Date: Sun, 18 May 2008 20:39:49 -0700
Reply-To: frankgrun <frankgrun@AOL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: frankgrun <frankgrun@AOL.COM>
Subject: Belly Pans for speed or MPG - Some Data
In-Reply-To: <00fb01c8b955$cc7900e0$6401a8c0@DJZL7KF1>
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I have just noticed these exchanges on the possible use of belly pans and the impact on Aerodynamics. I addressed this as recently as last July on the Vanagon list. I had been enjoying an exchange with Jim Akiba on actual available power with various engine transplants. From the archives, I reproduce below the text of that exchange on several approaches to reducing drag (and therefore increasing mileage and available power. The last paragraph addresses my belly pan experiments.:
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> Ha I liked your reply Frank. Excellent. We have multiple dyno charts all > taken from the same chassis dyno for a vanaru 2.2, tiico HO, 2 wbxers, 4 > different zetecs, 1 supercharged zetec, and coming soon e85 naturally aspirated > zetec, and e85 supercharged zetec.. as well as the upcoming turbo kit(the entire > exhaust tract is now complete)� Having all this info from the same dyno is > quite nice, and really the only way to do it. As Mark points out, it is the > "complete" inforamtion that is really required. BTW Frank, Clint(a listmemeber > and solidworks guru) is helping us model the vanagon so we can put it into a > CFD virtual wind tunnel... my intention is to build a collaspable colluder > cone (see through on top) to see how much fuel efficiency we can gain at cruise > with a simple strap on cone of reasonable size that is easy to produce and > inexpensive. I think given the low pressure area we always drag around behind > us, it may be a substantial improvement. > � > Jim Akiba > > > Jim,
Egad a reply to one of my rants! Exciting! I fully agree that results from the same dyno with the same operator seriously increase the quality of the data but more importantly the reality factor inherent in its comparative interpretation. For some time my son and I were developing a Solidworks based model of the Westfalia to tighten up the drag issues. Our power numbers come to second difference velocity curves taken at the Pomona International Dragway (when unoccupied by paying nutsos). We were also looking at FemLab for a far more detailed treatment.
In the course of these experiments, two areas showed enough promise to warrant some measurements. We looked at the rear drag profile and undercarriage sealing. For the rear drag issue, we modeled pressure zone profiles and put together three plastic hemi-balloons and taped then to the rear. I have to go back to my notes at the time but we reduced drag by 12 to 18%. In one test, the number was 33% but it couldn't be duplicated. (In the spirit of some tuners, I should have used this for an SAE paper). The upshot of this was that I got pulled over (we were testing at night on the Palmdale highway) by the CHP and got ticketed for occluded rear vision. Went to the court to fight the ticket. Had a full sheaf of papers. The judge listened attentively (!). He remarked that when younger he had had a bay window bus that just blew all over the road and asked if such mods would have any effect on stability. I offered that it was in the relm of possibility. I them looked around the courtroom to see blank stares for a mass of humanity that that must have thought I came from area 51. Finally the judge stopped the discussion, announced that this was the singularly most unusual anti-ticket response he ever heard, congratulated me for originality and dismissed the ticket with a warning about frightening other drivers at night.
The point here relates to the fundamental flaw. The rear balloon worked, we had an idea for speed related inflation, etc, etc, but no answer for rear vision. Practicality --- zip, but fun.
The second idea worked well and is a real solution. The open underside of the vanagon is a real drag component. We got a set of thin 0.0625 inch Al sheet stock and enclosed (frame rail to frame rail) and basically covered the underside from the front to the transmission. We added louvered slots for the radiator air flow, left the spare tire cover operable and stayed away from suspension components. Big decrease in drag - minimum 8% max 15%. 15% came when we added the Diesel sound box cover. Serviceability was a problem. The system has been removed as I complete the front to back dual muffler system for the TDi and the diesel fired heater. No temperature measurements, but we provide for air inlets at front by gas tank and at egress at the rear. Work in progress.
Frank Grunthaner
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