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Old 09-20-2018, 01:24 PM   #1 (permalink)
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Tonneau/half tonneau.....spoiler/half template

Hello again everyone.

I am enjoying reading my new book (Aerodynamics of Road Vehicles) and found the chart we have all seen on the aerodynamics of pickup trucks. I currently have a Ford Ranger with a soft full tonneau. According to the chart, there is a large gain (reduction in drag) to be had from adding a cab spoiler/wing as long as it attempts to fit the template. It only shows this with a half tonneau.

Does a spoiler/wing work just as well with a full tonneau (and didn't need to be drawn on the chart) or is the half tonneau the only want to see that amount of drag reduction? How does the extra cavity behind the cab from a half tonneau affect air flow as opposed to a full tonneau? Does it draw the flow downward to more approximate the template?

I would like to keep my full soft tonneau as it comes in rather handy and it's much better than the open bed.

The other question I have is regarding the type of spoiler/wing on the chart and how it may compare to something like a full aero cap cut in half. Something like this but shaped to better fit the template. For me it would allow better rearward visibility without having to engineer a weatherproof window to keep things dry. And if designed right I could still keep my soft tonneau.

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Old 09-20-2018, 02:44 PM   #2 (permalink)
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I did something similar on my Tacoma a while back.

https://ecomodder.com/forum/showthre...tml#post480010
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Old 09-21-2018, 10:49 AM   #3 (permalink)
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That's exactly what I'm thinking aardvarcus. Great work. Looks like you ended up with a 5-6% increase. I wonder if it would have been slightly higher combined with a bed cover of some sort.

I'm not a fan of the AST-2 shape from an aesthetic perspective though. I know it works but it's a little steep for my taste. Is it more effective than AST-1 or just another acceptable version of the template?
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Old 09-21-2018, 01:05 PM   #4 (permalink)
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hat-man,

I am not an expert, and hopefully the more experienced will chime in, but the AST-II is a little more aggressive than the AST-I but still well within safe conservative values. As you get more taper quicker, for a given fixed length the AST-II should perform better, but running the AST-I longer would allow the item to catch up. (e.g. it may take a 20" AST-I to match a 18" AST-II) You can always go shallower than either "template" but it will take longer to get the same effect.

I will warn you drag reduction gets addicting, and you may regret not doing more initially. It wasn't long after my half shell was finished that I started my full shell.
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Old 09-22-2018, 01:46 PM   #5 (permalink)
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cab wing

Quote:
Originally Posted by hat_man View Post
Hello again everyone.

I am enjoying reading my new book (Aerodynamics of Road Vehicles) and found the chart we have all seen on the aerodynamics of pickup trucks. I currently have a Ford Ranger with a soft full tonneau. According to the chart, there is a large gain (reduction in drag) to be had from adding a cab spoiler/wing as long as it attempts to fit the template. It only shows this with a half tonneau.

Does a spoiler/wing work just as well with a full tonneau (and didn't need to be drawn on the chart) or is the half tonneau the only want to see that amount of drag reduction? How does the extra cavity behind the cab from a half tonneau affect air flow as opposed to a full tonneau? Does it draw the flow downward to more approximate the template?

I would like to keep my full soft tonneau as it comes in rather handy and it's much better than the open bed.

The other question I have is regarding the type of spoiler/wing on the chart and how it may compare to something like a full aero cap cut in half. Something like this but shaped to better fit the template. For me it would allow better rearward visibility without having to engineer a weatherproof window to keep things dry. And if designed right I could still keep my soft tonneau.
In the original 1988 SAE Paper by Texas Tech,they listed the lowest drag lengths and angles for the cab-wing,depending on whether the truck was a short bed or long bed.
And to my knowledge,they were only tested with the half-tonneau.
I believe that this data is in the 'Seminars' on page-1 of the aero forum.
The theory behind the 1/2 cover,is that the low pressure from the swirling core of the vortex captured by the 1/2 cover,is communicated under the cover to the inner face of the tailgate,creating a high pressure differential across the gate,giving a forward 'thrust' if you will.
As to half of an aeroshell,AeroStealth and I tested a half-tonneau with the last half of an aeroshell installed on top,and realized an 8% improvement in HWY mpg.
We never tested the 'forward' half.
You might cobble something together with cardboard and duct tape,strong enough for testing.If you like the result,then make a permanent version.
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Old 09-22-2018, 03:06 PM   #6 (permalink)
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If I'm understanding you right Aerohead, the high pressure area is under the half tonneau and trapped there. And IIRC, isn't there an area of low pressure behind a vehicle, created as it punches a hole through the air. And a full aero cap helps make this low pressure area smaller.

