weird idea
 

Instead of hard underbody covers like coroplast or sheet allumium... how would reflective mylar sheets fare?

That'd be swimmingly easy to shove in there... give it a tight tug, tape or screw it down, and bam

it would be very easily chewed up by road debris. but the only way is to give it a try!

Flapping makes for bad aero.

I definitely agree you're going to need something more stiff and durable.

I've been pondering using UHMW in sheets. Rally racers use 1/4 inch think stuff to protect the underbody. I think the 1/8 inch would be plenty for normal use, and even thinner might work. It is expensive compared to 'free' coroplast though; I've found online ~$120 for a 4ft x 10ft piece.

http://i728.photobucket.com/albums/w...erbody28ff.jpg

http://teamjardevall.com/photos/2006/0611_02.jpg

Coroplast is the definite choice if you want environmentally-friendly, frugal underbody covers. Also, the stuff itself is tough. You would have to replace the UHMW once in a while, just like the coroplast. Why would you eco-mod your ride and forgo environmental advantages?

perhaps these woven plastic covers might have some use to create certain parts of an underbody. a light frame covered with tight strong fabric could provide a smoother surface than whatevers there now.

untill the beginning of WW2 many fighter planes like the british hawaker hurricane where at least partly constructed with fabric covered frames. mostely aft fuselages and controle surfaces. aereas that must have been subjected to considderable aerodynamic force.

so it's very possible to create a light strong and aerodynamic structure using fabric like materials.
i think the key would be to have a lot of lenghtwise ribs so the distance that needs to be spanned isn't to great.

Lightweight alternatives
 

If people are considering up level from Coroplast, they should go back to the sign company supply stores and look at different lightweight substrates. There are expanded PVC foam sheets (Sintra, Komatex, Celtec, etc) that is more durable, easily cut & routed and could even be heat-formed (possibly detrimental for Sunbelt ecomodders). If you want even more strength, the next level is aluminum-bonded/PVC panels (D-Lite, Alupanel, Alumalite, etc.)
Hopefully to create more 'permanent' installations...make sure, drain holes.

Even by the end of the war, a lot of aircraft had fabric-covered control surfaces. They were very light and quite well-understood, because "sticks and rags" had been the construction method of choice for aircraft since they had been invented.

The fabric on all but the earliest aircraft was "doped", painted with a very tough paint/glue that set up very stiff. And the fabric still required relatively closely spaced reinforcements (wood or metal ribs that the fabric was stitched to) to keep flapping to a minimum. Anyway, the doped fabric can be viewed as a very early form of composite material, where a woven substrate was supplemented by a stiff additive. Instead of fiberglass mat and epoxy, you had cotton cloth and dope.

It's a whole lot easier to work with coro-plast.

-soD

After you've got the fabric spread tight across the bottom of the car, then just hit it with a good coat of epoxy, and you've got instant form-fitted fiberglass belly pan! Might be hard to remove if you epoxy over the fasteners, though.