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-   -   Rear kammback spoiler (https://ecomodder.com/forum/showthread.php/rear-kammback-spoiler-38340.html)

eagle 05-07-2020 08:47 AM

Rear kammback spoiler
 
apart from the air - dam, splitter, engine under tray, I think the modification I am most probably going to do as soon as possible is a rear kammback spoiler since hatchbacks notoriously suffer at that region

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

https://iili.io/JcGtol.jpg

The cheap/basic edition fiestas have the small spoiler (beige car) that does not reach far back but does follow the lines of the rest of the body

https://iili.io/JcGbPS.jpg

I have already purchased the small version, which I am thinking I should extend to 60% percent of the rear window size -I have read that somewhere- and will keep the lines of the roof in order to create a kammback

something like this

https://iili.io/JcGDV2.md.jpg

should I follow the body lines with caution or could I go for an even steeper angle of attack? :rolleyes:

19bonestock88 05-07-2020 10:53 AM

I’d go another degree or two further down but longer, the longer it is the better it works.

Yippeekyaa 05-07-2020 10:56 AM

Would a similar concept work the same on a Prius rear spoiler? Have looked at mine and thought an extension that continues the downward angle would have an impact.

California98Civic 05-07-2020 11:18 AM

Yes. Modify that smaller kamm when it arrives. That's a good way to go, I think.

JulianEdgar 05-07-2020 06:41 PM

A longer extension that continues angled downwards will improve drag but increase rear lift. One that rises will not give the same drag reduction but will reduce rear lift.

Porsche recently released some good data on this for their Cayenne:

https://www.youtube.com/watch?v=cOpuT10kDPw

aerohead 05-11-2020 03:05 PM

longer
 
If the extension is elongated along the streamlined 'template' contour,and a side curtain is extended downwards to close the underside gap,such that creates the 'K' truncation,no separated flow will exists 'above' the aft-body,and all low-pressure,base pressure will be acting longitudinally,behind the vehicle as a drogue effect,with zero lift.Streamline bodies are incapable of generating lift.

JulianEdgar 05-11-2020 04:47 PM

Quote:

Originally Posted by aerohead (Post 623861)
If the extension is elongated along the streamlined 'template' contour,and a side curtain is extended downwards to close the underside gap,such that creates the 'K' truncation,no separated flow will exists 'above' the aft-body,and all low-pressure,base pressure will be acting longitudinally,behind the vehicle as a drogue effect,with zero lift.Streamline bodies are incapable of generating lift.

Did you look at the video I did on the Porsche Cayenne, linked in the above post? They quote real data from a real car which supports what I said. (And of course, there are plenty of other examples of this on other cars.)

And streamline bodies cannot create lift? Unless you are using a unique definition of what streamlining comprises, streamline bodies can certainly create lift!

aerohead 05-11-2020 06:26 PM

Porsche/lift
 
Quote:

Originally Posted by JulianEdgar (Post 623873)
Did you look at the video I did on the Porsche Cayenne, linked in the above post? They quote real data from a real car which supports what I said. (And of course, there are plenty of other examples of this on other cars.)

And streamline bodies cannot create lift? Unless you are using a unique definition of what streamlining comprises, streamline bodies can certainly create lift!

*The Cayenne's aft-body is mutilated,allowing a moment arm extending rearwards,beyond the greenhouse,of which the low pressure of the turbulence can act to produce lift.The Jaguar I-pace has the same flaw,as well as others.
*If the roof-line and greenhouse are extended,separation-free,along with the lower body,to the end of the vehicle,the entire wake is of uniform low pressure,and cannot act vertically to produce lift,as you've eliminated the moment arm.
------------------------------------------------------------------------
*A streamline body,by definition,is separation free.As a drag-minimum body,they are in the 'bucket',as Abbott and Von Doenhoff would describe it,at an angle-of-attack which is incapable of generating lift.Yes,the growing longitudinal cross-section does produce accelerated flow and lower pressure as per the Bournoulli Theorem,however,any' lift' generated by the differential there is cancelled by the high pressure attacking the forebody,and pressure recovery over the tail.They are a zero-lift shape.
I know that you are prejudiced about data from non-moving floor wind tunnels,but I'll mention that Spirit of Ecomodder measured essentially zero lift at DARKO,at 135-miles per hour,Cd 0.218 as a BEV, 128-mph speed potential according to one of the racing teams at Bonneville,and with a nearly 50-50 weight distribution,was rock solid cruising at 108-mph on the interstate highway in Arizona,in a crosswind.Everything done to the truck was inspired by Jaray's research of 1922.

