Wind blows azimuth arm with brakes on.

06 may, 2001

     On 6 may 2001 the azimuth was parked at az=214.4 degrees and the dome was at 19 degrees za with motors off and the brakes on. At 7:40:49 the azimuth started moving do to the wind. The wind was directly out of the south and the maximum velocity measured on jon hagen's wind monitor was 40 mph. The azimuth continued moving till az=280 degrees. It reached velocities of up to 1.84 degrees/second (about 5 times slew speed). The maximum acceleration was .14 deg/sec^2 (about 40% above the acceleration limit imposed by the computer). The operators began moving the dome downhill at azimuth 258 degrees. The dome was at 18 degrees by the time the azimuth stopped moving. The azimuth rail had been greased the previous week.

 The wind was out of the south and was relatively constant (it wasn't just gusts). If we assume that the wind was constant and that the two sides of the azimuth arm (dome,ch) present equal wind loads, then the net force on the azimuth is the differential wind load presented by the carriage house and the dome.  The maximum force during this time would be at azimuth = 270 degrees and gregorian = 19 degrees za (largest moment arm). The relative force on the azimuth would be proportional to:  cos(270-az)*sin(gr za)/sin(gr at 19 Degrees).
The plots show the 60 seconds of motion:

The cable were in no danger of breaking since the azimuth was on wrap 1. If the dome had made it around to 360 degrees then it would have oscillated about this position. If the azimuth had been on wrap 2 this would have broken the cables if it got to 720 degrees.

The azimuth stopped moving probably because of the variation of the wind and the resistance of the wheel flanges on the rails. Bringing the dome down 2 degrees made little difference.

During high winds we should probably park the dome at low zenith angle. It appears that it is more important to park the azimuth in a location of high flange/rail resistance than low zenith angle.

The grease applied to the rails benefits the wear of the wheels/rails but it does cause slippage on the rails and increases the danger of the azimuth getting blown by the wind.

processing: x101/010506/