Quote:
Originally Posted by Ecky
I'm not certain. A tonneau cover can simply cover a void, isolating the air under it, and cut drag. I suppose there is force transfer going on there, but it doesn't seem proportional and mostly would be preventing the venturi effect by separating high and low pressure air. I haven't fully wrapped my head around the physics of it though.
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Okay. Let me say it differently. The effectiveness of an aeromod is directly related to how well it can transfer force. I've made the bold claim that we want rigid aeromods, but the more accurate statement is: if we had two aeromods of the same basic shape, one rigid and one floppy, the rigid one will perform better. And there is a cap to the rigidity we need, i.e. the maximum amount of force that aeromod transfers onto the car frame under operating conditions.
Now, it's definitely the case that non-rigid surfaces can effectively harness aerodynamics. There are many examples: parachutes, kites, etc. And you are correct that a tonneau cover does, in fact, reduce drag. My statement is: if you put a rigid frame under the tonneau cover so that it more effectively transferred force onto the truck in addition to the fasteners, it would perform better. If you replaced the ribbed tonneau cover with an aerodynamically shaped bed cover that was made of rigid fiberglass or somesuch, it would perform even better. And if you calculated the total force on that cover due to aero and engineered it to withstand it, that would be the best it can do (rigidity-wise).
The reason I mention this is I originally had some "loose" elements in my design, expecting the wind to shape them into the optimal curve. Which it did. But because those elements were loose, I wasn't getting the full benefit of the aeromod, because the wind was spending it's energy into deforming my surface instead of into smoothing and diverting it's own flow.
Most importantly, I've now realized that the optimal forces that I want on the vehicle were the opposite of what I was expecting. An aerofoil has a big push back right along the tip of the nose... and then is
pulled outward by lower pressures at all other points. That means that surfaces (like my belly pan) which I though I needed to design to be pushed up I actually needed to make sure could be
pulled down. That requires a significantly different bracing strategy that I was using before.
Anyhoo, I hope that's clearer. I'm not saying loose elements do nothing; Champrius 3.0 was a loose design and it got me past 60 mpg. I'm saying tighter more rigid elements do better. I'm hoping that it gets me past 70 mpg this time around.