# Aerodynamics of sportscars

#### tormund

can anyone enlighten me with key aerodynamic features which are used to heighten the performance of sportscars.

thnx

#### Ranger Mike

Science Advisor
Gold Member
glad to
you talking about real race cars or door slammers?

#### tormund

no, talking more along the lines of ferrari, lamborghini, porsche, bugatti, etc. More the type of car that you really want to drive, but at the same time don't want to because its too nice

#### Danger

Gold Member
There's no such thing as a car that's too nice to want to drive.

#### Ranger Mike

Science Advisor
Gold Member
I agree..ifin you can't afford to see it come back on the hook don't race it..
I never liek to see the tow truck with the car dangling from it..but it happens, not a case of if..but when..cause it will happen...

Read the following 3 books published by Aero Publishers and written by Carroll Smith
Engineer to Win, Prepare to Win, and Tune to Win
also TV Motorsports published a good book , Inside Racing Technology by Paul Haney and Jeff Braun

Tune to Win gives the best basic introductory class on External Aerodynamics..
bottom line is any serious racer ( going over 100 mph) had better be aware of what it is and how it effects the car)

At road speed over 100 mph, aerodynamic drag is the most important limiting factor I straight line performance.

basic formula for automotive drag is
Drag (lbs.) = Drag Coeff. X (surface area in feet² ) X (velocity in mph)² / 391

unfortunately this number don't mean much so we have to translate pounds of drag into Horsepower required to over come it so we can better understand what we are facing..
Drag HP = Cd x Frontal Area x Velocity ³ / 146,600

the second factor of Aero is to channel the air to provide more downforce to better stick the tires..see previous posts from me titled Race Car Physics..it is all about tires..in fact, Indy cars theoretically have enough down force to literally stick to the ceiling at speed..

more on this later

#### Ranger Mike

Science Advisor
Gold Member
Frontal Area..pretty much fixed when car is designed..less is better but tire track width will ultimately determine the square footage
Coefficient of Drag- most team effort is to reduce this..again..designer sets this unless you are super good with fiberglass or carbon fiber and own a wind tunnel
Shape- myth buster...hypersonic aerodynamics and high speed shapes are not for us!
That stuff deals with compressible flow..no interest to us..we need large radiuses and gentle transitions. remember that these cars, especially mid engine types , don't like low drag shapes like the classic tear drop...not real practical..so the particle shape the car ends up with is going to generate some degree of aero lift. .this lift has to be compensated for with drag producing down force generators to cancel out the lift and add more down force.

in order to deal wit lift we need hi pressure area to form beneath the car and we have to prevent a low pressure area on the top of the vehicle.

one thing we have to eliminate as much as possible is letting any air flow UNDER the car
secondly..facing the fact that the top of the doorslammer is going to have air flow, nothing we can do about it..it is mandatory that this flow not separate..when this happens..we get low pressure and this makes lift.

more later

#### Ranger Mike

Science Advisor
Gold Member
Parasite Drag- took a long while but racers finally figured out but now they attack small increments of parasitic drag as fanatically as weight..i.e.. scrimping on grams to add up to pounds..drag caused by bumps, joints , protrusions all contribute to drag..an exposed bolt head on the needle nose of a FC car will produce tiny amount of drag..AND more important, flow will separate at the object and turbulence wake produced will propagate at the standard 20 degrees included angle until it reattaches ..if it does.
Hex head bolt C.d. = 0.80
round head bolt C.d. = 0.32
brazier pop rivet head C.d. = 0.04
countersunk machine screw C.d. = 0.02
flush countersunk rivet C.d. = .002

most critical areas to look for drag is forward 1/3 of the body plus the forward 30% and all of the underside of the wing.
trick is to delay flow separation to a point as far aft as possible
next aero down force

#### Ranger Mike

Science Advisor
Gold Member
study Bernoullis theorem ..this is the main principle involved.
race car wings function like propeller aircraft wings wit ha few differences
it is mounted " up side down" to produce down force instead of lift
is must operate both close to the ground and in dirty air...air disturbed by the vehicles passage through it and in close proximity to the ground
rules prohibit changing attack angle of the wing must be fixed in one position
there is a lot of engineering that goes into a successful wing design. Attack angle, Stall. Aspect ratio ( span/chord) Lift to Drag Ration , center of Lift

too much to go into detail here but the bottom line is you have to balance downforce with Drag. Another thing to consider is if you add downforce, you will have to change springs. ride height will change and you may risk bottoming out.
Another trick we use is a full bellypan sealing up the underside of the car and we add a diffuser to this in order to channel the air out and make more down force. It is basically a fiberglass tunnel to increase air speed under the car ( thus low pressure)
it is one of the compromises we have to make considering we can not prevent air from entering the area between the bottom of the car and the track...
these two things are a science in them selves and are the major components to adding down force to the car, once the car is made a smooth and slick as possible.

unless there is a question..i will leave this topic as it is

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