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Old 09-29-2020, 02:50 AM   #1 (permalink)
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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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Old 09-29-2020, 02:22 PM   #2 (permalink)
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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.
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Old 09-29-2020, 02:56 PM   #3 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
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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Old 09-29-2020, 04:01 PM   #4 (permalink)
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Quote:
Originally Posted by 19bonestock88 View Post
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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Old 09-29-2020, 04:28 PM   #5 (permalink)
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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.
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Old 09-29-2020, 04:38 PM   #6 (permalink)
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Tuft test top of boot / trunk and top of spoiler.
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Old 09-29-2020, 04:47 PM   #7 (permalink)
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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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Old 09-29-2020, 06:35 PM   #8 (permalink)
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Quote:
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Tuft test top of boot / trunk and top of spoiler.
Are you saying to just start with this?

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Old 09-29-2020, 07:25 PM   #9 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
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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Old 09-29-2020, 09:25 PM   #10 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
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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