bjbear wrote:
The 2-inches water column is pretty much what you should see. I am currently building a pitot tube to do some testing on my truck. I worked out the calculations this afternoon to see what kind of pressure range I would need and I found that at 60 mph, the pressure should be about 1.8 inches WC.
Also if measuring the air speed above the cab (assuming my musing about the Bernoulli effect are correct) having the low point static sensing side in the cab would give a misleading result. If the hypothesis is correct, the static air pressure will be reduced above the cab and therefore you need the static point there.
You are of course right - I didn't have my coffee that morning. The dynamic pressure at 55 mph is 7.7 lbs/ sq ft (not per sq inch!), so only 0.053 psi or around 1.5 inches water.
The formula for dynamic pressure is 0.5 * V^2*rho, V is velocity in ft/sec, rho (air density) is 0.002378 slugs/cu ft at sea level. The result is in lbs/ sq ft pressure.
If the air between cab and camper cabover is moving faster, the static pressure will be lower; if it is moving slower the static pressure will be higher according to Bernoulli . However the dynamic pressure (ram air pressure) will be higher in faster moving air. The two are inversely related.
