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  #1  
Old 10-06-2016, 03:22 PM
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snopercod snopercod is offline
 
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Location: Asheville, NC
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Default Tip: Wooden Prop Static Balancing Stands

In order to determine the source of some excessive vibration in my wooden prop, I built myself a couple of stands so I could re-check the static balance and tracking. This was supposedly done when I had this Aymar-Demuth prop refurbished, but I just wanted to check it myself. I built these stands in two halves for easy storage, and to allow spreading them apart or pushing them closer together as desired.

I made each stand out of 2x4s with both the base and height roughly 4 ft.



On top of each stand is a 4" piece of 1-1/2" x 1-1/2" x 1/8" aluminum angle, to which I bolted two Plastic Ball Bearings with Stainless Steel Ball, for 3/8" Shaft Diameter, 1-3/8" OD from McMaster-Carr at $7 each. Each one is rated at 45 lbs. and they spin freely.



I cut wooden spacer blocks to hold the two halves apart, and also serve as a base to check the tracking and length of each blade:



At first I used a piece of 1" tubing through the center hole of the prop, but I discovered that the hole in the prop was 1/32" off center in relation to the prop bolt circle, and also drilled at a slight angle:



The solution for the misalignment was to make up a pair of brackets with two 3/8" holes 4-3/4" O.C. with a 1/2" center hole exactly between the two in both dimensions. I did this on my drill press using my X-Y micrometer vice. Using a 1/1-2" aluminum bar with a 3/4" rod would have been better, but I didn't have any handy.



This setup turned out to be super sensitive. The slightest breeze would cause the prop to turn like a pinwheel, so to get any meaningful readings I had to close the hangar doors and even block off the gap under the door.

The blades were the same length within 1/64", but the tracking was off by maybe 1/8". It was a little out of balance but a coat of black spray paint on the back side of one blade fixed that. Yes, it's that sensitive.

By fitting AN960-6 flat washers in the holes in the spinner back plate, I could even check the balance of the back plate.

Sadly, although my prop is perfectly balanced now, the vibration still exists in flight - maybe due to the mis-tracking. Next step: Dynamic balance. If that doesn't help, then I guess I'm looking at a new prop.
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Old 10-06-2016, 05:06 PM
humptybump humptybump is offline
 
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Location: USA
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I used a similar setup after restoring my Aymar Demuth propeller. I used skate bearings. I created the false hub in CAD with a center shaft and 3D printed it. Out of curiosity I later 3D printed a false hub for my Catto prop.

https://vimeo.com/50006571
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Old 10-06-2016, 08:01 PM
thorpdrvr thorpdrvr is offline
 
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Location: San Antonio, TX
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If you balance with the blades at 3 o'clock and 9 o'clock it shows that the blades are equal weight. But the hub can be out of balance even with the blades in balance. In other words with the prop in the 3 and 9 position, the top of the hub may not be the same weight as the bottom of the hub. To check if the hub is balanced, you move the blades to the 1:30 and 7:30 position and check for any rotation. If the 7:30 blade goes down towards 6 o'clock, the hub is heavy on the top side. You can try the balance in different orientations, just put the blades in the 45 degree position relative to horizontal. If one side of the hub is heavier, just sand some off the heavy side or spray clear finish on the light side.

I have had my wood prop balanced at a prop shop and they used a hanging cable balancer. That type shows the balance of the blades and hub all at the same time. The rotational type of balancer you built requires the extra step to get the hub balanced.

