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Old 04-11-2017, 11:15 AM   #12
NN5I
Carl, nn5i
 
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Join Date: Sep 2011
Location: Tallahassee, FL
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Quote:
Originally Posted by Gashog View Post
As for wind load, I think if you did the math, you'd see that a 9' whip has a bit more wind load than an aerodynamic AC unit sticking up 12" off the roof.
Depends on what you call "a bit more".

Aerodynamically, it's a difficult question. There will be some wind load on the A/C unit, but it will be minimal because the air moving down the top surface is slowed by friction on that surface and by any other protrusions on that surface; and the wind load will be further reduced by any turbulence in the flow. Basically the wind load on the A/C ought to be quite small.

The whip, on the other hand, is in undisturbed laminar flow, sometimes at perhaps 70 to 80 mph. The wind load will be much larger, and is applied about 5 feet from the mount. That's enough moment (torque) at the mount to tear a petty big hole in the roof, or at least wrinkle the skin considerably. So you need a reinforcing plate, on which the whip is mounted, large enough to distribute that load. I'd think a four-foot square of 1/4" 6061-T6 aluminum plate would probably handle it OK.

Indeed, that "a bit more" would really be "a heck of a lot more".
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