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Prop Balance - How Much Weight to Add?

Piper J3

Well Known Member
We are balancing a RV-12 prop with DynaVibe. It's our first experience doing this. The unit is telling us 0.4IPS x 50 grams starting point = 20 grams weight.

Does that sound reasonable?
How much weight have folks added for balancing?

Thanks in advance...
 
Right... what I'm wondering is how much weight folks have used to balance the Sensenich ground adjustable prop on RV-12. I'd like to see if we are in ballpark...
 
Jim,

I ended up with about 19g on the back plate of my -12, drilled at a 9" diameter to get .03ips

Friday I did a Lycoming 320 with a Hartzell 2 blade. With no weight is was .43ips. The finished solution was 17.8g on a 10" diameter to get .04ips.
 
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RV-12 Sensenich - DynaVibe

Mine was 19.4 grams at the 9" diameter, for 0.08 ips final.
 
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The guy who did mine added one #6 machine screw to the spinner bulkhead. Hardly seemed worth the effort.
 
my buddy ended up adding just 1 washer under an existing spinner #8 screw. His readings were fairly good to begin with.

On mine, I started with 0.30-0.34 IPS and ended up adding 13g at a hole that was appox 3/4" from the spinner backplate edge. End result was about 0.04 IPS

Incidentally, I rent out my DynaVibe for $100 to any builder that needs it (up to a maximum of $1400.. as I'm looking only to recoup costs and not make money off it)
 
Mine was .50 after adding 27 grams it came down to .05 I used a 1/4" NAS bolt and 2 OD washers and lock nut to mid span of spinner bulkhead 180 degrees opposite of heavy spot...piece of cake.
 
gram scale

I just finished balancing the Sensenich prop on my RV12. before the balance it registered .17 in/sec after balance we got it down to .05 in/sec.
Now the rub! I added one #8 x 1/2" screw with a nylon lock nut at 4.5" from the center of the shaft. My gram scale said that the screw and nut only weighed 3.8 grams. Everyone else seems to be coming up with much higher weights to do the balance.
I hope the scale is not wrong because it is the same one I use to weigh the carburetor floats and they came in at 6.7 grams for each pair.
Any thoughts?
 
The weight added will depend upon your starting vibration level. Yours was already relatively low to start with, thus not requiring as much counter weight.
 
Tonight, we just finished a friends RV-12 prop balance. See post #1 above. Started at 0.46 IPS and we added a single 14.96 gram weight at 4-1/4" radius to achieve 0.06 IPS. We flew the plane for about 45 minutes and noticed the improvement...
 
I hope the scale is not wrong because it is the same one I use to weigh the carburetor floats and they came in at 6.7 grams for each pair.
Any thoughts?

I purchased this cheapo gram scale for $22 from Amazon. Since it was high-quality Chinese manufactured, I also purchased this $12 calibration weight set.

I have to say that for a $20 scale, it performed very well.. and for the extra $12, I have peace of mind that the scale is accurate (IIRC, it was +/- 0.002g or better every time I tested it). I'd recommend both of these as required tools when doing a balancing (0.001g resolution is definitely overkill. 0.1g is fine)
 
I'm sure someone much more knowledgeable than I am will come along and set things straight, but here are my thoughts:

If something is vibrating (shaking) it has to start at zero, accelerate to whatever the max velocity is, then decelerate to zero, and then repeat many times per second.

I'm guessing that the velocity change, which is acceleration, has something to do with this. If there were no vibration, there would be no acceleration, and the result would be zero ips (inches per second).

What I DO know, is that the smaller the number, the better the result. :rolleyes:

BTW, I don't think this takes into account any Torsional vibration, but I could be wrong.

What other unit of measurement would work?

I hereby reserve the right to delete this post later, if necessary!:eek:
 
Does anyone have an explanation for why vibration is expressed with units of velocity?

It's somewhat arbitrary. You could use velocity, acceleration, displacement, jerk, vibration dose value (VDV), furlongs per fortnight, etc. In my experience I would have expected acceleration units such as mm/s^2 in this application.

Piezoelectric accelerometers produce a voltage that is proportional to acceleration. The fft analyzer can integrate that to velocity, double-integrate it to displacement, or differentiate it to Jerk, etc. Software can further post-process the data into a human comfort metric such as VDV.

In the automotive industry we usually use velocity in mm/s for customer touch-points - seat track and steering wheel for example. This is because velocity correlates well to human perception in the frequency range of interest.

For source or transfer path measurements, such as engine vibration or engine mount frame attachment points, we might use acceleration.

If we want to estimate mount stiffness in-situ we might put accelerometers on both active and passive sides of the rubber and use displacement.

For transient events like engine start-up shake or shift quality we would use VDV.
 
I always assumed it was the total distance travelled. That is, the peak to peak displacement times cycles per second.
 
Prop Balance

I have balanced several. Did one today. 21 grams to get it to .04 IPS with the weight mounted to the backplate.

