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Balancing Works
Now this just makes my pacemaker flutter. David Moon read the articles we published on balancing; both the ones with amplitude only and the ones about the dueling Balancers. He provided the following, which I find intriguing and educational.
This correspondence is in regards to the excellent articles in EH, 07/2005 and 09/2005 on Single Plane Balancing using Amplitude Measurement; just in time for my tail rotor. Your follow up article Stu, 07/2006, is, as I said, excellent except for one thing, that $2000 - 3000 balancer. And you think you’re on a tight budget, retired, fixed income, so on and so on? Me too. Not to be foiled, I have a dial gauge, a lot of incentive and like the Russians, am on a tight budget.
Step 1: Safety, Safety, Safety.
Body parts come off very easily and quickly and don’t look good when glued back on. As with any rotating mass, shaft, rotor, etc, be extremely careful, mistakes are very unforgiving.
Step 2: Set-up fixtures as shown in pictures, reconfigure as necessary.
Attach one hp 220v with 3-½ inch variable speed pulley to a Hercules tower and pulley assembly to a T/R shaft coupling to the T/R assembly.
The direct connection of the shaft coupling, using the nylon star washer, transferred too much vibration from the motor and belt configuration, so I inserted the heavy duty hose as a coupler; worked great.
Test the set-up, slowly at first, less than 100 rpm, until you get used to it. After each run up, check all your mountings —every time. (Editor’s Note also count fingers if you are a bit clumsy).
Motor, Tower Pulley Assembly and Tailrotor Assembly
Step 3: My first balancing runs were to 1600 rpm. Too much vibration above that, not to mention I could not read the blur on the gauge.
1st Run – no weight - .021 inches
Step 4: 2nd Run - 18 gms at 0 degrees - .026 inches. First weight trial was 24 gms, at 0 degrees, no problem, but at 120 degrees, gauge was unreadable, so I backed off to 18 gms, worked great.
Step 5: 3rd Run - 18 gms at 120 degrees - .054 inches
Step 6: 4th Run - 18 gms at 240 degrees - .018 inches
Step 7: Select scale and do the math.
Scale = .001 inches = 100
Balance plate marked to keep track of test weight position
Therefore 1st run = .021 x 100 = 2.100 inches Radius
2nd run = .026 x 100 = 2.600 inches Radius
3rd run = .054 x 100 = 5.400 inches Radius
4th run = .018 x 100 = 1.800 inches Radius
Draw the base circle - 2.100 inches
Draw the 0 degree circle (# 1 position) - 2.600 inches
Draw the 120 degree circle (# 5 position) - 5.400 inches
Draw the 240 degree circle (# 9 position) - 1.800 inches
Find the common point -
Draw three lines from the intersecting arcs to each other, creating a triangle.
Draw three lines from the mid points of the previous three lines to their corresponding opposite triangle points.
Voila - From the intersection of these three lines, a point. From the base circle center to this point = 2.878 inches @ 284.48 degrees. If one could put 18 gms at this point, the rotor would be in balance @ 1600 rpm, but, we have to move it to the balance weight bolt circle.
A little math = 2.878 / 2.1 x 18 = 24.69 gms at the bolt circle. If one were to mount this mass (24.69 gms) on the bolt circle at 284.48 degrees, the rotor would be in balance @ 1600 rpm. Now we just have to move a corresponding amount of this weight to the # 10 and # 11 positions.
By paralellograming we get two more lines and ....some more math.
Draw a line parallel to the # 10 position line (270 degrees) and a line parallel to the # 11 position line (300 degrees) from the new point (2.1 inches @ 284.48 degrees) to the # 11 and # 10 lines respectively.
Balance plate marked to keep track of test weight position
From these intersect points to the bolt circle center are the scalar distances required to calculate the masses required at positions # 10 and # 11 on the bolt circle.
Mass at # 10 position (270 degrees) = 1.12 / 2.1 x 24.69 = 13.12 gms
Mass at # 11 position (300 degrees) = 1.05 / 2.1 x 24.69 = 12.35 gms
Did it work? You bet your sweet bippy!! The attached pictures show the rotor at 2200 rpm. One hp is not enough to drive the 822 gm SS (each) blades to 2700-3000 rpm; probably need 1 ½ hp, with 0 pitch. BTW, if one should reach in to adjust the pitch control arm (just out of curiosity) it really blows and sucks; if there’s anything loose laying around, you may be the recipient of a new implant — be careful.
Also, as part of the learning phase, the tail rotor was first balanced with no blades, just the mounting hardware, and secondly, some mock-up wooden blades I made (too much free time) (247 gms each) at 2800 rpm. Put your hand on your desk, that’s how smooth it was, just a faint whirring sound.
I’m so confident with this method — hand me a picnic table on a pole and I’ll balance it.
Whelp, gotta go. If you get a lead on main rotor blades, say eight - nine inches cord, 12 feet long, 6-6 ½ degree twist, NACA-0012, 8H12, 23012, or 23013 (clockwise rotation), bits, pieces, chips or chunks; I’m interested. Don’t worry about the grip ends or tip weights, I’m very innovative.
Hum, how about an 1/8 inch stainless steel 12 inches diameter disc with 1/4 inch holes every 10 degrees on an 11 inches bolt circle, mounted on the main rotor head just above the bearing end play cross-shaft — for balancing, maybe?
I like this new procedure.
Thanks again Stu for your excellent articles. You solved that one!!
C’Ya
Muskie
AKA - David S. Moon
muskie@ciaccess.com
I hope the above is useful. If questions arise, contact Stu @ (760) 377-4478 or eh@iwvisp.com
