Rear kammback spoiler

[aerohead]

Originally Posted by JulianEdgar

I am sorry, but that completely ignores the major reason that lift occurs on cars. The lift - especially rear lift - in most cars doesn't come from the wake (ie separated flow). It comes from the accelerating flows over the curved upper surfaces.



I am sorry, but that is wrong. An aerofoil has attached flow and can develop lift. You may choose to orientate a streamlined body (like an aerofoil) so it doesn't generate lift, but that is not the same as saying "streamline bodies are incapable of generating lift", as you previously wrote. In fact - of course - the best lifting bodies are streamlined.




I am sorry, but you are confusing cause and effect. You can be certain the upper surfaces of your car were developing lift. However, if the car in fact didn't develop overall lift, it would have been because of the low pressures developed under the car by its bellypan.

Note: all of this can be directly measured on real cars using a surface pressure disc (easy to make yourself), a Magnehelic gauge and a sealed reservoir.

*Firstly, Hucho appears to downplay apprehensions about 'lift' in passenger cars as related to stability. Below 100 km/h he says it's not an issue, even in crosswind. Hucho does emphasize the importance of lift in high speed sports cars and racing cars.
* I believe that the lift issues that you believe to be the major reason for rear lift would actually be attributed to separated flow over a horizontal portion of the aft-body, ahead of the transom. Squarebacks are incapable of generating rear lift. Proper fastbacks don't typically generate dramatic lift. Notchbacks are historically the worst offenders, and responsible for the commercial development of the rear spoiler, by Kamai, in 1982,beginning with a product for the BMW 2002.That spoiler merely extended the tail surface up into the inviscid flow, to provide re-attachment on the boot, while sequestering the low pressure of the greenhouse turbulence away from the 'base' of the car's transom, which would otherwise contaminate the entire wake, lower the base pressure, and increase pressure drag.
*The Cayenne and I-Pace both will create this separation with, zero chance of re-attachment, as they violate the limits of the Mair/Buchheim departure slope angles, necessary to allow for re-attachment.The plan-view section of the roof exposed to the turbulence is at the lowest static pressure, compared to the suction peak at the windshield header,and this low, acting over the 'lever arm 'spanning the distance to the tail creates the 'moment' which lifts the tail. Simply extending the roof to the back of the vehicle cancels the moment, leaving a higher base pressure, and lower pressure drag. On the Jaguar, this is part of the reason for the Cd 0.29,vs the Tesla S 0.26. The Porsche Taycan is close to the 'template' and enjoys Cd 0.25.
*On Spirit, the aeroshell and boat-tail decelerates the flow, and as per the Bernoulli Theorem, imparts a higher static pressure, removing the original separated, high-vorticity-induced low pressure from any surface 'over' the aft-body. This would be the same if no belly pan were present, as per the 1988 Texas Tech, SAE published research results. By the way, Spirit's belly pan was compromised at the time of wind tunnel testing, and probably would have rendered the diffuser useless, along with compromised underbody flow.

[JulianEdgar]

I am sorry, but this is your typical dump of misinformation, misunderstandings and plain outright wrong statements - as usual, dressed up in pseudo-scientific language.

Originally Posted by aerohead

*Firstly, Hucho appears to downplay apprehensions about 'lift' in passenger cars as related to stability. Below 100 km/h he says it's not an issue, even in crosswind. Hucho does emphasize the importance of lift in high speed sports cars and racing cars.

Well, perhaps read a bit more widely and recently than Hucho, second edition?

You say you have my book. Well then, read pages 179 - 182. Note specifically the reference to SAE 1999-01-0651 that shows how even small amounts of lift adversely impact stability, and to SAE papers 2009-01-004 and SAE 2015-01-1537 that again show how even small amounts of lift (especially rear lift) adversely affect car handling and stability.

But reading and driving are different things. I drive a car that, because of its modifications, develops downforce. It is easily measurable at 100 km/h. The car is significantly improved in its driving behaviour - eg cornering speed. In addition, it actually rides better because the sprung/unsprung weight ratio has effectively changed.

I might have believed what you say before I did the research for my book, and experienced the benefits of reduced lift for myself.

