spin balancing a tail rotor

Seems like you are describing a method for
balancing a 3-blade system since it requires
placing weights at 120, 240, and 360 degrees
successively but at the same radius.

There should be a similar method for a two-blade
system which involves placing the weights at
different radii and plotting the vibration using
the dial indicator.

Or am I missing something??

Don W.

Stuart & Kathryn Fields wrote:
Don: It turns out that the Russians were doing this, or something like it,
for some time
The method is basically make a run and record the peak dial indicator
reading. Add a weight to an arbitrary designated zero position. Make
another run and record that dial indicator reading. Now move that weight to
a position 120 degrees around from the zero position and make sure the
weight is installed at exactly the same distance from center that it was at
the zero position. Record the dial indicator reading and move the weight to
the 240 position, again at the same radial distance from the center and
record. Assign a graphical scale to the dial indicator readings and plot
first a circle representing the reading without a weight. Next plot a
second circle centered on the first circle at the zero degree position. Now
plot the next circle similarly at the 120 position on the first circle and
again the last circle centered on the 240 position. You should have an
intersection of all of the circles at one point. The distance from the
center of the first circle to this point represents that amount of weight
needed to correct the imbalance. The angle to this intersection represents
the angle measured from where you added the first weight to this
intersection point and designates the location to add the weight. In actual
practice you don't always get a point at the intersection but an small area.
The center of this area is used as the intersection point. I have used this
on the tail rotor of my helicopter successfully but used and accelerometer
instead of the dial indicator. The guys article in the magazine had color
plots of his graphical calculations.

Don W wrote:
Seems like you are describing a method for balancing a 3-blade system
since it requires placing weights at 120, 240, and 360 degrees
successively but at the same radius.

There should be a similar method for a two-blade system which involves
placing the weights at different radii and plotting the vibration using
the dial indicator.

Or am I missing something??

You can probably add the weights at whatever angle is most convenient
(if the rotor hub has a balance wheel with three holes evenly spaced at
120 degrees, or four at 90 degrees...) and then plot the circles
accordingly. My understanding is the solution will give you how much
weight to add and where (in degrees from your reference) to add it, and
that you add the solution weight at the same radius you added the test
weight (probably a washer or washers).

I gather the number of blades is irrelevant to this method. Besides,
adding and subtracting weights to two opposing blades only affects
spanwise balance (not chordwise, which is usually much smaller anyway).

You are pretty much correct. I did a conversion from three blade to two
blade in an article in the magazine. The catch is assuming the zero
position corresponds to a spot down the chord line of the blade, you have to
scale the weight up when you move it to the 90 and 270 degree spots to have
the product of the distance from the center times the weight to be the same
as that used at the zero position. Also the correction weight displayed in
the graphic solution assumes the installation will occur at the same
distance from center used in the zero position. If this is not true, then
it must be scaled to obtain the correct weight times distance product.
There is a minor problem using the dial indicator as the sensor for the main
rotor in that most main rotor balancing is done at flight speed and with the
helicopter in hover. It makes the reading of the dial indicator tough.
Also finding a stable mount for the dial indicator is more tricky than using
a velocimeter or accelerometer.
The graphical method is very handy when the phase angle measurements are
flaky, which does occur if you are anywhere near a critical speed of the
rotating system. I've experienced problems with balancing my tail rotor
using the phase angle measurements when the helicopter is setting on a
concrete or asphalt pad. On grass it is OK . The graphical technique
ignores the concrete issue.

Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478
(760) 408-9747 general and layout cell
(760) 608-1299 technical and advertising cell

www.vkss.com
www.experimentalhelo.com


"Jim Carriere" wrote in message
...
Don W wrote:
Seems like you are describing a method for balancing a 3-blade system
since it requires placing weights at 120, 240, and 360 degrees
successively but at the same radius.

There should be a similar method for a two-blade system which involves
placing the weights at different radii and plotting the vibration using
the dial indicator.

Or am I missing something??

You can probably add the weights at whatever angle is most convenient
(if the rotor hub has a balance wheel with three holes evenly spaced at
120 degrees, or four at 90 degrees...) and then plot the circles
accordingly. My understanding is the solution will give you how much
weight to add and where (in degrees from your reference) to add it, and
that you add the solution weight at the same radius you added the test
weight (probably a washer or washers).

I gather the number of blades is irrelevant to this method. Besides,
adding and subtracting weights to two opposing blades only affects
spanwise balance (not chordwise, which is usually much smaller anyway).

Jim Carriere wrote:
Don W wrote:

Seems like you are describing a method for balancing a 3-blade system
since it requires placing weights at 120, 240, and 360 degrees
successively but at the same radius.

There should be a similar method for a two-blade system which involves
placing the weights at different radii and plotting the vibration
using the dial indicator.

Or am I missing something??


You can probably add the weights at whatever angle is most convenient
(if the rotor hub has a balance wheel with three holes evenly spaced at
120 degrees, or four at 90 degrees...) and then plot the circles
accordingly. My understanding is the solution will give you how much
weight to add and where (in degrees from your reference) to add it, and
that you add the solution weight at the same radius you added the test
weight (probably a washer or washers).

I gather the number of blades is irrelevant to this method. Besides,
adding and subtracting weights to two opposing blades only affects
spanwise balance (not chordwise, which is usually much smaller anyway).

I was missing something. I didn't realize that
there was a balance wheel on the hub. It makes
sense now.

Don W.