The Underbody (flat belly pan is suboptimal - explanation & solutions offered)

[AndrzejM]

I see that discussion went in a very serious direction so I just want to share an idea for fabricating dimples to a bellypan. Why just not drill or cut the holes in a coroplast or aluminium and close those with anopther piece of coroplast?



1 - bellypan
2 - additional layer of coroplast
3 - hole

What do you think of that?

[redyaris]

More trouble than its worth...

[AndrzejM]

Yeah probably you are right

[UltArc]

I disagree on that, being more trouble than it is worth. It would be easy to just make belly pan, and then drill the wholes. Take extra pieces, and attach it to the back. I might try it just because I want to do two layers anyway, for stability.

Though I do not think.it will make a difference.

[Otto]

Originally Posted by AndrzejM

I see that discussion went in a very serious direction so I just want to share an idea for fabricating dimples to a bellypan. Why just not drill or cut the holes in a coroplast or aluminium and close those with anopther piece of coroplast?



1 - bellypan
2 - additional layer of coroplast
3 - hole

What do you think of that?

Easy to do and might work aerodynamically if the edges of the holes were contoured or beveled. Otherwise, the sharp edges of the drilled dimple holes may make drag worse. Or better. Who knows without specific testing?

Since Corplast can be formed somewhat when heated, I'd thought about using a heated heavy iron ball (shot put heated in the oven) and place it on the flat Coroplast where I want dimples. The hot and heavy iron ball softens and distorts the Coro so the ball sinks down into it, forming the dimple. Repeat for other dimples.

Otherwise, I was talking to a guy yesterday who told me the Navy now paints its ships' hulls with a crinkle finish paint, essentially the same stuff as sold in rattle cans at the hardware store. Reportedly, the crinkle finish paint texture emulates shark skin, dampening turbulence or keeping it attached, reducing drag. Anybody heard of this? Thoughts?

[Smokeduv]

Why not use some vacuum forming instead of the heated iron ball? You can make all the dimples in just one run, then cut the coroplast to form the bellypan, maybe?

[freebeard]

1 - bellypan
2 - additional layer of coroplast
3 - hole

What do you think of that?

If you were making a bump instead of a dimple, the double thickness at the attachment point would help. As shown, the fastener would only hold the patch and all the stress would be on whatever holds the coroplast together.

I'm with redyaris.

[mcrews]

I ran a dimpled material on my first front belly pan.......no effect.
see thread link in sig.

[Telco]

Hello, new member here who came looking for help on a project and who beleives in giving back. And, I have an interest in this anyway as I have a full size reg cab truck with ginormous cavities under the bed. An underbody cover would likely be a huge help for my truck's fuel efficiency.

I was also going to suggest the holes in one sheet/second sheet as a backer. The heated iron ball in each hole would eliminate the rough edge problem. And, no need to worry about stress on what fastens the two sheets together if they are attached with glue across the entire section. This will both help stiffen the two sheets together and spread any stress across the entire pair of sheets. I would also go at it by cutting the holes, then gluing the sheets, then applying the iron balls. Vacuum would likely not work well unless the corplast were heated first anyway, because the corplast would need to be able to stretch and it's not going to do that at room temp. Since heat must be applied anyway, might as well get the iron balls out.

For the fellow with the Suburban and the overheating issue due to the inadvertent spoiler, what about putting a wing right in front of that piece with the flat to the bottom and the curve to the top? A wing generates lift by putting a low pressure area over the top, so the plane is actually sucked up into the air from the top as much as lifted from the bottom. This would help draw air out of the engine bay, but would not generate so much lift that it would be a problem for a Suburban since the vacuum would be filled with engine bay air.

Hope some of this helps.

[Otto]

Use the dimple cavities as mounting points for the zip ties, Dzus fasteners, sheet metal screws, etc.. Keeps the fasteners from sitting in the slipstream and adding drag. Might save, say, a thimble full of fuel over the life of the car. Borrow an iron shot put ball from the local high school track coach. If fender washers come in concave dish shapes, maybe heat them up to melt/distort the Coro to make the dimples and also as a hard point for mounting.

I still think a corrugated surface might be pretty good, as used on the Ford Trimotor plane in the 1930s, with ridges and valleys aligned with the slipstream. I'll see what my Hoerner aerodynamics book says about such surfaces, which may dampen lateral wake from rough spots. Home Depot and garden supply places sell corrugated (sine wave) sheets of translucent fiberglass in sheets up to 12' as I recall, for cheap.

Back to the point of discussion of the OP: How exactly does a rough and irregular belly (or any other part) of a body moving through a fluid benefit from such shape, vs. a smooth surface? Why is the belly any different to the flow than the flow over a smooth roof vs. convertible top down roof, or flow over smooth windows vs. open windows? Seems to me, flow is flow and the air molecules don't know or care if they're going over, past, or under the car, but definitely like smooth and gradual shapes more than jagged ones. Please explain why the belly would be any different, or where in Nature one finds any example of irregular/rough being any better.