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).