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Old 10-24-2014, 04:52 PM   #1
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Pressure on windshield?

We were heading to the west and I got to thinking. How much pressure is the windshield experiencing when going 60mph. I know the angle and how big it is and the wind speed and direction makes a difference. I tried to look it up but I am not intelligent enough to use all the possable varriables. Just a rough idea would be interesting to know.
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Old 10-24-2014, 05:28 PM   #2
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There are 3 basic variables to consider when estimating drag force on an object that has velocity relative to the wind: Velocity, area, and Drag coefficient. In addition, the density of the air is also important.

The drag coefficient of a flat plate inclined at 30 degrees or so (like a windshield) is about .9 for speeds that RV's travel at. The area (reference area or the projected area of the windshield) of a medium sized windshield is about 6 ft. X 2 ft. or 12 square feet.

The dynamic pressure acting on the windshield is a function of velocity and density is calculated by 1/2 X density X velocity squared. For sea level and standard temperature (70F or so) the density of air is about .077 lbm/cubic ft. and 60 MPH is about 88 ft./second. So, using a calculator the dynamic pressure is about 9.21 lbs./square ft. (must use 32.2 ft/sec/sec conversion factor to get lb. from lbm.)

9.21 lb/square foot X 12 square feet X .9 Cd = about 100 lbs of force. For a larger windshield, like 8 ft. X 3 ft. the force is double this about 200 lb.
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Old 10-25-2014, 12:46 AM   #3
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So what your saying is there is just over 1 oz. pressure per square inch at 60 miles per hour. I would have thought it would have been more than that but you learn something new every day. Thanks for your input.
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Old 10-25-2014, 03:26 PM   #4
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Quote:
Originally Posted by Doctor DEF View Post
The drag coefficient of a flat plate inclined at 30 degrees or so (like a windshield) is about .9 for speeds that RV's travel at. The area (reference area or the projected area of the windshield) of a medium sized windshield is about 6 ft. X 2 ft. or 12 square feet.
Those numbers look like they're for a typical car windshield, not a motorhome. Motorhomes typically have vertical, or almost vertical, front caps, not slanted at 30 degrees, and the windshield is typically about 1/2 the total front cap area - considerably more than 6' X 2'.
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Old 10-25-2014, 04:51 PM   #5
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Old 10-26-2014, 08:39 AM   #6
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If there's enough pressure, then why doesn't my windshield pop back in when a corner decides to push out??
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