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Old 11-21-2018, 07:15 PM   #1
atm0706
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Default Wingless rear down force for wagons??

Today while daydreaming I had this idea of a way to increase down force on a track driven wagon without adding a stupid looking wing to the roof. I got the inspiration from race cars and hyper cars with flow through aero, which seems to be really effective.

I also pretty much have no real knowledge in aerodynamics, so this may just be a stupid daydream after all. Hoping someone more knowledgeable can chime in as to whether or not this could work.

The idea is basically to cut a really big naca duct type hole in the roof, and the duct the inside rear of the car so that the air flows through a hole cut in the hatch, and out the back of the car. Obviously you would want complete aero, not just this, but I wonder if paired with a front splitter and flat bottom would work in the real world.

I have attached a crude little doodle just to help show the idea.

Alex

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Old 11-29-2018, 12:43 PM   #2
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Your NACA duct is backwards. Lots of cars use this principle, but for engine cooling in mid-engine vehicles, but you may be able to get some downforce out of this design.
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Old 11-29-2018, 01:16 PM   #3
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yeah
this is lift
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Old 11-29-2018, 01:55 PM   #4
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I assume when you get the wagon, you give up on any downforce.

If you want the air to go through the wagon from the roof, you would need a massive scoop bigger than the wing you're trying to replace.
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Old 11-29-2018, 05:56 PM   #5
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As if cutting a hole in the roof and hatch as well as adding an air tunnel is not as bad as a big wing?
IDK maybe remove the wing off track?
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Old 11-29-2018, 08:30 PM   #6
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Quote:
Originally Posted by car_freak85 View Post
Your NACA duct is backwards. Lots of cars use this principle, but for engine cooling in mid-engine vehicles, but you may be able to get some downforce out of this design.
Yeah lol, I realized that after I posted and looked at more pictures of naca ducts.

Quote:
Originally Posted by jasonwrx86 View Post
As if cutting a hole in the roof and hatch as well as adding an air tunnel is not as bad as a big wing?
IDK maybe remove the wing off track?
Not in my personal opinion, its mostly invisible from the profile of the car. I don't mind big wings on sedans/coupes, but they look so cringy on wagons.

Quote:
Originally Posted by Hyper View Post
yeah
this is lift
Again, I am by no means an expert, but I would like to understand how it would create lift vs downforce vs no effect.
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Old 11-29-2018, 09:46 PM   #7
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Nice read here on NACA ducts.

Quote:
A few words on implementing your own NACA duct as a means of induction to your race car.

-1: design is very important. The duct is designed to be efficient with the correct wall angles (sharp), base slope, and width-to-depth ratio in relation with speed.

-2: The duct needs to be installed in an area of high pressure. A leading edge of a car is a great place.

-3: Don't buy garbage. Edges on the slopes have to be sharp, otherwise the flow would separate, reducing the duct's efficiency.

-4: NACA duct is only useful in applications where you really don't want much air, at least not as much as you hoped, and certainly less than you expect.

-5: If you want any air at all, make sure the NACA duct is placed in a region with a positive pressure gradient; i.e., put it where the air sees the body as increasing in size, not constant or decreasing.

-6: They don't work well, or at all, when placed on the negative pressure area.
Something like you want *may* be doable, but not with a NACA duct on your roof (low pressure area), I would guess. I'd try a properly set up rear diffuser before I hacked the isht out of my car.
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Old 11-29-2018, 11:37 PM   #8
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OK, I am not an aero engineer, but I fly and understand some aero and I own a wagon as well.
What I did was leave snow on the top in winter. Sorta light snow, then drive. Look at the shape of snow on the top after a drive.
Air coming off the hood/windshield, then the OEM moonroof deflector made the snow sorta smooth beyond the rear OEM wing.
I found I could add a small aluminum angle under the deflector to pull it down a bit, then add some turbulators/vortex generators to the deflector to get airflow to the stock wing.
Taking the stock wing off and installing a raised wing (so, into airflow) can add downforce.
Your choice on route.

Without the deflector, I will guess the top rear of the roof is still close to negative pressure, so your design is still wasted.

