2000 GMC FastBack AeroCap Project
 

Hi All.

Great job on your aero cap Fubeca!!! :thumbup: ( http://ecomodder.com/forum/showthrea...ead-11611.html )


For as much as it cost it looks terrific.

This is my first post here to this forum. I have a 2000 GMC Sierra ext cab 4x4 very very similar to Fubecas Silverado truck. I am going to be driving it across the country again in February from Montana to Vermont. I have been sickened by the lousy highway mileage I have gotten in the past, 14-15 mpg . (I do have something of a lead foot:eek:, which, I'm looking to cure on my upcoming trip by giving myself an extra days travel time, and plan to average 62 MPH or so instead of 72-78 MPH)

I have been thinking on my own how to build an aero cap since I know the drag in the back of the truck is a huge mpg killer.

Here is my twist on this, I have been thinking about removing my tailgate and building the aero cap down to the floor of the truck bed to the point where tailgate meets the bed. In my mind, this eliminates the huge square area being dragged behind the vehicle. It will look like heck I'm sure, and I'll have 2 vertical “fins” sticking up where my aero cover drops below the top of the cargo box. In my mind I don't see the fins as being much of a factor aerodynamically since the wind on the side of the truck would seem to be traveling parallel at this point on the outside. And the down sloping wind will just move around the fins.

My reasoning for removing the tailgate and dropping the mod all the way to the floor has to do with 2 things.
1st. Aircraft streamlining always seems to pay much more attention to how the air leaves the back of the airframe, look at the gentle taper of the rear of an airliner compared to the fairly steep slope of the front.
2nd. In observing the behavior of a tarp I have used as a temporary tonneau cover, I notice that the air really tries to mash it down in the area 2 to 3 feet in front of the tailgate. This indicates to me that the air really wants to go there and will remain attached to the aero cover.
3rd. I know based on reading other forums and from a Mythbusters episode, removing your tailgate does not help your mileage, but this is just removing the gate without putting something in the bed to fill the "void".

By doing this, I think it will eliminate a large area of vacuum which gets formed at highway speeds behind the tailgate itself creating a significant drag load.

Again, I know this is not a look for everyone. Factors influencing my decision are:
1) I don't care how it looks or what anyone thinks.
2) I don't need to haul anything in the back of my truck.
3) I can remove it and store it at my destination, so the aero mod will only be used for my 6000 miles of cross country highway driving.

I'm thinking of making it out of dimensional lumber for a frame and using 1/4" OSB for the skin. Although this build has me considering using shrink-wrap and an OSB space frame kind of a thing.

So what do you all think?? :confused:

Hello ChazInMT and wecome to EM.

I moved your thread to keep Fubeca's thread clean and give your thread the attention it deserves.

There are a few aero experts on this forum and I'm not one of them but will say that removing your tailgate and sloping the aerolid back to the bottom of the bed will probably be too steep of an angle and will cause separation. If you get separation, you won't see any benefit in drag reduction. You should do an ecomodder forum search on streamline template. For a rough guide, 11 to 15 degree is about the max angle you can go depending on the height of the truck. Again, I'm no aero-expert but an ecomodder lurker.

btw, great build! It's giving me more inspiration on my tacoma aerolid.

seminars
 

I recommend you read the seminar on rooflines.It will explain the science of attached flow and explain why your idea will only disappoint you if you go that steep.

Where do I find these seminars you speak of?

An aerocap needs a curve at the beginning, and you can save some length by moving that job to a bulge on the roof, making one smooth line from windshield to cap. Leaving fins at the side is not so good, because air will be trying to spill over, since it has no tapering in to do at the sides. I'd be inclined to leave the tailgate on, and extend the cap at least to the end of the lowered gate, with a flat back. One way to get quick curves and framing is to use large-bore plastic sewer pipe for the lengthwise edges. With a maximum angle, you need rounded edges in a crosswind. Anything you can do to smooth out the front end with an overall cover/grille block will help too. If you won't need to pop the hood, some outdoor window shrink-wrap, taped on, might be nice. Some stuffing would give it a better shape.

