The Fastback bloc

[JulianEdgar]

Originally Posted by freebeard

Well, thanks for correcting me, but it isn't just a good idea, it's Cunninghams Law.



https://xkcd.com/386/

1) Calling something a law gives it no further credibility

2) A Wiki - an editable body of knowledge - is nothing like a discussion group. Here people don't edit out their mistakes, misunderstandings, etc. All the material stays present to cause confusion as people further down the track try to follow threads.

3) To be successful, the approach that you suggest requires a very large number of people knowledgeable about the topic. In car modification, it also helps a great deal if people have actually done a lot of testing and development of what is being discussed.

I reiterate: I think suggesting that posting incorrect information is a good idea is really dumb.

[freebeard]

Trapped (I am) in a world I never made.

A Wiki - an editable body of knowledge - is nothing like a discussion group.

Did you know there exists an EcoModder Wiki? The Recent Changes page shows no edits in the last 30 days if I understand. Maybe that is the domain for you. I kinda like the banter and Tu quoques.

[California98Civic]

Originally Posted by freebeard

Well, thanks for correcting me, but it isn't just a good idea, it's Cunninghams Law.



https://xkcd.com/386/

Hahaha!

Batman is not in anyone's aero cult. A hero's work is never done.

[JulianEdgar]

Originally Posted by kach22i

Another thing said over and over again in this forum over the years is that tuft attachment does NOT always mean good low drag airflow above. The many reasons why have seemed reasonable to me when patiently explained by Phil/Aerohead and others.

Who said that it did?

What tuft testing shows very clearly is whether the flow is separated or attached.

So arguing that cars that have sharper downwards curves than the The Template will have separated flow (as Aerohead constantly states) is demonstrably wrong.

And, as a result of that, most of Aerohead's theories (eg lift, required height of rear spoilers) are also wrong.

[kach22i]

Originally Posted by JulianEdgar

I am pretty amazed that you are now nominating another template for rear add-ons. This is just continuing the problem!

The whole idea of a templated shape for add-ons is flawed - at minimum, flow attachment will depend on the thickness of the boundary layer, which depends on what is happening ahead of the template extension.

Again, flawed as any 3D template is and using only the latter half in 2D is, it's better than nothing at all for the novice experimenter.

The starting point is not flawed if it gets one to a working end point.

I have commented before about what I describe as an engineer's affinity for efficiency. This affinity for chasing an idealized solution often ignores less efficient designs which may lead to narrow thinking.

The attraction of the Morelli design for me and others is that it dares to color outside of these standard coloring book lines.

It is outside the box that we learn more about the box.

Thank you Julian for describing the differing boundary layer thickness dependant on the body forefront.

I have struggled to understand this before and your comments click into place many lose ends that went underexposed and undetermined in this forum in the past.

[aerohead]

Originally Posted by JulianEdgar

Yes, Aerohead has been here before - a long list of cars for which he claims an identical rear profile. Yes, really.

In a minute he'll get to to the theory that spoilers 'reach up' to flow that has become separated - something that has not been the case since sedans of the 1990s....

Look forward to your graphics.

[aerohead]

Originally Posted by California98Civic

But it is not a CFD study. The author used pressure taps on a 1:4 scale model developed by BMW/Audi and tested it in a wind tunnel. The flow visualizations were done with paint on the body in the wind tunnel. Everything about the two shapes is standard except the slope of the roof/backlight/trailing edge.

That the modest change in the slope angle and curves on the fastback yields higher pressure readings on the roof is interesting. If the pressure readings along the backlight and decklid drop quickly to lower than the notchback, might it be possible that this particular fastback becomes too fast after the roof AND that the pillars are not optimized well?

If the vortices are the problem and the fastback shape is otherwise known to have more potential for reducing drag and lift when undertrays, diffusers, spoilers, and fins are used, then why not explore reducing the vortices off the pillars by tapering and radiusing the fastback's full rear body contours more? We might be able to get Cd possibly under 0.20 and lower lift too.

That kinda is a description of the XL1, which even narrowed the rear track. The XL1 was speed limited possibly because of higher lift over 100 MPH. But lift at high speeds could be dealt with through other optimizations or active aero. We'd have an ultra low drag body and no lift problems at 150 MPH.

I would love to find a contemporary study in automotive aerodynamics where some grad student tests an abductively reasoned thesis like this.

