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#1
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We recently received an article for our magazine where the author performed
a balance of his tail rotor on a bench using an electric motor and a dial indicator instead of an electronic balancer. I was impressed with his thinking outside the box. If there is any interest in this out there, I will see if the author would be willing to discuss his technique here. -- Stuart Fields Experimental Helo magazine P. O. Box 1585 Inyokern, CA 93527 (760) 377-4478 ph (760) 408-9747 publication cell |
#2
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Sounds neat. I'd like to hear how he did it. It
seems that the dial indicator would tell you how good the balance was, but would not be very good at telling you which side was heavier. Don W. Stuart & Kathryn Fields wrote: We recently received an article for our magazine where the author performed a balance of his tail rotor on a bench using an electric motor and a dial indicator instead of an electronic balancer. I was impressed with his thinking outside the box. If there is any interest in this out there, I will see if the author would be willing to discuss his technique here. |
#3
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Don: It turns out that this guy used the information in an article written
for our magazine by Larry Meidell , who according to Chadwick is a supremely talented helicopter vibration analyst, and that technique does not require phase angle information. Evidently the technique was originated, used and written up by Russians who did not have access to an electronic balancer at the time. I have used the same technique on some balancing that I have done using instead of the dial indicator just the accelerometer without using any of the phase angle information. It works. The technique involves a bit of graphics and drawing circles. Our September 2006 issue of the Experimental Helo magazine had this guys article and the original article by Meidell was in the July 2005 issue, with a follow on article written by me on the adaptation of the technique to a two rotor system which appeared in the Sept 2005 issue. If you have access to these issues of Experimental Helo, there is a fairly complete story on the process. -- 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 "Don W" wrote in message om... Sounds neat. I'd like to hear how he did it. It seems that the dial indicator would tell you how good the balance was, but would not be very good at telling you which side was heavier. Don W. Stuart & Kathryn Fields wrote: We recently received an article for our magazine where the author performed a balance of his tail rotor on a bench using an electric motor and a dial indicator instead of an electronic balancer. I was impressed with his thinking outside the box. If there is any interest in this out there, I will see if the author would be willing to discuss his technique here. |
#4
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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. -- Stuart Fields Experimental Helo magazine P. O. Box 1585 Inyokern, CA 93527 (760) 377-4478 ph (760) 408-9747 publication cell "Don W" wrote in message om... Sounds neat. I'd like to hear how he did it. It seems that the dial indicator would tell you how good the balance was, but would not be very good at telling you which side was heavier. Don W. Stuart & Kathryn Fields wrote: We recently received an article for our magazine where the author performed a balance of his tail rotor on a bench using an electric motor and a dial indicator instead of an electronic balancer. I was impressed with his thinking outside the box. If there is any interest in this out there, I will see if the author would be willing to discuss his technique here. |
#5
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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. |
#6
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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). |
#7
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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. |
#8
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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). |
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