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Old 12-12-2016, 12:52 PM   #31
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The Chaparral 2J and the Brabham BT46b comes to mind! You wouldn't need much to start, you just want to get the most air to exit by the cut in the bumper and to keep air from entering by the sides. That way it would increase inside bodyshell airflow.
And more recently, F1 cars used to have blown diffusers using exhaust gas. Then it became outlawed.
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Old 12-12-2016, 12:55 PM   #32
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And more recently, F1 cars used to have blown diffusers using exhaust gas. Then it became outlawed.
Oh yes those are more recent examples, for what I gathered at the time the exhaust gases were to keep the diffuser air from reaching stall speed. Neat.
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Old 12-12-2016, 01:37 PM   #33
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It can assume a shaped bottom too, stationary like the car not getting lower from downforce?
The inside of a car, I think, is going to be a very strange thing aerodynamically. The lip inside the front of the body, before the bumper, the bumper support, then the gaps around the suspension arms, and finally the gap behind the rear diff.

Air will be flowing across that, and there will be boundry layer effects. Especially with the proximity to the stationary ground. The body, and undercarriage is going to try to drag air across the ground....

Underbody is a complex place, but a place that you can find a lot of downforce, for very little drag.
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Old 12-12-2016, 02:09 PM   #34
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The inside of a car, I think, is going to be a very strange thing aerodynamically. The lip inside the front of the body, before the bumper, the bumper support, then the gaps around the suspension arms, and finally the gap behind the rear diff.

Air will be flowing across that, and there will be boundry layer effects. Especially with the proximity to the stationary ground. The body, and undercarriage is going to try to drag air across the ground....

Underbody is a complex place, but a place that you can find a lot of downforce, for very little drag.
Indeed!

If most of the air exits through the bumper cut then most of the air will remain low until the end of the rear lower arms, maybe at chassis height knowing that above the air will encounter more resistance/less airflow. For sure those gaps will slow down airflow and create turbulence, I am going to try to make the lower front bumper support be inside the foam and not bellow.
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Old 12-12-2016, 02:14 PM   #35
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There are also some air pumps running inside the body. The wheels are going to stirr things up quite a bit, plus motor and drivetrain... And some cars even have ~actual fans~.
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Old 12-12-2016, 02:37 PM   #36
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Yep, would be better to simply close the underside and have a little intake to cool electronics...Like the TRF415 had but way smaller.

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Old 12-13-2016, 04:24 AM   #37
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The Bugatti Chiron also has two of its exhausts exiting vertically downward either just upstream or right in the diffuser itself.

I've got a few ideas I'm going to try for fun, I just need to figure out which body I want to chop up. One would be fans blowing air across the ESC and motor, with lexan ducts which direct the air towards slits in a rear diffuser. Another would be fans sucking air out of the body and blowing directly through diffuser slits. Both of these would be done with a full lexan under tray sealing the body, and ending in the diffuser.

The third, and most ambitious, would be a fan or two in conjunction with a big curved piece of lexan on the inside of the body. Think upward curving interior tray. This tray would start low and curve up high, ending right by the top corner of the trunk lid. Then the entire rear surface of the body would have vents. I think that would be the best chance to maximize the speed of the air under the body. You can set up the fans to increase the air speed, and have them switchable. How cool would it be to switch on an extra channel for added downforce when you'd like it.

The Aston Martin- Red Bull AM-RB 001 has some interesting aero. The central "body" is a teardrop shape.

I'm a mechanical engineer by trade but more on the designing things out of hunks of metal side. I remember a bit of fluid mechanics from hs and college, but it still seems like a black art. Since my teens have always been into trying to implement cool real car tech into RC. I always wanted to hook a wing up to a third channel where I could dial in downforce with a turn of a knob, but never did that. Even that seems pretty easy now lol. But anyway I'm always up for experimenting and will enjoy attempting a few of these things even if they don't prove useful. Because even if they don't that's an interesting result.

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Old 12-13-2016, 12:12 PM   #38
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The tear shape is the best at keeping the boundary layer from detaching the body and disrupting airflow.
Interesting designs, I would do the fans sucking air directing the stream at the bumper opening just for a start. Run the body as low as you can get to seal at speed.