Originally Posted by aerohead

* I believe that the lift issues that you believe to be the major reason for rear lift would actually be attributed to separated flow over a horizontal portion of the aft-body, ahead of the transom. Squarebacks are incapable of generating rear lift. Proper fastbacks don't typically generate dramatic lift. Notchbacks are historically the worst offenders, and responsible for the commercial development of the rear spoiler, by Kamai, in 1982,beginning with a product for the BMW 2002.That spoiler merely extended the tail surface up into the inviscid flow, to provide re-attachment on the boot, while sequestering the low pressure of the greenhouse turbulence away from the 'base' of the car's transom, which would otherwise contaminate the entire wake, lower the base pressure, and increase pressure drag.

I am sorry, but this is just rubbish. (1) The lift being described is nothing to do with separated flow. (2) Squarebacks can create lift. As Barnard specifically says in his book, even a brick can develop lift. Fastbacks are notorious for lift. (3) Notchbacks are not historically the worse offenders, and of course as most people know, the first spoiler was fitted to the classic Porsche 911 in 1973 (page 195 of my book). The 911 was a high lift shape, just as you'd expect with that long upper body curve.

Originally Posted by aerohead

*The Cayenne and I-Pace both will create this separation with, zero chance of re-attachment, as they violate the limits of the Mair/Buchheim departure slope angles, necessary to allow for re-attachment.The plan-view section of the roof exposed to the turbulence is at the lowest static pressure, compared to the suction peak at the windshield header,and this low, acting over the 'lever arm 'spanning the distance to the tail creates the 'moment' which lifts the tail. Simply extending the roof to the back of the vehicle cancels the moment, leaving a higher base pressure, and lower pressure drag. On the Jaguar, this is part of the reason for the Cd 0.29,vs the Tesla S 0.26. The Porsche Taycan is close to the 'template' and enjoys Cd 0.25.

I am sorry, but this is all just rubbish. Have you ever measured pressures on a car? Obviously not! There is attached flow on these cars (eg Cayenne) to the trailing edge of the roof - the roof is not exposed to turbulence. There are plenty of wind tunnel videos around of the latest Cayenne, showing just this. In fact, the idea that the flow separates on the roof of these cars is just laughable.

I can see now why some of the misinformation is spread here: it appears that you just make up theories, completely without evidence, and then happily apply them!

[California98Civic]

Originally Posted by aerohead

The clock has run out an the store where I do internet is closing.
I'll be back tomorrow and we can extend our exploration.If you have Hoerner's book I recommend it.

The Tao Master.

To the OP, this all amounts to: order that spoiler and modify it. Tinker, test, learn--consult what others say. And most importantly... report back!

[JulianEdgar]

Originally Posted by aerohead

* I believe that the lift issues that you believe to be the major reason for rear lift would actually be attributed to separated flow over a horizontal portion of the aft-body, ahead of the transom. Squarebacks are incapable of generating rear lift. Proper fastbacks don't typically generate dramatic lift. Notchbacks are historically the worst offenders, and responsible for the commercial development of the rear spoiler, by Kamai, in 1982,beginning with a product for the BMW 2002.That spoiler merely extended the tail surface up into the inviscid flow, to provide re-attachment on the boot, while sequestering the low pressure of the greenhouse turbulence away from the 'base' of the car's transom, which would otherwise contaminate the entire wake, lower the base pressure, and increase pressure drag.
*The Cayenne and I-Pace both will create this separation with, zero chance of re-attachment, as they violate the limits of the Mair/Buchheim departure slope angles, necessary to allow for re-attachment.The plan-view section of the roof exposed to the turbulence is at the lowest static pressure, compared to the suction peak at the windshield header,and this low, acting over the 'lever arm 'spanning the distance to the tail creates the 'moment' which lifts the tail. Simply extending the roof to the back of the vehicle cancels the moment, leaving a higher base pressure, and lower pressure drag. On the Jaguar, this is part of the reason for the Cd 0.29,vs the Tesla S 0.26. The Porsche Taycan is close to the 'template' and enjoys Cd 0.25.
*On Spirit, the aeroshell and boat-tail decelerates the flow, and as per the Bernoulli Theorem, imparts a higher static pressure, removing the original separated, high-vorticity-induced low pressure from any surface 'over' the aft-body. This would be the same if no belly pan were present, as per the 1988 Texas Tech, SAE published research results. By the way, Spirit's belly pan was compromised at the time of wind tunnel testing, and probably would have rendered the diffuser useless, along with compromised underbody flow.

I was thinking about this - how could someone have developed a theory about lift which is so comprehensively wrong? But I think I now have it. The clue was in the mention of notchbacks having highest lift, and the rear spoiler on the BMW 2002.

