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Old 02-04-2010, 08:04 AM   #1 (permalink)
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Wake field of pickup

I've been following several discussions that touch on this area but I thought it should have it's own thread. I'd like to focus this discussion to pickup trucks.

Several members have fabbed some very nice aero caps for their trucks. There may not be 100% agreement on which is truly best, nor what "best" means from one person to the next. There does seem to be consensus opinion backed by MPG numbers showing improvement over an open stock bed. (Contrast that with great confusion about which produces least drag: tailgate up, tailgate down, tonneau cover, cab-matching toppers provide.)

I have read considerable discussion about side view profile as a design tool (starting point) for aero caps, but very little about top view profile by comparison. So I see that as one area of improvement in our knowledge base.

I would like to suggest some ideas to help quantify and compare designs and applications. Every truck creates a wake in the field of air molecules it plows through. That wake is effected by shape and speed of the truck. I think it could help to stick some numbers to that shape. For example, how far behind would a tailgater benefit appreciably? Call it draft zone.

I have other discussion points but I'll stop here for now.

Cheers
KB

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Old 02-04-2010, 06:52 PM   #2 (permalink)
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Good Idea for a thread KamperBob.

Here is a link to a thesis published by a Naval Officer who used CFD study to determine an optimum aerodynamic shape for an aerocap. I had posted this link some time ago here on ecomodder and many found it informative.

It is interesting to read what this report says about a conventional camper shell. It is also interesting what fuel efficiency percentages were realized by the author, who built his own aerocap for this study.

Hope you find the report informative.

Bondo

http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf
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Old 02-04-2010, 07:15 PM   #3 (permalink)
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quantify

Quote:
Originally Posted by KamperBob View Post
I've been following several discussions that touch on this area but I thought it should have it's own thread. I'd like to focus this discussion to pickup trucks.

Several members have fabbed some very nice aero caps for their trucks. There may not be 100% agreement on which is truly best, nor what "best" means from one person to the next. There does seem to be consensus opinion backed by MPG numbers showing improvement over an open stock bed. (Contrast that with great confusion about which produces least drag: tailgate up, tailgate down, tonneau cover, cab-matching toppers provide.)

I have read considerable discussion about side view profile as a design tool (starting point) for aero caps, but very little about top view profile by comparison. So I see that as one area of improvement in our knowledge base.

I would like to suggest some ideas to help quantify and compare designs and applications. Every truck creates a wake in the field of air molecules it plows through. That wake is effected by shape and speed of the truck. I think it could help to stick some numbers to that shape. For example, how far behind would a tailgater benefit appreciably? Call it draft zone.

I have other discussion points but I'll stop here for now.

Cheers
KB
KB,historically,drag coefficient has been the 'language' of aerodynamics.All elements of aero drag are represented within this number.
In 1963,researchers at GM published an SAE Paper which quantified the direct relationship between drag coefficient(Cd) and mpg,for City,Highway,Combined.
Since Cd is the most recognized indicator of shape-efficiency,we might be wise to respect it's use rather than attempt to re-invent the wheel by creating a new 'language.'
Tail-gating,in addition to being illegal in all states,would endanger all attempting to quantify it's affects.That would be better left to a test track as Myth-Busters used.
In the Seminar sticky,there is a complete discussion of the role of Cd to mpg.
As to "plan-taper" of the aeroshell.If one follows the template design tool,for a long-bed pickup you could expect a little over 21% drag reduction and 13% mpg improvement at 70-mph.
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Old 02-04-2010, 08:18 PM   #4 (permalink)
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I'm surprised they went with the plaster...I think the expanding foam would make a more reasonable lightweight material to shape with. I just might experiment with replacing my current cab high fiberglass topper with an expanding foam based one...
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Old 02-04-2010, 08:48 PM   #5 (permalink)
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Quote:
Originally Posted by aerohead View Post
Tail-gating,in addition to being illegal in all states,would endanger all attempting to quantify it's affects.That would be better left to a test track as Myth-Busters used.
Thanks for replying but you misunderstood. I did not encourage anyone to tailgate! (sigh)

Fact is, others WILL tailgate behind you whether you want them to or not. Try to "drive smart" (ie, light foot) and you too might find your efforts hampered by tailgaters. It seems like 9 times out of 10 the person tailgating me is not intentionally drafting just paying more attention to a conversation with someone via the cell phone. Why they don't just pass I do not know. I have developed strategies to discourage them. Slow down, temporarily use hazards, even take the unoccupied passing lane (in which case they invariably resume the speed with which they overtook me and proceed on their merry way).

Regardless of why they fall in behind and stay there, or how exactly you get rid of them, meanwhile they draft you. Knowing the length of your tractor beam could be a useful tool for dealing with tailgaters.

As an engineer and scientist I am hopelessly addicted to data and analysis. I would also point out that history is full of innovators who either ignored or (heaven forbid) dared to challenge conventional thinking. Low and behold some accomplished things their predecessors knew was not possible. Being tolerant of creativity and enthusiasm need not be an annoyance or threat; it could be an opportunity.

Cheers
KB
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Old 02-04-2010, 08:54 PM   #6 (permalink)
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Quote:
Originally Posted by bondo View Post
Here is a link to a thesis published by a Naval Officer who used CFD study to determine an optimum aerodynamic shape for an aerocap. I had posted this link some time ago here on ecomodder and many found it informative.