Here's the seminar thread: http://ecomodder.com/forum/showthrea...html#post28158

And the index of seminars...

http://ecomodder.com/forum/showthrea...ists-7118.html

All right then, I've spent the last 3 days thinking hard about this, and have come to a few realizations.

1st, I no longer feel that the vacuum created by the large area of the tailgate is the number 1 enemy.
2nd, it seems the main thing to avoid is turbulence. The energy which goes into making air swirl about or spin like tornadoes behind the truck are really what we need to be aware of.
3rd, a clean release line at the back of my cap/truck is preferable to a radius style release.

I have made 40-50 movies using Flow Illustrator to test out my theories and to check on what the going wisdom I have seen on the site look like.

My idea of extending my roofline down would be valid if I had a 2d truck, almost no turbulence is seen at the rear with a very clean release. But as fate would have it, the GMC Sierra pick up which I own, is indeed, part of the 3d world.

One of the things which had me seriously second guessing the "Leave the sides sticking up in the back" theory was the idea that the air on the outside rear of the bed would be getting sucked into the low pressure area created on the inside rear, this is a set up for some major vortex creation. I got to thinking I could add spoilers to counteract this down inside the box like the wing tips on airliners, then I remembered, I'm a freakin amateur, trying to design these things is beyond my capabilities. So In order to keep it simple and still do myself some good, I have decided on keeping everything above the box. I still want it to be as low as possible at the top of the tailgate, and I want the front facing profile to match the back of my cab.

Now I have come to consider the details. One of the things that just seems not quite right with Bondo, Sepp, and Fubecas builds are how the sides remain fairly vertical all the way to the back of the truck. This creates a large down sloping area on top of the cap, with long boundary areas on each side. Even in some of the tuft test images I've seen the side air wanting to roll up to the top. If this is generating a vortex or turbulent flow on the back corners, it is certainly an efficiency robbing problem. Even Bondo reports dirty air here. I’d be curious to see how paper party streamers (think “Just Married”, pop cans tied to strings optional) or longer strands of yarn would behave at highway speeds if they were taped to these designs on the top of the caps above the tail lights.

So, in my laying awake trying to combine things that I believe to be true, on image popped into my head. How do we taper in the sides? Keep the profile tight on the front? Try to account for the vortex generation on the corners? Certainly someone has thought of all this?

Enter:
Attachment 5301


So this is what I’m basing my design on. The air coming from the side window areas can taper into the center a foot or so along the side, this allows that air go where it wants to without having to leap over the edge to do it. Any vortex creation that tries to get started on the corners will be stopped dead be the horizontal triangle areas in the rear corner areas of the cap.

Obviously, since my need for downforce generation at high speed is nonexistent, the rear wing thinga mahjig has to go….maybe I could sell it on ebay.

My trick now is going to be in the production side of this. I really like Fubeca’s construction techniques. I have a similar set of woodworking tools. My only questions are how readily will the thin plywood will accept compound bending and how do I get the shrink wrap plastic to hold into the concave area where the horizontal triangle area meets the fastback hump?

I’m really not thinking at all about fiberglass because of the expense and my never having messed with it. I have been a builder for 9 years and am quite the finish carpenter so I think I’ll stick with the wood.

Has anyone tried to incorporate these ideas into an AeroCap before?
Does anyone have any other ideas on how to protect the wood?
Also, what did Fubeca use as a heat source to shrink the plastic?

I plan to make this as 1 piece and keep the weight down through engineering.

I’d add a LED strip 3rd tail light in the area under the tailgate and above the bumper to address any legal concerns regarding covering my 3rd stop light.

So, What do you think???? Any and all suggestions or comments at this point are welcome.

thinking hard
 

Profile drag IS pressure drag.The turbulence is what creates the low base pressure against the back of the truck.The pressure differential between there,and the forward stagnation point at the front IS your drag.
The side/roof vorticity is created as the roof/side flow meeting at different velocities,as where a tributary meets a river.
Eliminating this would require that the cab roof and entire bed of the truck be discarded and completely re-designed from the ground up.
Even Kamm's cars had this,you can see the tuft testing in the thread on FLOW-IMAGES.
Your idea for the plan taper,as in the Nissan photo is okay.I've had this since 2005 and it gives me a 13% mpg improvement vs 10%.
There will still be vorticity.You can see mine in the Aerodynamic Streamlining Template thread.
With respect to weather-proofing the wood,I can only recommend something like West Systems epoxy.I've tried many of the polyurethane spar varnishes,from cheapest to most expensive,I think they're all crap.

