My airplane is ready!

[FlyingMonkey]

Or just about, anyway.  It has been at Lockwood aviation in FL for a couple of weeks.  I had done:

 

* Annual condition inspection

 

* Trutrak Digiflight II VS autopilot installed (originally from Wm.Ince's CTSW). 

 

* OAT probe moved from bottom of fuselage to correct position in wing inspection port.

 

 

Along the way Lockwood found:

 

* A SB on one of the autopilot servos, they told me about this before I even showed up so I got the servo fixed by Trutrak before arriving.

 

* They got me a free new fuel pump, apparently Rotax has some free upgrade going for us troglodytes still using the original style pump.

 

* I had them check the prop balance, and it was way out...max spec deviation is 0.2, mine sat at 0.66, so they are balancing that out.

 

I will have more info on their findings once I get down there, but so far I have been very pleased with their service and level of communication during the work.  I'll update this thread after I pick up the airplane.  Once the prop is balanced Lockwood is going to test fly it and I'm driving down Sunday and will pick it up Monday (or Tuesday if the weather is bad). 

 

 

[Anticept]

FYI the unit of measurement for prop balance is inches per second, and .2 is the industry standard. If able, getting to less than .1 is preferred.

[FlyingMonkey]

FYI the unit of measurement for prop balance is inches per second, and .2 is the industry standard. If able, getting to less than .1 is preferred.

 

So then, as I suspected, 0.66 just...sucks.

[Anticept]

So then, as I suspected, 0.66 just...sucks.

 

No, that's actually not that terrible for such a small prop and engine. That would be atrocious for a larger engine. IPS is a measurement of acceleration, and as we know from physics, the amount of resultant force also needs a mass variable. It matters so much to really large prop engines (500 hp or more, both recip and turbine), that the FAA considers it a major repair to dynamically balance such an installation if there are no manufacturer balancing instructions.

[FlyingMonkey]

Lockwood said they found an incorrectly placed balancing weight on the prop assembly. Removing that brought the balance from 0.66 to 0.226. Adding a new weight in the correct position got the balance to 0.054. I should definitely notice that difference! Hoping to fly the airplane back Monday if weather permits.

[Doug Hereford]

Anticept,

Actually, according to the current consensus as I understand it, the FAA considers dynamic propeller balance (I prefer to think of it as "rotating mass" balance) to be an "Airframe" major alteration. Here is a practical reason for this approach (Standard Airworthiness):

As A&P's, we are not allowed to perform major repairs or alterations to propellers. Repair stations also have to be appropriately rated. Typically, the dynamic balance procedure involves adding weights to a convienent place near the largest rotating mass component (propeller area). This is done by adding trim weights to the spinner bulkhead. In 99% of cases, the spinner bulkhead is an Airframe part.

To refer to this procedure as prop balancing, is mis-leading in my opinion. Actually what we are doing when we are performing a balance, is attempting to balance a propeller (and its rotating accessories) to a particular engine (and its rotating accessories).  We are actually not performing propeller maintenance/alteration, or engine maintenance/alteration at all. We will typically alter the spinner bulkhead by drilling a hole (holes) and adding weights (standard hardware), to bring the total balance vector into acceptable limits.

Major alterations do require approved data (standard airworthiness). So far, the FAA has considered balance equipment instructions to be in that category, as well as service info from the propeller mgf's. There are also a few airframe MFG who publish data for their specific aircraft. This would also be approved.

In the case of SLSA, we cannot perform this alteration without manufacturer approval, and in accordance with their procedures. ELSA, game on!

All,

I am not a good sales person for dynamic balance, however I feel that the absolute most important thing that can come from this procedure is a specturm analysis report (summary of vibration levels at various frequencies). I like to think of this like oil analysis. One report is a snapshot, and virtually worthless. Several reports constitute a signature for the machine. At this point, trends can be monitored, and potential maintenance issues can be predicted, and addressed on-condition.

 

Andy,

If I were you, I would see if Lockwood has such a report on your aircraft, and request a copy for your historical records.

