Aerodynamics advice neede - Ducktail+Wing?

[VerusEric]

Originally Posted by 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

I'm not sure what I can add to the conversation at this point as it is all speculation unless tested and verified.

There can and likely will be a combination (spoiler/wing) that will aid performance, hurt performance, and offer little to no change to performance.

This is precisely why we tend to stick to numerically testing components through analysis. I know it may not be everyone's cup of tea, but we do have a reason behind it all .

I would believe what Porsche did is beneficial to whatever their goals are for the rear aero of their cars. They have the budget/knowledge to run CFD/wind tunnel testing on the combo.

Thanks,
Eric

[OND]

Dude, I've got something for you. I just got curious and spent 1 week learning OnShape and OpenFOAM to create a simplified CAD model of the BRZ and perform some CFD modeling at ~90mph, comparing different cases of wings/spoilers.

The lift and drag numbers are obviously not representative of the actual car, but I think the relative comparison might give you a rough idea about how different combinations behave.

To sum up, having that aggressive spoiler actually helped with downforce, but caused more increase in drag, so the efficiency (L/D) of the whole setup was the worst. The wing setup by itself had the best efficiency and pretty good downforce, so that might actually be the way to go unless you are looking for the max possible downforce at the rear.

Results:


Velocity fields and streamlines:

Stock


Wing


Wing+Spoiler


You can see the large spoiler is actually decreasing the flow velocity above the trunk and displacing all the streamlines upwards, so the bottom side of the wing isnt working as effectively as possible. There is also a large separation zone at the tail end of the car with wing+spoiler combo, which usually indicates high drag. DISCLAIMER: These comments are coming from someone who doesn't do any professional aero work, so take them with a grain of salt.

[Spuds]

Read this.

https://www.quora.com/How-much-downw...n-acceleration

[Mr Nerd]

I have Autodesk Flowdesign. If you don't know what is it then looking up and send me a laser scan of your car or a 3d object file

Sent from my Galaxy S8+ using Tapatalk

[Gunman]

Originally Posted by Spuds

Read this.

https://www.quora.com/How-much-downw...n-acceleration

That article is awful....there are some bits of fact in there, but in general, it's awful.

[Spuds]

Originally Posted by Gunman

That article is awful....there are some bits of fact in there, but in general, it's awful.

The sky is blue... There are some bits of white in it, but in general, it's blue.

While you are entitled to your opinion, I'd recommend being more specific if you'd like to debate the content.

Perhaps you are referring to the lack of mathematics or detail? Yes this is a simplified explanation tailored to those not educated in aerodynamics. The article only takes into account 2d flow as I recall, ignoring lateral velocity. Also ignores boundary layer, laminar vs turbulent flow, etc. Who cares. For the sake of this audience and subject, I feel it's adequate.

I mostly linked it because of the wind tunnel streamline pictures which say much more than any cfd screenshot. Idk who the author is, but I didn't see anything glaringly wrong, though I didnt read that intently.

[OND]

Well, for one, the Plymouth was not a dumb beauty and afaik it was banned from Nascar due to its effectiveness.

[Gunman]

Originally Posted by Spuds

The sky is blue... There are some bits of white in it, but in general, it's blue.

While you are entitled to your opinion, I'd recommend being more specific if you'd like to debate the content.

Lets start with the title,

"How much downward force does a spoiler produce, and is it significant enough to make a positive impact on acceleration?"

Acceleration? ok, for a drag car maybe, but for a road course type car, the wing, or spoiler is not there for acceleration, but downforce, ie aero grip in corners.

An automotive airfoil is shaped like an upside-down airplane wing -- it deflects airflow upward to generate down-force on the rear of the vehicle.

Using a wing on a car as an example, the air is accelerated on the underside, creating low pressure, which pulls the wing down, ie negative lift, aka downforce.

This is why so many older vehicles had idiotically-ineffective spoilers or wings. Like this dumb beauty:

This guy address the article: https://oppositelock.kinja.com/the-s...-or-1704941408

Just to make it extra clear what I'm saying here: spoilers that stick up above the roof of the car are blisteringly stupid.

So now he is talking about the Superbird, but calling it a "spoiler", when it is a wing...he forgot his own words:

AIRFOILS ARE NOT SPOILERS AND SPOILERS ARE NOT AIRFOILS. Ok? Don't confuse the two, or you will look dumb on the internet.

A race car designer will put the wing as high as the rules allow, in an attempt to get clean, non turbulent air. The wing will work better, and be more efficient. Better efficiency will allow a less aggressive airfoil, and less drag.

Got it? Good. I'm tired of the internet being so consistently wrong about this.

Yeah...should take his own advice. He has some good info, but it's sprinkled among some not so accurate info.

It's just not a good article. I'd suggest a copy of Katz book, "Race Car Aerodynamics: Designing for Speed", or attending Paul T. Glessner's seminar, if you can.

[Mr Nerd]

All of the last few posts are dead on. But what we are all aware of but clearly not remembering is that aerodynamics are objective. No one here can give you ANY good advice without testing your particular set up.

Sent from my Galaxy S8+ using Tapatalk

[Spuds]

Originally Posted by Gunman

Lets start with the title,
Acceleration? ok, for a drag car maybe, but for a road course type car, the wing, or spoiler is not there for acceleration, but downforce, ie aero grip in corners.

Im pretty sure the title was using sarcasm and or being intentionally misleading to pull in folks who think "wings make me faster yo".

Using a wing on a car as an example, the air is accelerated on the underside, creating low pressure, which pulls the wing down, ie negative lift, aka downforce.

Excellent! You just hit one of my pet peeves. Sorry for the following explanation/rant/etc.