Old cars, like that BMW 2002, had flow that separated at the end of the roof. Therefore, yes, lift was in part created by the separated flow and the low pressure associated with it acting on the trunk lid / rear window. And in that case, the spoiler did in fact work in the way stated.

But of course, major flow separation at the rear of the roof of a sedan hasn't occurred in any aerodynamic cars made since, about, 1990. So for the last 30 years, this mechanism of lift production has been irrelevant.

To attempt to apply it to the Cayenne (etc) is as absurd as I previously stated. In fact, it is easy to show how absurd it it. The measured lift pressures on the upper surfaces of these cars is lower than the wake pressure...

So, there's a good example of the perils of not keeping up with aerodynamic understanding, and applying to current cars a conceptual model that works only with old-shape cars.

[California98Civic]

Originally Posted by JulianEdgar

I am sorry, but this is your typical dump of misinformation, misunderstandings and plain outright wrong statements - as usual, dressed up in pseudo-scientific language.

Originally Posted by JulianEdgar

I am sorry, but this is all just rubbish. Have you ever measured pressures on a car? Obviously not!

Julian, there is no call for derision and its really distracting.

[JulianEdgar]

Originally Posted by California98Civic

Julian, there is no call for derision and its really distracting.

Yeah I get a bit cross when people are spreading such misinformation, and in doing so, wasting the time and energy of lots of people who follow it.

[j-c-c]

Originally Posted by eagle

Fiesta STs or SESs have this spoiler (red car) which has a little upwards inclination and also two holes at the sides Im guessing so the turbulent air can escape (?)

So nobody addressed your guess?
I was thinking more to assist to keep road dust off the rear window out of the range of the wiper.

[aerohead]

Originally Posted by JulianEdgar

I am sorry, but this is your typical dump of misinformation, misunderstandings and plain outright wrong statements - as usual, dressed up in pseudo-scientific language.




Well, perhaps read a bit more widely and recently than Hucho, second edition?

You say you have my book. Well then, read pages 179 - 182. Note specifically the reference to SAE 1999-01-0651 that shows how even small amounts of lift adversely impact stability, and to SAE papers 2009-01-004 and SAE 2015-01-1537 that again show how even small amounts of lift (especially rear lift) adversely affect car handling and stability.

But reading and driving are different things. I drive a car that, because of its modifications, develops downforce. It is easily measurable at 100 km/h. The car is significantly improved in its driving behaviour - eg cornering speed. In addition, it actually rides better because the sprung/unsprung weight ratio has effectively changed.

I might have believed what you say before I did the research for my book, and experienced the benefits of reduced lift for myself.




I am sorry, but this is just rubbish. (1) The lift being described is nothing to do with separated flow. (2) Squarebacks can create lift. As Barnard specifically says in his book, even a brick can develop lift. Fastbacks are notorious for lift. (3) Notchbacks are not historically the worse offenders, and of course as most people know, the first spoiler was fitted to the classic Porsche 911 in 1973 (page 195 of my book). The 911 was a high lift shape, just as you'd expect with that long upper body curve.




I am sorry, but this is all just rubbish. Have you ever measured pressures on a car? Obviously not! There is attached flow on these cars (eg Cayenne) to the trailing edge of the roof - the roof is not exposed to turbulence. There are plenty of wind tunnel videos around of the latest Cayenne, showing just this. In fact, the idea that the flow separates on the roof of these cars is just laughable.

I can see now why some of the misinformation is spread here: it appears that you just make up theories, completely without evidence, and then happily apply them!

I'm just attempting to compensate,for the sake of lurkers,your misinformation,misunderstandings, and plain outright wrong statements.I don't mean to jeopardize your book sales,and self-created reputation as an authority on automotive aerodynamics.
Donald Rumsfeld has said:'There are known-knowns,known-unknowns,and unknown-unknowns.'
Sometimes your comments suggest that you're attempting to punch above your weight class.
I believe that you mean well,and I applaud your efforts to educate.
I don't believe that you have as good a grip on physics as you might presume.I'm pretty certain that you don't understand lift.I'm absolutely certain that you do not understand the implications of flow separation as associated with local pressure,along with their location on the body,as pertains to lift.You keep repeating the same fallacy every time you mention lift.
*Perhaps Barnard isn't the sharpest knife in the drawer.There is counterfactual evidence to his claims to be considered. Did you?
*The rear spoiler on the 911 does not function for the reasons you submit.You can compare the 911's aft-body downslope contour with the 2020 Taycan,and then tell us why the Taycan can drive safely at 160-mph without a rear spoiler.
*All your comments about the performance of your Insight modifications are subjective. Accelerometer data would put some numbers with your testimony.3rd-party testing is always appreciated.I own the 2000,5-spd Insight.I'm okay with the way it performs in stock form.A boat-tail helps.We have an associate registering in the 80-mpg range by utilizing this mid-1930s FKFS technology.