1. Build a basic pole system and tarp it. Easy.

2. Why even have a truck if you dont haul anything.

Thanks for the tips Aloha.

Unfortunately, I can't afford to buy a regular car right now, I was a builder for 8 years and now travel around the country working at Nuclear Power plants during scheduled outages. So, I am forced to use my truck to travel. All my crap fits inside the cab of the truck, and if I did need to put something in the back, I could, it would just be a bit of a pain.

A pole and tarp system sounds OK to test a theory I suppose, but I plan on driving 6000-7000 mostly highway miles in the coming months. I don't think I could engineer a pole & tarp thing to hold up. Not to mention the noise coming from such a thing would likely drive me nuts.....or I suppose you could say, More nuts.

rough. too bad you cant sell the truck for something better on gas.

the tarp isnt too bad if its tight enough i guess

http://ecomodder.com/forum/showthrea...-cap-5640.html

http://ecomodder.com/forum/showthrea...aero-1224.html

http://ecomodder.com/forum/showthrea...-cap-3539.html

Aerohead, other readers,

I'd like to share some of my expertise as a boat builder in payback for the
aero knowledge you share with us every day. (BTW, many thanks.) :thumbup:

There is no way to permanently stabilize the surface of fir plywood. No
matter what you put over it, even a layer of epoxy impregnated fiberglass,
in time it will "surface check." That means the face ply will crack and pull
apart, up to 1/8 inch. It will tear the fiberglass. You can put on multiple
layers of 'glass, but the cost and weight get prohibitive.

A better choice for a wood-only construction would be ply made from okume
or sepele, these are standard marine construction materials. The are quite a
bit more expensive, and they do surface check, but the checks look like
razor blade cuts. The are easily filled-in/repaired during a second paint job.
One nice feature of these plywoods is you can sometimes get it in 5 ft by 10
ft sheets. The cost can be surprisingly high, but a standard sheet is 32 sq ft,
and these are 50 sq ft, 56% greater area.

For a truck cap, I would recommed HDO Overlaid fir ply -- High Density
Overlaid. It has one or both faces covered with a layer of resin impregnated
film, heavy kraft paper I think. The key here is the layer is fully water tight,
and the face ply doesn't check. It takes paint well, no sanding, and the grain
in the ply is pretty much subdued.

A heavier duty version of this is Signal Overlay. It's the stuff the big
overhead freway signs are made of. It is fir ply with a hardwood face ply,
and a HDO on top of that. Excellent paint surface, no grain "show through."

Odd Radiation on Merlin
 

[QUOTE=Rokeby;152783]Aerohead, other readers,

I'd like to share some of my expertise as a boat builder in payback for the
aero knowledge you share with us every day. (BTW, many thanks.) :thumbup:

There is no way to permanently stabilize the surface of fir plywood. No
matter what you put over it, even a layer of epoxy impregnated fiberglass,
in time it will "surface check." That means the face ply will crack and pull
apart, up to 1/8 inch. It will tear the fiberglass. You can put on multiple
layers of 'glass, but the cost and weight get prohibitive. "

There are ways to defeat epoxy encapsulation, but it isn't easy if you work indoors and follow the instructions. Fir ply is seldom bright finished, but it has always worked for me.

wood
 

Rokeby,would you use a sanding-sealer and then a marine grade paint,as one might use on an all-wood Chris Craft?

Aerohead,

I can't tell which one of the options I gave you are asking about. :confused:

So I'll address them in turn.

One point on using any type of plywood. Plywood failures typically start at
panel edges that aren't part of a properly executed "two step" epoxy joint.
This due water getting into the end grain in the interior plys. This due
careless prep of the edges. You have to seal up the end grain so it can't suck
up water. This take 3 or 4 coats of straight epoxy. Keep on coating until
when you sand lightly with 100 grit paper there is no indication of the end
grain... smooth and even. Only then is it time to begin the painting preps.