Here again is the link to the article (I previously posted it in post #58):
https://ecomodder.com/forum/showthre...tml#post630095

A properly contoured fastback would respect the degree of pressure regain tolerable to the turbulent boundary layer, so as to prevent triggering separation. That's it's job.Nothing more. And there'd be no attached longitudinal vortices.
Speed-limiting the XL1 may have had to do with what a major automaker had to say about their Cd 0.25 car of the 1960s , which never made it into production.
The argument was that, if you lower drag, the speed increases, then you need bigger brakes to stop it, better and bigger tires to withstand the braking, which increases loads on the chassis, which now needs to be beefed up to withstand the greater stresses, which adds weight, requiring more horsepower, in a never-ending spiral.
The first solution came in overdriving. By putting such tall gearing, say in 3rd, 4th, and 5th gear, the engine would never be allowed to reach a torque which would support an unacceptable speed. My CRX was engineered this way. The rpm splits between gears are so high, the engine just falls on its face shifting to a higher gear. My 1st-gen Insight is the same. Your stuck in 3rd gear.
Today, electronic fuel injection will just shut off fuel delivery at any predetermined set point a carmaker likes.
Somewhere there's an unregulated top speed published for the XL1. I want to say that it was 112-mph, but I'd want to check.
It's very easy to kill front lift with an airdam. Rear lift on the BEETLE was zero. I'd be very surprised if the XL1 had significant rear lift at all. Even if it were 'neutral' the diffuser could produce enough downforce for stability, as in the D-B, M-B C-111 III of 1978, which was running in the 250-mph range.

[aerohead]

Originally Posted by JulianEdgar

Yes, Aerohead has been here before - a long list of cars for which he claims an identical rear profile. Yes, really.

In a minute he'll get to to the theory that spoilers 'reach up' to flow that has become separated - something that has not been the case since sedans of the 1990s....

1) On page 281, Hucho discusses ' steep tapering.... is limited as the resulting flow separation increases air drag.'
2) And in discussing the use of wings on race cars on the same page, he comments, ' it is also possible to use a rear spoiler, the decisive feature being the relative height of separation in relation to the rest of the body.'
Hucho's words, not mine. Something I directed at you specifically many months ago.
I suppose bashing aerohead is more entertaining than learning about the topic you bash him for. Some sort of retarded, circular logic, devoid of introspection, writers without a clue get addicted to?

[jakobnev]

https://www.youtube.com/watch?v=6QAiA7doiW0

[aerohead]

Originally Posted by Vman455

The pressure coefficient at the rear of the roof does not matter, in and of itself. Lift is determined by the effect of the pressure over the entire surface area. This figure clearly shows that the notchback achieves higher peak pressure than the fastback, at the base of the rear windshield, while the figures comparing lift and drag show that the total pressure on the rear of the cars is higher on the notchback than on the fastback.

Pressure can be thought of as a scalar, while the surface area is represented by an area vector A. Think of the surface area of the car body as a bunch of small area "cells," each a vector dA normal to the surface at that point, with a certain discrete pressure p acting on each cell (directly proportional to the pressure coefficients in the figure). Multiplying each surface area "cell" (vector) by the pressure acting on it will give you a force, with a direction also normal to that bit of surface (since dF = pdA, the resulting force points in the same direction as the area). Sum all these force vectors together (integrate pdA over the whole surface) and you'll get one force vector representing the force acting on whatever part of the car body you chose as the area--the rear part, in this case. Break that force vector into its components along the x, y, and z axes and you have drag (or thrust) in x, side force in y, and lift (or downforce) in z acting on that part of the car (along whatever arbitrary axes you choose). Do this over the entire surface area of the car and you will get the total drag, total side force, and total lift. The aerodynamic forces result from the local pressures which result from the local velocities which result from moving the car.

Is that what you're asking?

The plan-view of the two vehicle rear ends does nothing to inform us about the rest of the geometry of the two shapes. Conditions are everything.
The Polestar-I notchback is Cd 0.375.
Take a look at the Polestar-3 Precept. There's a reason for the change. Something Elon Musk knew all along.
Hucho gave us a half-dozen shapes with Cd 0.15. There's not a notchback in there. Nor any vehicles below Cd 0.15.
If there's a spoiler on a car, it means that there was a mistake with the original design and they've put a BANDAID on it.
Yes, a Euro-spec Mercedes-Benz CLA 180 Blue Efficiency, notchback and one of the BMWs is Cd 0.22. I've gone lower in my pickup truck.
Really low drag comes only from a fastback, and not just any garden variety. Hucho is crystal clear about it.