[aerohead]

Originally Posted by JulianEdgar

I was thinking about this - how could someone have developed a theory about lift which is so comprehensively wrong? But I think I now have it. The clue was in the mention of notchbacks having highest lift, and the rear spoiler on the BMW 2002.

Old cars, like that BMW 2002, had flow that separated at the end of the roof. Therefore, yes, lift was in part created by the separated flow and the low pressure associated with it acting on the trunk lid / rear window. And in that case, the spoiler did in fact work in the way stated.

But of course, major flow separation at the rear of the roof of a sedan hasn't occurred in any aerodynamic cars made since, about, 1990. So for the last 30 years, this mechanism of lift production has been irrelevant.

To attempt to apply it to the Cayenne (etc) is as absurd as I previously stated. In fact, it is easy to show how absurd it it. The measured lift pressures on the upper surfaces of these cars is lower than the wake pressure...

So, there's a good example of the perils of not keeping up with aerodynamic understanding, and applying to current cars a conceptual model that works only with old-shape cars.

*On the other hand, Mitsubishi Lancer and Subaru WRX notchback sedans became poster childs for high speed instability and rear lift, only mitigated by the addition of VGs and wings.
*Until you understand the implications of what the local pressure is at the separation line on top of the Cayenne,and what it would mean to move that separation line all the way back to the rear of the car,there would be no point discussing the degree of lift it can telegraph over the rear of the car.I recommend a brush up on Bernoulli.

[JulianEdgar]

Originally Posted by aerohead

I'm just attempting to compensate,for the sake of lurkers,your misinformation,misunderstandings, and plain outright wrong statements.I don't mean to jeopardize your book sales,and self-created reputation as an authority on automotive aerodynamics.

Self-created authority? I am just a humble amateur working at home - never said anything else.

Originally Posted by aerohead

I don't believe that you have as good a grip on physics as you might presume.I'm pretty certain that you don't understand lift.I'm absolutely certain that you do not understand the implications of flow separation as associated with local pressure,along with their location on the body,as pertains to lift.You keep repeating the same fallacy every time you mention lift.

But my statements about lift are supported by the actual measurements that can be made on real cars. Your statements are not.

Originally Posted by aerohead

*Perhaps Barnard isn't the sharpest knife in the drawer.There is counterfactual evidence to his claims to be considered. Did you?

Richard Barnard is a world-renowned aerodynamicist. He started his career as an undergraduate aeronautical engineering apprentice at Handley Page Ltd, and later became Principal Lecturer and Postgraduate Research Tutor at the University of Hertfordshire, and is currently a Visiting Research Fellow. He is also a founder member of the UK Wind Engineering Society, and a Fellow of the Royal Aeronautical Society (FRAeS). He has been a consultant for numerous companies, including Jaguar and Rolls-Royce Bentley. Richard is the author of Road Vehicle Aerodynamics and has written many technical papers.

Um, I think I would back Dick's understanding of aerodynamics over yours....

Originally Posted by aerohead

*The rear spoiler on the 911 does not function for the reasons you submit.

Oh yes? Refer to Page 195 of my book, that shows via tuft testing the Porsche rear spoiler doing exactly what I state.

Originally Posted by aerohead

You can compare the 911's aft-body downslope contour with the 2020 Taycan,and then tell us why the Taycan can drive safely at 160-mph without a rear spoiler.

I haven't seen a tech paper on the Taycan yet, but I'd be pretty sure we'll find it has a very good underfloor that develops low pressure.

Originally Posted by aerohead

*All your comments about the performance of your Insight modifications are subjective. Accelerometer data would put some numbers with your testimony.3rd-party testing is always appreciated.

It doesn't need a vehicle dynamics expert to understand that if you have measurable downforce, and the car's mass has barely increased, then tyre grip is improved.