Try some soft snow on the top and drive it a bit, see what the snow smooths to. You will see for sorta cheap.
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Old 11-30-2018, 02:42 PM   #9
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Quote:
Originally Posted by Charlie-III View Post
OK, I am not an aero engineer, but I fly and understand some aero and I own a wagon as well.
What I did was leave snow on the top in winter. Sorta light snow, then drive. Look at the shape of snow on the top after a drive.
Air coming off the hood/windshield, then the OEM moonroof deflector made the snow sorta smooth beyond the rear OEM wing.
I found I could add a small aluminum angle under the deflector to pull it down a bit, then add some turbulators/vortex generators to the deflector to get airflow to the stock wing.
Taking the stock wing off and installing a raised wing (so, into airflow) can add downforce.
Your choice on route.

Without the deflector, I will guess the top rear of the roof is still close to negative pressure, so your design is still wasted.

Try some soft snow on the top and drive it a bit, see what the snow smooths to. You will see for sorta cheap.
Thanks for your input! Interesting observations. My wagon doesn't have a moon roof, and is currently in pieces lol, so any snow driving/testing will probably have to wait till next year.

This idea wasn't something I actually plan to do, just an idea. My wagon probably won't see a ton of track time after its built, but I've always wondered if there were ways to increase down force without a huge wing on the back, or course in addition to a splitter and rear diffuser which are certainly available already.

Last edited by atm0706; 11-30-2018 at 03:01 PM.
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Old 11-30-2018, 03:00 PM   #10
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Quote:
Originally Posted by oaklandish_WRX View Post
Nice read here on NACA ducts.



Something like you want *may* be doable, but not with a NACA duct on your roof (low pressure area), I would guess. I'd try a properly set up rear diffuser before I hacked the isht out of my car.
Thanks for that.

It makes a lot of sense. I think the only *maybe* doable way would be with a scoop in addition to the duct as someone else also mentioned. maybe similar to a Ford RS200, although that roof scoop is for an intercooler not downforce...
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Old 12-01-2018, 08:55 PM   #11
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What's the purpose of this exactly? A hole in the roof of a street car?

If you want downforce on a wagon, the voltex diffuser (or the whole kit) is about all you're going to get.
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Old 12-04-2018, 10:32 PM   #12
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seems to me the purpose of a wagon is to have a fun sleeper/ utilitarian car to street. if you want down force get sedan? cutting holes in the tail gate and roof eliminates the awesomeness of being able to carry stuff in your car/ take it camping ect...
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Old 12-05-2018, 03:23 PM   #13
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Quote:
Originally Posted by vicious_fishes View Post
What's the purpose of this exactly? A hole in the roof of a street car?

If you want downforce on a wagon, the voltex diffuser (or the whole kit) is about all you're going to get.
Lol why would you assume it would be a street car given the context of the post? It definitely would not see the street ever again if this happened.

I already own the wagon, so if I ever wanted to make it a full track day car, I would make it work, rather than buying a sedan just to throw a wing on. The wagon is already swapped and was not cheap. I wouldn't want to start over again. A splitter and diffuser/flat bottom would probably be plenty anyways just for hobby track car. Not trying win unlimited awd at gridlife time attack.
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Old 12-05-2018, 04:25 PM   #14
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You can take a look at this thread. I wouldn't want to do this to my car but it would be tempting if I tracked it more.

https://forums.nasioc.com/forums/sho....php?t=2764189
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Old 12-05-2018, 06:10 PM   #15
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Two issues there, first, the top of the roof is low pressure like was mentioned, also flow gets more turbulent the farther back along the roof you go. What that means is a duct like that will not really draw in much air. Air wants to go from high pressure to low pressure, or needs to be physically deflected.

So to get air into your duct you either need to "catch" it and turn it downward, which means lift, or you need to have the inside of the duct at a lower pressure than the roof of the car, which also means lift.

A modification of that ideal that could work, though, would be to, say, cut the whole roof off and make a new curved teardrop shaped piece that met the top of the bumper. This would greatly reduce drag and overall lift and then an wing shape across the top would add downforce. From the side the car would still look like a standard wagon.
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Old Today, 01:04 PM   #16
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a full under body skidplate/tray that connected to a diffuser at the rear will increase downforce on a wagon.
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