The Fastback bloc

[JulianEdgar]

Originally Posted by California98Civic

Again you miss the point. I am not your research assistant, and I have nothing invested in convincing you of anything.

Mmm, well if you believe "that a properly streamlined body with a fastback would produce less lift than the notchback" and can produce no evidence whatsoever to support that, it's just as well you're not my research assistant!

I like to go on published research and personal measurement - as with all areas of car modification, there's a million erroneous theories around... and they are especially prevalent on discussion groups.

[JulianEdgar]

Originally Posted by aerohead

I'm forever the student, and I value actionable information, and correction, if handled factually, tactfully and diplomatically.
Having read your book, I take technical issue with specific aspects of your handling of lift.
After providing, literally, chapter and verse counterfactual evidence to your view, from Hucho, you continue to cling to 'folk knowledge' for why, I do not know.
And since you obviously do not have a complete command of the topic, I can only laugh at at your reactive behavior when dosed with reality.
Operating within an information void, I can see why the real world would seem foreign to you. What's most troubling, is that you're obviously not interested in the complete narrative on lift, attacking anything outside your myopic understanding as heresy. Your aerodynamicist friends didn't do you any favors when they signed off on your book.

So your post summarised: no new evidence, no new data, no new measurements... no new insight.

After providing, literally, chapter and verse counterfactual evidence to your view

Er no. I do know a thing about writing expert-reviewed chapters, and I can say with some certainty you've not written any chapters, let alone had them reviewed by experts before and after publication.

But I don't really care that your beliefs about car aero are, to say the least, rather stange. I care that you mislead others about the topic.

[JulianEdgar]

Originally Posted by aerohead

Researchers that are WAY above your pay grade have done the measurements and published before we were even born. It would be STUPID on my part to duplicate such an expensive undertaking, only to arrive at the same result.
CAR and DRIVER tested a 'template' car at the A2 Wind Tunnel. It produced zero-lift. My 'template' pickup truck measured zero-lift.
You've never experienced a 'streamlined' car before, so you wouldn't have a first-order-reality experience with them. That, however, does not prove their inexistence.
Exceptio probat regulam.

I am glad that you put streamlined in inverted commas, because of course we have seen how you have your own, er somewhat idiosyncratic definition of what streamlined means. (For people new to the discussion, Aerohead doesn't believe an aircraft wing is streamlined - because apparently, streamlined shapes don't produce lift. Truly.)

The A2 wind tunnel? I am afraid it's a joke - read the book you have on wind tunnel testing and see the parts about blockage factors. Read a bit more widely, and see the stuff about moving floors and wheels.

As to researchers having already done the measurements, I don't recall even one peer-reviewed, published example you have been able to provide that shows a 'template' shaped car having zero lift. That is, front and rear lift coefficients being zero.

Not even one example.

I won't hold my breath, because of course no examples exist.

[JulianEdgar]

Originally Posted by aerohead

* The streamlined body represented by the 'template' is a fastback.
* A streamline body, taken to its conclusion, possesses zero flow separation.
* A streamline body recovers all static pressure, less what's lost to surface friction drag.
* The static pressure acting on a streamline body is nearly identical at nose and tail. It's incapable of generating lift.
* You've never analyzed a streamline body.
* None of your measurements are germane to streamline bodies.
* Hucho is a champion of streamline bodies, the only path to really low drag. Germany thought enough of his work that it's in their national museum.

I am sorry but the idea a streamlined shape develops no lift is supported nowhere. It's kinda so obvious that one doesn't even know where to begin. An aircraft wing, well designed, has zero flow separation. But it develops lots of lift (I need to state that, lest Aerohead has a theory that planes don't fly because of lift.)

Again, where is even one peer-reviewed, published example that shows a 'template' shaped car having zero lift? That is, measured front and rear lift coefficients being zero?

After all, if this has all been well known since the 1930s, there must be heaps of cars you can show us as examples, no?

(Standby for Aerohead to change the proposition, write a huge screed of irrelevancies, misquote Hucho 2nd edition, or say that it's all a fashion conspiracy that car manufacturers don't produce slippery cars. Then after that, California98Civic will post that he truly believes.)

[JulianEdgar]

Originally Posted by aerohead

The Taycan, while certainly a 'fastback,' is not a streamline body. It's a composite, with the greenhouse independent of, and glommed onto the main body.
The roofline of the Taycan is a bit too aggressive, there's too much pressure regain for the length of the roof, compromising the boundary layer. The spoiler helps reach out to the streamline contour. It's not as good as simply lofting the end of the body.
Upper surfaces of the body, flanking the greenhouse represent very little deceleration, with high velocity, low pressure. Nothing like a streamline body, in which the body would boat-tail from the same point as the roof apex, building pressure the entire length of the aft-body.
I believe that a close reading of Doctor Wolf supports everything I mention. It's at home, not at hand.

Classic example of Aerohead changing the proposition. Now it's not about fastback cars, now it's about - well... I don't know quite know what.

Can someone explain?

