may be their largest?

117” I think, coupled to a PT6-67 engine at 1700 SHP.

The Hartzell outperformed it, and was much better suited to conditions where the aircraft was going to, but the customer chose the MT because it looked cool.

 

 

[Hank]

4 hours ago, A64Pilot said:

This is the MT prop I have the most experience with and may be their largest?

117” I think, coupled to a PT6-67 engine at 1700 SHP.

The Hartzell outperformed it, and was much better suited to conditions where the aircraft was going to, but the customer chose the MT because it looked cool.

Fugly planes need a cool prop, right?  

[A64Pilot]

I guess it’s fugly, but not any fuglier than the AH-64. We worked with an Air Force unit in the late 80’s. they were interested in how we “did” nights. One commented on how he now understood now why we flew so much at night, we were too ugly to be allowed out in the daytime.

‘Later we found out that they were the F-117 squadron, now talk about an ugly airplane. I wish I knew then what they flew.

 

[spistora]

Looks like a well armed crop duster.

[EricJ]

36 minutes ago, spistora said:

Looks like a well armed crop duster.

Maybe it has to deal with horselubber grasshoppers or palo verde beetles or similar critters.

[Skates97]

23 hours ago, carusoam said:

Great pirep Will!

Sounds like the prop came off once, and got put back on 180° out of phase....

Something to consider each time the prop gets removed...

There must be a way to mark a reference.... for knowing years from now...

PP thoughts only, not a mechanic...

Best regards,

-a-

My AP/IA uses a super high tech method. He makes a couple marks with a sharpie so he knows how to line it back up again. I have similar marks on the edge of the spinner so it goes back on the same way it came off. 

[Hank]

10 hours ago, Skates97 said:

My AP/IA uses a super high tech method. He makes a couple marks with a sharpie so he knows how to line it back up again. I have similar marks on the edge of the spinner so it goes back on the same way it came off. 

My spinner, the little pieces that close off the openings that slides over the blades, and the root of each blade are all numbered in sharpie to put back together the same way, and preserve the balance. It's difficult to confuse 1, 2, 3 . . . .

Not sure what's behind the prop, it's only been off once to replace the alternator belt since I bought it in 2007.

[201Steve]

19 hours ago, EricJ said:

19 hours ago, spistora said:

Looks like a well armed crop duster.

Maybe it has to deal with horselubber grasshoppers or palo verde beetles or similar critters

Texas hog control

[Hank]

3 hours ago, 201Steve said:

Texas hog control

We got wild hogs here in Sweet Home, too . . . .

[201Steve]

4 hours ago, Hank said:

We got wild hogs here in Sweet Home, too . . . .

It is a recreational sport in Texas to mow down the hogs from a helo with an AR15

[Will.iam]

21 hours ago, Skates97 said:

My AP/IA uses a super high tech method. He makes a couple marks with a sharpie so he knows how to line it back up again. I have similar marks on the edge of the spinner so it goes back on the same way it came off. 

Maybe but i think when the previous owner removed prop boots because one side had a tear I don’t think the prop was rebalanced and I suspect that it’s been out of balance ever since.