I have balanced many large radio control wood props where the blades are in balance but the hub is not.
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Old 10-07-2016, 07:06 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by humptybump View Post
I created the false hub in CAD with a center shaft and 3D printed it.
You have a 3D printer? I'm jealous! You did it the right way to use the hub for a center. So why did you change from the Aymar-Demuth to the Catto, if I may ask?
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Old 10-07-2016, 07:11 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by thorpdrvr View Post
If you balance with the blades at 3 o'clock and 9 o'clock it shows that the blades are equal weight. But the hub can be out of balance even with the blades in balance.
I see what you mean. Thanks for that information. After flying with my static-balanced prop, it was still vibrating a little. So I removed the fiberglass spinner and flew once around the pattern. Amazingly, it was better. I had already balanced the aluminum back plate and it's hard for me to see how a lightweight spinner could make that much difference. Hopefully the upcoming dynamic balance will take that out.
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Old 10-07-2016, 08:12 AM
BobRv4 BobRv4 is offline
 
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Try getting the track closer before doing the dynamic balance.
You can bias the torque on the prop bolts to move the tips.
The dynamic balance will take care of span and cord balance, you can tell where the issue is by the clocking angle.
Also you can check the pitch of each blade, to make sure they are both equal.
This type of vibration shows up in flight, if it gets worst under high loads, like climb under full power, or goes away when you are in a slight dive, zero thrust, then look at the blades. Good luck.
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Old 10-07-2016, 09:05 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by BobRv4 View Post
Try getting the track closer before doing the dynamic balance. You can bias the torque on the prop bolts to move the tips.
Could you elaborate on that a little bit? My prop is supposed to be torqued to 17 ft.lb. Should I tighten the nuts a little tighter on the side with the leading blade?

Quote:
Originally Posted by BobRv4 View Post
...you can tell where the issue is by the clocking angle.
Good idea. I'll ask the guy with the balancing equipment about that.

Quote:
Originally Posted by BobRv4 View Post
...you can check the pitch of each blade, to make sure they are both equal.
I could certainly do that with my prop stands, but I haven't yet. I'm kind of afraid of what I might find. This prop is almost 20 years old and has been stored for most of that time. If it's warped, what can I do (except order a new Catto)?

Quote:
Originally Posted by BobRv4 View Post
This type of vibration shows up in flight, if it gets worst under high loads, like climb under full power, or goes away when you are in a slight dive, zero thrust, then look at the blades. Good luck.
I'll take more detailed notes next time I fly.

I tried clocking the prop in all possible orientations WRT #1 TDC and the best that I found was the 10:30-4:30 position. I think that's what most folks recommend for a 4 cylinder engine.
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Old 10-07-2016, 04:27 PM
BobRv4 BobRv4 is offline
 
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Default Answers

1. Loosen all nuts, tighten the leading side to 20lbs, I torque mine to 35, so 20 is fine. Then tighten the other ones, try 17 on these see if that helps. If that doesn't help very thin gasket paper or stainless steel tape under the one side of the prop. So you are placing a shim under one half of the prop hub.
2. By the blades it's span 90/270 is cord.
3. Unless you want to re carve maybe waste of time.
4. Bring #1 up on compression then move to TDC. Put prop vertical, move CW one bolt hole so prop is at 1 o'clock, looking aft. When the engine shuts off the prop should end up at 10 oclock, in the perfect position to prop it.
Hope this helps. Let me know if you have more questions.
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Old 10-08-2016, 05:40 AM
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snopercod snopercod is offline
 
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Thanks Bob, I'll try torquing the bolts as you suggested and see if I can fix the tracking. As a last resort, I'll shim it. I didn't clock my prop for hand propping because it's a high compression engine and I doubt if I could. My prop is currently mounted one lug CCW with #1 at TDC. I'll let you know.
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Old 10-09-2016, 01:09 PM
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snopercod snopercod is offline
 
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I checked the tracking and it was perfect. I guess the last time I torqued the prop a couple days ago, I must have used the magic sequence. The airfoil shape at the tips was also identical. So I believe I've done all I can do prior to the dynamic balance.

Speaking of dynamic balance, I found a good reference: ACES Systems Guide to Propeller Balancing. The part that jumped out at me was where they said if your Lycoming engine has a "long" prop extension (mine has a 4" extension), then the balance weights need to be placed on the spinner back plate rather than the ring gear. The last time I had a balance done, the guy put the weights on the ring gear.
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