It is interesting to read what this report says about a conventional camper shell. It is also interesting what fuel efficiency percentages were realized by the author, who built his own aerocap for this study.

Hope you find the report informative.

Bondo

http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf
Bondo, that looks like some tasty reading.

Thanks a bunch!
KB
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Old 02-04-2010, 10:58 PM   #7 (permalink)
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Quote:
Originally Posted by KamperBob View Post
Thanks for replying but you misunderstood. I did not encourage anyone to tailgate! (sigh)

Fact is, others WILL tailgate behind you whether you want them to or not. Try to "drive smart" (ie, light foot) and you too might find your efforts hampered by tailgaters. It seems like 9 times out of 10 the person tailgating me is not intentionally drafting just paying more attention to a conversation with someone via the cell phone. Why they don't just pass I do not know. I have developed strategies to discourage them. Slow down, temporarily use hazards, even take the unoccupied passing lane (in which case they invariably resume the speed with which they overtook me and proceed on their merry way).

Regardless of why they fall in behind and stay there, or how exactly you get rid of them, meanwhile they draft you. Knowing the length of your tractor beam could be a useful tool for dealing with tailgaters.

As an engineer and scientist I am hopelessly addicted to data and analysis. I would also point out that history is full of innovators who either ignored or (heaven forbid) dared to challenge conventional thinking. Low and behold some accomplished things their predecessors knew was not possible. Being tolerant of creativity and enthusiasm need not be an annoyance or threat; it could be an opportunity.

Cheers
KB
The only thing I feel I should note is that tail gaters usually decrease the aerodynamic load of the lead vehicle, which is beneficial, or this is at least the way I understand it.

Unless the shape is somewhat streamlined to begin with, of course. There is obviously, as with everything, an equilibrium point, where it could either be a moot point for them to tailgate, or even possibly have a negative effect on the drag of the leading vehicle.
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Old 02-05-2010, 09:49 PM   #8 (permalink)
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Quote:
Originally Posted by bondo View Post
Bondo, do you have any idea how many hours you cost me since yesterday? (wink)

This research went a great distance towards answering some of my questions. It also created some new ones. For example, I'm trying to puzzle out the elevation at which the horizontal slice planes were taken.

Anybody know what's up with the blunt triangular protrusion in front of the tailgate in figures 54, 55, 66 and 67? Not the underbody; I know they slice the axles. I gather they are somewhere between the top and bottom of the box. Hopefully they were all taken at the same elevation.

I'm guessing the open bed detail neglected wheel wells. The eddies inside the bed seem to confirm that. It's a minor detail if focusing on covers. Still, I wonder what effect it could have on the baseline and relative difference.

Yup, I definitely see more hours staring at these plots.

Cheers
KB
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Old 02-06-2010, 02:23 PM   #9 (permalink)
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misunderstood

Quote:
Originally Posted by KamperBob View Post
Thanks for replying but you misunderstood. I did not encourage anyone to tailgate! (sigh)

Fact is, others WILL tailgate behind you whether you want them to or not. Try to "drive smart" (ie, light foot) and you too might find your efforts hampered by tailgaters. It seems like 9 times out of 10 the person tailgating me is not intentionally drafting just paying more attention to a conversation with someone via the cell phone. Why they don't just pass I do not know. I have developed strategies to discourage them. Slow down, temporarily use hazards, even take the unoccupied passing lane (in which case they invariably resume the speed with which they overtook me and proceed on their merry way).

Regardless of why they fall in behind and stay there, or how exactly you get rid of them, meanwhile they draft you. Knowing the length of your tractor beam could be a useful tool for dealing with tailgaters.

As an engineer and scientist I am hopelessly addicted to data and analysis. I would also point out that history is full of innovators who either ignored or (heaven forbid) dared to challenge conventional thinking. Low and behold some accomplished things their predecessors knew was not possible. Being tolerant of creativity and enthusiasm need not be an annoyance or threat; it could be an opportunity.

Cheers
KB
Thanks KB,now I'm on the proper page.
I've no solution for tailgaters.I refer to them as 'alonaphobes',incapable of being by themselves on the open road.
The fun thing about streamlining is that,if we do our work correctly,there will be no drafting possible,as a following vehicle would feel the full effect of the air behind our vehicles.
As to innovation and creativity,I'm all for that.I'm also for fully exploiting everything that has already been thought up since 1922,but never put on the road.
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Old 02-06-2010, 02:28 PM   #10 (permalink)
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possibly

Quote:
Originally Posted by Christ View Post
The only thing I feel I should note is that tail gaters usually decrease the aerodynamic load of the lead vehicle, which is beneficial, or this is at least the way I understand it.

Unless the shape is somewhat streamlined to begin with, of course. There is obviously, as with everything, an equilibrium point, where it could either be a moot point for them to tailgate, or even possibly have a negative effect on the drag of the leading vehicle.
Hoerner reported,that in the case of two perfectly streamlined bodies,that when 'tail-gated',the drag of the lead body increased due to destruction of the boundary layer over the aft-body.No NASCAR drafting for low-draggers!

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