Would there be any benefit in "venting" some of that high pressure under the half tonneau (say something like 20%) through a couple of openings in the tailgate (something like two 2" pvc fittings) into that low pressure area to further reduce it?

In my mind I'm seeing a cab wing and half tonneau like in the chart, with two openings at the lowest part of the tailgate nearest the bed. Or even crazier yet, turning the area under the half tonneau into some sort of a modified De Laval nozzle.

p.s. I think my mind has finally wandered way out of the box having to be on the couch with this broken foot. LoL
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Old 09-22-2018, 03:11 PM   #7 (permalink)
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Ok after re reading that maybe it's low pressure under the tonneau and a higher pressure (still low but not as low as under the half tonneau) behind the truck at the tailgate, causing the pressure differential and the idea of 'thrust"?
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Old 09-22-2018, 03:45 PM   #8 (permalink)
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thrust

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Ok after re reading that maybe it's low pressure under the tonneau and a higher pressure (still low but not as low as under the half tonneau) behind the truck at the tailgate, causing the pressure differential and the idea of 'thrust"?
*Without the tonneau cover,there's nothing but turbulence back there.All the kinetic energy is lost forever,and there's no chance for any re-attachment.A lot of drag and a lot of lift.
*The tonneau provides a surface of re-attachment for the separated flow,allowing for a much smaller wake,and it kills most of the rear lift.
*The half-tonneau is a peculiar animal.It allows a locked-vortex to be captured behind the rearward facing step of the cab.
*The inviscid (streamline flow) follows over the vortex as if it were more a solid body,strikes the tail end's upper surface,really killing lift.
*The extremely low pressure of the 'eye' of the vortex is transferred to the inner face of the tailgate.This inner face is at a lower pressure than behind the gate,creating a pressure differential which 'pushes' the gate forwards,something which doesn't happen with a full tonneau.
*If you cut ducting through the gate,you lose the differential,and the 'push.'
*Also,air would actually go through the hole.from the back to the front,tripping up the flow and creating drag.
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Old 09-22-2018, 04:20 PM   #9 (permalink)
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Thanks Aerohead. I get it now. Low pressure under the tonneau and higher behind tailgate. I'm assuming that the cab end of the half tonneau needs to be open to the bed to work correctly. It's kind of a shame if that's true. It defeats the practical purpose of the cover keeping the elements out. Full aero cap becomes the better drag reduction/practical option, but also the most labor intensive.

Is half tonneau a figure of speech, or is it covering 50% of the length of the bed? My mind is trying to configure something that is open to the bed and can still keep out most of the weather.
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Old 09-22-2018, 05:34 PM   #10 (permalink)
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50%

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Originally Posted by hat_man View Post
Thanks Aerohead. I get it now. Low pressure under the tonneau and higher behind tailgate. I'm assuming that the cab end of the half tonneau needs to be open to the bed to work correctly. It's kind of a shame if that's true. It defeats the practical purpose of the cover keeping the elements out. Full aero cap becomes the better drag reduction/practical option, but also the most labor intensive.

Is half tonneau a figure of speech, or is it covering 50% of the length of the bed? My mind is trying to configure something that is open to the bed and can still keep out most of the weather.
GM,who patented the half-tonneau, illustrated their drag curve as a function of cover length,and you can see that the drag minimum occurs when the length is 50%.So yes,it really is a 'half' tonneau.
And it is the forward portion that's open to the atmosphere.

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