Angle for rear wing?

Paros

Angle for rear wing?

Post by Paros » Sat Jun 22, 2019 6:13 pm

How many degrees are you using for your rear wings?

I have a APR wing.

Paros

Re: Angle for rear wing?

Post by Paros » Wed Jun 26, 2019 8:36 pm

?

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Re: Angle for rear wing?

Post by John Scherrer » Thu Jun 27, 2019 10:58 am

Hello Paros!

You probably won't get an answer as most people adjust their wings to what looks reasonable by eye and then change it to see if they can notice any difference. I suspect nobody measures the angle !

Also, this optimum angle will vary for different design wings, different manufacturer etc and whether they also have a front wing or not.

So there's no real answer to your query.
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Re: Angle for rear wing?

Post by Winmoz » Sat Sep 21, 2019 3:43 pm

I had a play with mine and found that I got a lot of heat into the front tyres on track if I had it about in the mid point. I tried it on full and to be honest I was worrying too much about flex and stressing the wing so I didn’t really drive aggressively or notice a big difference.

So in conclusion, a stickier front end was experienced at about 50% attack.


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Re: Angle for rear wing?

Post by hamtt » Mon Sep 23, 2019 4:14 pm

Winmoz wrote: Sat Sep 21, 2019 3:43 pm I had a play with mine and found that I got a lot of heat into the front tyres on track if I had it about in the mid point. I tried it on full and to be honest I was worrying too much about flex and stressing the wing so I didn’t really drive aggressively or notice a big difference.

So in conclusion, a stickier front end was experienced at about 50% attack.


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That doesn't quite right (I'm assuming you were still talking about the rear wing).

A rear wing is *supposed* to give you downforce, ie, push the car down at the rear, so the tyres are pushed in to the ground more and in turn more lateral force is needed to make the tyres slide sideways, thus providing you with more "grip".

The engine/gearbox, the heaviest part of the Atom is ahead of the rear wing but behind the front wheels. It's fairly safe to assume to centre of gravity will be somewhere around the engine/gearbox area.

This means if a force DOWN was applied at the rear wing, the car will rotate around the centre of gravity, so the front wheels will actually lift up slightly, giving you less grip at the front.

You could argue that because there's less true mechanical grip at the front due to less force being applied to them, they are sliding around more which in turn is heating the wheels (from the friction) and thus allowing the tyre compound to reach optimum operating temperatures and in return giving you more grip. However, this is highly unlikely and in practice the car would be like a dog to handle and the front tyres would just cool down again once the sliding had stopped, taking you back to square one.

With regards to the "angle of attack", if you're looking to generate true downforce from your wings then the AoA shouldn't be your first consideration. What's more important is the cross-sectional profile of the aerofoil used and ensuring "clean air" reaches them. Wings aren't meant to create downforce (or lift) by being pushed down themselves from the air that's hitting them (if that was the case you could just stick a flat plank of plywood at 45 degrees and "catch" the most amount of air as possible), but rather by splitting the air and causing a difference in pressure. When air hits the leading edge at the wing it gets split above and below the wing. According to Bernoulli's principle this air must meet together at the trailing edge of the wing at the same time, so if an aerofoil can be designed so that the air has to travel more distance on one side (and thus go faster), and a smaller distance on the other side (and thus go slower), there will be less pressure on the faster side and more pressure on the slower side. This difference in pressure then either creates downforce or lift.

Ideally you want a lower AoA as possible to limit the amount of drag. Drag increases exponentially with speed so as you go faster, the drag caused by a steeper AoA will increase exponentially and be more noticeable.

The biggest issue with the Atom is that no clean air reaches the back. You can visibly see this by how much the rear wing vibrates inconsistently even if you're going in a fast straight line. What ideally you should be seeing is the wing vibrating at slower speeds when aero doesn't work and as the speed increases the wing starts to stabilise to the point its pretty much not moving as it is pushed down fully.

As mentioned above, you'll get better results with taller uprights than fiddling around with the AoA. Secondly, you really need to be going 70mph+ for the aero to really start working, and there's not many tracks in the UK where you'll get that opportunity to see the full affect of downforce.

The front wing is a lot more effective. But that has it's own other issues.

The rear wings definitely do make a difference, but that difference is unmeasurable, unpredictable, and unrepeatable. At best, they make the back end behave slightly more when it snaps out. But they don't do what a proper rear wing should do in their standard set-up. The Atom simply isn't a slicks'n'wings car.
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Re: Angle for rear wing?

Post by Italianpaul » Mon Sep 23, 2019 4:30 pm

Hi Hamtt

Thanks for the very comprehensive explanation of the benefit/disadvantages of a rear wing.

I think if your summary is correct,(and I think it has a lot of validity) then most of the members will probably not bother with either front or rear folis.

I know it has changed my specification for a 4.

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Re: Angle for rear wing?

Post by Winmoz » Mon Sep 23, 2019 7:57 pm

Hammy,

You are correct. I was referring to my front wing as well as my rear. With mor aggressive AoAs I increased my Cd and managed to generate more heat in my tyres. I was interested in improving turn in so had to increase the downforce at rear to compensate for the increase at the front.

With the front AoA increased, but the back neutral, I experienced a very lively rear end.

The CFD analysis I’ve just had done (albeit on a non accurate CAD model) does how that the rear wing is too low to really catch clean airflow but this doesn’t Mary up with what I experienced on track.


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Re: Angle for rear wing?

Post by Winmoz » Mon Sep 23, 2019 7:58 pm

Apols spell correct changed Hamnt to Hammy.

