I imagine it would be challenging to keep a huge surface area like that from "balooning" into the low pressure around it at speed (like some closed convertible tops do on the freeway).
Wouldn't that actually be a desired effect? The shroud at the back is to fill in the low pressure area back there. Why would a similar filling of the low pressure area above the roof not be desired?
Make it stiff enough to not flap, put it in motion, and let the wind sculpt it as it cures and hardens into a retained shape.
I was just pondering on what materials I could possibly use to clean up airflow on a pick-up. Basicly, just a tonneau cover and belly pan. I'm using plywood for a half tonneau now, but it sags a little under it's weight.
IIRC, my train of thought started with paper mache.
I know airframes are pretty skeletal. Since flight isn't my goal, I can be more conservative with support. Maybe use wire fencing under the sheet.
I thought of the monokote, but is it tough enough to take a stone kicked up into the wheel well?
I'll take a look into home built aircraft, but I really need to look into suorces of coroplast first.
Trollbait, just throwing this in for ideas
What you described for your first post is exactly how professionals build custom speakerboxes. Pretty simple concept, but pretty heavy and a bit expensive. What they usually do is start with a felt type material and stretch it to the desired shape, than soak it in fiberglass resin. Once hardened, they reinforce it with fiberglass for strength. Than use bondo to give it smoothness.
For a cover though, it would take a lot more work and in the end it may cost the same to buy a premade one.
The beauty of fiberglassing is that it is actually the resin and hardener that results create the shape. If you'd like you could use any type of material from clothing to cardboard. People only use fiberglass because it has the best strength to weight ratio. Better than carbon fiber.
A friend of mine covered his velomobile with the sturdy shrink wrap plastic that boat yards use. There are bales and bales of that stuff thrown out every spring when the boats are uncovered, around here anyway. I picked up some for him to re-use.
If one could avoid contact with the hot exhaust plumbing, the marine grade shrink material should handle the occasional rock ding.
Vibration and flutter might become an issue at highway speeds?
This could make your little gassaver into the world's most economical
On a somewhat related topic...
Toyota experimented with non structural panels on one of the cars they ran at Le Mans in the late 1960's.
The rear spoiler was made from papier mache (yeah like you used to make in school) and survived speed of about 300 kph (180 mile per hour) so it can be made to work.
Another thing to consider. Many of the WW2 era aircraft still had fabric on some of the control surfaces. Even the F4U Corsair had fabric on the rear control surfaces. With proper framing they should work if they were good enough for 400+ MPH.