I wouldn't use fir plywood... too much trouble downstream.

On HDO and Signal HDO, you don't need or want to sand, that will just fuzz
up the overlay. Go straight to a good grade oil based, "alkyd," primer. Two
thin coats dry faster/harder than one thick one. Lightly smooth/skuff the
surface with 120 grit abrasive, "sand paper" to most folks. ;)

Then on to high quality alkyd, or one part polyeurethane, top coats. Marine
grade paint is considered the best. But at least when it comes to alkyd paint,
there is general belief that "porch paint" is the same as marine grade.
Sand with 220 between coats.

Painted surfaces that will see service out in the real world will last longer,
and require less downstream upkeep if allowed to dry for 30 days before
being put into service... yeah, not likely. But a week at least.

As to the finish on a classic Chris-Craft type hull, that would be a whole
different world. First off, the hull may be a single layer, or even laminated
of 2 or 3 layers. For the most part it would be treated as solid wood. Some
folks would start with a single purpose sanding sealer. Others start with a
25% varnish, 75% thinner first coat. This just to get the fuzzed up wood
fibers to stand up for sanding, subsequent "prep" coats at 50/50 and then
25/75 thinner/varnish. Further "full strength" varnish coats until the grain is
filled, as many as it takes. (Actually the varnish is thinned 10-15%, and
plasticicers and drying agents are used to doctor the mix to ambient
conditions.) Sanding at this stage with 150.

Only when grain is filled do you start counting varnish coats. How many?
How much time/money do you have to spent? At least 20, 30? Abrasives
gradually get less gritty; 320, 400, 600. Most folks stop there, higher grits
tend to polish as opposed to smooth. Could take 15,000 sheets on a 30 ft
hull. :eek:

Down stream, you'll need to do some top coating every year. Varnish just
disappears, it ablates. How many coats depends on UV levels. You want to
be slowly building up, never thinning the total film thickness over time.

Sort of like the 50's era "hand rubbed lacquer" auto finishes. :cool:

And that's the short course! :rolleyes:

1st Drawing of AeroCap
 

Here is a basic line drawing of the FastBack AeroCap I'm thinking to build.

http://ecomodder.com/forum/attachmen...2&d=1263227286

Chaz: line drawing didn't show up. Were you posting a link or attaching an image?

I see the drawing now. I like it! This is what I was looking to build but I couldn't come up with easy/cheap way to do it on my budget and timeline.

I am excited to see your progress - I think my version 2.0 will look more like this.

Thanks Fubeca! I'll take a bunch of pics. Your build has given me the inspiration for the construction technique. I am going to go buy a roll of butcher paper, or craft paper and I'll trace each part as I make it. This way I can either copy the work, or have a basis to tweak it.

fastback
 

I've been enjoying mine since 2004,I think you'll like your's!

Just read your Post.
 

ChazInMT,

Just read you post on the aerocap you were thinking about building. Your sketch is very good and I know how it is when you have a good idea and just don't have the time to build it. Believe me, I also know how it is to travel long distances in your work. An aerocap would save you alot on fuel.

Back to your sketch. It reminds me alot of one a friend of mine designed several years ago. He lives here in Little Rock and is in the aircraft design biz. Your sketch looks alot like his design which I always thought was pretty cool. I scanned the rendering he did and post it now.

Good luck with building your cap and if I can lend any advice on the build, just PM me and I'll be glad to help if I can. It is great to see such an interest in aerocaps and I have a great deal of respect for anyone who has built or plans to build their own aerocap.

Bondo

ChazInMT:

Your drawing looks a lot like my fairing.

What to use for Shrink Wrap?
 

OK, I been thinking hard about how to cover my AeroCap. I've come down to 2 alternatives.

* 6 mil black polyethylene from Hardware store, or
* 7 mil White Marine grade polyethylene from a ebay store that sells it by the foot.

$25 cost if I use 6 mil black Polyethylene from my local lumber yard, it's enough to cover my cap 7 times, 10' x 100'. ( I wouldn't put 7 layers on, I could screw up 6 times and still have one shot is what this means)

Good
It's cheap.
I think the black will look good on the back of my truck.

