How much surface wobble is ok?

[meanderingthemaze]

I was thinking of building some body parts, ie: fenders, hood, trunk cover... with a stretched canvas or other fabric. But I'm wondering how the wobble of the surface (over time and stretching, or just at high speeds) will affect aero. I'm guessing it will not be good. But again, this whole aero thing is not always intuitive to me. And I get it wrong a lot.

I could also seal it with a stiffener, or even almost do a very thin fiber glass treatment with some resin or something comparable. But, I'm trying to keep weight as low as possible. On the other hand, I know sometimes the weight is relatively insignificant compared to reduced drag.

This application is a custom built electric car for general purpose, mostly commuting. Let's say 70% Street/30% Highway. Max speed of 70mph. Considerable stop and go traffic, but it will have regenerative braking, so that will help a bit with recovering some energy.

Thanks.

[moorecomp]

Fabric covered aircraft, using polyester, heat shrunk fabrics, routinely last 20+ years even when stored outside. Check out Ceconite.

[meanderingthemaze]

Coool, fascinating, thanks for that lead. So, I guess I wouldn't be having any wobble after all.

But, I'm also trying to control costs. So, hopefully it is within budget.

[mort]

Hi meanderingthemaze,
I'll add that hobby type airplane models use doped silk for the lightest air foils, or tissue paper as the low cost substitute. Also plastic films or woven cloth like Mylar, Aerokote or Koverall.

-mort

[meanderingthemaze]

Somehow I don't think tissue paper will hold up. I did think of paper mache though. Definitely cheap. Thanks.

[aerohead]

Before World War-II,the first dry lakes 'Streamliner' to race in this new SCTA class was built by 19-yr-old ,Los Angeles carpenter, Bob Rufi ,who built body sections of doped fabric over wood/chicken-wire substrate.

[meanderingthemaze]

It sounds like the technology of fabric and doping has come a long way since pre-WWII times. No longer as flammable. Easier to work with.

This is a pretty good thread about this topic.
Anyone built a doped-fabric car body? - THE H.A.M.B.

But I still wonder about a straight stretched canvas application. Would that be bad?

[meanderingthemaze]

Can't help myself, this is pretty cool.
Cars with fabric body skin coming our way? (BMW, different) - Automotive -Sports cars, sedans, coupes, SUVs, trucks, motorcycles, tickets, dealers, repairs, gasoline, drivers... - City-Data Forum

[aerohead]

Originally Posted by meanderingthemaze

It sounds like the technology of fabric and doping has come a long way since pre-WWII times. No longer as flammable. Easier to work with.

This is a pretty good thread about this topic.
Anyone built a doped-fabric car body? - THE H.A.M.B.

But I still wonder about a straight stretched canvas application. Would that be bad?

Of course,low drag is usually associated with exotic compound curves but there's no reason you can't do pretty well with more simple faceted panels.
You could do shaped foam transitions where absolutely necessary,more simple panels fore and aft of those.

[meanderingthemaze]

The goal here is to come up with a simple-to-construct, cheap, light, aerodynamic panels, which aren't a hazard, such as being extremely flammable. They need not be very strong, only strong enough to hold up under head winds on the highway, and everyday driving conditions, NOT collisions. The strength of the chassis is all going to be in the tube frame. Passenger safety will come from that. The panels are only to reduce drag, which is why it would be great if they didn't weight anything.

Maybe I should posit the question this way. Would stretched (non-doped) fabric be better than nothing? If so, how much better, as compared to a tube frame (think dune buggy)?