Thread: Biggest gains are at the back???
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Old 01-13-2012, 06:22 PM   #30 (permalink)
aerohead
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Apology to NUNA folks
Quote:
Originally Posted by Ken Fry View Post
Hi Aerohead,
Good observations. I imagine you are thinking about more than you are actually writing. For example, you mention the wheel-less Aptera/Morelli shape which (several sources agree) is a .05 -.06 shape, as is a typical bomb, torpedo, etc. But what you did not mention is that the actual CD with wheels, suspension, ground proximity and drive shafts is at least .15, and the X Prize test results suggest a little higher than that -- although I don't have an accurate frontal area.



Sailplanes are all about lift/drag, and fuselage drag is a small part of the overall drag. Induced drag is the big part. If you were writing only about the fuselage, then there is a lot more than just skin drag. It would be simple to make a sailplane with a cylinder for a fuselage, but such a shape produces, (for the fuselage alone) a Cd close to 1. So I think your statement is a little shorter than it needs to be to be valid.





The MG, Bonneville streamliners, the .143 Sunraycer, etc all taken together show that there is something about them that is dramatically different than a Malibu -- it takes more than tapering the back to make a really low Cd.



I think you meant to say, perhaps, that sub .12 Cd's can result from wheel streamlining if everything else has been optimized. The MG's wheels are already quite well streamlined, and the body is extremely well streamlined, front and rear, but still it is not sub .12. The Aptera had well streamlined wheels but is a long way from sub .12. The VLC has well-streamlined wheels but is also a long way from .12

As of 2010, the .143 number for the Sunraycer still stood as the lowest recorded at GM.



This makes it sound as if one merely puts a boat tail on a car according to the template, and a Cd of .12 results. I imagine that's not what you intended. My old Citroen SM was a reasonably good fit for the template, but gained very little from extending the rear into a full boat tail, vs the existing Kamm back. Its Cd was around .30 either way.
Hi Ken,I caught your comments a few days ago and wanted to do some homework before I responded.Good comments,thanks!
*With respect to Aptera,Hucho's Cd 0.05 was for a 'normal' ground clearance.I understand that as a condition of X-Prize competition,that each vehicle would be evaluated for Cd.I've not kept up and no member has volunteered any data,so I have no idea what Aptera's Cd ended up at.
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* with respect to the sailplane,I must tell you that I shudder every time a member interjects aeronautical aerodynamics into road vehicle aerodynamics.
Hucho reminds us that fully attached flow is presumed for aircraft and the same cannot be said for road vehicles.
*Abbott and Von Doenhoff would remind us that 'data' for aircraft are valid only for 'flight conditions',i.e.,no turbulence,no ground-effect,and also that everything on an aircraft is a parasite beyond the wing.Since the fuselage is presumed in attached flow,then it can have no pressure drag,only skin friction.
The sailplane,aside from parasitic drag would experience induced drag do to lift.
*If members were talking about a fuselage as a road vehicle body then separation would have to in the discussion.
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*with respect to Bonneville streamliners and Sunraycer,the emphasis is on minimum frontal area of course,along with separation-free flow,and directional stability.The emphasis I'd like place with them is their similarity with respect to aft-body wake turbulence elimination via boat-tailing.
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*with respect to the Chevy Malibu,it's Hucho that tells us that the only way we can get it to Cd 0.12 is with boat-tailing.Walter Lay of the University of Michigan proved the premise in 1933 after wind tunnel investigation in which varying degrees of boat-tail were added to a model passenger car.Three of his 'cars' achieved Cd 0.12.
*And it is my contention that if you're looking for Cd 0.12 that you'll never get there without a boat tail.Of course,everything upstream has got to be clean and Hucho's the one implying that generally,the typical current day automobile has a good enough front to exploit the tail.
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* with respect to the 1971-1974 Citroen SM,I only have a brief article about one that owner Jerry Hathaway ran at Bonneville at 154 mph on an estimated 250 bhp.There is a photo of the car from the side.Under the 'Template' it appears that the flow is compromised at the very top of the backlight,with the rear slope going to 26-degrees (Ford Pinto,Datsun 240 Z).
The flow should separate right there with no hope of re-attachment.It's just too steep.Adding a tail,extending the original achitecture of the car's rear would stuff the wake,helping a bit,but would never establish clean flow.
The SM,at factory length,with a 'Template' roofline would be capable of Cd 0.19.
Hope this data helps.
PS I forgot about the wheel/tire streamlining.And yes,once the major body modifications were accomplished,it would be time to go after the wheel/tire drag of which Hucho reports,can constitute half the drag of a low drag car.
And we don't have much to work with except the solar cars,as they're the only ones which actually operate at real world road speeds in turbulent boundary layer flow.
An apology to NUNA folks.When Ken posted the photo of the NUNA my brain went to the pac II super mileage car.(early dementia!)
I'd just recently been re-visiting an article about Dr.Andrew Franks at University of California,Davis,California campus,and a couple of super mileage cars they'd built(Side Effects & Shamu ).These are for SAE's Super Mileage competition which requires an average speed of 15-mph (24 km/h).
Anyway,my pea-brain envisioned NUNA-5 as one of these cars,when solar cars actually are 'full-size' cars and DO operate in a turbulent boundary layer flow(the real world).
I also beat up on NUNA's wheel fairings,and after 'looking' at the appropriate vehicle find no fault in its design.

So I want to say I'm sorry for the insult,and for casting confusion.I've sent off the Seattle,Washington for an ACME Corrective Hat and will wait impatiently for its arrival.
Last edited by aerohead; 01-16-2012 at 05:25 PM.. Reason: delete incorrect NUNA data
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