EcoModder Forum Excellent paper on rear spoilers

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 09-29-2020, 02:50 AM #1 (permalink) Banned   Join Date: Nov 2017 Location: Australia Posts: 2,060 Thanks: 107 Thanked 1,594 Times in 1,131 Posts Excellent paper on rear spoilers A collection of SAE aero papers arrived today (SAE SP-1600) and it contains a very interesting paper on rear spoilers. The paper is 2001-01-1267, written by Hyundai engineers in 2001. A few things make it interesting. First, it covers squarebacks, notchbacks and fastbacks. Second, it was written when Hyundai had only just got their full-size wind tunnel. You can see the engineers struggling a bit with the relationship between wake size, attached flow giving downwash (that they call the coanda effect), rear lift and drag. Because I think it was all new to them, they did some really interesting 'basics' test. One good example is plotting pressures around a notchback (and one with a 'modern' rear window angle, too) with and without a front undertray and rear spoiler. (I tried to scan these diagrams to post here but the reproduction quality is pretty bad.) But as one example, a rear spoiler on the trunk / boot clearly improves flow past the front undertray. They also have very clear measurements of reduction in rear suction peaks at the end of the trunk / boot lid giving reduced drag and rear lift, and a very interesting section where on a squareback, they trial a gap between a rear roof extension / spoiler and try different spoiler angles, all correlated against drag coefficient. They also go out on what today would be regarded as 'on a limb', drawing a very tight mathematical correlation between rear coefficient of lift (note: not overall CL, but CLr) and increases in drag. But - and again this is very interesting - they then give no less than 61 examples of measured data in the wind tunnel to show real world examples of their mathematical relationship. There's plenty of scatter in the graphs but you can see what they're getting at. (Their relationship? CD = CD value (no rear lift) + 0.8996 *(CLr)^2) A bit like with wings on cars, when you go looking for SAE papers on spoilers there aren't a lot around - and I wasn't aware of this one until today. For people measuring pressures, it's an especially interesting paper.
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 09-29-2020, 02:22 PM #2 (permalink) Ecomodding amateur     Join Date: Nov 2015 Location: Indiana Posts: 656 The Van - '95 Chevy Astro Cl V8 Swapped Team Chevy 90 day: 9.5 mpg (US) The new bike - '17 Kawasaki Versys X 300 abs Motorcycle 90 day: 64.68 mpg (US) The Mercury - '95 Mercury Tracer Trio Team Ford 90 day: 37.77 mpg (US) Thanks: 119 Thanked 238 Times in 171 Posts I have always heard the difference between spoiler and wing is that a wing has airflow/opening under neath it. A spoiler is just like a lip or extension on the car. Is that correct? I wanted to make sure my terminology was the same before asking more questions. __________________ Věci získávají svou skutečnou krásu pouze ve vztahu k našemu životu -Jiří Mucha (1915-1991), Publicist 1973 Fiat 124 Special 1975 Honda Civic CVCC 4spd 1981 Kawasaki KZ750E 1981 Kawasaki KZ650 CSR 1983 Kawasaki KZ1100-A3 1986 Nissan 300zx Turbo 5 spd 1995 Chevy Astro RWD (current project) 1995 Mercury Tracer 2004 Chevy Astro AWD 2017 Kawasaki VersysX 300 https://www.youtube.com/channel/UC6s...LulDUQ8HMj5VKA
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Quote:
 Originally Posted by M_a_t_t I have always heard the difference between spoiler and wing is that a wing has airflow/opening under neath it. A spoiler is just like a lip or extension on the car. Is that correct? I wanted to make sure my terminology was the same before asking more questions.
That’s what I gather
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 Originally Posted by 19bonestock88 That’s what I gather
On cars I define wings as: those devices that have attached airflow over both surfaces, with these surfaces shaped to provide a lower pressure on the bottom surface compared with the top surface.

There aren't many wings fitted as standard to cars.

A spoiler, on the other hand, is an attachment that causes a change in flow and, from that, a change in pressures - usually a change in pressure both on the attachment itself and also on other panels of the car.