@Nerobro, yesterday night I had another prophecy thanks to you! I was thinking why most race cars had the wheel arch behind the front tire cut or non existent (see left of Lada Racing stickers on the door). Then it hit me with you saying about the tires and boundary layers. The engineers want that air behind the tire out and away from entering the wheel arch. I thought they would want the air going around the wheel smoothly to reattach as fast as it could.
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Old 12-13-2016, 12:26 PM   #39
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There's a few reasons for wheel arches like that. Closing angles behind objects of more than.. I think 6 degrees cause flow separation, and are just not worth the material you make them of. What you're seeing there is the wheel flare being carried back to a convienant panel line, then instead of trying to re-attach flow, they're going with a sharp cut. This causes what flow there is to separate cleanly. The area behind the flare is also open to the wheel volume, providing an air supply to fill in under the flow separation. This acts to suck air out of the wheel well.

Some cars go so far as to completely eliminate the wheel arch behind the wheel. It's a very nasty place to try to put any ~useful~ bodywork. So if there's the option, they often just eliminate it. That sharp edge along the top of the trunk, serves the same purpose, flow wise. Having a sharp edge to release the air attached to the body helps reduce drag significantly.

I need to buy some sheets of lexan. I want to see what putting an underbody on my car will do... Sadly most of the classes require "complete" rear bodies on the cars, making under body air evacuation tricky.
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Old 12-13-2016, 12:57 PM   #40
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Yep I think I've read 6 degrees somewhere too. Thanks for taking the time to explain, I enjoyed it. Now that you've wrote about flow separation in the rear I remembered the Kamm style rear ends. Also sorry the pic URL had an error, corrected.

An underbody would be a good start, maybe working with air evacuation that's why some drivers run fans sucking air from the motor? That wouldn't be a bad place to place a fan blowing almost straight at the bumper opening.
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Old 12-29-2016, 09:55 AM   #41
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Had the closed undertray mod on my mind for several years now and finally found the time and courage to try it. The whole thing weights 105g and is made of 1mm thick Makrolon. Not perfect, but this was what was available. The car now perfectly matches the minimum weight rule of 1350g. Not perfect either is where I try it: On an Indoor track. Outdoor would be better suited for the higher speeds...
Enough whining! Here some pictures:







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Old 12-29-2016, 11:46 AM   #42
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Great work as always @wtcc.

Mind if I give you some pointers to make it more effective?
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Old 12-29-2016, 11:50 AM   #43
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That looks great. My only questions involve the side trunks leading to the rear wheels. "real" cars end up using air dams to move air outside instead of up into the rear wheel area.

That's a very, very, hard area to figure out airflow in. So... give it a whirl. :-) I can't wait to hear a report back.

Last edited by Nerobro; 12-29-2016 at 11:52 AM. Reason: Concerns wasn't the right word.
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Old 12-29-2016, 12:30 PM   #44
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Of course you guys can point out weaknesses of my build

It is my first shot in this area. I had limited time and material to build. Goal is to verify if working on aerodynamics on a 1/10 rc undertray makes sense or not. The chassis movement is in extreme angles compared to 1/1 cars. So laptimes will show if there is any improvement. If there is then it is worth to invest more time and money.
I hope my undertray is good enough to influence the laptime. If not I remove it and still have a nice day

By the way: the body will come down to close gap between body and undertray side profile (see pics). The front splitter will be attached to the Dodge Dart splitter with tape.
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Old 12-29-2016, 01:48 PM   #45
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Not weaknesses, think of them as opportunities to improve we want to extract maximum performance now we know how it works. Basically there's two important things: the closer (and within limitations, the bigger) the parts that create low pressure are to the ground the more downforce it will create and the less foreign air enters that zone the better. You want to keep the side skirts as low as they can be, if you have to raise roll centres and use stiffer springs do it. You also want some air to enter by the front and exit fast at the rear. The front splitter can be as high as 4-5mm off the ground, the skirts theoretically should touch the track surface in all their extension and the rear diffuser seems too extreme, half of that would be a good start as you won't be running high speeds. Check attachments to see examples.

Concerning your work, lower the body around 1-2mm than the undertray, rake the undertray so the rear sits higher than the front and raise the tip of the wings like the example attached. If the body is sucking the dust from the carpet you know it's working

Also, that was the pitfall of the Lotus 78, the air path was disrupted by the rear suspension arms/axles,etc... The 79 corrected that.
Attached Thumbnails
Little aero secrets-term106_001.jpg   Little aero secrets-diffuser_blown_senna__mexico_1991_mclaren-honda_mp47a_big.jpg  
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