Understanding Race Car Aerodynamics

[AkumaNoZeta]

I've owned the book Race Car Aerodynamics for a while but never got too far into it because I always got lost in Chapter 2. But I'm about halfway done with it now after deciding I'll read through it and just do my best to get a general idea and not giving up after not understanding the small details. I just want to know if I got some of this right. Concave shapes create higher pressure while convex shapes create lower pressures. The longer something is the less drag it has. Laminar boundary layers have less surface drag but seperate easier while turbulant boundary layers have more surface drag but stay attached longer which in the back of the vehicle can cause less total drag. If you have the beginning of the wing taper towards the stagnation point it allows you to have more attack angle without detaching the airflow but works for cars because they are constant and not for planes because they're always adjusting lift. With the "V" shaped wings, if the ends go towards the front it creates more lift in the center therefore good for cars but planes have the ends go towards the rear because they make more lift at the ends and that makes it easier for a plane to turn since they use the airflow over the wings to manuever. End plates on the end of the wings pretty much just make it so air doesn't creep off the ends and around the wing. Vortex generators turns a laminar boundary layer into a turbulant one so it stays attached longer allowing you to create more attack angle on the wing. These are the things I want confirmation on whether I have it right or wrong, where I'm at the rest I either understand or I don't feel I need to worry about (such as the windtunnels because I'll never afford anything like). Ok, now everybody tell me how wrong I am, I can take the criticism.

[dr_hunt]

Correct I believe in all accounts.

[AkumaNoZeta]

Pertaining this to an S30, where does the laminar layer turn turbulant? Would having a cowl with you vent holes cut into it build up even more pressure in front of the windsheild reducing a little of the lift? Would placing vortex generators on the roof right in front of the rear window create a turbulant boundary to keep it attached to the rear more and without a wing or a spoiler result in less drag with the consequence of more rear end lift? My main objective is drag reduction because I do want MPGs (wieght reduction in there too) since this will be my daily driven car....one day....hopefully, but I also want to reduce as much lift as possible for mental security mainly but more stability as speeds as well while racing. So far my plan is to have the Subtle Z kit modified a little and a belly pan. I've been thinking of a full belly pan in 3 pieces, front, rear, and middle. I figure if I can reduce the drag with the wingless and spoilerless application of vortex generators on the roof I can drive it around and if I encounter rear lift problems that are really noticable then I'll get one of those APR wings (I think the company's name is APR but not 100 on that). I've also been thinking of side skirt/rocker panel modifications for better aero too but trying to keep the car as pretty as possible too, close to original lines and everything. Some ideas but I need to draw them out to see if I like them enough or not. Sorry about the rant, but I've been thinking about this for a long time.

[thehelix112]

Grim,

 

Sounds about right to me, though its been a while since I read it. Great book isn't it? I thoroughly enjoyed reading it. Simon McBeath's book and the aero section in Milliken & Milliken are also worth the read.

 

When you say / are you talking about delta wings?

 

Also, I thought it was termed camber in reference to the curvature of the profile of a wing (tapering towards stagnation point).

 

The general concept I took away from the book was that of pressure gradients. Air does not like to flow `uphill' (as in down the back of the hatch), and needs energy to do so. A turbulent flow around this region is more highly energized, and this energy is used to flow up the hill. When the energy is depleted, the flow will detach.

 

Also I would pay close attention to the wind tunnel results in this sub-forum. Lots of great empirical evidence in there. I don't know whether you are against spoilers for aesthetic reasons, but from what I can see, the results support the theory that a wing/spoiler reduces drag by promoting air flowing under the car to flow upwards and fill the gap at the back of the car.

 

Tests 10, 11, 12 with whale tail, MSA short, and SDI 5.25" spoilers show drag reductions to 103.5hp, 98.7hp, 99.0hp respectively.

 

However the highest reductions in drag come from blocking off frontal openings, which others have theorized (correctly I think fwiw) reduces the buildup of high pressure in the engine bay by limiting airflow into it.