JulianEdgar 05-11-2020 06:54 PM

Quote:

Originally Posted by aerohead (Post 623885)
*The Cayenne's aft-body is mutilated,allowing a moment arm extending rearwards,beyond the greenhouse,of which the low pressure of the turbulence can act to produce lift.The Jaguar I-pace has the same flaw,as well as others.
*If the roof-line and greenhouse are extended,separation-free,along with the lower body,to the end of the vehicle,the entire wake is of uniform low pressure,and cannot act vertically to produce lift,as you've eliminated the moment arm.

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.

Quote:

Originally Posted by aerohead (Post 623885)
------------------------------------------------------------------------
*A streamline body,by definition,is separation free.As a drag-minimum body,they are in the 'bucket',as Abbott and Von Doenhoff would describe it,at an angle-of-attack which is incapable of generating lift.Yes,the growing longitudinal cross-section does produce accelerated flow and lower pressure as per the Bournoulli Theorem,however,any' lift' generated by the differential there is cancelled by the high pressure attacking the forebody,and pressure recovery over the tail.They are a zero-lift shape.

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.


Quote:

Originally Posted by aerohead (Post 623885)
I know that you are prejudiced about data from non-moving floor wind tunnels,but I'll mention that Spirit of Ecomodder measured essentially zero lift at DARKO,at 135-miles per hour,Cd 0.218 as a BEV, 128-mph speed potential according to one of the racing teams at Bonneville,and with a nearly 50-50 weight distribution,was rock solid cruising at 108-mph on the interstate highway in Arizona,in a crosswind.Everything done to the truck was inspired by Jaray's research of 1922.

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.

aerohead 05-11-2020 07:03 PM

wrong
 
Quote:

Originally Posted by JulianEdgar (Post 623888)
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.

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.

aerohead 05-12-2020 01:47 PM

ignores/wrong/sorry
 
Quote:

Originally Posted by JulianEdgar (Post 623888)
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 05-12-2020 06:34 PM

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.


Quote:

Originally Posted by aerohead (Post 623956)
*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.


Quote:

Originally Posted by aerohead (Post 623956)
* 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.


Quote:

Originally Posted by aerohead (Post 623956)
*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 05-12-2020 08:41 PM

Quote:

Originally Posted by aerohead (Post 623890)
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 05-12-2020 08:50 PM

Quote:

Originally Posted by aerohead (Post 623956)
* 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 05-12-2020 09:21 PM

Quote:

Originally Posted by JulianEdgar (Post 623999)
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.

Quote:

Originally Posted by JulianEdgar (Post 623999)
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 05-12-2020 09:59 PM

Quote:

Originally Posted by California98Civic (Post 624019)
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 05-12-2020 11:23 PM

Quote:

Originally Posted by eagle (Post 623451)
Fiesta STs or SESs have this spoiler (red car) which has a little upwards inclination and also two holes at the sides :confused::confused: Im guessing so the turbulent air can escape (?)

https://iili.io/JcGtol.jpg

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 05-13-2020 12:57 PM

wrong
 
Quote:

Originally Posted by JulianEdgar (Post 623999)
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 05-13-2020 01:10 PM

since 1990
 
Quote:

Originally Posted by JulianEdgar (Post 624016)
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 05-13-2020 05:40 PM

Quote:

Originally Posted by aerohead (Post 624077)
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.

Quote:

Originally Posted by aerohead (Post 624077)
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.


Quote:

Originally Posted by aerohead (Post 624077)
*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....

Quote:

Originally Posted by aerohead (Post 624077)
*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.

Quote:

Originally Posted by aerohead (Post 624077)
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.