Do yourself a favor. Don’t even bother to balance using weights under the spinner screws. The balance solution will change when you move the weight to the spinner backplate. You end up just doing it all over again. Take the time to drill holes and install #10 nut plates before hand. Put one at each screw location (10 total) leaving enough room to install an AN970 washer. The balance task will be fast and easy and you will end up using 2 or 3 of the holes.

I had balanced this prop earlier using weights under the spinner screws. The airplane owner decided to move the weight to the backplate. He did the calculations taking into account the difference in the arm and came up with the weight to make the same moment as the spinner mounted weights. We did a test run with the weights relocated. The recorded IPS was .26. This is more complicated than it seems. In the end, he ended up with holes in three locations.
 
You must have done something wrong with the calculation in the move or rotated the spinner etc. I have done many balances from Rotax to Lycoming. The move to the backplate or flywheel has never changed the ips more than .01.

I agree that having locations (nut plates) on the backplate would simplify the process but that is a lot of up front work to save a little time once a year.
 
I agree that having locations (nut plates) on the backplate would simplify the process but that is a lot of up front work to save a little time once a year.

I don't think I'd trust nut plates with fast spinning weight. Better to ream hole for correct AN bolt and then split weights (washers) front and back of spinner plate. The washers will trap the aluminum backing plate and distribute load evenly and bolt will not move in reamed hole...
 
I have balanced several. Did one today. 21 grams to get it to .04 IPS with the weight mounted to the backplate.

Do yourself a favor. Don’t even bother to balance using weights under the spinner screws. The balance solution will change when you move the weight to the spinner backplate. You end up just doing it all over again. Take the time to drill holes and install #10 nut plates before hand. Put one at each screw location (10 total) leaving enough room to install an AN970 washer. The balance task will be fast and easy and you will end up using 2 or 3 of the holes.

I had balanced this prop earlier using weights under the spinner screws. The airplane owner decided to move the weight to the backplate. He did the calculations taking into account the difference in the arm and came up with the weight to make the same moment as the spinner mounted weights. We did a test run with the weights relocated. The recorded IPS was .26. This is more complicated than it seems. In the end, he ended up with holes in three locations.

You must have done something wrong with the calculation in the move or rotated the spinner etc. I have done many balances from Rotax to Lycoming. The move to the backplate or flywheel has never changed the ips more than .01.

I agree that having locations (nut plates) on the backplate would simplify the process but that is a lot of up front work to save a little time once a year.

I agree.
We have done 30+ prop balances on RV-12's using a single weight repositioned onto the spinner bulkhead after calculating an offset (we use an Aces balancer which calculates it for you)

I don't think I'd trust nut plates with fast spinning weight. Better to ream hole for correct AN bolt and then split weights (washers) front and back of spinner plate. The washers will trap the aluminum backing plate and distribute load evenly and bolt will not move in reamed hole...

I agree with this as well. The spinner bulkheads are made from much softer 5052 aluminum. Because of this, I always install balance weight with a flat washer on each side of the bulkhead to maximize distribution area. In my opinion, drilling a bunch of unneeded holes in the bulkhead to install nutplates is just increasing the chance of developing cracks.
 
I don't think I'd trust nut plates with fast spinning weight. Better to ream hole for correct AN bolt and then split weights (washers) front and back of spinner plate. The washers will trap the aluminum backing plate and distribute load evenly and bolt will not move in reamed hole...

I think either way will work fine.

A nutplate has 2x 3/32 rivets for attachment. Combined cross sectional area is .0552 sq.inches and shear strength is 26,000 psi. The rivets in shear can hold 1435 lbs (not accounting for the extra holding ability that dimpling/countersinking imparts, or the friction cause by tightening the balance bolt down). Between the nut plate and the extra balance weight, you might be at 100 grams.

At 8" and 2700 RPM, a 100 gram mass exerts a load of 365 lbs, a safety factor of almost 4. The safety factor will be higher if you account for the friction and dimpling. Nuts and bolts will work just fine, but I don't think there's a reason to be scared of nutplates in this situation either.

EDIT: Just saw that Van's posted while I was typing this up. I guess if the home base suggests nut/bolt then thats the way to go.
 
does phase angle move?

Thanks for your explanation, BobbyLucas. I like that you said it was arbitrary--now I don't feel like I was missing something basic.

Time for another basic question, though. Does phase angle tend to change as you go though the trial solutions to arrive at a final balance? I can't think of why it would, but I've got enough grey hair to know that the world is more complicated than I think it is.

Does it make sense to use two holes that bracket the initial location so small angular deviations can be handled with slightly different weights? (e.g., analyzer says 0.5 IPS @ 180°, drill holes at 150° and 210° and start with 10 gm?)

thanks in advance,
-dbh
 
Remember that the unit us telling you where the HEAVY spot is, so you add weights 180 degrees from that angle, assuming you don’t have weights at that location to remove.
 
Does it make sense to use two holes that bracket the initial location so small angular deviations can be handled with slightly different weights? (e.g., analyzer says 0.5 IPS @ 180°, drill holes at 150° and 210° and start with 10 gm?)

thanks in advance,
-dbh

FWIW, I split locations in 98% of my balance jobs, unless you get extremely lucky and happen to have an existing hole in the location you need, splitting wts gives you the ability to move things around a bit. If you drill a single hole where you think you need it and you're off then you stuck.
 
