Commercially produced aerodynamic pickup bed cap

[Bicycle Bob]

Originally Posted by tomi_k

the shape will be 3D surface, for sure, just need to keep in mind also how to ensure that the cap door will be usable / practical and ergonomic... and finally the cap door needs to be "waterproof"....

So initial framing is just to make the same for the final product, final product is targeted to be glass fiber cap as "classical" caps are....

Fiberglass lends itself to different shapes than plywood. There is no graceful way to get from one to the other.

[JulianEdgar]

Originally Posted by Bicycle Bob

Fiberglass lends itself to different shapes than plywood. There is no graceful way to get from one to the other.

Yes - I suggest that you just do mock-ups to test first, without worrying too much about 'getting things right' (eg the underlying framework) for the final one.

If you expect to have to try 4-5 different iterations, then you won't lock yourself into something that may well not be so good.

I'd probably start with bent steel tube (cheapest small diameter I can find) as the frame, and use tape and plastic corrugated board as the outer cover.

[tomi_k]

Visit in local HomeDepot... corrugated plastic sheet is more expensive than similar thickness plywood... pros and cons on both options.... leaning towards plywood + foam...

Also tried to bend 5mm plywood... its nothing... bends what need to be done would be fine to make with that.... considering length of the cap vs. transitions needed... it would be probably fairly easy to "bend" that plywood... hence leaning towards that....

[tomi_k]

IMG_02 : Has "straight but curved" sidewalls, 3D curved top surface
IMG_03 : Has 3D curved side and top surfaces -> towards teardrop shape -> top surface is matching to the cabin's top roof surface and extrapolated to the length of the cap. Side surfaces will taper gradually towards end of the cap forming teardrop shape yet still blends into the tail (in height) -> no increase on tail section...

Framing under is just to give a base for the plywood (simple shape)... foam on top of the plywood makes finer shapes...

[Bicycle Bob]

Basically, you are developing a buck to pull a fiberglass mold from. You can make it from any kind of garbage that can be polished and is strong enough to support wax, wet cloth, and a paint brush.
If you are planning on a single skin construction, and don't want any of the usual grooves, steps, or ridges to add stiffness, you can make a separate molding to produce an internal support, just like you find on a hood or trunk lid.
The "right" material is the one you most enjoy working with, provided it can make a suitable shape. I've done a lot of detail work using wall patching compound, and then hardened its surface for polishing with Polyvinyl Alcohol, a release agent no gentleman's molding shop should be without. Shellac also works. Final polishing can be done on the mold surface itself.
Material choice is important. Ten percent of the homebuilt aircraft that are begun are finished by the first builder, ten percent by the second builder, and ten more by the third builder, who gives up on 70% of them. You have to enjoy the work, not just the vision.

[freebeard]

I'd never find it again, but there was a SEMA Mustang show car in a video that had a hood that four-bars backward to over the windshield. That would work well in reverse for your situation.

[tomi_k]

trialing how to make twisted surfaces but still missing "teardrop" tapering (looked from top)... but got an idea how to make it because of this....

https://ecomodder.com/forum/member-t...-211524-01.jpg

[tomi_k]

Something like this... but will need to check how contours of the truck goes and do some estimations of how much "curvature" could be....

Blending all surfaces nicely to the back of the bed (not increasing wake area) and keep transitions of surfaces following closely to the truck's overall shape is the target.... Once that is established, then tweaking surfaces here and there is much easier... taking baby steps while learning....

https://ecomodder.com/forum/member-t...-211524-02.jpg

[JulianEdgar]

Don't forget that if you extend the top edge of the shape past the tailgate you can then add a spoiler without increasing wake area. That is then likely to reduce drag and lift.

Also, you don't necessarily want a beautifully curved surface from the top to the sides, because this will encourage flow around this corner and so likely increase the strength of trailing vortices. The Tesla Model S has low vortex formation off the rear pillars - note their shape.

[tomi_k]

Thanks for insights, JulianEdgar - appreciated! The idea on that top edge (between side and top surface) rounding is something like that on the Tesla... nothing drastic...

If the edge is top edge is left too "sharp" and as the top surface drops down anyways, air on side wall travels straight (looked from side) while on the top air follows the top surface with higher speed... which creates pressure differential and air movement start to go "up" on side surfaces and over the edge... if I have understood correctly.... Thats what I try to "avoid"...

Example on the Aerolid.... (and copyrights belongs to the Aerolid)...