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Old 05-07-2020, 07:47 AM   #1 (permalink)
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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 Im guessing so the turbulent air can escape (?)



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



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



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

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Old 05-07-2020, 09:53 AM   #2 (permalink)
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I’d go another degree or two further down but longer, the longer it is the better it works.
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Old 05-07-2020, 09:56 AM   #3 (permalink)
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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.
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Old 05-07-2020, 10:18 AM   #4 (permalink)
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Yes. Modify that smaller kamm when it arrives. That's a good way to go, I think.
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Old 05-07-2020, 05:41 PM   #5 (permalink)
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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:

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Old 05-11-2020, 02:05 PM   #6 (permalink)
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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.
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Old 05-11-2020, 03:47 PM   #7 (permalink)
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Quote:
Originally Posted by aerohead View Post
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!
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Old 05-11-2020, 05:26 PM   #8 (permalink)
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Porsche/lift

Quote:
Originally Posted by JulianEdgar View Post
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.
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Old 05-11-2020, 05:54 PM   #9 (permalink)
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Quote:
Originally Posted by aerohead View Post
*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 View Post
------------------------------------------------------------------------
*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 View Post
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.
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Old 05-11-2020, 06:03 PM   #10 (permalink)
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wrong

Quote:
Originally Posted by JulianEdgar View Post
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.

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