Test 14 blocked the lower half of the front, reducing drag to 88.8hp, Test 18 blocked the lower half of a MSA Type III airdam, reduced futher to 86.4hp.

 

The results and their pics are worth the read.

 

Dave

[AkumaNoZeta]

I dont remember terms like that so easily, gotta go over it a few times before it sticks. I just got past the section on the spoilers reducing drag yesterday (I read it during breaks at school because I get too distracted by tv and internet at home). My car came with the 3-piece spoiler and I really do like how it looks but...I don't know, I just think it looks nice wing/spoiler-less but that opinion is based on seeing one photo of a Z without and hundreds of Z with them. What I would like is some nicely squared up photos of a car with the Subtle Z wide-body from the front, side, rear, and top but I think thats a little too much to request from someone, mainly the top view.

[dr_hunt]

Pertaining this to an S30, where does the laminar layer turn turbulant?

 

Laminar and turbulent flow is a function of velocity. Using your understanding of what shapes cause high and low pressure you should be able to deduce exactly where on the car that would happen.

 

Would having a cowl with you vent holes cut into it build up even more pressure in front of the windsheild reducing a little of the lift? Would placing vortex generators on the roof right in front of the rear window create a turbulant boundary to keep it attached to the rear more and without a wing or a spoiler result in less drag with the consequence of more rear end lift? My main objective is drag reduction because I do want MPGs (wieght reduction in there too) since this will be my daily driven car....

 

If your looking for MPG's then speed is your killer here. As speed increases almost exponentially so does drag.

 

 

one day....hopefully, but I also want to reduce as much lift as possible for mental security mainly but more stability as speeds as well while racing. So far my plan is to have the Subtle Z kit modified a little and a belly pan. I've been thinking of a full belly pan in 3 pieces, front, rear, and middle. I figure if I can reduce the drag with the wingless and spoilerless application of vortex generators on the roof I can drive it around and if I encounter rear lift problems that are really noticable then I'll get one of those APR wings (I think the company's name is APR but not 100 on that). I've also been thinking of side skirt/rocker panel modifications for better aero too but trying to keep the car as pretty as possible too, close to original lines and everything. Some ideas but I need to draw them out to see if I like them enough or not. Sorry about the rant, but I've been thinking about this for a long time.

 

I think it is the general consensus that most cars have not experienced much of a problem until speeds are in the 150+ category.

 

Not saying that you can't achieve some good results at speeds lower than that but there is a cost/benefit line there somewhere. That is the more time and $$$ you spend yields little results. That was the purpose of the windtunnel tests, to see what is the best bang for the buck so to speak.

[ZZZeee]

"...best bang for the buck."

 

DOE!!

[260DET]

In addition to what has already been said, lowering the car as much as is practical reduces drag and improves stability by reducing the amount of under car air and the effective frontal area. Maybe its always assumed that this is an important point but its hardly ever mentioned.

[AkumaNoZeta]

I know the ride height has a lot to do with the efficiency of the design, I figure I want about 3" since it is a road vehicle. I also realize that aerodynamics plays almost no role for me since the car would spend most of its time around the posted speed limit. I don't know why I'm so interested in it, I think there's a part of me that secretly wants to be an engineer. Usually I spend a few hours thinking about the aero modifications and then punch myself while saying "Whats the point, the car wont see those kinds of speeds" but I cant help but keep on thinking about it. So even though I know it's pretty much pointless for my application the thought process and eventually design/build of it will greatly entertain me. I drew a nice picture of what I would like, as soon as I get back home I'll reinstall my all-in-one on my computer and scan/post the pic.

[thehelix112]

Grim,

 

Looking forward to the pic.

 

I'm unconvinced that the lower ride height reduces the frontal area. Its covered for a current LMP1 car in the latest RaceCar Engineering I have at home so I'll check there. My thinking goes that lowering the vehicle increases ground effects which increases downforce. If you're increasing downforce without also reducing regions of low pressure towards the rear (which I think is the case here?) then I don't see how it couldn't add drag. But I'm not an expert, and will probably prove myself wrong when I post the LMP1 stuff when I get home (unless someone else has it handy?).