And my CFD model ran at 50m/s


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Re: Angle for rear wing?

Post by Hedge » Wed Oct 02, 2019 12:34 pm

Italianpaul wrote:Hi Hamtt, I know it has changed my specification for a 4. Paul
I’d give the factory a call about this, were I you. They can tell you definitively about the aero on a 4 vs previous iterations.

Cheers,
Hedge

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Re: Angle for rear wing?

Post by Winmoz » Sun Oct 06, 2019 7:17 pm

Plus, and this is a big point. Wings look cool.




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Re: Angle for rear wing?

Post by hamtt » Mon Oct 07, 2019 1:50 pm

Hedge wrote: Wed Oct 02, 2019 12:34 pm
Italianpaul wrote:Hi Hamtt, I know it has changed my specification for a 4. Paul
I’d give the factory a call about this, were I you. They can tell you definitively about the aero on a 4 vs previous iterations.

Cheers,
Hedge
The rear wing on the Atom 4, as I've seen from the rendered images, will still be pretty much ineffective for true aero, unless they change something significantly or it's raised significantly higher.

CFD analysis on the Atom will be very very inaccurate. How air behaves around the tubular structurs and nooks and crannies on the Atom will be pretty much nearly impossible to emulate by CFD. Furthermore, the error in one calculation will be amplified when that result of that calculatin is taken in to account for the next calculation, so where vortices interect with other vortices, the error will be significantly huge.

CFD only works well with flat/linear/large surfaces, even then there are a lot of assumptions. Such as that the air will hit the wing in a certain manner or certain direction, or the density of the air. In reality the air never behaves as in ideal modelled conditions.

For good rear aero you need a significant body on the car, to route and direct the air how you want it to be when it hits the wing. The air on the Atom by the time it's reached the driver, never mind behind that, is so turbulent and messy that it would be impossible to streamline it again by the time it hits the wing, no matter how much you play about with the design of the rear of the car. Look at aeroplanes, the most aerodynamic onjects designed. Even they, when they encounter turbulent air or the wake left behind another aircraft start falling apart. This is why they have to wait a 2-3 minutes before attempting to land or take off following another plane, they have to allow the air to settle back down again or teh aero just doesn't work.

The math involved in CFD is very complex. This is one of the reasons why weather forecasting is so difficult, and one of the uses of the original supercomputers was this. Trying to do CFD on your average computers on anything besides fairly basic shapes will produce inaccurate results. Look at even the F1 teams on the lower budgets (was it Lotus a couple of years ago?) that try to rely on CFD alone for design, the results never match up.

The ONLY way to get real results of how well the aero is performing for the Atom is to stick pitot tubes across the wing and take it out on the track, or at the very least using a decent size scaled full model of the car in a wind tunnel.
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Re: Angle for rear wing?

Post by phil4 » Mon Oct 07, 2019 2:03 pm

hamtt wrote: Mon Oct 07, 2019 1:50 pm The ONLY way to get real results of how well the aero is performing for the Atom is to stick pitot tubes across the wing and take it out on the track, or at the very least using a decent size scaled full model of the car in a wind tunnel.
Like this?
image.png
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Re: Angle for rear wing?

Post by Winmoz » Mon Oct 07, 2019 2:07 pm

Hammt,

I agree that CFD is tricky and the models are only ever that, models. Real life may or may not be better or worse depending on your point of view.

I do think that, with even with some commoditised hardware, you can get some indicative information from the test data.

What I have learned from the basic model so far:

Atoms are not in any way aerodynamically optimised. A Cd of .82 is about what you’d get from a truck.

The rear wing needs to be clear of the dirty air for it to work.

The front wing can generate downforce and a lot of drag.

From on track experimentation:
The AoA the warmer I can get the tyres and the less scrub/slip I get in corners.




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Re: Angle for rear wing?

Post by hamtt » Mon Oct 07, 2019 2:19 pm

phil4 wrote: Mon Oct 07, 2019 2:03 pm
hamtt wrote: Mon Oct 07, 2019 1:50 pm The ONLY way to get real results of how well the aero is performing for the Atom is to stick pitot tubes across the wing and take it out on the track, or at the very least using a decent size scaled full model of the car in a wind tunnel.
Like this?

image.png
No in that picture they've basically stuck a grid of pitot tubes behind the wheels to analyse how the air is coming over the wings. This is probably either to see how the front winglets are throwing the air over the wheel or to see how the air is coming behind the wheel so they can design the side of the car accordingley.

To measure the effectiveness of the rear wing you would have to stick pitot tubes INSIDE the wing facing up and also facing down, and span them across the wing. You would then measure the pressure as a function of speed in various conditions and repeat several times to get some sort of idea of how much downforce you're generating and how consistant or predictable it was.
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phil4

Re: Angle for rear wing?

Post by phil4 » Mon Oct 07, 2019 2:29 pm

hamtt wrote: Mon Oct 07, 2019 2:19 pm No in that picture they've basically stuck a grid of pitot tubes behind the wheels to analyse how the air is coming over the wings. This is probably either to see how the front winglets are throwing the air over the wheel or to see how the air is coming behind the wheel so they can design the side of the car accordingley.

To measure the effectiveness of the rear wing you would have to stick pitot tubes INSIDE the wing facing up and also facing down, and span them across the wing. You would then measure the pressure as a function of speed in various conditions and repeat several times to get some sort of idea of how much downforce you're generating and how consistant or predictable it was.
Gotcha, I was meaning in a general sense... since until earlier this year I'd have had no idea what you were talking about, thought a pic like that might help others too.

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