Lift

[JulianEdgar]

Originally Posted by JulianEdgar:

Aerodynamics of Road Vehicles, 5th Edition, 2016, Page 449:

"The lift forces acting on a vehicle are extremely significant in road dynamic terms, since they can influence self-steering and braking behaviour, and thus the driver's ability to control the vehicle."

Originally Posted by aerohead

Context? 250-mph in a McLAREN Speedtail on the autobahn, or 70-mph in a Chevy Spark?
70-mph is 70-mph.

They stated it as I have written it. If they thought it relevant only to a select class of vehicle, no doubt that would have been stated too. It's in the chapter 'Aerodynamics and driving stability' - not for example, in special chapters on sports cars or motorsport.

[freebeard]

At 250 mph the driver will have tunnel vision. I don't know if it scales at the square of the speed.

I do know I get velocitized on Interstate 5.

[JulianEdgar]

Aerodynamics of Road Vehicles (5th edition - Pages 298 - 299) has a crystal clear diagram and description of rear lift variation with squareback and fastback configurations.

The diagram:



Note how lift decreases as the shape moves from fastback to squareback ie a fastback shape has higher lift.

The description:



Note how the explanation has nothing to do with flow separation but instead to do with the airflow wrapping around that rear fastback curve.

So this is of course yet more evidence that Aerohead's theory of lift on modern car shapes is completely wrong.

(I should add that the book defines a fastback as having attached flow to the trailing edge of the car.)

[aerohead]

Originally Posted by JulianEdgar

Originally Posted by JulianEdgar:

Aerodynamics of Road Vehicles, 5th Edition, 2016, Page 449:

"The lift forces acting on a vehicle are extremely significant in road dynamic terms, since they can influence self-steering and braking behaviour, and thus the driver's ability to control the vehicle."




They stated it as I have written it. If they thought it relevant only to a select class of vehicle, no doubt that would have been stated too. It's in the chapter 'Aerodynamics and driving stability' - not for example, in special chapters on sports cars or motorsport.

If so, then the only explanation, with respect to the USA market would be, the higher allowable speeds compared to pre-1995 ( 85-mph vs 55-mph ), higher overall vehicle heights of pickups, CUVs and SUVs, plus their high drag / lift coefficients.

[aerohead]

Originally Posted by JulianEdgar

Aerodynamics of Road Vehicles (5th edition - Pages 298 - 299) has a crystal clear diagram and description of rear lift variation with squareback and fastback configurations.

The diagram:



Note how lift decreases as the shape moves from fastback to squareback ie a fastback shape has higher lift.

The description:



Note how the explanation has nothing to do with flow separation but instead to do with the airflow wrapping around that rear fastback curve.

So this is of course yet more evidence that Aerohead's theory of lift on modern car shapes is completely wrong.

(I should add that the book defines a fastback as having attached flow to the trailing edge of the car.)

1) the diagram depicts a squareback and notchback.
2) for a fastback vs notchback vs squareback comparison, please see Hucho's 2nd-Ed Figure 5.9, page 221.
3) the notchback has a higher CL at 0.5085, than the fastback, at CL 0.4500. The squareback does, clearly have the lowest CL, 0.2390.
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4) if the air WAS actually 'wrapping' the fastback, it would have the highest static pressure available, and lowest possible lift, simply as a function of the Bernoulli theorem, by definition. This is what your team of five have overlooked in their perusal of your book.
5) For the VW Polo / Golf / Scirocco, prismatic body type research, the Cd minimum was at the 'template'.
6) as the slant angle increases, vortex drag is being introduced, which steadily increases until maximum @ 30-degrees, then 'vortex-burst' transforms to wake to a squareback wake, large, but vortex-free.
7) ' The highest drag ... results from vortex drag.' Hucho, page 149, 2nd-Ed.
8) Your Dr. Thomas Wolf said the rear spoiler on the Porsche ' creates a stronger negative pressure gradient.' This would be impossible unless the air wasn't 'wrapped'.
9) the issue is the co
10) and going back to Hucho's comment about aerodynamic drag of road vehicles, it's a function of pressure drag. And pressure drag is a function of separation. Separation is a function of body shape.
11) total drag includes vortex drag, which is implicated in Cd.
12) if you want to see the 'template' at work, consider the Cd 0.31 Aston Martin Cygnet, compared to the Cd 0.40, 1964, Porsche 911. It's all about the aft-body.

[JulianEdgar]

Originally Posted by aerohead

1) the diagram depicts a squareback and notchback.

