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  #21  
Old 06-21-2021, 01:52 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by ksouthar View Post
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.
Quote:
Originally Posted by seagull View Post
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)

Quote:
Originally Posted by Piper J3 View Post
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.
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Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
FAA/DAR
RV-6A (aka "Junkyard Special ")
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  #22  
Old 06-21-2021, 02:45 PM
agent4573 agent4573 is offline
 
Join Date: Aug 2019
Location: Mountain view
Posts: 602
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Quote:
Originally Posted by Piper J3 View Post
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.
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  #23  
Old 06-28-2021, 10:00 AM
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dbhill916 dbhill916 is offline
 
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Location: Westerville, OH
Posts: 181
Question 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
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  #24  
Old 06-28-2021, 10:21 AM
Bob Y Bob Y is offline
 
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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.
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  #25  
Old 06-28-2021, 10:49 AM
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Walt Walt is offline
 
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Quote:
Originally Posted by dbhill916 View Post

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.
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EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags, MTV-9 prop
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  #26  
Old 06-28-2021, 10:51 AM
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BobbyLucas BobbyLucas is offline
 
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Quote:
Originally Posted by dbhill916 View Post
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.
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Last edited by BobbyLucas : 06-28-2021 at 11:53 AM.
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  #27  
Old 06-28-2021, 12:32 PM
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Walt Walt is offline
 
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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.
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EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags, MTV-9 prop
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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  #28  
Old 06-29-2021, 10:48 AM
JwWright57 JwWright57 is offline
 
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Location: Charlotte, NC
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Man you guys are talking "martian"! I was hoping that the balance process would be somewhat understandable to a novice but I guess not!
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  #29  
Old 06-29-2021, 12:59 PM
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BobbyLucas BobbyLucas is offline
 
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Quote:
Originally Posted by JwWright57 View Post
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?
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  #30  
Old 07-05-2021, 04:18 PM
JwWright57 JwWright57 is offline
 
Join Date: Dec 2016
Location: Charlotte, NC
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Quote:
Originally Posted by BobbyLucas View Post
  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?
#7 i have nailed!
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