Thanks for your explanation, BobbyLucas. I like that you said it was arbitrary--now I don't feel like I was missing something basic.

Time for another basic question, though. Does phase angle tend to change as you go though the trial solutions to arrive at a final balance? I can't think of why it would, but I've got enough grey hair to know that the world is more complicated than I think it is.

Does it make sense to use two holes that bracket the initial location so small angular deviations can be handled with slightly different weights? (e.g., analyzer says 0.5 IPS @ 180°, drill holes at 150° and 210° and start with 10 gm?)

thanks in advance,
-dbh

"Somewhat arbitrary." ;)

In theory, with a simple rotating single-plane imbalance (imagine a simple spinning disk on a short shaft sitting on your desk - not our airplanes) with no error in the measurement system and with infinite precision/accuracy in correction-weight mass and placement, you could go from any level of vibration to zero on the first try.

In practice, there are limitations imposed by the measurement system, available correction masses, and possible placement locations, among other sources of error and noise. Long story short, yes, every time you place a correction mass on the system you're changing the mass distribution and therefore the imbalance magnitude and possibly location(phase).

  • Place a mass in the exact correct location, but a little too light, and the phase will not change
  • Place a mass in the exact correct location, but a little too heavy, and the phase will change 180 degrees
  • Place a mass a few degrees off target and the phase will change
  • Adding mass at 0 degrees is the same as removing mass at 180 degrees, so you never strictly need correction masses opposite each other (unless you just can't get the exact mass needed I guess)
  • Also remember that imbalance (g-cm, for example) is mass x distance from axis of rotation. The further away from the rotation axis the mass is placed, the more effect it will have

For your last question, I will defer to those that have gone though the process on our planes. (Looks like Walt answered as I'm typing) My engine and propeller have not shipped yet. I will say, that if you split the masses you will need more total mass, and the further apart you split them, the less effective they will be, requiring even more mass.

Sorry for the long-winded response. :eek:
 
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And just to add a bit more to it, no prop/spinner/mounting system is exactly like any other for example a FP prop vs a CS and a million other variables.

Most pro balancers like the Microvib I use and ACES use a test weight on the second run to characterize the effect of the added test weight. Basically you're telling the computer exactly how much weight you added and the location you added it, then the box is able to calculate the effect of the test weight and recommend the final wt/location based on that characterization.

I can do the average balance in 3 runs, if you have to add a lot of weight then often times a 4th run is required to tweak the solution.
 
Man you guys are talking "martian"! I was hoping that the balance process would be somewhat understandable to a novice but I guess not!
 
Man you guys are talking "martian"! I was hoping that the balance process would be somewhat understandable to a novice but I guess not!

  1. Mount accelerometer and phototachometer
  2. Run engine and record data
  3. Install test weight
  4. Run engine and record data
  5. Remove test weight and install recommended correction weight
  6. Run engine and record data one last time to verify that you nailed it the first time
  7. Have an adult beverage

Better? :D
 
  1. Mount accelerometer and phototachometer
  2. Run engine and record data
  3. Install test weight
  4. Run engine and record data
  5. Remove test weight and install recommended correction weight
  6. Run engine and record data one last time to verify that you nailed it the first time
  7. Have an adult beverage

Better? :D
#7 i have nailed!
 
I'm getting ready to do the prop balance. The mechanic that is going to do this has a DynaVibe. He has done several Lycomings and Continentals, but not a Rotax. I just observed him doing a Lycoming. There was a convenient bolt in the top of the case to mount the accelerometer.
I have not removed my top cowl yet, but trying to remember the layout. Is there a good place to mount the pickup and accelerometer? Or better yet, how have you guys done it?
 
Getting ready to balance prop, my mechanic uses Chadwick Helmuth equipment and has no RV-12 prop balance experience. Can anyone share photo of equipment mounting and Chadwick experience I can share with him prior to work?
 
Getting ready to balance prop, my mechanic uses Chadwick Helmuth equipment and has no RV-12 prop balance experience. Can anyone share photo of equipment mounting and Chadwick experience I can share with him prior to work?

The Chadwick is going to mount the same as the Dynovibe, mount the accelerometer as close to the prop as possible and the optical sensor 6 - 12” back from the prop close to 12 o’clock.
 
Dick, I wasn’t as successful as Walt with placing the reflective tape on the back of the spinner backplate. The surface was too reflective so the optical sensor couldn’t distinguish the tape from the spinner. I had to move it to the base of the prop. Turns out I needed to place a weigh at the location of where the tape had been located anyway, so it worked out better on the prop for me. Just sharing my experience.
 
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I did have that problem in a different way. The spinner backplate is flat grey so it didn’t reflect but when I added the final weight which consisted of a large washer my recheck was crazy. I finally realized the washer was randomly sending a second reflection. A touch of grey paint on the washer brought the readings back to normal.
 
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