Yes, you're right - my mistake. Otherwise, my point remains - flow wrapping around those upper curves create lift.

2) for a fastback vs notchback vs squareback comparison, please see Hucho's 2nd-Ed Figure 5.9, page 221.
3) the notchback has a higher CL at 0.5085, than the fastback, at CL 0.4500. The squareback does, clearly have the lowest CL, 0.2390.
-------------------------------------------------------------------------------------

That diagram is for cars (Type 31 Volkswagen fastback, squareback and notchback) that are 59 years old!

Flow patterns on modern car shapes (eg notchbacks) are quite different. This is a primary cause for your confusion - car shapes (and their flow patterns) have changed, but you act as if they have not.

[JulianEdgar]

Originally Posted by aerohead

4) if the air WAS actually 'wrapping' the fastback, it would have the highest static pressure available, and lowest possible lift, simply as a function of the Bernoulli theorem, by definition. This is what your team of five have overlooked in their perusal of your book.

I wish you wouldn't keep saying this - saying it repeatedly does not make it true. The Insight is a good example:



Are you still trying to suggest that the low pressures over the roof and hatch are caused by separated flow?

[MeteorGray]

Quote: "Flow patterns on modern car shapes (eg notchbacks) are quite different. This is a primary cause for your confusion - car shapes (and their flow patterns) have changed, but you act as if they have not."

Aerohead, do you acknowledge this, or do you refute it?

It's hard to argue points without a basis of common principles.

I have a hard time following all this, but it's interesting nevertheless.

[JulianEdgar]

Originally Posted by MeteorGray

Quote: "Flow patterns on modern car shapes (eg notchbacks) are quite different. This is a primary cause for your confusion - car shapes (and their flow patterns) have changed, but you act as if they have not."


I have a hard time following all this, but it's interesting nevertheless.

If you strip away the obscure language, it's actually easy to understand.

Old shape cars (eg notchbacks) had flow that separated at the end of the roof. So, my old W123 Mercedes shows flow separation from the end of the roof:



Note the tufts on the rear window and boot / trunk lid show separation (they're whirling around, not lined up in flat rows.) Basically, the wake extends to the height of the roof.

Conversely, modern cars have attached flow across the boot / trunk:



Note the tufts on the rear window and boot / trunk lid show attached flow (they're not whirling around but instead are lined up in flat rows.)

As far as I can ascertain, Aerohead believes the flow pattern shown on the Jaguar isn't real, in part apparently because the rear contours of the Jaguar dip more sharply than his Template. He believes that the flow is really separating far forward. (In fact, he has said at least once that it is separating at the windscreen header rail, believe it or not.)

Now to lift. If we look at the old Mercedes, we can see the boot / trunk lid is in separated flow, so in effect it is within the wake. The wake is a low pressure area and this low pressure is bearing on the boot / trunk lid, so causing lift. So in the old cars that Aerohead likes citing, in fact separated flow was causing lift.

But this theory is obviously quite invalid for modern car shapes (there's almost no separation), so where is the lift coming from? It is coming from the airflow wrapping around (ie attached to) the upper curves, so generating low pressures.

Here those low pressures can be seen on Jaguar CFD (hotter colour = lower pressures):



Not the low pressures all across that curved roof and part way down the rear window. The blue areas show that in fact Jaguar claim positive pressures on the boot lid (achieved by the rear spoiler).

My Insight shows it well - note the measured lift across the upper curves. The Insight has attached flow on all upper surfaces. (Length of arrows shows magnitude of force.)




So:

• the shape of the car determines the airflow pattern
• car shapes have changed a lot in the past 60 years
• theories of airflow need to take into account what is happening on current shape cars

Unfortunately, Aerohead has built entire theories on faulty and / or outdated premises, and then extrapolated them to the point where a great deal that he writes is completely wrong. (Not everything, but a lot.)

He then denigrates the real experts. Who? Well, he has made adverse comments here on a number of aerodynamicists, including the head of Porsche aero, the head of Jaguar aero, an F1 aerodynamicist, a former Tesla aerodynamcist - and so on.

So he doesn't read current aero literature, and doesn't want to learn from real experts.

That's not a good combination for giving people advice!

[freebeard]

Old shape cars (eg notchbacks) had flow that separated at the end of the roof.

The Type III Notchback will always be 'new' to me.



The [rear] trunk lid is remarkably short. I'm just holding my hands in the air but it seems like it was less than three feet. Not much to lift on.

Since I've got my albums open, here's my favorite Type III. I knew the guy, he used to deliver eggs by the dozen to me in it.