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.