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Tip: Wooden Prop Static Balancing Stands

snopercod

Well Known Member
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.

u0thbI.jpg


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.

eHCzuw.jpg


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:

iaYGFU.jpg


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:

93AvuA.jpg


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.

Rj9snk.jpg


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.
 
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
 
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.
 
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?
 
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.
 
Balance

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.
 
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?

...you can tell where the issue is by the clocking angle.
Good idea. I'll ask the guy with the balancing equipment about that.

...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)?

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.
 
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.
 
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.
 
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.
 
Aces

I use the aces 2020 to balance props, with very good luck.
Always use the ring gear, since I don't want to to drill holes in the back plate, and good place to start a crack. I think you are fine with the ring gear. If you wanted the weight on the backplate, you would first use the ring gear then when you get a good solution transfer the weight to the backplate. You need to adjust the weight based on the distance from the center of the crank. More distance, less weight, less distance more weight, moment arm is the reason. I would first try and get you to move the prop before I'd balance it. To me that is just a know starting point for me. I forgot you can also put weight on the prop bolts but that usually takes a bunch of weight, plus you only have 60 degrees or 6 holes to work with.
 
I sure appreciate your advice here because I've been floundering around for a while. So you think I should clock the prop one lug CW of #1 TDC rather than CCW like I did? I guess I'll try that if I'm not happy with the present clocking next time I fly. Sadly, the guy who was going to dynamic balance my prop with the ACES 1015 backed out. I emailed ACES and asked them if there was anyone in the Asheville, NC area with a 2020.

I was out your way about a year ago (with a different prop). I stopped for Oxygen in Tucson, then popped over to Marana to visit a friend. Marana Low Pass. I love the desert!
 
Zero friction is a lot easier to achieve (and a lot cheaper) with a 'knife edge' balancer.
http://www.featherlightppg.com/wp-content/uploads/2012/04/MG_5268.jpg
http://www.aquacraftmodels.com/hardware/includes/tuning-images/aqub9765ai04m-lg.jpg

All you have to do is get the two knife edges parallel and level in both directions; no bearings needed.

Try the google for more details.

Balance weight location, when dynamically balancing: There is no need to drill the back plate to add weights. The simple technique is to use longer spinner-attach screws, and add the weights (washers/locknuts) to the screw(s) behind the nutplate ('inside' the flange of the back plate).

FWIW,

Charlie
 
Prop

Would I change the prop? Won't hurt, may not help, but for me it's a known starting point. I've balanced helicopters where I give up, take all weights off, zero everything start from scratch, and within a couple runs get it smooth.
to me it just elimates one variable.
You really are wasting time balancing just the prop, soon as you add the bolts, backplate, spinner, extension it all changes. Bolts can change weight by a gram or two, which will screw up the balance. Think of balancing a tire, then you mount it on the rim? Stop at FFZ next time and the balance is on me.
 
You really are wasting time balancing just the prop, soon as you add the bolts, backplate, spinner, extension it all changes.
I was surprised just how sensitive this whole process is. With my balancer I could have installed all the bolts/nuts/washers, crush plate, and spinner back plate...and even the prop extension if I had a longer center rod. If I end up doing this all over again, I'll do it that way.

Yesterday I emailed ACES and one of their techs called me this morning. He is going to send me the name of someone in Concord, NC (90 NM away) who owns a 2020 and (hopefully) be willing to do the balance. (Thanks for the offer. You've been very helpful!)
 
Prop

You are very welcome.
If it's Mike L. at aces tell him Bob from Falcon has been talking with you.
He's very good at It!
The only way I know to check track is with it on the engine.
I think you could build something to balance the whole works, but it's easier to balance on the aircraft. Good luck, send me the ips and clocking when you balance, would be interesting.
 
That's true, but you can't check tracking that way can you? Total cost for my balancer was about $50, more or less.

As Bob said, you can't check tracking in the balancer; only when it's actually mounted on the engine. (And don't forget that the mount has built-in offset; measure to a single reference point.)
 
Prop

Get everything right before balance. Try and balance at cruise RPM, this is usually hard with a fixed pitch prop, since most won't Rev that high. Just try to use full throttle.
But to answer the question, won't tell me anything, I won't even worry about it.
Most props, have a sweet spot they run the smoothest, some have areas to avoid (2100-2200 example). So balance it at one rpm, closest to cruise as you can then call it good.
When you have it balanced, do a baseline run write all runs down.
Reset the box and remove all weights, if there are no weights start run 2, don't reset box.
Add weight, if it's over 12 grams (or so) split weights.
Should take 3 to 4 runs, if all the stars are aligned!
Let me know!
 