Quote:

Originally Posted by aerohead (Post 624077)
*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.

JulianEdgar 05-13-2020 05:45 PM

Quote:

Originally Posted by aerohead (Post 624079)
*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.

What separation on the roof of the Cayenne? Have you looked at the wind tunnel video with smoke streams produced by Porsche? Just as one would expect, it doesn't show any separation on the roof.

freebeard 05-13-2020 09:06 PM

Won't someone think of the OP, eagle, and take it elsewhere? I suggest the form of a rap battle.

Quote:

Im guessing so the turbulent air can escape
My thought drift to the Merkur XR4Ti with it's biplane spoiler.

http://www.curbsideclassic.com/wp-co...9/merkur-4.jpg
https://ecomodder.com/forum/showthre...tml#post267188

Just mounted at the top of the hatch and pancaked somewhat.

I know this is off-topic, but maybe those other two can take it off to another thread. I ran across it looking for a Merkur pic.

Behold, a fastback Porsche coupe with Template conformity, and a Coanda effect from the engine rear mounted heat pump.

http://farm7.staticflickr.com/6005/5...6ba2b578_o.jpg

California98Civic 05-14-2020 12:01 AM

Quote:

Originally Posted by freebeard (Post 624179)
... My thought drift to the Merkur XR4Ti with it's biplane spoiler.

http://www.curbsideclassic.com/wp-co...9/merkur-4.jpg
https://ecomodder.com/forum/showthre...tml#post267188

Just mounted at the top of the hatch and pancaked somewhat....

I love that spoiler. I also suspect that the roof spoiler the OP is describing might not be so much to let air escape as it is for "tuning" the flow in some way. For example, Subaru apparently spent a lot of time and money tuning the position of those silly looking vortex generators you see on the STi from a fee years ago. The goal, I read, was to direct flow to the substantial wing on the rear decklid.

JulianEdgar 05-14-2020 02:59 AM

Quote:

Originally Posted by California98Civic (Post 624191)
Subaru apparently spent a lot of time and money tuning the position of those silly looking vortex generators you see on the STi from a fee years ago. The goal, I read, was to direct flow to the substantial wing on the rear decklid.

That was certainly the goal on the original Evo Lancer, and they did (according to Mitsubishi's modelling) work a bit.

Left with, right without:

https://i.postimg.cc/XqMVy2ht/Figure-6-57.png

It would be interesting to see the modelled pressures under the wing.

j-c-c 05-14-2020 11:51 AM

Quote:

Originally Posted by freebeard (Post 624179)
Won't someone think of the OP, eagle, and take it elsewhere? I suggest the form of a rap battle.


My thought drift to the Merkur XR4Ti with it's biplane spoiler.

http://www.curbsideclassic.com/wp-co...9/merkur-4.jpg
https://ecomodder.com/forum/showthre...tml#post267188

Just mounted at the top of the hatch and pancaked somewhat.

"Biplane spoiler" or Hybrid aero device, or Wing supplemented with a spoiler, which can be rather dubious in that one can deduct effectiveness, depending on orientation to each other, from the other, unless cosmetic enhancement is the primary goal, in that it sells cars and earns profits, from the punters, IMO.

And I also don't see a lot of commonality with OP's pictured "guess" example.

JulianEdgar 05-14-2020 06:16 PM

Quote:

Originally Posted by j-c-c (Post 624223)
"Biplane spoiler" or Hybrid aero device, or Wing supplemented with a spoiler, which can be rather dubious in that one can deduct effectiveness, depending on orientation to each other, from the other, unless cosmetic enhancement is the primary goal, in that it sells cars and earns profits, from the punters, IMO.

And I also don't see a lot of commonality with OP's pictured "guess" example.

Yes, I have always wondered how this was supposed to work. Has anyone ever seen a tech paper on this type of design? It would be really interesting to see some tuft testing.

freebeard 05-14-2020 08:55 PM

I'll bet we could/should ask aerohead. ;)

MHR1294 05-14-2020 08:58 PM

To OP

I had a 2001 Volvo V70 T5 which had a high mount spoiler with a "hole/slot" in it.