 

That and I seem to recall hearing about F1 cars working to get the ride height as high as possible down the straights to reduce drag. Though I've no idea where I heard that.

 

Dave

[PalmettoZ]

I've owned the book Race Car Aerodynamics for a while but never got too far into it because I always got lost in Chapter 2. But I'm about halfway done with it now after deciding I'll read through it and just do my best to get a general idea and not giving up after not understanding the small details. I just want to know if I got some of this right. Concave shapes create higher pressure while convex shapes create lower pressures. The longer something is the less drag it has. Laminar boundary layers have less surface drag but seperate easier while turbulant boundary layers have more surface drag but stay attached longer which in the back of the vehicle can cause less total drag. If you have the beginning of the wing taper towards the stagnation point it allows you to have more attack angle without detaching the airflow but works for cars because they are constant and not for planes because they're always adjusting lift. With the "V" shaped wings, if the ends go towards the front it creates more lift in the center therefore good for cars but planes have the ends go towards the rear because they make more lift at the ends and that makes it easier for a plane to turn since they use the airflow over the wings to manuever. End plates on the end of the wings pretty much just make it so air doesn't creep off the ends and around the wing. Vortex generators turns a laminar boundary layer into a turbulant one so it stays attached longer allowing you to create more attack angle on the wing. These are the things I want confirmation on whether I have it right or wrong, where I'm at the rest I either understand or I don't feel I need to worry about (such as the windtunnels because I'll never afford anything like). Ok, now everybody tell me how wrong I am, I can take the criticism.

 

 

Hey Grim-

The "V" shape wings you are talking about I think are what is called swept wing design for aircraft. These are simply a much faster aircraft wing than a straight wing. Swept wings have less lift than straight wings and higher stall speeds, this is why jet aircraft take off and land at much higher speeds than prop driven straight wing aircraft. This has nothing to do with the aircrafts turning ability though as you stated.

 

The end plates on the wings you are talking about are called winglets on an airplane and are only there to reduce drag created by vortices off of the wing tip. Again these are common to swept wing planes.

[dr_hunt]

Hey Grim-

The "V" shape wings you are talking about I think are what is called swept wing design for aircraft. These are simply a much faster aircraft wing than a straight wing. Swept wings have less lift than straight wings and higher stall speeds, this is why jet aircraft take off and land at much higher speeds than prop driven straight wing aircraft. This has nothing to do with the aircrafts turning ability though as you stated.

 

The end plates on the wings you are talking about are called winglets on an airplane and are only there to reduce drag created by vortices off of the wing tip. Again these are common to swept wing planes.

 

I think your talking about two different parts of the plane. He is talking about the tail section and the v shaped portion of the tail that appears on some planes I believe.

[260DET]

I know the ride height has a lot to do with the efficiency of the design, I figure I want about 3" since it is a road vehicle. I also realize that aerodynamics plays almost no role for me since the car would spend most of its time around the posted speed limit. I don't know why I'm so interested in it, I think there's a part of me that secretly wants to be an engineer. Usually I spend a few hours thinking about the aero modifications and then punch myself while saying "Whats the point, the car wont see those kinds of speeds" but I cant help but keep on thinking about it. So even though I know it's pretty much pointless for my application the thought process and eventually design/build of it will greatly entertain me. I drew a nice picture of what I would like, as soon as I get back home I'll reinstall my all-in-one on my computer and scan/post the pic.

 

Its interesting stuff, thats why us HybridZers do our own thing, as I said on a racing forum 'anyone can walk into a showroom and buy a fast lap time'.