Bad
I'm afraid of 2 things,
- Will it heat shrink well enough? The guy in the store tore a bit off and we put his lighter to it and it shrank, so I think I'm OK here.
- On hot sunny days, will the heat from the sun cause it to get real loose and baggy?
- On hot sunny days will it heat up my bed area too much & stress the wood shell?

For $30 I get enough to cover my cap once, 10' X 16', with the marine grade 7 mil, and I have a 5 x 10 chunk left over to play with. (More like I play with the 5 X 10 chunk to get my technique down then do the job for real)
For $40 I could get a chunk that's 16' x 16' and I could wrap my cap twice.

Good.
- It's UV protected to last 1 year.
- I know for sure it'll work. (98%)
- It's white, so it will heat up less in the summer, keeping the bed area cooler and being less stressful on the substructure.
- I think it will be more stable thermally and less likely to get baggy on me on a Sunny 95F day.
- It is 1 Mil thicker & probably better quality.

Bad.
- It costs more.
- If it doesn't work, I'm really out $30-40. ( I could always use the black poly for something)
- The white color won't look bad, but I don't think it will be as stylish as the black. Although I could paint the white with Krylon Plastic Fusion paint ($20)


What do you all think? Should I not be such a freakin cheapskate and get the white? Or go with black, knowing I could always tear the black off and get the white later if the black fails me?

My vote is for black :)

It is cheaper, you have more chances to get it right and I think it looks better.

I thought about going with white (same stuff you are looking at) but it would have taken too long to get for my timeframe. I don't really think the boat stuff is significantly better quality.

One question, how are you planning to get the plastic to stay tight to the contours near the sides at the back where the flat section meets the curve? I couldn't think of an easy way to make shrink plastic work with that design (hence, my current cap doesn't taper in at the sides).

Thanks for the reply Fubeca.

I am going to make the concave part in one piece and the flat part in another. Then I will wrap them individually and put them together.

Like this:

http://ecomodder.com/forum/attachmen...1&d=1263879233

Kinda..Maybe. I think it will work. The flat part will be about this size & shape to act as a support platform for the back 4 feet. It will also provide a fair bit of "anti spread" strength at the caps mid point. Over engineered? :eek: Probably :rolleyes:.....it's kinda my thing. :cool:

I have changed the back of the concave part to have a 3" flat area on the aft end. I did this for ease of construction. I couldn't get my head around making it taper flat and still have something to nail everything down to. This way I can make the last "rib" out of a chunk of 2x4 for my final tie in.

I don't think it will cost me any efficiency at all.

Status Report:

I have the ribs all made, I plan on making the flat parts for the ribs to rest on tomorrow and putting together the flat deck too. I'm using 1/2" OSB for everything so far to keep the weight down. I have 2 sheets of 1/4" CDX for the skin, it's pretty rough but it was half the cost of anything else they had at the lumber yard which I would consider using. I may just need to plaster on a few glops of Elmers wood filler to fill in some knots in the CDX, or not. I don't think they'll be seen through the shrink wrap. I have taken some pictures which I'll post when I get the frame together and before I put the sheeting on.

I changed the shape of the top profile of the cap, I was going to make it rounded to match the roofline of the truck, but since I have a fairly pronounced aero shape when viewed from the side I thought trying to get the 1/4" CDX to compound bend would be difficult. So now I basically have a 2 foot wide flat area on the very top of the cap which will go from stem to stern. On each side of that is a 1 foot wide flat spot that tapers for the final 3 feet, it is about 10 degrees sloped in relation to the 2 foot wide area. I figure it'll make the application of the sheeting a cinch and still match the profile of the roofline within 1/2 inch or so. See the illustration of the rib profile for a better idea.

http://ecomodder.com/forum/attachmen...1&d=1263879056

Thanks for your input. I suppose I can keep a bunch of the Black Poly in my truck and if it gets too loose & baggy I could always cut new pieces to overlay and slap staple the crap out of them, it would look like hell but would work.