Interesting thing happened, you could see the giant swirls in the mirror generated when it was raining in the spray. You could see 2 big swirls forming on the rear window coming down from the upper middle an spreading out. So looking from the rear, that's clockwise on the left and anti-clock on the right.

With the hole covered up, the dirt left on the rear window was more uniform spread. Fuel consumption was a little bit worse too, however the scale only reads to the nearest 0.1mpg (imp) so 0.4mpg improvement might not be as significant when you take in other factors.

so, I don't know what it was doing, but it was definitely doing something. :)

Perhaps it's performing a similar function on the fiesta?

JulianEdgar 05-14-2020 09:02 PM

Quote:

Originally Posted by MHR1294 (Post 624250)
To OP

I had a 2001 Volvo V70 T5 which had a high mount spoiler with a "hole/slot" in it.

Interesting thing happened, you could see the giant swirls in the mirror generated when it was raining in the spray. You could see 2 big swirls forming on the rear window coming down from the upper middle an spreading out. So looking from the rear, that's clockwise on the left and anti-clock on the right.

With the hole covered up, the dirt left on the rear window was more uniform spread. Fuel consumption was a little bit worse too, however the scale only reads to the nearest 0.1mpg (imp) so 0.4mpg improvement might not be as significant when you take in other factors.

so, I don't know what it was doing, but it was definitely doing something. :)

Perhaps it's performing a similar function on the fiesta?

Interesting. Trailing vortices that rotate (viewed from behind) clockwise on the left and anticlockwise on the right are indicative of the body developing lift.

MHR1294 05-14-2020 09:13 PM

Quote:

Originally Posted by JulianEdgar (Post 624252)
Interesting. Trailing vortices that rotate (viewed from behind) clockwise on the left and anticlockwise on the right are indicative of the body developing lift.

I think I got that right, it was in the mirror. If I still had the car I'd love to go and test it.

The droplets would run down from the upper middle of the screen, and spread towards the lower outer edges. So whatever direction that is!

The spoiler did angle down too, I'm sure I've got a picture of it somewhere I'll go have a dig.

JulianEdgar 05-14-2020 09:17 PM

Quote:

Originally Posted by MHR1294 (Post 624256)
I think I got that right, it was in the mirror. If I still had the car I'd love to go and test it.

The droplets would run down from the upper middle of the screen, and spread towards the lower outer edges. So whatever direction that is!

The spoiler did angle down too, I'm sure I've got a picture of it somewhere I'll go have a dig.

That's pretty typical of trailing vortex direction - 99 per cent of cars develop lift. However, it's pretty rare to be able to see it so clearly in the rear vision mirror.

I had a look for some pics of the spoiler but wasn't sure precisely which one it was, so I'd be interested.

MHR1294 05-14-2020 09:19 PM

Best images I can find, it's not easy to see it's shape in Yellow

https://forums.mightycarmods.com/fil...photoid=808285
https://forums.mightycarmods.com/fil...photoid=808286

Fat Charlie 05-14-2020 10:49 PM

Ideals notwithstanding, given the preexisting condition of having an 09 Fiesta, the best way to find out is to try.

JulianEdgar 05-15-2020 04:36 AM

Quote:

Originally Posted by Fat Charlie (Post 624260)
Ideals notwithstanding, given the preexisting condition of having an 09 Fiesta, the best way to find out is to try.

Absolutely. An hour of testing beats a week of forum discussion, every time.

JulianEdgar 05-15-2020 04:41 AM

Quote:

Originally Posted by MHR1294 (Post 624258)

Thanks for that.

My guess - and that's all it is - was that the slots were for changing flow patterns to reduce dirt deposition, as you have said.

It's not often mentioned in discussion groups, but there is a lot of professional aerodynamic tech literature on 'soiling' - how dirt covers cars. In fact I was reading something the other day that implied the air curtains on trucks are largely for this - to keep the doors clean so signwriting isn't obscured. Looking at the trucks I've seen since, the air curtains do all seem to be aimed at the doors...

aerohead 05-15-2020 11:40 AM

Have you ever measured pressure....?
 
