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.
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.
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.