 

 

Doug Hereford

[Anticept]

Anticept,

Actually, according to the current consensus as I understand it, the FAA considers dynamic propeller balance (I prefer to think of it as "rotating mass" balance) to be an "Airframe" major alteration. Here is a practical reason for this approach (Standard Airworthiness):

As A&P's, we are not allowed to perform major repairs or alterations to propellers. Repair stations also have to be appropriately rated. Typically, the dynamic balance procedure involves adding weights to a convienent place near the largest rotating mass component (propeller area). This is done by adding trim weights to the spinner bulkhead. In 99% of cases, the spinner bulkhead is an Airframe part.

To refer to this procedure as prop balancing, is mis-leading in my opinion. Actually what we are doing when we are performing a balance, is attempting to balance a propeller (and its rotating accessories) to a particular engine (and its rotating accessories).  We are actually not performing propeller maintenance/alteration, or engine maintenance/alteration at all. We will typically alter the spinner bulkhead by drilling a hole (holes) and adding weights (standard hardware), to bring the total balance vector into acceptable limits.

Major alterations do require approved data (standard airworthiness). So far, the FAA has considered balance equipment instructions to be in that category, as well as service info from the propeller mgf's. There are also a few airframe MFG who publish data for their specific aircraft. This would also be approved.

In the case of SLSA, we cannot perform this alteration without manufacturer approval, and in accordance with their procedures. ELSA, game on!

All,

I am not a good sales person for dynamic balance, however I feel that the absolute most important thing that can come from this procedure is a specturm analysis report (summary of vibration levels at various frequencies). I like to think of this like oil analysis. One report is a snapshot, and virtually worthless. Several reports constitute a signature for the machine. At this point, trends can be monitored, and potential maintenance issues can be predicted, and addressed on-condition.

 

Andy,

If I were you, I would see if Lockwood has such a report on your aircraft, and request a copy for your historical records.

 

 

Doug Hereford

 

Advisory Circular 20-37e, page 28:

 

 

 ( c ) For aircraft or propeller manufacturers that provide procedures for dynamic balancing of the propeller in their maintenance manuals or instructions for continued airworthiness, propeller balancing is not considered a major airframe alteration.

 

(d) When approved aircraft or propeller manufacturer’s procedures are not available, there are other acceptable dynamic propeller balancing procedures. These include, but are not limited to the Chadwick-Helmuth Publication No. AW-9511-2, entitled “The Smooth Propeller”, and ACES Publication No. 100-OM-01, entitled “ACES Systems Guide to Propeller Balancing”. Dynamic balancing of propellers using FAA-approved or -accepted dynamic propeller balancing procedures is not considered a major propeller repair unless the propeller static balance weights are altered or when using the Chadwick-Helmuth or ACES type documents on propeller installations of 500 horsepower or more.

 

 

According to that advisory circular, it could be either or, depending on circumstances. I find it very odd how ( c ) is worded though, it implies that any kind of balancing without manufacturer instructions are major alterations???

 

As for balancing the prop, I have permission from FDUSA to use the Chadwick Helmuth or ACES procedures.

 

Also, I believe that Chadwick Helmuth and ACES both say if a prop is more than 1.25 ips out of balance, it should be taken off and sent in.

[GlennM]

IPS is a measurement of acceleration, and as we know from physics, the amount of resultant force also needs a mass variable.

IPS are units for velocity not acceleration.

[Anticept]

IPS are units for velocity not acceleration.

 

In this case we're measuring the rate of acceleration, and could care less about the velocity. I know that mathematically, It should be "inches per second per second" or IPS^2, but for some odd reason prop balancing just uses plain old IPS. Maybe the squared is implied from the fact the manuals state we are measuring acceleration.

 

At least, that's how I was taught. It could very well be that I was taught wrong, but I'm not sure. I don't know angular velocity physics as well as I would like, and I know the math gets a little odd vs linear velocities.