I am university trained in both aerodynamic and hydrodynamic measurement.My senior project (thesis) involved the aerodynamic streamlining of my VW Transporter,which yielded an increase in highway fuel economy,from 27-mpg,to 35-mpg.For nearly 6-years,I earned my daily bread making these measurements.I orchestrated the wind tunnel testing of the Cd 0.11 Becker-Lyon,BMW,LSR motorcycle streamliner.I hold two USFRA speed records.Since 1986,I've probably been part of only a rarified community of motorists incorporating a relative-wind-seeking,slewing pitot-tube/airspeed indicator.
Since my projects mimic vehicles which have already been analyzed by simultaneous recordings of 42-pressure tap/manometer readings,drilling my car full of holes,and bankrupting myself purchasing hardware would be folly.I've been happy enough to spend thousands of $ on 3rd-party testing.
In 1951, Sighard Hoerner reported that the average passenger car had a frontal-area-based coefficient of lift of 0.40.At 50-mph,this translated to 60-pounds lift,on a car of 4,500-pounds.
The 1935 Jaray-Adler Sportwagen was measured at Cl= 0.20,which would generate 30-pounds lift under the same conditions.
Spirit of Ecomodder indicated a Cl= 0.00591.At 135-mph she generated 8-pounds of downforce.For a 4,220-pound vehicle this is essentially a 'zero-lift' vehicle. Modelled in part,after the FKFS K-cars,which FKFS reported as 'neutral',as far as lift is concerned.
There's a reason Hucho wasn't concerned with passenger car lift,as of 1987.Can you imagine why? And can you tell the audience what has changed about Earth's atmosphere,or physics since then,that we should obsess over it now?

aerohead 05-15-2020 11:54 AM

streamlined bodies and lift
 
On page 22,Fig.1.12.,of Hermann Schlicting's 'Boundary-Layer Theory',you'll find a complete pressure profile of a streamline body,presented originally by Fuhrmann.
You'll notice the positive static pressure acting on both the nose and tail,allowing for zero lift.
On page 23,Fig.1.14. you'll find a complete pressure profile of a Zhukovskii wing section,demonstrating a zero-lift condition (just as the 118-families of wing sections listed in Theory of Wing Sections) ,after the research of A. Betz.
If 'the exception proves the rule',then,when you make statements about the existence of lift for wings and streamline bodies,you'll want to include all the qualifying caveats/conditions.

aerohead 05-15-2020 12:18 PM

humble,etc.
 
Quote:

Originally Posted by JulianEdgar (Post 624138)
Self-created authority? I am just a humble amateur working at home - never said anything else.



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




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....



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



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.



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.

*The 'tone' of your voice takes on the color of anything but humble.Have you listened to yourself?
*My statements about lift are grounded in fluid mechanics. And specifically with respect to road vehicles.
*Let me know when Handley Page,Jaguar,Rolls-Royce,or Bentley debut a Cd 0.12 automobile under Bernard's tutelage and he'll have my attention.
*If you're looking under the car for stability,you're looking on the wrong side of the car. It's all about the roof profile.
*Why are you introducing downforce? Are you trying to finish off the Great Barrier Reef all by yourself?

aerohead 05-15-2020 12:21 PM

Cayenne roof
 
Quote:

Originally Posted by JulianEdgar (Post 624139)
What separation on the roof of the Cayenne? Have you looked at the wind tunnel video with smoke streams produced by Porsche? Just as one would expect, it doesn't show any separation on the roof.

There's no reason in discussing it until you demonstrate a handle on the Bernoulli Theorem.Bernard should have told you that.

aerohead 05-15-2020 12:24 PM

Ford of Cologne's Bi-wing spoiler
 
Quote:

Originally Posted by California98Civic (Post 624191)
I love that spoiler. I also suspect that the roof spoiler the OP is describing might not be so much to let air escape as it is for "tuning" the flow in some way. For example, Subaru apparently spent a lot of time and money tuning the position of those silly looking vortex generators you see on the STi from a fee years ago. The goal, I read, was to direct flow to the substantial wing on the rear decklid.

Ford's patent is available.There's no reason to speculate on it's premise,you can read it directly from the inventors.


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