Tonneau With Steps
 

I have a makeshift aerolid. It is suboptimal in my view. Too steep (17 degrees) and I'm blind as a bat to the rear, thus married to big side mirrors. But at the same time, it was a 1.5 MPG improvement over a hard flat commercial tonneau, so the thing had some virtue. Therefore I'm on the lookout for something different: Better smoothing of air flow and better visibility.

Back in younger days I had a Z-28 that had a rear window cover that was a bunch (8 - I think) of stepped louvers over the rear window. Mostly, it was there to keep snow off the rear window.

This is what I'm thinking:

I still have the flat fiberglass tonneau. Why not fot it with three of four flat plates stepping down from the top of the cab. The plate would be paralell to the tonneau, so could see out; and the steps would approximate a aeroshell.

The top of my cab would be about 22 inches above the top of the tonneau. I'm thinking three 12 gauge aluminum plates. Only the top plate need ride close to the top of the cab. The lower ones would progressively begin farther back from the cab and would be attached to the tonneau with some sort of strut.

My only worry is the truck winds up looking like a Fokker DR1. But, then, it is the Red Baron.

Might work as long as the "virtual" angle is shallow enough... and to fix that if needed you could hang a shelf out the back. I don't know if it would work better/worse/same if the sides were enclosed.

Were I to build an aerocap I'd put a one or two step rear window in it.

go for it
 

Dave,I'd be very inclined to believe that you'll see some benefit.My original shell ( pre-goats ) had a step for the 3rd brake light and another for a vertical rear backlight.It was still good for 13%.
Both Ferrari and Porsche have used the multi-step backlight louvered cover.
And Ford's patented bi-wing spoiler ( Probe-III,Sierra,Merkur XR4Ti ) is kind of a cruder version,but with good effect.
Your steps should provide for re-attachment and creation of locked-vortices which will help smooth the outer flow.
And thinking about Frank's comment,if your cab-to-bed tailgate ratio is 'fast',the rear shelf would help shape the vortex closer to the 'template' curve.
I'd do it.;)

I'm a little confused here.

1. BigDave, I thought you had a 3/4-ton diesel. Am I confusing you with someone else?

2. Phil, Dave feels 17-deg slope is too steep but that's less than 22. Any comments on that?

I'm not sure if filled or open sides would be better. If you can compare both and report results, that would be awesome!

Thanks
KB

Hi All,

All pickup trucks should have rear-view cameras and sonar sensors anyway. Just add one. They are cheap enough....

Big Dave -

In another thread I made this for aerohead, and I think he approved it :

http://www.youtube.com/watch?v=-MobBDsznZE

CarloSW2

Interesting! Steady state fields look very similar.
http://kamperbob.com/images/aero/KammBack-Fine.png
http://kamperbob.com/images/aero/KammBack-Coarse.png

Cheers
KB

Kamper Bob:
1. Mine's always been a one-ton. You must have me confused with someone else.

2. I have no intention of going to 22 degrees. I have to step down a total of 22 inches in an 80 inch run.

3. I'm gonna do open sides. I'm looking for some visibility so I can cut down or eliminate my side mirrors.


CFG:
Thanks. That's what I'm looking for. I'm thinking two flat plates attached parallel to the top of the hard tonneau. Use the top of the tonneau as the zero datum, I thought the first one might be set about 14 inches above the datum and the rear one 7 inches above the datum.

I'm thinking 14 gauge aluminum with some welded longitudinal stiffeners. I'll also need stronger gas struts for the lift mechanism as the tonneau top gets heavier.

Dave, thanks. It IS you. Right, full not three-quarter ton. I knew it was heavier than half. I'm seeing that in your sidebar stats now. It was actually your thumbnail that threw me. The cap on back looks familiar but for some reason the truck looks like a mini. That's where I went wrong. No insult intended it just confused me.

I'm also puzzled by your angle comment. Why do you say 17 degrees is too steep?

Maybe I'm just not having an off day. (shrug)

Cheers
KB

results
 

KB,this is going to be all speculation on my part and here's how I'm looking at it.
If we place Dave's truck under the template and look at the slope of the tangent at the tailgate,we might find that,while short of Mair's 22-degree maximum angle,we're to 'soon' along the template line to support 17-degrees.
If you look at Mair's boattail in the archives,you'll notice that he doesn't hit 22-degrees until at least a full diameter ( 2-truck body heights ) behind the point of max. roofline camber.
So 17-degrees may be two 'fast.'
As far as the sides are concerned,I'm going out on a ledge.They should behave exactly like a roofline excepting that the flow is less energetic and cannot support as radical a divergence as the roof and even with a windtunnel,would require very careful integration so as not to introduce attached vortices where they 'blend' into the roof sides.
The other thing is the fact that Dave's will be an 'open' structure,something we don't have a lot of published data on.
I do have Ford's patent for their bi-wing spoiler and they go into the aerodynamics of it's performance.Even though their spoiler has 'open' architecture it can still develop some tasty bound-vortices which are capable of modifying the outer field flow in a beneficial manner.
I've found about a dozen cars which have used the 'multiple-rear-facing-step' backlight cover and it appears that they take the bound-vortex from the macro,to the micro level.
I have a very strong suspicion that Dave will see a benefit but I have no way to put any quanta with it.
When Lamborghini,Ferrari,Porsche and others adopt this technology as they have,you've got to figure that they did not arrive at the decision in a haphazard fashion.I've no crystal ball but I'm giving it my thumbs-up.
One upshot is that Dave will 'know' at some point and have something to report.