After reading last week’s editorial about Manchester’s proposed windmill bylaw, I became curious. How did turbine height become the most important measurement for setbacks?
At first glance, it doesn’t seem unreasonable. The model zoning bylaw from the Mass. Division of Energy Resources includes guidance for such a condition (presumably in case the tower falls over).
But what is the goal of setbacks in the first place? Safety? If safety of abutters is a goal, then a critical measurement to use is the “fling” or “throw” distance. A Business Week article last August cited a number of instances of blade pieces ripping off and landing several hundreds of feet away from the turbines. Isn’t it more likely for a blade tip to tear off in gusty winds, or to have built-up ice flung off a blade and hit someone in the head, than a falling tower?
The throw distance can be calculated from the blade length and the rotation speed (rpm) at which the blades turn, using formulas found in a high school physics textbook. For a turbine with blade length of 50 feet, rotating at 20 rpm (about one revolution every three seconds), the throw distance is about 342 feet. The speed of the piece would be more than 72 mph. And as the rotor rotation speed increases, the throw distance increases exponentially.
Imagine walking your dog only 300 feet away and being struck by a torn-off piece of blade going 72 miles per hour.
Maybe the club’s general manager should count his blessings that the proposed bylaw is focused on turbine height.
LEE ANNE KOWALSKI
Bellevue Avenue, Gloucester
10 March 2008
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