I have a Nov. 1 appointment at Extreme Aviation at 8A7 (NE of Charlotte) to get my prop balanced with an ACES 2020 unit. I spoke for a moment with Brian Meadows about prop clocking and he recommended what you did: One lug CW at #1 TDC. I asked him whether that was for hand propping or some other reason. He replied that it was so the prop was in the best position too accept the power pulse from the engine. I'm a little unclear on that concept, but will re-clock the prop as you and he recommend before flying over there.
 
Prop

Not sure 100% why the prop is clocked like that but it works for me.
Good luck let me know what happens.
 
Not sure 100% why the prop is clocked like that but it works for me.
I flew the plane yesterday with the prop clocked at the 10:30-4:30 position. There was vibration at ~1300 RPM and a little at 1800 RPM during the runup. At full power during takeoff and climbout, it wasn't too bad. (I was in the pattern with a B-17!)

All things considered, I would fly with it that way, but when I got back to the hangar I removed the prop and spinner and re-clocked it one lug Clockwise to the 1:30-7:30 position at #1 TDC. I haven't flown with it that way yet, but that's the way the balance shop wanted it.

So after over a month of checking everything I could think of and clocking the prop every which way, I've got it back to the original position. Sigh...
 
yep.

I have a Nov. 1 appointment at Extreme Aviation at 8A7 (NE of Charlotte) to get my prop balanced with an ACES 2020 unit. I spoke for a moment with Brian Meadows about prop clocking and he recommended what you did: One lug CW at #1 TDC. I asked him whether that was for hand propping or some other reason. He replied that it was so the prop was in the best position too accept the power pulse from the engine. I'm a little unclear on that concept, but will re-clock the prop as you and he recommend before flying over there.

You well be fine with this appointment. We have a Prince 6879 on a 195 Hp. IO-360 that we have balanced the back plate the middle plate, the spinners as a unit, and the prop, both ways and on a bearing stand. What has been said by Bob is correct, once you put the bolts, screws and washers in it changes ever so slightly. You still need to do the dynamic balance after all that. I will try to be there to set in on your balance. Harmonics is a funny thing, what has a small vibration at 1300 RPM's may be just perfect at 2300 RPM's. Ours does. Hope to see that big smile on you face after the fact.
Yours, R.E.A. III #80888
 
Hope to see that big smile on you face after the fact.
Me too. This whole thing has been very depressing for me. When I "finished" my plane in 2014, I thought then I could just go fly it. Silly me. I've had to deal with one problem after another and after fixing each one I keep thinking, "This will be the last thing I have to deal with." This time for sure :confused:

Looking forward to meeting you in a couple weeks. Hopefully the weather will cooperate.
 
Two days ago I installed the prop in the "recommended" 1 o'clock-7 o'clock position, first lug CW from #1 TDC. So today I rechecked the bolt torque (it was fine), put the spinner and cowling back on and went flying for 0.8 hrs. The vibration was just terrible - much worse than before - and at all RPM up to 2750. It was real bad at 1600.

So I intend to drop back to the last good position - the first lug CCW from #1 TDC - where the vibration wasn't that bad. The balance guy will just have to deal with it.
 
Prop

Ok, please keep us updates after the balance.
I really don't understand what's going on, with that big of change due to clocking.
Good luck.
 
Update - no dynamic balance yet

Update: My Nov. 1 prop balance didn't happen; I was unable make the 40 min. flight to the shop due to low clouds and fog over central NC pretty much all day. We rescheduled for yesterday but as I was strapped into my plane just about to fire her up, my cell phone rang. It was Brian from Extreme Aviation telling me that he had a family emergency and wouldn't be able to balance my prop that day. So now I'm waiting to hear back from him. For over two weeks the flying weather has been almost perfect (except for Nov. 1), but we're about to enter a bad period so I don't know when I'll get this done. Maybe somebody is trying to tell me that I should just buy a Balance Master instead?

In the meantime, I'm flying with the prop clocked in the 11-5 position (one lug CCW of #1 TDC) and the vibration isn't terribly bad.
 
I have another possible method to static balance a prop which I think may be more sensitive.

It should work for 3 blade props too. The idea is to suspend the prop by a mono-filament fishing line. The fishing line offerer virtually no resistance to bending, and thus a near perfect balance point. Have the prop suspended in the horizontal plane at its centerline. That supporting point can be a circular disk, centered on the prop hole, to which the fishing line is attached at its center. This method will reveal any imbalance for a 2 or 3 blade prop. For a 3 blade, it will reveal the direction of the imbalance even when not along a particular blade. Also this will not have starting friction or bearing friction as might the roller arrangements.
 
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