Aerodynamics advice neede - Ducktail+Wing?

[MSTiFK8R]

Hi pals!

I've a BRZ with Rocketbunny v.2 kit, which comes with an aggressive ducktail spoiler and recently installed a wing on the trunk together with the ducktail.

The car itself is a track tool with ~500hp, custom splitter and full flat underbody and aggressive rear diffuser, which ends the underpanels nicely.

Ducktail+Wing combo look VERY unusual to the eye, still thinking if that couple could have some future, so needing your TECHNICAL ADVICE. Let the cosmetics sit aside, that's too personal.

Technically, pls, will this combo allow faster laptimes or they just ruin the hella of all aerodynamics.

If one could illustrate the flow path in this case will be my hero )

Yours, Mike


BTW, GT3RS has the same combo for some reason, as long as the Cayman GT4 also

[swarb]

Without data there is no real answer.
By product of down force is drag.
Without data there is no real answer.
Top speed vs down force. Too many variables.
Without data...

[Gunman]

Tough call without a tunnel, of CFD.

The wing should be more efficient, better L/D. Spoiler should have a higher drag penalty. Then the question of the interaction...will the spoiler choke the under wing flow? will the combo move the Cp too far aft, ie more than you can dial out with the wing angle of attack?

[MSTiFK8R]

Originally Posted by Gunman

Tough call without a tunnel, of CFD.

The wing should be more efficient, better L/D. Spoiler should have a higher drag penalty. Then the question of the interaction...will the spoiler choke the under wing flow? will the combo move the Cp too far aft, ie more than you can dial out with the wing angle of attack?

okay, now how that could be checked/tested without a wind tunnel?)

spoiler choking the underwing flow is my main concern

but still, having the wing is helping to reduce drag and improve downforce anyway, which is supposed to result in faster laptime...theoretically

[8RZ]

The difference is, Porsche has put R&D when building their cars whereas that BRZ just slapped together parts that looked similar in hopes of achieving similar results.

[OND]

Based on my nonexistent professional aerodynamics experience and the total lack of data, I am 110% sure that spoiler is too aggressive for optimal use with the wing.

Maybe get a TRD spoiler instead? Someone should be willing to trade theirs for that badass Rocketbunny one of yours.

[Gunman]

Originally Posted by MSTiFK8R

okay, now how that could be checked/tested without a wind tunnel?)

spoiler choking the underwing flow is my main concern

but still, having the wing is helping to reduce drag and improve downforce anyway, which is supposed to result in faster laptime...theoretically

You could try yarn tufts, flowvis (or engine assembly lube), but that will only get you surface flow, and attachment. A pressure rake would be ideal, but outside a tunnel, you are dealing with changing environmental conditions. You couldn't accurately reproduce back to back tests.

Wing won't reduce drag, it should have less than the spoiler, but it will add drag.

[norcalpb]

That spoiler is more of a gurney flap imo which will add a bit of drag.

I would get a TRD spoiler because any air that's not going over the wing needs to GTFO the back of your car as fast as possible. I am in no means an expert on aero but I could see an actual spoiler reducing the turbulence of the wake behind your car at speed.

[retoocs]

Throwing a bunch of aero parts on a car won't make it go faster around the track. You don't seem to understand why you even put a wing on a car. The goal isn't to generate the most amount of down force. The goal is to generate the minimum amount of down force necessary to get around a particular track.

[Icecreamtruk]

Really good answers in this thread. I'll try to add something to it.

From a lot of reading and other people's data:

For the wing to work properly, you need higher velocity air under it, and lower velocity on top of it. The bigger this differential, the more downforce it generates. The bigger the angle on the wing, the more drag it creates but the more differential we have (so more angle = more drag and more downforce).

Spoilers work a little differently. In your case, its a more of a gurney, which increases pressure on top of your trunk by creating a almost stagnant pocket of air. Small downforce gain, lots of drag. But it also allows the air from the top of the vehicule to stay separated from the bottom for longer, so the turbulences behind your car are farther back, which reduces drag. So all in all, small downforce, small to medium drag after considering it all.

Spoilers like those in the porsches have a way lower angle, they dont work like a gurney flap they work more like an extension to the trunk, giving the benefit of reducing drag by increasing the distance of the turbulence behind the car, but without adding much (if any) drag to it. They do not disturb airflow velocity under the wing, so they can work with it without any problem.

