Drag per aft-body length/roof angle

[Xist]

Originally Posted by aerohead

It was Sir Issac Newton who said about his contributions,that he was merely standing on the shoulders of giants.
There have been do many altruistic researchers who've devoted their lives to the betterment of mankind,that it would be crime not to pay forward their contributions,especially for those who are gone.
Abraham Lincoln said,"To know and not tell makes cowards of men."
We're so fortunate here at EcoModder.com to have so many, who contribute so much of their human capital,it's a privilege to be part of it.

Just say "You're welcome!"

[aerohead]

I found another photo,from NASA,of notchback drag and linked it up at page-1.
Looks like NASA is taking aero into US classrooms.A good thing.

[aerohead]

I finally got around to some images which may help to visualize the concept of our vehicle's effective fineness ratio.
*From Hucho,we see that the vehicle is resolved into itself,plus it's mirror image.It's effective fineness ratio includes the vehicles length,divided by the height of the vehicle PLUS it's mirror companion.

*From the higher Reynolds number curve it's evident that the drag minimum occurs when the effective fineness ratio is 2.5:1.
*Ellipsoids and streamline bodies of revolution share identical drag coefficients for identical fineness ratios.Here is a drag table for streamline bodies in free flight.
*The 2.5:1 body has the drag minimum.NOTE: these are not frontal area-based Cds.But you see that the numerical minimum occurs with 2.5:1.
*For a frontal area-based coefficient of aerodynamic drag for this body in free flight you'll find Cd 0.04.
*As the body moves into ground proximity it's drag doubles.
*A 'half-body' possesses this 'doubled' drag,or Cd 0.08.
*Adding skinny wheels.tires brings drag up to Cd 0.12.

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Here is a 3-car montage of Hucho's Beetle,plus one of Kamm's K-cars,and Walter Lay's Model# 22 in ground reflection.

*At each 'leap' in Effective fineness,the car gets it's drag cut about in half.
*Here,I've superimposed a rough outline of Jaray's ideal contour

*As the 'void' is filled in under the 'Template' the drag reduces by means of wake reduction.We can't eliminate it altogether due to the half-ring of turbulent boundary layer sloughing off the tail of the car,but it's enough to get us to Cd 0.12,plus mirrors,features drag,and cooling system.
*With wheel fairings you're looking at Cd 0.10,to 0.08.
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*So when you're considering Hucho's 'elongation' for drag reduction,bear in mind that you've got to deal with 'both' your cars.

[freebeard]

With my brass caliper and the screen (reduced to Normal from an eyeball friendly size) the Kamm K-car is 123% the length of the Beetle (I love that choice for baseline). Assume a 12ft length for your typical bumperless Beetle body and that makes the K-car 14.76ft. That's close to my own estimation of 3 additional feet (i.e., 15ft) to boat tail the body.

The full boat tailed body is untenable. Suppose for a moment that the K-car body has a Magic Butt Trumpet. Rather than a taper to a straight line, the trailing edge is a half-circular arc and the underside is half an exponential horn. It could even transition between square 'twixt the rear wheels and circular at the tail.

[aerohead]

Originally Posted by freebeard

With my brass caliper and the screen (reduced to Normal from an eyeball friendly size) the Kamm K-car is 123% the length of the Beetle (I love that choice for baseline). Assume a 12ft length for your typical bumperless Beetle body and that makes the K-car 14.76ft. That's close to my own estimation of 3 additional feet (i.e., 15ft) to boat tail the body.

The full boat tailed body is untenable. Suppose for a moment that the K-car body has a Magic Butt Trumpet. Rather than a taper to a straight line, the trailing edge is a half-circular arc and the underside is half an exponential horn. It could even transition between square 'twixt the rear wheels and circular at the tail.

yes,there's been a complete consensus over time that the full length is a non-starter.Jaray wouldn't even do it,and he developed the 'combination' form as a compromise,joining a vertical airfoil section to a horizontal one.
Unless the tail was 'active,' it would be completely impractical.
As to the 'butt trumpet,' we ought to have a sketch so everyone's on the same page.Like Bucky Fuller,I'm reluctant to approach discussing a concept,unless I've got a model or drawings to help others visualize it.

