The body shape of minimum drag

[euromodder]

Originally Posted by ERTW

Is a 20' long 2 passenger vehicle practical?

No, and we're having very few of them on the road.
20' might work in the US, it won't work here.

The basic shape is an idealisation. The better the basic shape, the better the final car. One of Morelli's stipulations was that it have practical internal volume for passengers and their baggage (his main reason for avoiding the tear drop).

Same goes for the teardrop - you'll get most of its benefits even if you cut it short a bit.
But you get more practically useful volume - we just need to rearrange things compared to our current cars.

Smaller more efficient engines need less cooling, take less space, and are lighter so they could go back to where they caused some issues in early teardrops : in the back.
It would help unload today's overburdened front axles .
(driven wheels + engine weight + most of the driver / front passenger weight in typically lightly loaded cars).

Or as Mercedes did in the previous B class and Toyota in the iQ : tilt the engine and put it up front, low down. PLenty of space in a bulbous teardrop nose.

Electric drive solves the whole issue instantly.

Like the MB bionic car pointed out, current frames are inefficient.

The Bionic shows that you can have practically useful volume and good aero together.
The boxy new B class puts it into practice : 0.24 is a good value for this kind of car.

[Ladogaboy]

Originally Posted by Sven7

Are we talking about this? It's so difficult to understand many things on here without images.


http://ecomodder.com/forum/showthrea...hape-3746.html

Looks like he copied the profile of a dolphin.

[NeilBlanchard]

As near as I can tell, this early model of the Boxfish / Bionic has a Cd of 0.095:



Here's a series of screen captures from a video of the same model, that are the only other images I know of:



And the Bochum University SolarWorld GT car has a Cd of 0.137.

[wheel_of_steel]

These papers aren't even mentioning some pretty important aero considerations, namely:

-lift produced
-cross wind stability
-wind noise
-centre of pressure changes due to changes in pitch

Low drag is great, but they would be sacrificing real world fuel economy if the powertrain and suspension have to be compromised to achieve it. The driver would be fighting the car all the time and probably waste fuel in the process.

I think that is why cars like the insight and prius are still pretty 'aerodynamically conservative', so to speak.

[aerohead]

Originally Posted by Ladogaboy

Looks like he copied the profile of a dolphin.

There is a (Richard) von Mises airfoil section with reflexed-camber tail, kinda similar to Morelli's form.
Morelli has borrowed from NACA (now NASA) for his computational fluid dynamics and he has probably been acquainted with this section.'Could have been an inspiration.

[aerohead]

Originally Posted by wheel_of_steel

These papers aren't even mentioning some pretty important aero considerations, namely:

-lift produced
-cross wind stability
-wind noise
-centre of pressure changes due to changes in pitch

Low drag is great, but they would be sacrificing real world fuel economy if the powertrain and suspension have to be compromised to achieve it. The driver would be fighting the car all the time and probably waste fuel in the process.

I think that is why cars like the insight and prius are still pretty 'aerodynamically conservative', so to speak.

*The CNR 'banana' car produces zero lift
*The CNR 'banana' car is one of the most stable forms ever tested,requiring no stabilizing fins.
*The CNR 'banana' car,by default,would be very quiet,as it has no flow separation over its entire length
* The CNR 'banana' car was critically designed with CP/CG considerations as a premise for its creation,and remains an extremely stable design under all wind conditions for pitch/roll/yaw.

[ERTW]

Yaw stability is handled pretty well on modern cars. Any better is a bonus, and imo not a driving factor for design. I CFD Phil's aero template, expecting high lift, and it was CL 0.069 or so. Even the 2.5:1 ellipse is very low. Even though they appear to be wings, optimal shapes are better in every way than non optimal shapes (maybe I'm stating the obvious).

[wheel_of_steel]

Okay, I didn't read enough into that article.

I do counter that all of those benefits fall apart when wheels, mirrors and drivetrain components are attached - it might be even worse than a conventional 'non-optimal' design.

[ERTW]

An SAE paper is available for sale online for $16. There's lots more said in the paper that I won't post.

Morelli's newer paper 2000-01-0491: a new aerodynamic approach to advanced automobile basic shapes. There's so much information, that I have to break it up into several posts.

"tests in scaled down models gave, in the best cases: Cx ~ 0.15.

Taking into account that the frontal area of a wheel Sw, is approx. one twentieth that of the body, and assuming Cxw ~ 0.5 the drag coefficient of an isolated wheel, the interference coefficient, CxI, results:

CxI = Cx - CxB - Cxw (Sw/S)*(4/3)
= 0.15 - 0.07 0.5*(1/20)*(4/3) = 0.047

The factor 4/3 stays for the number of wheels (4) multiplied by the area ratio of the wheel not covered by the body (1/3)."

Morelli doesn't define CxB, so I'm unsure of its relevance. My main interest is that Cxw can estimate drag due to wheels. Instead of the 1/20 assumption, it should generally be Afw/Af (the ratio between wheel frontal area and total frontal area). the 1/3 term should be Afw/Af. Since the tire area is width times ground clearance; hG*tire width/Afw.

i.e. 0.5*Afw/Af*hG*tire width/Afw

this simplifies to 0.5*hG*tire width/Af.

To minimize wheel drag, you need to have minimum ground clearance, minimum tire width. Although a larger frontal area minimizes the wheel drag contribution, it hurts overall drag. Wheel fairings hide the tires, without increasing overall frontal area. Lowering ground clearance also affects drag in mysterious ways.

Morelli also cites a paper which analyzed centrifugal fans on the rear wheels. They saw a drag reduction of 18-20% - substantially higher than simple wheel fairings.

[aerohead]

Originally Posted by wheel_of_steel

Okay, I didn't read enough into that article.

I do counter that all of those benefits fall apart when wheels, mirrors and drivetrain components are attached - it might be even worse than a conventional 'non-optimal' design.

I don't have enough data to agree with your assessment or not but I certainly understand your train of thought.
In 1976,when the 1/2-scale CNR body was tested,after slicing successive pieces off the back,they were able to see Cd 0.161,in ground proximity,but without wheels.
In 1978,when they got financing for the full
scale project,when wheels were added to the body,the Cd jumped to 0.35.
Much work was done with the cooling system and wheel integration to get back down to Cd 0.205 or so,and they figured that when all features were added,that they'd end up with around Cd 0.24.
Here we are in 2012 and we're finally at a point where we could purchase a car with Cd 0.24.So Morelli deserves some credit.
When I GOOGLE'd for 'Aptera drag coefficient/frontal area' a website reported it at Cd 0.11.I don't recall the Af.
Love the car or hate the car,Cd 0.11 is around 1/3rd of typical Cds,or 66% lower.On a frontal area -based comparison,at 55 mph,we'd be looking at a 33% HWY MPG improvement.
With the lower frontal area we'd be looking at even larger savings.
It's certainly body-in -white ecomodding!