 

Just one thing, using dot points is a reader friendly way to set out a series of facts/propositions, rather than face us with a wall of words

[AkumaNoZeta]

Oops, sorry. I didn't mean for my posts to be so hard to read. I just type what comes to mind, maybe I oughta start writing first and then organize/proofread it. lol, I should just take English in school again

[Daeron]

maybe I oughta start writing first and then organize/proofread it. lol, I should just take English in school again

 

Engrish...

 

This is a very helpful tactic, indeed.

 

I have had this issue myself... until it was politely pointed out to me..

 

Hitting "enter" a couple of times never hurt anyone

[AkumaNoZeta]

I finally got my drawing scanned, sorry it took so long. It's not a technical drawing, I just traced a photo of the white Subtle Z car that I found a long time ago. The photo is around here somewhere. And I just "customized" it. I'm gonna get around to doing a front, side, rear, and hopefully some top and bottom drawings done. I wish I had my Z to look off of but it's back in Arkansas and I'm in school in Pennsylvania. I'm gonna try this in a list now:

 

Custom air dam based on popular models, smooth concave surface whole way around blending into front fender

 

Gurney flaps in front of wheel wells to help pull air out of wells (read about that on an online article about stock car racing aero)

 

Custom grill, simple design but I want it denser, and opening blocked from the sides instead of the bottom in an attempt to increase cooling effiency of intercooler (SR20 swap in mind)

 

On hood, scoop for engine intake on driver side and semi-circular louvers on passenger side to extract underhood air/heat

 

Vents eliminated on cowl to build up pressure more infront of windsheild and I don't think I need the vents anyways because I always have my HVAC set to recirculate the air from inside the cabin, I never have it set on vent...thats how it works, right?

 

Engine exhaust coming out of fender so I dont have to work around an exhaust system for making a flat underbody belly plates

 

Plate coming out of bottom on side of car to prevent air to go underneath the car from the sides

 

Splitter, headlight covers, aero mirrors

[Michael]

The subject of laminar to turbulent transition is exquisitely complex. Please don't believe that a few general principles are enough to give accurate estimates of boundary layer physics. That said, cars are incredibly "dirty" aerodynamically, and the extent of laminar flow for a S30 Z is negligibly small.

 

I would seriously doubt that one could perform enough aerodynamic enhancements to obtain measurable mileage improvement while driving a "legal" speeds.

[AkumaNoZeta]

Well, if you look at my drawing, I didn't really change too much of it to lower drag, in fact I probably made it a lot worse. But I think it may be good enough to not make it noticable when crusing but help keep the car from becoming too unstable at speeds. I'm not sure if my ideas are good or not, thats why they're on here as questions.

[JMortensen]

I would seriously doubt that one could perform enough aerodynamic enhancements to obtain measurable mileage improvement while driving a "legal" speeds.

I think C&D proved you wrong on that one back in the 70s. I know I've seen the article online somewhere, I looked but couldn't find it. Hopefully someone will point us to a link. If I recall they put a spook and a spoiler of their own construction on the car and lowered drag, increased downforce, and gained mpg.

 

Grim, your gurneys in front of the wheels can be seen on the new Z06 Vette. Splitters are known to work, although exactly what downforce you'll get from yours depends on the design and the car.

 

I wouldn't do an intake scoop on one side and an exit on the other. I'd do exits on both sides and then put the intake in the cowl or in the rad core support.

 

The vents in the cowl aren't significantly affecting the high pressure stagnant area in front of the cowl if the fresh air vents in the car are closed. By using some of the pressure to move air whether it be into the engine or passenger compartment I think you'll see increased flow over the windshield.

 

I think the goal for most of us is going to be to increase downforce at the expense of drag, unless you're Tony D and going for a land speed record.

[AkumaNoZeta]

So if I have the same type of louvers on both sides it would be a lot better? I don't think I could put the intake at the cowl because that would be a very sharp bend. Maybe if I do a V-mount with the intercooler and radiator and have the intake inside of the V? The ducting for that would be a little more difficult but get nice cold air for the intake as well improve cooling to both the heat exchangers, right?