Dynamic Prop Balance

[bumper]

ips stands for inches per second, and is a measure of vibration amplitude. A linear accelerometer is installed, hopefully near the front of the engine and near centerline. Then a photo-optical sensor is installed to sense a piece of reflective tape on the prop. The computer then knows how much vibration amplitude there is, the prop rpm, and where the peak vibration amplitude is in relation to the reflective tape. It can then recommend a "trial weight" size and location. When the trail weight test is run, the computer will then recommend a new (and hopefully much more accurate) trial weight and location.

 

The aircraft should be faced into the wind and with no obstructions upwind to cause turbulence in the airflow through the prop. Best results are obtained when max wind is less than 10 knots, less than 5 is even better. Some props are more susceptible to turbulence interference than others (the Stemme S10-VT prop comes to mind as being particularly touchy).

 

I purchased my Aces 2020 computer balancer back when the cost almost 5K . . . there are doubtless cheaper good alternatives now, though the 2020 still works just fine.

 

Renting or loaning out a balancer can be a dubious proposition. Drop a linear accelerometer (the device that measures vibration amplitude) and it's toast - - and they aren't cheap.

 

Normally, weights are not attached to the prop. It is acceptable practice to *properly* attach weights to the spinner back plate. 

 

When doing the test runs, it's often most expedient to simply attach the weights to the existing spinner attach screws (or longer screws if required). Then there's a formula to calculate the additional weight required as the final weight will be installed at a lesser radius.

 

bumper

[Shadrach]

ips stands for inches per second, and is a measure of vibration amplitude. A linear accelerometer is installed, hopefully near the front of the engine and near centerline. Then a photo-optical sensor is installed to sense a piece of reflective tape on the prop. The computer then knows how much vibration amplitude there is, the prop rpm, and where the peak vibration amplitude is in relation to the reflective tape. It can then recommend a "trial weight" size and location. When the trail weight test is run, the computer will then recommend a new (and hopefully much more accurate) trial weight and location.

 

The aircraft should be faced into the wind and with no obstructions upwind to cause turbulence in the airflow through the prop. Best results are obtained when max wind is less than 10 knots, less than 5 is even better. Some props are more susceptible to turbulence interference than others (the Stemme S10-VT prop comes to mind as being particularly touchy).

 

I purchased my Aces 2020 computer balancer back when the cost almost 5K . . . there are doubtless cheaper good alternatives now, though the 2020 still works just fine.

 

Renting or loaning out a balancer can be a dubious proposition. Drop a linear accelerometer (the device that measures vibration amplitude) and it's toast - - and they aren't cheap.

 

Normally, weights are not attached to the prop. It is acceptable practice to *properly* attach weights to the spinner back plate. 

 

When doing the test runs, it's often most expedient to simply attach the weights to the existing spinner attach screws (or longer screws if required). Then there's a formula to calculate the additional weight required as the final weight will be installed at a lesser radius.

 

bumper

In my case, we kept chasing the ideal location...add weight, run... add weight to opposite side...run, add weight to opposite side, etc...

 

I discovered 2 issues afterwards; all of which may have been related:

 

1)  loose injector (like chattering around, not even finger tight loose)

2) cracked case

 

I theorise that the loose injector may have caused an intermittent intake leak and the resultant vibes may have caused the crack over time (crack was just aft of the generator pad).

 

I look forward to getting the new engine done when the weather warms up! 

[aaronk25]

In my case, we kept chasing the ideal location...add weight, run... add weight to opposite side...run, add weight to opposite side, etc...

I discovered 2 issues afterwards; all of which may have been related:

1) loose injector (like chattering around, not even finger tight loose)

2) cracked case

I theorise that the loose injector may have caused an intermittent intake leak and the resultant vibes may have caused the crack over time (crack was just aft of the generator pad).

I look forward to getting the new engine done when the weather warms up!

A cracked case oh man......bad news...can't believe the cases don't hold up better! What do you think culprit is with these cases cracking????

[Shadrach]

A cracked case oh man......bad news...can't believe the cases don't hold up better! What do you think culprit is with these cases cracking????

 

Time and vibration.  I think that older non-counterbalanced 360s can be even more vulnerable due to the weight of the generator.  It's a rigid mount, and all of the that vibration get's transmitted directly to the mounting pad on the lower part of the passenger side case half...and right at the parting flange.  I was devastated at the time.  880SMOH on an engine that was seemingly running well until it started bleeding oil on the heat exchanger. I did get to see that the engine was in good shape at tear down (cam, lifters and crank were all in good shape) Under $4K for the IRAN all in, excluding my labor and it's running better than ever.

[aaronk25]

We'll not to bad then especially since as long as it was apart you hit to look at all the parts!

Not that your looking at selling but a bird like yours would almost guarantee a potential buyer that the stuff in the lower part of the engine is in good shape......

[Awqward]

Seth, there is an IA from Binghamton NY area that comes to your airport. I believe he comes down to MD. He will also supply a 337 to go with if required.  IIRC it was about $325 if you can line up several planes for his trip.

 

Al

Is a 337 required? Ie is balancing considered a Major Alteration or Repair??

[Awqward]

Just going through the maintenance records I see that the prop was dynamically balanced in April 2010 with a reading of 0.05 IPS....must explain why it feels so smooth....any ideas on how often the balance should be checked?

[bumper]

In my case, we kept chasing the ideal location...add weight, run... add weight to opposite side...run, add weight to opposite side, etc...

 

I discovered 2 issues afterwards; all of which may have been related:

 

1)  loose injector (like chattering around, not even finger tight loose)

2) cracked case

 

I theorise that the loose injector may have caused an intermittent intake leak and the resultant vibes may have caused the crack over time (crack was just aft of the generator pad).

 

I look forward to getting the new engine done when the weather warms up! 

Assuming conditions are reasonable (.i.e low wind, no turbulence, airplane into the wind - and "set up" and balance procedures are done properly), then being unable to achieve a satisfactory balance can be indicative of mechanical problems, a broken motor mount, bad damper, something untoward going on in the engine can cause issues. I once couldn't get a decent balance on a C185. Unbeknownst to the owner,  the previous owner had a weld repair done on a cylinder and that weldment intruded significantly into the combustion chamber, apparently raising compression well higher than the other jugs . . . that is what he told me after the fact, I didn't get a chance to look at it though. Some balancers will display a percentage of error, or interferance, on the balance screen. On mine that will typically be 3% to 5% if all is well.

 

If you have a good balance done, and nothing changes, no grease slinging, prop dings filed out etc. it should stay smooth 'till it gets rough :c)

 

bumper