[hat_man]

I guess what I was seeing my warped and twisted mind was sort of a hybrid. A half tonneau like discussed earlier in this thread. The rearward section of a truck bed covered at the height of the stock Ranger bed rails, with the addition of the Ridgeline side sail panels and Ridgeline bed rail section above the stock Ranger bed rails.

Aerohead was explaining (an I hope I have this right) that the low pressure area created behind the cab is transferred (or extended?) to the area under the half tonneau creating a small pressure differential across the tailgate. Lower pressure inside the bed and higher pressure outside the bed. I am assuming that it works best in conjunction with a wing attached to the cab. My thoughts earlier in this thread were that there might be some small gain with having the half tonneau and no wing. The half tonneau is an easy project where the wing is possibly more involved. I was going to do the half tonneau and as I get back to work I will have a nice A-B test platform (with the 60 mile drive each direction) to test it on. That would give me some time to try and design a wing arrangement and then test that too.

Those plans led my to thinking about how adding side panels (in addition to the half tonneau and the wing) and extending them (sloped) to the tailgate might affect the results. If I have the thinking right the air moving across the roof creates the low pressure area behind the cab. I wasn't sure how the air moving across the doors/windows interacted with this low pressure. Does it help create it (A+B=AB) or does it disturb the creation of the low pressure and lessen it (A+b=Ab), but still create a low pressure area where AB>Ab>0. (Maybe that should be < as we are talking about lower pressure and not higher pressure, but I think you can see what I'm getting at.) If you sip through a straw with a crack in it you can still maintain enough pressure to move liquid through the straw. But if you seal the crack there wouldn't be a pressure loss and would create a lower pressure in the straw. So my thought was IF the "side air" was like a "crack" in the low pressure area behind the cab then would the addition of the side/sail panels be like sealing that "crack" and create a slightly lower pressure area behind the cab?

In theory, the red and yellow lines are half covers. The trapped area with the red "cover" would be smaller in volume than the trapped area with the yellow"cover". I also couldn't quite figure out how this would affect the pressure differential created across the tailgate. 35psi in a tire is one thing, but the same 35psi in a 36" diameter pipe is quite another . Air escaping from the tire is fairly harmless even at the moment of escape, but in the pipe would probably sever an arm off. I didn't know if the math worked the same in reverse. Given a specific air flow across the roof, is the pressure differential across the tail gate directly/indirectly proportional to the volume of trapped air being acted upon? And does the addition of side/sail panels increase/decrease or have no effect on the volume of trapped air?

Again, I have no plans at this point other than the half tonneau and no wing, and recording those results as soon as I get back on the road to work. The cab wing is definitely on the table, but at a much later date after recording how the half tonneau alone works. I'm just trying to learn how all these other variables interact with each other to expand my knowledge base.

We know that a full aero cap is the way to go, and there are some excellent threads showing other members work and their FE improvements. I don't think anyone has tried the half tonneau and wing arrangement yet. I think I saw someone had done just the cab wing and saw some improvements. I thought I would be the unofficial test bed for the half tonneau idea and eventually add the wing to track the gains/losses of both.