Progress Update
 

Status Report:

Thought I'd post some pics.

http://i48.tinypic.com/ephesn.jpg

Here is the Rib Template I had made at Kinko's. Since I designed everything using Microsofts Draw program, I thought it would make my life a bit easier to have a full scale drawing to work from. I was able to scale it up perfectly from a 1" = 1' scale drawing I made in a word program. I converted this into a pdf file and the Kinkos guy was able to print it on his 36" roll feed printer/plotter thingy. $9, well worth it!

http://i50.tinypic.com/4in15f.jpg

I didn't want to cut the template up, so to make life more difficult on myself, I made patterns out of it by tracing some Tyvek house wrap I had laying about.

http://i48.tinypic.com/f0rcit.jpg

http://i47.tinypic.com/200wu2d.jpg

I then traced the tyvek templates onto the 1/2 osb. This was my first trace and I was trying to use tape.....not so good, I ended up using my slap stapler for the rest to hold the pattern in place.

http://i45.tinypic.com/2rz5r3o.jpg

Then I cut the ribs out with my Mk 47 jig saw.

I spent a good number of hours sorting the ribs out with a belt sander to get the shapes all matched up. Then I started on the base for the ribs to rest on.

I'm using all 1/2" osb to save weight. I think the frame when complete won't be very sturdy, but once it is sheeted with the 1/4" plywood, I think everything will work together to make it quite tough.

I had a $20 home made tonneau cover which I used as a platform in place of building off of my truck bed. Today I was able to dry fit the ribs onto the base to see what sort of shape I had. The following are pictures of this dry fit, everything is just sitting there loose, no nails or glue or nothin yet. I'm happy so far.

I'm a bit worried about the plywood for the sides, I know I can kerf the back with my circular saw to help it bend into shape. What I'm afraid of is when I bend the plywood around the back AND I twist from 70° to 40°, is the wood going to want to "dive down"?...Just thought of how to solve the problem. I'll make another Tyvek template and play with that, whew.

I'll post more pics as I progress.


http://i45.tinypic.com/flc7r6.jpg

http://i49.tinypic.com/288p8x3.jpg

http://i50.tinypic.com/2ppjais.jpg

http://i49.tinypic.com/i1mmb5.jpg

http://i50.tinypic.com/2qtzhmo.jpg

http://i46.tinypic.com/2vb0o3p.jpg

http://ecomodder.com/forum/member-ch...t-back-qtr.jpg

I'm very excited to see this progress. For some reason - I can't see any of the pictures but it sounds like you are making very good progress. I think your attention to details will make your top much more finished looking than mine!

I see them now - Looks great! I look forward to seeing your technique with the plywood covering. I had a hard time getting even the minor compound curves bent in mine.

Very nice!

Great photos. Definitely giving me some inspiration on my aerocap build (when I get to it). I've been tossing around the idea of compound curves or not. Please document the plywood attachment. I'm hoping to add another element of having it open like a hatchback.

So far, great build. Looking forward to the progress.

Added to the project library (EcoModder Project Library - EcoModder.com) .

Enjoying watching this come together.

Nice cross sections!
 

That is going to be one sharp looking aerocap when you are done. It is alot of fun building your own aerocap. Fubeca has definitely set the standard on how to construct one for a minimum cost.

Progress Update
 

Today I finally got a part of the "Skin" made to put on the rib frame....and naturally the first thing one must do at this point is chuck it on the back of the ol rig and see if I've made any galactic errors before continuing on.

Took some snaps, thought I'd share them for those who are interested.

I think it looks good, I initially had the front frame pulled back an inch er so at the top, that wasn't going to work. So I fixed it post haste. I think I'll bend the corners in to close the gap a bit, it should end up being about 1 1/2" on avg, no real concern I don't think. Part of me wonders if the gap won't upset the flow a little bit and help it stay attached better.

