Quote:
Originally Posted by bmack2
Thanks charlie,
I looked at the pages you said, wouldnt the best cap be the one with the lowest drag coefficient?
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Yes, and NO. At 8° you'll be within 1% of the improvement potential, technically more like 4% of the total gain, but still, you've made a tremendous improvement. If you drop it further, the lift coefficient starts going back up. Also, and most really important here, your design is very basic, you have no curves, therefore, if you try and push it, you will likely end up with more drag.
Quote:
Originally Posted by bmack2
I know that laminar flow depends on velocity so this would probably be what youre talking about forming vortex where this test in the paper would not?
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The vortex will generate at 5 mph, speed is not important to whether and aero feature will function, at least not until you reach several hundred mph. To directly address your question, Laminar flow will never occur on your truck. It is, and always will be, turbulent. The vortex formation I refer to is not illustrated in the paper, it is more like long horizontal tornadoes being dragged behind like 300 ft party streamers off the tops of your tail lights.
Again, 8° gives you a tremendous gain in aero efficiency, within a few % of ideal, and the most utility with regard to under cap capacity. It also stays away from the potential to go too far and start giving the gains back. I know you want to do the most you can, and it does intuitively seem as though 11° to 13° will "Shrink the wake" more and thus be more efficient, but this is NOT true. Efficiency has to do with disrupting the air as little as possible as we drive through it, the less we move it, the less energy it takes.
I look at the relatively puny amount of air a fan moves, and consider the amount of electric energy required to move it. It really does take some power to move air. Consider that when you drive your truck down a quiet country road on a calm day. The air above the road before you arrive is just sitting there, then Voooom....you ram through it. If you're in a VW XL1, that air will likely just waft about a bit and return to being calm in short order. When you drive a pick up through it, you really stir up the air and cause it to move quite far from where it started out, this displacement, or movement of air is where your energy is going. A vortex requires a lot of energy to establish itself and therefore creates a lot of drag. 60% of what we do when we design aero shapes is to avoid the vortex.
Please trust me, I have thought about these things long and hard for the better part of 4 years, if you want to optimize your cap, you'll have an 11" height in the back. 5° would give you a 15" rear height, 11° will have 8". If this were my project, I'd build it with a 13" height, and curve the top and sides, and know I've done the best I could, zero doubt. The minimization of the lift coefficient to me has a huge draw, since you can't optimize both, aim for the middle.