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Old 05-29-2020, 12:24 PM   #15 (permalink)
aerohead
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variety

Quote:
Originally Posted by JulianEdgar View Post
So if the template can be scaled in the way you describe, isn't it basically just an angle? The angle from the front the windscreen header rail to the rear of the car?

And then when we look at cars with angles that differ like the 2020 Tesla Roadster and 2019 McLaren Speedtail, isn't it just a range of that angle?

You say "It has had very good effect nearly all the time, on a variety of vehicles, in a variety of tests but usually testing has been limited to somewhat longterm fuel economy tracking."

I'd expect that to be the case if the flow remains attached - and that's great. But what I cannot see is why this is somehow regarded as the best shape.

It's like the person who says a particular airfilter gave good engine power - but hasn't compared it to any others.

I just posted this in another thread. It's from Hucho 2nd ed p.201. It shows how a variety of shapes can be used to give the same low drag.



I think the idea that there is a 'best shape' is a bit simplistic - and especially in the real world, the thickness of the boundary layer (that in turn will depend on what is happening on the forward section of the car) will help determine if the flow remains attached.

As to swimming upstream - that's something I am very used to doing, especially in the often group-think environment of an online discussion group!
* Below 250-mph,the bulbous nose of the 'template' is of the lowest drag, and provides for extremely good outward vision,which is lacking in some of the other VW shapes.
* The 'template' is modeled upon the lowest drag streamline body of revolution,at Cd 0.04. It's in Hucho, page 61, in Hoerner's drag table.
* In ground reflection, it actually produces Cd 0.08.
* Adding wheels raises it to Cd 0.13.
* Adding the diffuser drops it back to Cd 0.12.
* Goro Tamai's full wheel skirt package allows it to approach the Cd 0.07- 0.09 which Hucho says could be achieved,for exactly the reasoning I've shared. It's just off-the-shelf-technology.
It may not be the 'best',but it will generate Cd 0.12 and lower,depending upon how well one does with the wheels.
Since some of the folks here expressed an interest in really low drag, I thought it could help save precious time and resources.All I can say is,that it works.

* The 'template's' aft-body is modeled on the shortest streamline body of revolution, which does not violate the Mair/ Buchheim aft-body taper angle of 22-23-degrees downslope.
* Since the sides of the body are essentially the same contour as the roofline, the body is more like two wingtips joined together, with so much transverse curvature that pressures self-equalize along the path to the rear,eliminating spikes.
* Spirit of EcoModder.com is based upon it.
* Spirit is zero-lift.
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Last edited by aerohead; 05-29-2020 at 12:26 PM.. Reason: typing
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