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 09-29-2020, 04:28 PM #5 (permalink) Ecomodding amateur     Join Date: Nov 2015 Location: Indiana Posts: 656 The Van - '95 Chevy Astro Cl V8 Swapped Team Chevy 90 day: 9.5 mpg (US) The new bike - '17 Kawasaki Versys X 300 abs Motorcycle 90 day: 64.68 mpg (US) The Mercury - '95 Mercury Tracer Trio Team Ford 90 day: 37.77 mpg (US) Thanks: 119 Thanked 238 Times in 171 Posts OK. I so wanted to find out if the stock spoiler(?) on my car does anything at all. Right my plan was as follows: 1. tuft test it 2. pressure puck on the top, maybe bottom, but might be a tight squeeze and therefore possibly inaccurate. I don't have a direct picture on hand right now, but these should help. I have tufted the back window and given the distance from the spoiler to the glass I don't think it will be in attached flow. __________________ Věci získávají svou skutečnou krásu pouze ve vztahu k našemu životu -Jiří Mucha (1915-1991), Publicist 1973 Fiat 124 Special 1975 Honda Civic CVCC 4spd 1981 Kawasaki KZ750E 1981 Kawasaki KZ650 CSR 1983 Kawasaki KZ1100-A3 1986 Nissan 300zx Turbo 5 spd 1995 Chevy Astro RWD (current project) 1995 Mercury Tracer 2004 Chevy Astro AWD 2017 Kawasaki VersysX 300 https://www.youtube.com/channel/UC6s...LulDUQ8HMj5VKA
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 09-29-2020, 04:38 PM #6 (permalink) Banned   Join Date: Nov 2017 Location: Australia Posts: 2,060 Thanks: 107 Thanked 1,594 Times in 1,131 Posts Tuft test top of boot / trunk and top of spoiler.
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 09-29-2020, 04:47 PM #7 (permalink) Banned   Join Date: Nov 2017 Location: Australia Posts: 2,060 Thanks: 107 Thanked 1,594 Times in 1,131 Posts Here's two diagrams from the paper. Notchback with and without small rear spoiler ("kick-up"). Note that with spoiler there is a: - major pressure increase on boot / trunk lid - major reduction in suction peak due to clean separation - small reduction in drag and larger reduction in rear lift In one sense none of this is startling - the pressures are what you can measure yourself on cars that have attached flow on the trunk / boot. But at the same time it's good to see such clear evidence of the changes possible.
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Quote:
 Originally Posted by JulianEdgar Tuft test top of boot / trunk and top of spoiler.

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Quote:
 Originally Posted by M_a_t_t Are you saying to just start with this?
Yes, it's all pretty horrible isn't it?! (Maybe there's just some flow reattachment on the spoiler - maybe.) Would need to see proper length video to be sure.

It might be better to add roof and C-pillar extensions and get clean separation at those points. I can't see you being able to do a lot on the trunk lid without doing absolutely radical modifications.

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Quote:
 Originally Posted by JulianEdgar Would need to see proper length video to be sure. It might be better to add roof and C-pillar extensions and get clean separation at those points. I can't see you being able to do a lot on the trunk lid without doing absolutely radical modifications.
Do you mean like this?

I was thinking it might be cool to try and replicate the style of a lotus elise rear window area with the over hang and wrap around. My trunk isn't as long as the elises, so not quite the same. Probably end up looking more like a del sol with a roof extension.

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Věci získávají svou skutečnou krásu pouze ve vztahu k našemu životu
-Jiří Mucha (1915-1991), Publicist

1973 Fiat 124 Special
1975 Honda Civic CVCC 4spd
1981 Kawasaki KZ750E
1981 Kawasaki KZ650 CSR
1983 Kawasaki KZ1100-A3
1986 Nissan 300zx Turbo 5 spd
1995 Chevy Astro RWD (current project)
1995 Mercury Tracer
2004 Chevy Astro AWD
2017 Kawasaki VersysX 300