While that is one way of looking at it (Bernoulli method), it is not entirely accurate. If you trace the lineage of Bernoulli's principal, you get back to Newton's second law, F=MA (among other work of course). In that case, stating that a wing is simply deflecting air is actually more accurate than your statement. Airfoils are just the most efficient physical means of doing that.

Proof: Hold a piece of paper horizontally by one edge so that it is curved downwards and blow over the top. Lifts right? Bernoulli's principle, faster flow means ? Now do the same thing, but blow on the bottom of the paper. According to the Bernoulli's principal we were all taught in highschool (yes me too) it should pull down further, because the air underneath the paper is moving faster and has lower pressure. Spoiler alert (pun is necessary): That's not what happens.

This guy address the article: https://oppositelock.kinja.com/the-s...-or-1704941408

Fair enough. He done messed that one up real good.

So now he is talking about the Superbird, but calling it a "spoiler", when it is a wing...he forgot his own words:

I took it to mean that a device meant to be a spoiler shouldn't be put waaaay up in the air. I dont think he was specifically referring to the superbird at that point in the article, but we all see things differently. Granted, you are correct that it is confusing. Not going to defend the dude's writing skill, or lack thereof.

Original Article: AIRFOILS ARE NOT SPOILERS AND SPOILERS ARE NOT AIRFOILS. Ok? Don't confuse the two, or you will look dumb on the internet.

A race car designer will put the wing as high as the rules allow, in an attempt to get clean, non turbulent air. The wing will work better, and be more efficient. Better efficiency will allow a less aggressive airfoil, and less drag.

Your statement is correct, but Im not entirely sure he said otherwise. He also calls "Wings" "Airfoils" now that I look at it closer. All my aerodynamic pet peeves are coming out on this one but Ill spare everyone the rant... this time....

Anyway, I agree its not the best information the world has to offer on the subject, but I still think its worth looking at for the mildly interested.

I'd suggest a copy of Katz book, "Race Car Aerodynamics: Designing for Speed", or attending Paul T. Glessner's seminar, if you can.

I've heard good things about the book from a colleague, but never read it myself. +1 on getting some in-depth literature/education if the OP (or anyone interested in this) wants to spend the time learning it.

[Myriad]

I found the linked article very interesting. Correct me if I'm wrong, but it appears a daily driver, who might take a car on a relatively straight highway and not race around a track, would benefit from a low rear lip spoiler to decrease drag. A track car, who makes tight turns at high speed, would benefit from a higher wing to keep his rear planted. Having both, who knows? There is no advantage to having a bare trunk.

As someone who is just planning on daily driving it, would the stock 86/frs and pre-2017 BRZ spoilers be sufficient? Is the new 2017 BRZ/860 spoiler an improvement at all, or is it more like a downforce-producing wing? It technically looks like a wing, but it is very low to the trunk level, so perhaps it still functions as a solid low rear lip spoiler. I'm assuming they made this replacement because it is better at reducing drag than the old stock designs. (It certainly looks cool). My old 99 Integra and 95 Celica have/had this similar raised profile that you could at least slide your fingers under. What difference would a solid lip have from one with a space underneath? From the article's diagrams it appears either one would deflect air over the top of the car with less drag, as long as it doesn't get too high. Thoughts?

Myriad.

[churchx]

My bet is that there is not much difference between pre & MY2017 spoiler efficiency, and most probable reason for change is looks. If you care about drag, worth also checking if yours has body underpanels fitted. Step further - something like velox's rear diffuser & diff cover plate. Another step - fitting small non-adjustable race side mirrors .. wait .. last or last two steps might be a bit too far for just daily driven car , as gains in drag reducing at legal speeds won't be that big for it to pay off quickly and they also may reduce practicality slightly. But at the end .. this car main function is to provide driving fun, and very probably it may be driven eco-mindedly not that often, even if just DD and on public roads only , +many install race upgrades due looks. Well, to each his own. Everybody has their own priorities, their own preferences, their own budget.

[Gunman]

Originally Posted by Spuds

Excellent! You just hit one of my pet peeves. Sorry for the following explanation/rant/etc.

While that is one way of looking at it (Bernoulli method), it is not entirely accurate. If you trace the lineage of Bernoulli's principal, you get back to Newton's second law, F=MA (among other work of course). In that case, stating that a wing is simply deflecting air is actually more accurate than your statement. Airfoils are just the most efficient physical means of doing that.

It's actually both Bernoulli and Newton, mixed with some other...thus the reason they call it "wing theory".
https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html

Interesting bit on the pre 2017 spoiler on the Subaru, it reduced Cd compared to the bare deck lid. I really wish they would have included downforce numbers, or lift changes. Generally a wing, or spoiler will increase drag, but I suspect the OEM spoiler is working more like a gurney, and just tripping the air to help fill in the wake.

[Spuds]

Originally Posted by Myriad

I found the linked article very interesting. Correct me if I'm wrong, but it appears a daily driver, who might take a car on a relatively straight highway and not race around a track, would benefit from a low rear lip spoiler to decrease drag. A track car, who makes tight turns at high speed, would benefit from a higher wing to keep his rear planted. Having both, who knows? There is no advantage to having a bare trunk.

I believe you have a correct understanding of the basic principle.

Spoilers are used to separate (spoil) flow from a body under desirable circumstances (aircraft use these as well). I can't speak for increasing or decreasing drag in this application, but the pre-17 spoilers should reduce lift at high speed.

Wings on the other hand are used to generate a force from flow. This WILL result in increased drag due to conservation of energy, but applies actual force to whatever you attach it to.

Spoilers reduce aerodynamic forces (at least in one direction, as drag may or may not increase). Wings increase aerodynamic forces.