[Doug Hereford]

I don't know much about physics, but according to my three year old, the villain in Despicable Me 2 was named "Vector". He commits crimes with both direction and magnitude. IPS without phase is useless for dynamic balance, regardless of what it is called.

 

Doug Hereford

[Anticept]

I don't know much about physics, but according to my three year old, the villain in Despicable Me 2 was named "Vector". He commits crimes with both direction and magnitude. IPS without phase is useless for dynamic balance, regardless of what it is called.

 

Doug Hereford

 

The phototach senses the prop position during the rotation, and the device solves where to place the weights based on the vibration. The computer tells us what position on the propeller to install or remove the weights as well as the IPS measurement. As you said, neither direction nor magnitude are useful without the other, but that's why once we are done, we mark the prop, spinner, and bulkhead for orientation, and record the final IPS measurement in the logbook.

 

Generally, if the IPS exceeds 1.25, it's a strong indication that there is much more wrong with the assembly than just an out of balance prop. I believe the Chadwick Helmuth manual states that the prop should be sent in for examination if it's that bad.

[Doug Hereford]

Yep,

That's how mine works. Isn't IPS technically just speed? I think if I was bored, I could convert the speedo in my truck to read IPS. Phase equals direction, IPS equals magnitude....................................Vector.....................................Freeze Ray!

 

Doug Hereford

[Anticept]

Hi Doug!

 

Check this out about vibration:

 

http://www.ccohs.ca/oshanswers/phys_agents/vibration/vibration_measure.html?print

 

Just reading the first section, "How can you measure vibration?", is enlightening. And yes, IPS is just constant velocity, which is why I had said a couple posts before, it should be IPS squared.

[Doug Hereford]

Yea,

I'm not buying it. But really to me it is completely unimportant. I actually don't think IPS are acceleration or velocity. I do think that velocity comes closer to defining it from a practical sense. To me IPS are just jiggilies. If one is performing a dynamic balance, the goal is to get to the lowest number of jiggilies.

 

Doug Hereford

[Anticept]

Right, but if you don't understand the measurement of the jigglies, how do you know if there's a bigger problem than just an out of balance prop?

 

If they want to call it IPS, I'll use IPS. If they want to call it jigglies per two pi rad, I'll use that. Someday I'll sit down and learn a bit more about the physics behind it.

[Doug Hereford]

This where the spectrum analysis report I mentioned earlier comes in. If Mr Morden can get one of these reports for his plane, it will show vib levels (jigglies) at various different frequencies (RPM's). One report isn't very helpful (similar to a single oil analysis report), but if the dynamic balance procedure is repeated again at some convenient time in the future, a jiggly comparison can be made, and potential problems predicted and corrected on-condition.

That would be one way to know if there is a problem  bigger than just a prop balance issue.

If the balance procedure was conducted at 2200 RPM, and vibration levels were high at 1100 RPM (for example), one might suspect something in the valve train. Other frequencies could be observed and compared to known rotational ratios for the specific engine (ie gear box on 912's), and those potential issues could also be predicted. In my experience, engine mount problems usually give forth order vibration level changes, but if I have a vibration that moves around, I also suspect mount issues or propeller static weight shift. 

Anyway, in my opinion dynamic balance has its greatest value as a diagnostic tool. Turbine smooth is just a nice side effect.

 

Doug Hereford

[FastEddieB]

A while back a Cirrus owner wrote an app to measure vibration levels, though I'm not finding it now.

 

Did find thus, however:

 

https://itunes.apple.com/us/app/vibration/id301097580?mt=8

[FlyingMonkey]

Regardless of all the technical aspects involved, the difference between a (now) properly balanced prop and an improperly balanced one is pretty dramatic.  I used to be able to see the "mushroom" and my throttle lever vibrating during flight, now they are perfectly calm.  I'm also sure all that vibration was taking a toll on structural parts and fasteners as well.  I highly recommend that anybody who has not had their prop balance checked to have a reputable shop do it...I had no idea what I was missing!