A wing that is close enough to the trunk will produce the effect of both, a spoiler, and a wing. But since there is a lot less air passing thru it, it generates less downforce than being put higher up. So if you want to maximise downforce while reducing drag, you either put the wing really close to the trunk, or use the wing together with a small spoiler with a very small angle (if any, think of drag spoilers, almost flat). Porsches have the second option here.

The worse case scenario would be the wing high up with a very steep angled spoiler or gurney flap on the trunk. You generate massives amount of drag and only moderate (or even small) amounts of downforce.

All of that is from theory, lots of reading on aerodynamics and CFD.

From my own data playing with my wing angle (APR GTC-200). My car does not have a trunk spoiler of any sort but does have front splitter and side splitters (horizontal skirts). Negative angle on my wing (around -2) allows me to have around 10mph faster speed at the end of the straight vs max angle (around 15). The car is neutral mid corner on corners above 60mph or so, loose in corners below that, slight understeer in corners above that speed with negative wing angle. The car understeers like a pig at max angle on anything but hairpins. Highest lateral Gs were achieved when the car was understeering a lot with max wing angle, but fastest lap times were with the wing at negative angle. Havent had enough time to play in between.

[ichitaka05]

Originally Posted by 8RZ

The difference is, Porsche has put R&D when building their cars whereas that BRZ just slapped together parts that looked similar in hopes of achieving similar results.

That BRZ, prob for the looks. This in other hand, it's all function, no show...

[MSTiFK8R]

Originally Posted by Icecreamtruk

Really good answers in this thread. I'll try to add something to it.

From a lot of reading and other people's data:

For the wing to work properly, you need higher velocity air under it, and lower velocity on top of it. The bigger this differential, the more downforce it generates. The bigger the angle on the wing, the more drag it creates but the more differential we have (so more angle = more drag and more downforce).

Spoilers work a little differently. In your case, its a more of a gurney, which increases pressure on top of your trunk by creating a almost stagnant pocket of air. Small downforce gain, lots of drag. But it also allows the air from the top of the vehicule to stay separated from the bottom for longer, so the turbulences behind your car are farther back, which reduces drag. So all in all, small downforce, small to medium drag after considering it all.

Spoilers like those in the porsches have a way lower angle, they dont work like a gurney flap they work more like an extension to the trunk, giving the benefit of reducing drag by increasing the distance of the turbulence behind the car, but without adding much (if any) drag to it. They do not disturb airflow velocity under the wing, so they can work with it without any problem.

A wing that is close enough to the trunk will produce the effect of both, a spoiler, and a wing. But since there is a lot less air passing thru it, it generates less downforce than being put higher up. So if you want to maximise downforce while reducing drag, you either put the wing really close to the trunk, or use the wing together with a small spoiler with a very small angle (if any, think of drag spoilers, almost flat). Porsches have the second option here.

The worse case scenario would be the wing high up with a very steep angled spoiler or gurney flap on the trunk. You generate massives amount of drag and only moderate (or even small) amounts of downforce.

All of that is from theory, lots of reading on aerodynamics and CFD.

From my own data playing with my wing angle (APR GTC-200). My car does not have a trunk spoiler of any sort but does have front splitter and side splitters (horizontal skirts). Negative angle on my wing (around -2) allows me to have around 10mph faster speed at the end of the straight vs max angle (around 15). The car is neutral mid corner on corners above 60mph or so, loose in corners below that, slight understeer in corners above that speed with negative wing angle. The car understeers like a pig at max angle on anything but hairpins. Highest lateral Gs were achieved when the car was understeering a lot with max wing angle, but fastest lap times were with the wing at negative angle. Havent had enough time to play in between.

So much thanks for this detailed approach for you and the guys above.

I will try to make some measurements and flow testing.

I know the essense now. Cheers, bros!

[DIY]

I have done a little aero testing in the past with a limited budget and got some great results. What I used was a string pot potentiometer mounted on the suspension to measure changes in ride height at certain speeds.

This was on a 05 wrx and what I found was the car would lift with the factory aero. I then built my own front splitter and air dam and added a Cusco GT rear wing.

The front splitter created considerable downforce, the rear wing at high attack angle was taking away almost all of this downforce. I backed it off and got a good balance of downforce front and rear.

Without these measurements it would have been very hard to get those results. In my experience rear downforce is actually pretty easy to get if you buy a good quality rear wing. The front end is where you will have trouble maximising downforce.

Good luck.

[MSTiFK8R]

I think its the right time for @VeloxEric to chime in

I know how mad these guys are when it comes to CFD..

Would you share your thoughts on this combo pls?


BTW, nice vid showing wind tunnel testing of Porsche with ~same combo