Quote:
Originally Posted by jimepting
OK, my above post was a bit shakey on factual data, so I've gone back to Hucho to provide some real quotes.
On the subject of trusting "old" data, refer to page 25 and the description of the Tatra 87. Lange measure a model of this car at Cd=.244. Quoting directly from Hucho, "The drag figures published for this car caused a lot of confusion, and are a good example for how "old" figures must be handled......But the correct figure turned out to be Cd=.36 as measured in 1979 in the large wind tunnel of Volkswagen AG with an original car."
Page 26 of Hucho provides a very interesting comparison between the theoretical and the real. Midpage Hucho describes the development of a model, including wheels, by Lange which was "long and lean" and measured at Cd=.16 in the VW windtunnel. When this shape was translated into a real world car, the Porsche 911 Carrera, the Cd came in at Cd=.33. Huco comments that the Lange model must be considered in the light that it "was completely smooth, with no recesses for windows, no real wheel houses, and no undercarriage." I would add that it had no radiator opening and no mirrors. That hardly makes for a practical Prius or Insight.
To me the bottom line is that I personally need to have a practial goal of aeromodding in mind. The GM Aero, Huco p.57, perhaps shows what a idealized prototype can achieve, Cd=.14(by their own measurements) without mirrors, but no modder is ever going to achieve that shape with an existing car. I can see lowering the Insight Cd to the order of Cd=.20-.22, but I don't see modders getting any better than that. We don't have the option of doing away with window openings, wheel openings, undercarriage, and perhaps mirrors. JMHO
But, perhaps I degress. Your original point was that the Insight and Prius were too steep, to early on. I really don't have the basis to object to this assessment. but I thought it had been decided when 3 Wheeler was designing his boattail that his overall profile met the template. That would imply that the Insight angles were within spec. I guess I'll have to go back and read that thread again. I look forward to seeing some tuft testing on the actual car, after he finishes. I would only point out that the determination of the "high point" of the car has a rather profound impact on departure angles and that the criticality of the high point location is not really know for real world mods. Don't mean to offend and I think the conversation is really worth having. Perhaps Metro will be able to do some tuft testing and that would certainly help in answering this question. I would do it myself, but currently I'm very busy with some more critical things like repairing a bent racecar for sale and FINALLY getting my complicated taxes finished.
As always, thank you very much for your insightful thinking and your long term interest in aero topics. We couldn't do without you  |
The funny thing about the early numbers is that some,when tested in modern tunnels yield numbers actually lower than originally published ( Jaray small car,Cd 0.235,1920s,Rumpler saloon car,Cd 0.28,1924,Klemperer 'minivan'1921,Cd 0.15.
* Jaray's Cd 0.13 pumpkin seed of 1921 goes off 'Template' @ 32%,and descends to an exit slope of 26-degrees,violating what Mair would discover later in 1968 to be a limit of 22-degrees maximum slope for attached flow.
* In 1933,Walter E. Lay stays on 'Template' for the whole enchilada and nets Cd 0.12,a limit for a non-wheel fairing car.
* In 1947 you'll find pumpkin seeds hiding below Boeing PB-1W aircraft.
* In 1949 you'll find 'em under Grumman AF-2 Guardians.
* In 1953 the US NAVY begins a second look at fish shapes with USS Albacore,begins to break every underwater record,and sets the stage for all modern nuclear submarines.
* In 1957 the MG EX 181 borrows the pumpkin seed to set land speed records at Bonneville.
* Same year,the 'Template' pumpkin seed hitches a ride atop Grumman's E-1(WF-2 Willy Fudd) Tracer.
* 1961,Mickey Thompson goes to Bonneville with "The Pumpkin Seed."
* 1968,W.A.Mair establishes the optimum boat-tail angle at no more than 22-degrees,achieving that angle at no sooner than one diameter body thickness behind the point of boat tail origin.
* 1981,VW uses 'Template' with 'Drop Shape' for Cd 0.16 @ 42% aft-body.
* 1981,VW uses 'Template' with 'Flow Body' for Cd 0.15 @ 46.5% aft-body.
* 1981,VW uses 'Template' with 'Blunt Body' for Cd 0.15@ 43% aft-body.
* 1987 Chevrolet Express Cd 0.195
* 1987 Vesta II Cd 0.187
* 1987 GM Sunraycer Cd 0.89-0.12.
* 1990,ITworks CRX, Cd 0.235
* 1991,GM HX3,Cd 0.258
* 1994,Honda Dream II,Cd 0.10
* 2004 Team Corvette C5-R
* 2008 M-B,Bionic car,'Boxfish',Cd 0.19
* 2009 VW 1-liter,Cd 0.195
* 2010 Lamborghini Murcielago
* 2010 Cadillac CTS Wagon
* 2010 Ford GT
* 2010 Ferrari 458 Italia
* 2010 Ferrari 599XX
* 2012 Porsche 918
* 2012 Lamborghini Esto Elemento
* 2011 Honda Type R
* 2011 Nissan Leaf
* 2011 Nissan Leaf NISMO race car
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As to Hucho and the origin of the 'Template" it requires a full reading of the text.I can tell you that the 'Template' is hiding in full view.You just have to be looking for it.
I gave some details in the 'Template' thread.
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I'll be trying the full tail in September.It's an unknown quantity until then.And it's obviously longer than need be.It's just that it's where I'll be sleeping,so I made it accordingly.