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Old 09-03-2010, 04:34 PM   #21 (permalink)
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If it is 3"+, then Jim doesn't have to do proverbial back handsprings to raise his car high off of the ground?!

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Old 09-03-2010, 05:06 PM   #22 (permalink)
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Morelli's Cds

Quote:
Originally Posted by Jim Bullis View Post
aerohead,

No, the Morelli shape is not a miracle shape, but the discussion of aerodynamics backed up with test data is quite excellent in the actual 1982 paper by Morelli.

But in that paper, Morelli does not proclaim this shape to be a great answer in the real world, though it gets a Cd of .047 for the low to the road position. He goes on to discuss the essential changes that would be needed, including wheels, and this pushed Cd up to .19, again, as I recall, for wind tunnel tests of that form.

My appraisal is that the Morelli shape is quite inefficient from the volume perspective, even though Cd defined on frontal area is very low.

It might be of interest that Cd for airships in the old test data is defined on a volume basis, specifically, the cube root of volume squared to get area like dimensions.
Jim,does Morelli go into any of his wind tunnel protocols?
Back in the late 70s the CNR model car created quite a stir with Cd 0.161.
In later articles about the full-scale development car,it was reported that this form achieved this low drag and then "when wheels were added and the car lowered onto the ground the drag jumped to Cd 0.34.
With additional work,and with a cooling system and cabin ventilation they believed that the car might achieve in the range of Cd 0.201-.23.
This is the reason I've had guarded enthusiasm for the new form.
No one else has ever achieved a 3-D technical system which scored lower than Cd 0.04 in free stream,above critical Reynolds Number,and that would translate to Cd 0.08 in ground image reflection,after Jaray/Prandtl.
Morelli's shape is a 3-D variation on the Von Mises wing profile with reflexed-camber tail.
According to boundary layer theory it should be impossible for the flow to remain attached under the tail.It violates everything ever investigated for diffusers on cars with excellent onset flow from ahead and with perfectly smooth bellies.Unlike the wing in 'flight' condition,there is no free stream below the body to pump momentum into the boundary layer.The wing is operating at zero separation and is governed by skin friction,Morelli's form would be governed by pressure drag should separation occur.
The 'banana' car had pontoons blended into the body which acted in part to communicate base pressure forward under the car to cancel lift and also had a ducted rear bumper to also expose underbody flow to the wake's low base pressure.
The Morelli shape as depicted possesses none of these features.
I don't know how it would work.Western Washington State University's Viking series of cars used a milder version of this form and never achieved remarkable Cds.
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Old 09-03-2010, 05:25 PM   #23 (permalink)
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source

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Originally Posted by 3-Wheeler View Post
I wouldn't mind if Jim also elaborated on the correct shape for the Aerohead Streamlining Template as well.

Or at least a good source for a high quality rendering.

Thanks, Jim.
Jim,the 'Template' is based on a 2.5:1 streamline body of revolution.I generated my image as a photo-enlargement from Hucho's drag table.
I concentrated on the aft-body,as Hucho and all the other PhDs say the fore-body is of lesser importance in light of today's auto bodies.
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Old 09-03-2010, 05:37 PM   #24 (permalink)
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Cds

Quote:
Originally Posted by Jim Bullis View Post
aerohead,

No, the Morelli shape is not a miracle shape, but the discussion of aerodynamics backed up with test data is quite excellent in the actual 1982 paper by Morelli.

But in that paper, Morelli does not proclaim this shape to be a great answer in the real world, though it gets a Cd of .047 for the low to the road position. He goes on to discuss the essential changes that would be needed, including wheels, and this pushed Cd up to .19, again, as I recall, for wind tunnel tests of that form.

My appraisal is that the Morelli shape is quite inefficient from the volume perspective, even though Cd defined on frontal area is very low.

It might be of interest that Cd for airships in the old test data is defined on a volume basis, specifically, the cube root of volume squared to get area like dimensions.
Jim,if you can snag a copy of Sighard Hoerner's book,Aerodynamic Drag,he's got formulas to convert Cds from surface area,volume,and frontal area.
He also does a minor drag breakdown on airship 'Los Angeles' ( LZ 126 ) designed by Paul Jaray,perhaps similar to the Akron.
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Old 09-03-2010, 06:50 PM   #25 (permalink)
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Google found this elsewhere on this site: Morelli's design all tweaked for the road. Road & Track did a feature on it.
1978 CNR - Pininfarina - ??????????? - ????? 100000 ?????????? ???????????
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Old 09-04-2010, 12:17 AM   #26 (permalink)
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Neil,

Audi thing shown does not follow what I call Morelli rules as far as the elliptical cross section requirement. Thus, air moving toward the low pressure region under the vehicle will traverse a most un-aerodynamic ridge, and again when it flows upward toward the rear the same issue comes about.

However, the Avatar pretty much meets my definition of struts holding the body apart from the wheels. Somehow it looks more like a fashion design effort pretending to be aerodynamic, but maybe not.
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Old 09-04-2010, 12:20 AM   #27 (permalink)
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bicycle bob,

It might be that this is a Morelli design, but it bares scant resemblance to the Morelli shape that accomplished such good aerodynamic performance.

We have to get the Morelli paper of 1982 somehow, in order to see the difference. Again, Prof. Thacher's book on solar cars discusses the important points. But that also is under copyright.
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Old 09-04-2010, 12:26 AM   #28 (permalink)
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aerohead,

Hoerner's book is also informative but not all that free of the conventional automobile straight-jacket.

I think the Los Angeles is another airship along the lines of the 1906 Fuhrman (Prandtl's student) airship. They really seem about the same, and the only reason I specifically use the USS Akron zrs4 is that the testing reported by Freeman 1934 is by far the most complete and the best explained. It also was done at a scale where the actual test body was about the size of a car, so no real scaling is even needed. That report is free and very well detailed.
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Old 09-04-2010, 12:33 AM   #29 (permalink)
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Neil from 3:34 pm.

Jim (that is me) has to do handsprings because he is not using a Morelli shape; the USS Akron is the basic shape with only some tweaks according to Morelli's guiding principles.

The Aptera does not have to do handsprings because it is almost exactly a Morelli shape,except for the rear wheel which is on centerline so it might not cause too much of a problem. However, the Aptera is plenty long, but still has no rear seat, since the Morelli shape requires the underside curve that cuts into where the floor would need to be.
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Old 09-04-2010, 01:17 PM   #30 (permalink)
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Right, the Morelli shape, and the Aptera are specifically meant to provide side-by-side seating. And the earlier Aptera prototype "Zen" had a Cd of just 0.11 -- I think that will be hard to beat.

The only other car with actual wheels on the ground that has an even lower Cd is the Mercedes Bionic model, which has a Cd of 0.095:



This is the basis for my CarBŒN Concept EV Open Source Project.

The basic boxfish model had a Cd of 0.06:



It has a nearly square cross section and so the interior volume is very usable.

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