To Perforate or Not To Perforate
 

I am considering a custom tube frame project, from the ground up, and I was planning a belly pan. To save a little weight (maybe not too significant) I was thinking of perforated aluminum sheet metal.

But the question is...is perforated worse aero than totally smooth? I would guess that it is worse, but sometimes things aren't what you'd expect with aeromods.

No good, would nearly negate it, might actually increase drag.

My semi-edumicated guess would be perforated would be about the worst aero thing you could use there.

You could cover the perforations with a thin sheet of plastic.

i assume the bellypan itself isn't structural and therefor if you want to save weight, just use thinner aluminum... i've used very thin aluminum on mine and it holds up perfectly. or perhaps some kind of fabric like on ww1 aircraft could work(although that might come out heavier than the thinest aluminum) corrugated plastic is as always a good idea too.

i'm not sure what the requirements are for what your building, will it race , mingle with traffic or whatnot.

weight reduction will give you most gains in situations where you have to stop and accelerate a lot (city traffic) while aerodynamics will pay of more on the highway... i presonally think that the added weight of a lightlweight undertray, will be more than canceled out by the aero gains.

a more difficult engineering exersize might be to make (parts) of the undertray a structural element... like the stressed skin on most WW2 fighter planes allowed the use of thinner internal structures (or for those to be perforated) wich imho would see bigger weight savings while allowing perfect aero. brace wires might also be an interesting idea to make certain structures more rigid while allowing the heavy structures to be lighter.

on the other hand there's something to be said for over engineering as well when it comes to safety etc... so it all depends where the priorities are

Look up interference drag.

Perforated is practically identical to solid as far as aero goes. Once air gets to a certain speed or pressure the small holes basically act as if they are not there.

This.

Is why window screen makes a perfect grille block on the highway, and allows normal cooling in red light traffic.

Youll probably save more weight by skipping a meal when you get in the car than you will by using perforated sheets.

I'd wager there's a significant difference between the behavior of a perforated surface perpendicular to flow and one parallel to flow.

What about the boundary layer, frank? Are you thinking that because it's not a solid, the boundary layer "weeps through", and doesn't keep flow separated from the actual surface?

How can there be a boundary layer over a hole in the first place?

In order for this concept to work, the panels would have to be in perfect alignment with the airflow, not possible.

There are many ideas and concepts that would and could work in controlled environments within strict parameters, however in the real world they do not.

air molecules like anything else will normally follow the path of least resistance which is normally not through perforation in a panel. At speed they'll be acting opposite of what you think.

it would however be nice if we could define the size of perforations were talking about here

my apologies for the lack of punctuation my phone doesn't support it via talk to text

Air would either go into the holes and bounce back out, causing drag.... Or try to flow straight and the bernoulli effect would suck air out of the holes interrupting smooth flow.

that specifically depends on the difference in pressure between area where the flow is versus the area being covered by the perforated panels, as well as on air speed and the size of the perforations.

I however am far from an expert on the matter.

I'd speculate that a perforated floor would have flow into the cabin in some areas AND out of the cabin in some areas AND flow "along" the floor... too much going on. At any rate I think that the flow would be disrupted by the perforations in whatever axis... vs say a perforated grille block which is pretty much just oncoming (perpendicular) flow, and when the oncoming velocity gets high enough the mesh starts to behave as a solid with much of the air simply going around it because that is easier than going through it...

my thinking here is the high speed flow over a perforated surface creating movement in n out of the perforations at any rate would create a highly turbulent area thick enough to be considered a boundary layer without actually being textbook definition of one.

to answer the original posters question the weight reduction probably would not offset the difference in aerodynamics between a flat smoooth panel and a perforated panel of the same parameters.

Wow, quite the interesting discussion generated by this question. I probably should have given more specifics, but some of those specifics are not decided yet (ie: what's above the belly pan, what is the cabin going to look like, etc). Also, I don't think the discussion would have been as informative.

I didn't think it was quite as straight forward as it might have seemed. It does depend on several factors and now I can design better with those in mind.

FWIW, it's a general purpose passenger commuter type car. Battery electric vehicle with an attempt to make super light and the more aero the better.

My goal is to get 100-120 range on lead acid battery technology and keep hard costs under $10k. I'm even considering a PVC frame, but need to think hard about the structural integrity in collisions on that one. Or maybe a hybrid frame.

Anyway, this leads me to the next question which I'll post in another thread to keep the discussions relevant to the post heading...

How much surface wobble is ok?

Air will weep through, even when it's perpendicular to the airstream, even at very high air speeds.

Examples ?
The rear F-4 splitterplates.
They are nearly perpendicular to the airstream.

F-15 movable intake ramp.
Here the holed intake ramp faces the airstream at high speeds.

That can only happen if the flow had greater pressure than the ambient on the opposite side of the membrane. If the pressure of the flow is equal, turbulence should create a boundary layer. If the difference is negative (highest purdue on the non-flow side), there should be an extraction of purdue as the flow passes each hole, which, by itself, will keep the flow from actually making contact or being able to permeate the surface.
Perpendicular, obviously the flow will have more force against the membrane than if it were parallel, so a higher ambient pressure behind the membrane would be necessary to prevent flow through.

Just use thinner sheet & have it bead rolled to increase the stiffness. You can roll an overlapping lip onto the piece in the rear so the seams don't catch air either.

Don

...just remember, holes let air "in" and strength "out."

a little tip to reinforce thin sheet is to fold the edges up creating a lip that's 1 or 2 cm high, where the thin sheet itsef is can be quite floppy it's amasing how much stiffer the structure gets when you fold all 4 edges and they cant flex anymore.

easy cheap and saves weight

Good call.

perforation
 

I have nothing on perforated bellys except a recent article from CAR and DRIVER on a Ferrari racer with dual electric-fan suctioned diffuser operated up to 150 mph,and disenabled above that speed.
Millions were spent on this cars development.One might presume that when Ferrari puts its prestige on the line that they've looked down every avenue for performance.A fully perforated bellypan was not on their menu.