OK.... after a bit of a delay due to some family issues, I was able to do some testing with a Pitot Tube above my truck cab.
As mentioned in previous posts, I was wondering if there was a Bernoulli effect going on between the cab and camper overhang, and if so, what was the effect.
What got me thinking about this was that I had observed that there were a lot more bugs accumulated on the center of the front bulkhead of the camper than on the front. This suggested to me that the air flow above the cab was stronger in the center and therefore would exert more force (drag) at this point.
Front of overhang
Center of bulkhead
So, as suggested by some, I decided to test this hypothesis by building a Pitot Tube and taking some measurements.
The Pitot tube has a static port on the side and I used a Phidget analog differential pressure to USB device to measure the pressure differential and then calculate an air speed. I validated the results by testing the unit held about 3 feet away from the side of my car. The measurement was consistent with the speed of the vehicle (and yes, my wife thought I was weird as she drove me down the highway hanging out the window!!)
I then mounted it on my truck and camper and with the help of my grandson, we ran it up and down the highway while he took readings.
At first, I thought the results failed to support the hypothesis. However, after some further study, I realized what was happening is that the air was piling up in front of the bulkhead creating a high pressure area and therefore increasing the drag.
Note that as actual speed increases, the measured speed increases but the differential to actual speed gets larger and larger. I am interpreting this to mean that as speed increases, there is a resultant increase in the build-up of air in front of the bulkhead.
Conclusion:At this point, I think this data (and other observations) supports Sleepy's (and abslayer's among others) design. Although I love the idea of the air tabs, I just do not see that they would give as good a result. They may help by creating turbulence that will tend to move the air to the sides, but not sufficiently to warrant using them exclusively.
I plan on doing some further testing, and have an idea for a different air dam design that will utilize a few air tabs which I will share at a later date.