Reading it again, I think it might be Aerohead telling Porsche how they could have made things so much better by following a mythical streamline body - one that apparently has low drag and low lift!

Gosh, what a shape that would be - and yet, as far as I know, no-one has ever produced such a road car with this measured data (low drag and zero lift) available.

All these professional car company aerodynamicists, yep, they're just stumbling about in the dark waiting to hear from Aerohead.

[aerohead]

Originally Posted by JulianEdgar

Classic example of Aerohead changing the proposition. Now it's not about fastback cars, now it's about - well... I don't know quite know what.

Can someone explain?

Reading it again, I think it might be Aerohead telling Porsche how they could have made things so much better by following a mythical streamline body - one that apparently has low drag and low lift!

Gosh, what a shape that would be - and yet, as far as I know, no-one has ever produced such a road car with this measured data (low drag and zero lift) available.

All these professional car company aerodynamicists, yep, they're just stumbling about in the dark waiting to hear from Aerohead.

1) Porsche used it on the 1997 911 GT1, 918 Spyder rear buttresses.
2) Daimler-Benz used it the next year on their 1998 M-B CLK GTR.
3) McLaren will use Jaray's 1922 Cd 0.13 pumpkin seed contour for the Speedtail.
4) BMW's i8 is right on it.
5) Honda's Insight-II
6) Hyundai's IONIQ
7) KIA Stinger GT
8) M-B Bionic Boxfish
9) Renault Vesta-II
10) Ford Probe-V
11) GM's Vauxhall/ Opel Flextreme
12) Volkswagen XL1
13) Cambridge University's CUER solar racer is quite close.
14) Toyota's Prius is close.
15) Nissan LEAF is dead on.
16) Chevrolet Volt.
17) Chevrolet Bolt
18) Chevrolet SPARK
19) Mitsubishi Mirage 'G'
20) Mitsubishi i-MiEV
21) Volkswagen 'Flow' body is very close.
22) De Tomaso Pantera
23) DeLorean
24) Tesla Roadster-II
The list goes on and on.
---------------------------------------------------------------------------------- Then there's quite a list of cars in which the rear spoilers mysteriously ascend upwards, then terminate exactly at the 'template.' Ferrari, Ford, Aston Martin, Jaguar, Lexus, Citroen, Corvette, Alfa Romeo, Audi, Lotus, Peugeot, Porsche, Saleen, Peter Brock, Carrol Shelby, Bristol, ........................
When it really matters, they'll lower themselves enough to use it.

[JulianEdgar]

Originally Posted by aerohead

1) Porsche used it on the 1997 911 GT1, 918 Spyder rear buttresses.
2) Daimler-Benz used it the next year on their 1998 M-B CLK GTR.
3) McLaren will use Jaray's 1922 Cd 0.13 pumpkin seed contour for the Speedtail.
4) BMW's i8 is right on it.
5) Honda's Insight-II
6) Hyundai's IONIQ
7) KIA Stinger GT
8) M-B Bionic Boxfish
9) Renault Vesta-II
10) Ford Probe-V
11) GM's Vauxhall/ Opel Flextreme
12) Volkswagen XL1
13) Cambridge University's CUER solar racer is quite close.
14) Toyota's Prius is close.
15) Nissan LEAF is dead on.
16) Chevrolet Volt.
17) Chevrolet Bolt
18) Chevrolet SPARK
19) Mitsubishi Mirage 'G'
20) Mitsubishi i-MiEV
21) Volkswagen 'Flow' body is very close.
22) De Tomaso Pantera
23) DeLorean
24) Tesla Roadster-II
The list goes on and on.
---------------------------------------------------------------------------------- Then there's quite a list of cars in which the rear spoilers mysteriously ascend upwards, then terminate exactly at the 'template.' Ferrari, Ford, Aston Martin, Jaguar, Lexus, Citroen, Corvette, Alfa Romeo, Audi, Lotus, Peugeot, Porsche, Saleen, Peter Brock, Carrol Shelby, Bristol, ........................
When it really matters, they'll lower themselves enough to use it.

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....

[Vman455]

Originally Posted by California98Civic

Lift is one of the things we're interested in here. However, it is not THE thing. For modding purposes and general learning about about aero, I am curious about the pressure coefficient and how it relates to lift.

Here is the Figure 5. What, aerohead or anyone, do I misunderstand when I read this as demonstrating that you were right to say a fastback shape would have a greater pressure coefficient at the rear of the roof?

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?

[JulianEdgar]

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.

Excellent explanation.

Force = pressure x area, and so you can have a low pressure over a high area, or a high pressure over a small area, and the forces are the same.

That's why measuring overall lift / downforce is better than trying to integrate all the pressures x areas. (However, measuring pressures can be very useful when tuning modifications, as it's a bit easier than measuring downforce / lift.)

[freebeard]

Force = pressure x [normalized] area... following Vmann455.

Following Permalink #64 fastbacks suffer in crosswinds. Which tend to be vortex streets.

Cadillac wasn't far wrong with that 1949 tail light. Shoulda been bigger and finer.