[Xist]

Originally Posted by aerohead

butt trumpet

This is all that I have. Anyone useful?

https://www.youtube.com/watch?v=KB1wkFE9AnI

[freebeard]

...we ought to have a sketch so everyone's on the same page....

Apparently.

I'll try it in Wings3D. It's pretty simple in concept—full body of revolution truncated. Take a torus' inner 'core' and stretch it (actually, Scale Radial) to make a trumpet shape. Graft it into the truncation (carefully counting edges to make Put On work) and half-body the whole thing. Easy peasy, it should take ....hours.

Meanwhile what about this?


http://www.roadandtrack.com/new-cars/a3821/retro-1945-bugatti-veyron/

45° is the optimal windshield angle and the Beetle body could get there by cutting a slit across below the windshield, slant chopping the rear and removing a foot or so above the C-pillar. A steering column drop and chopped seat bases might help. Unfortunately, a MBT would go right where the VW engine is.

Less headroom and ground clearance equals greater fineness ratio. It needs for the side scoop to go all the way to the drip-rail, like the new Bugatti Chiron.


EDit: Well that didn't take long, mainly finding the squircular aeroform on an old thumb drive. I cut corners by just inverting the tail and re-contouring it.



Hopefully, you can see how the thickness of the bottom rear edge can contain the rear wheels. The vein running up the middle bottom is like R. B. Fuller's 'air keel' in reverse.

[aerohead]

Okay,thanks!
From fluid mechanics,any diverging nozzle should never have a wall angle exceeding 7-to-8-degrees,or separation/turbulence will occur.I suffered from this at Bonneville by mimicing a Mustang GT diffuser (which I had to close off completely with cardboard and duct tape).
The 'trumpet' is a good technology though if tuned in a tunnel.
Recently,the Bochum University team used it on their WSC car

In 1978,Morelli used it on his CNR concept,creating wheel fairings to enclose the diffuser,helping to achieve zero-lift.

The 2nd-gen Prius has a fine 'trumpet'.

[freebeard]

From fluid mechanics,any diverging nozzle should never have a wall angle exceeding 7-to-8-degrees,or separation/turbulence will occur.

maybe the trumpet needs a mute.

The Bocham University car is the inspiration, but it gives up a lot of interior volume. Maybe a flat truncation or box cavity with a downsized MBT that's only a fraction of the size? I was thinking it would create a channel or tributary that would gather the under-car air and suck it out the back.

Here's what I was thinking in 1995, fan powered:

[aerohead]

Originally Posted by freebeard

maybe the trumpet needs a mute.

The Bocham University car is the inspiration, but it gives up a lot of interior volume. Maybe a flat truncation or box cavity with a downsized MBT that's only a fraction of the size? I was thinking it would create a channel or tributary that would gather the under-car air and suck it out the back.

Here's what I was thinking in 1995, fan powered:

*The Bochum car is just a 2-seater,and the diffuser doesn't affect the passenger compartment.I've been in the car and it's a flat floor up front.
*Any ductwork will have more surface area and friction than the area of the car it's shunting air from.
*And this area which is losing airflow is now subject to separation since we've taken all the isoenergetic stream filaments away from it and moved them into the duct.
*The duct inlet is probably at a lower pressure than it's outlet,so we would have to put a blower inline,or the flow would be backwards.
*And the blower would require horsepower equal to the CFM,at the delta-P within the duct,plus wall friction losses.
*The duct has also created additional drag which will have to be overcome.
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*The only 'solution' which has been verified so far is just a simple,proper,diffuser,of low divergence,incorporated with boat-tailed wheel fairings,as in the Prius.
*Anything else will require more power than it saves.
*Only a over-unity power supply will help us.