I will work on the rest of the skin now, it isn't going to be easy for sure. The way everything tapers down to essentially a point at the back along with the radius corners is going to make for some very interesting looking 1" strips. The sides are contoured to match the truck shape, curve in, and twist from front to back. I can't decide whether to use the 1/4 CDX or run out and get a more workable/flexible 1/4 mahogany sheet. My fear is that the mahogany will disintegrate on contact with water if anything should rip the shrink wrap skin I put on. I suppose 2 cans of spray paint would help prevent that. The mahogany costs $8 more. I wish I could use coroplast, but these sides are critical to the structure of the cap.


http://ecomodder.com/forum/member-ch...aight-back.jpg

http://ecomodder.com/forum/member-ch...t-rear-qtr.jpg

http://ecomodder.com/forum/member-ch...t-rear-qtr.jpg

http://ecomodder.com/forum/member-ch...wing-taper.jpg

http://ecomodder.com/forum/member-ch...wing-curve.jpg

http://ecomodder.com/forum/member-ch...ed-gap-bit.jpg

Looking good. Nice to see somebody else is hardy enough for winter work. I'd just like to correct the usage of the term "laminar." That is flow in which the air just distorts smoothly, with each bit maintaining its distance from the surface. It usually extends from the front stagnation point to the first scratch or change in contour. To maintain it back as far as the mid-point on a shape at our regular speeds, you need a single, perfect contour and no blemishes.
The concern about the gap mentioned here is to maintain Attached flow. In general, minor roughness aids this by thickening the turbulent boundary layer, although a bad contour can initiate separation where it is close to coming off anyway.

ChazInMt,

You are facing the same issues in covering your 'cap that traditional boat
builders do when planking a hull; How can you quickly, accurately, and
repeatably find the true correct shape of planks that bend and twist to
conform to the ribs?

The process is called "spileing" and it is a purely mechanical way to generate
the needed shapes accurately -- no special tools, no computer needed. :thumbup:

Here is a place to start: Spileing .

Thanks Bob, I was thinking attached but wrote laminar. I don't think there is a whole lotta laminar going on with the shape of this truck. I changed my post.

You're most welcome. I've also been called a grammar fascist, but I'm mostly interested in technical clarity, since we do so much education here.

Plywood glue is usually quite waterproof, although cheap door skins are sometimes flawed in several ways. In boat building, it is sometimes given a mild compound curve and called "tortured Plywood" but those techniques don't work for this shape. Any wooden construction can be made stable and waterproof with a good coat of epoxy. The popular WEST brand is an acronym for Wood Encapsulation Stabilization Treatment, or something similar.
If you wanted a fiberglass shell, one easy way is to cover frames like those with C-flex, a fiberglass cloth with resin-cured thin fiberglass rods woven in in one direction. The frames can be retained as structure or just used as a mold.

I was planning on setting my circular saw to 1/8 inch deep and making kerfs in the back in the direction (or two) that I wanted the wood to bend. I know it isn't the best way to do it but I don't think it will affect the strength that much if I fasten it well. This is just a "proof of concept" cap anyway. I'll be happy to use it until June. So epoxy coating is not really a priority. If it significantly improves my mileage on the road, I'll make a more robust version with some kind of hatch back door thing and windows.

I'm fairly convinced that this is a good direction to start with by tapering in the sides and having a curved arch top profile. Most folks don't taper the sides and I think really only end up improving the airflow over 2 or 3 feet down the centerline of the truck bed. By taking the time to do this, I'm managing all the air above the bed which is about 8 linear feet of the slipstream (22" tall sides + 52" wide roof). After reading a bunch of technical papers today over in the "General Efficiency" forum, I'm getting a little concerned about lift forces that might be generated at highway speeds (with a head wind, passing a truck and 82% humidity). I don't think it will significantly affect handling, I just worry that the cap will come loose and take a short flight. I'm going to double the hold down system I was originally planning to avoid having to fill out a EAA membership, although I'm certain Burt Rutan would love having me in the ranks.

planning
 

The cap will "explode" if not strong enough to withstand the delta-p across it.
And it would "leave" if allowed,so good idea on extra hold downs.
The plan-view curvature will cut extra drag.You'll experience a bit of cross-talk where the rails and cap meet.It cannot be avoided short of a bed re-design.Kamm suffered this on his K-cars,don't worry about it.
As far as bow shock-waves from 18-wheelers,etc.,she should be rock-solid.The cap will provide you downforce and you'll swear there's a load in the bed the way she rides.
Put in a Plex side window on the passenger side if you can.Backing out of a parallel parking space can be terrifying without it.
Even a slit at the rear will also give you some vision you'll enjoy.
Tensile strength is your friend.
P.S.If you're going anywhere near goats,make it VERY strong!