Aeromods must transfer force to be effective
 

That means they should be rigid and firmly affixed. I originally thought that getting the shape right was the most important thing. It turns out we want both, but the force transfer is paramount. An aeromod that isn't transferring force isn't doing much, and might even be counterproductive.

More thoughts and pressure stats here:
https://www.instagram.com/p/Cel1DD-L1K1/

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.

rigid shape
 

The 'big-boys' call it aeroelasticity.
Forces acting on an under-engineered profile can actually deform it into a lifting panel, which subsequently rises to an angle of attack aggressive enough to get to burble point, then stall. The panel falls below the critical angle which initiated lift, and the 'system' begins the lift/stall cycle all over again, leading to the cyclic deformation known as 'flutter', which can bring an aircraft down.
Depending on the material, the degree of deformation, and the natural frequency, or harmonics of that frequency, the panel will just self-destruct.
There are YouTubes of model, Lockheed C-130 Hercules coming apart in a wind tunnel.
The easiest solution, yet the most difficult to fabricate, are 'compound' surfaces, where they curve in three axes. Like a hen's egg. The strongest and lightest structures have compound surfaces .
Most contemporary automobiles have some degree of compounding to their sheet metal. Strong and light!;)

A worst case would be a hoop and tarpaulin cover on a military truck.

I was driving through Northern California in the 1970s when as I met a Lincoln convertible the top tore loose at the header and turned into a roll-back as we passed.

That's the most readable post I've seen from you to date. No bullet points, and something resembling paragraph organization.

Last weekend I pulled a 16ft canoe on a trailer, mostly utilizing the canoe as extra storage, but also hoping to take it on the lake. I had all 4 interior sections filled with totes and tents, then covered the whole thing with 2 tarps. I tried to get them as taught as I could, but there was still some flutter at the end since it tapers so much creating that compound curve.

Seemed to be pretty aero though, as flat cruising didn't appear to suffer much in the way of fuel economy loss.

Next purchase I'm looking at a hitch mounted cargo platform for those instances where I just need more space, but not a boat. I'm torn between a sturdier looking lower profile, or the utility of higher walls.

https://i.pinimg.com/474x/cd/eb/65/c...57b3faec1b.jpg

https://www.uhaul.com/MovingSupplies...932&media=6255

In aeronautics there's really two kinds of drag

Induced drag and parasite drag

Everything related to induced drag would be load bearing this is the drag from the airfoil form

Parasitic drag could be load bearing. But it can also not be any more complicated than keeping air out of cowlings and spaces. Or not installing some thing onto the fuselage that protrudes into the slipstream. It is actually OK to use tape on commercial aircraft in some of these instances

A good example on our cars would be anything perpendicular to the airflow will need to be rigid enough to not flutter which would be worse than it not being there

Anything parallel to the airflow like a wheel skirt also doesn't need to flutter but at the same time sees much lower aerodynamic loads

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.

3mm abs engine undertray will fail at about 180km/h speed unless its not reinforced.

Install few metal L-bars to make it stronger and you are good to go faster.

tonneau cover
 

As far as automakers are concerned, pickup trucks are just large sedans, without a trunklid.
Flow separates off the rear of the roof, and there's nothing to reattach to.
There's no pressure recovery.
High drag and high lift.
A tonneau cover is essentially a 'trunklid.'
If it's sufficiently close enough, vertically, and sufficiently 'far' longitudinally, it provides a surface for reattachment.
A vortex is captured on top of it, and the outer free stream will skim over the 'locked-vortex' as if it were a solid structure, plus 'touch' the back of the cover before separating.
If 'smoked', you'd see streamline filaments diverging as they decelerated down the 'contour' of the vortex, picking up pressure as they lost velocity.
This is the drag reduction.The higher pressure is communicated to the base, behind the tailgate, raising the base pressure, which lowers the pressure drag, the largest component of aerodynamic drag; and why we streamline.
And since slower, higher pressure air impacts the rear of the tonneau, it also kills most of the rear lift.
A 'half-tonneau' works better due to reattachment, plus the low pressure of the vortex core telegraphing under the cover, to the forward face of the tailgate, increasing the pressure differential between the front and rear face.
GM has the US Patent on it.
If you watch a vinyl cover, you'll notice a dip at the rear where air is attacking it.
At the front, behind the cab, it will look like Yoda's under there trying to get out, creating an upwards bulge in the fabric. Highs and lows.

This video gives a good feel for the relative pressures around a typical modern car, which directly translate into the forces one should account for.
https://www.youtube.com/watch?v=n1Hro6QpnRE
Enjoy!