F1technical.net

I found this aero-data for Ferrari P4/5 by Pininfarina:

Cx 0,340
Czf -0,123
Czr -0,025
Frontal Area (m2) 1,906

I was impressed and don't know why such a supersleek car has a Cx that high. It is not the first coupè I found with higher drag figure than common saloon cars. With my rudiments of aerodynamics I would state a coupè to have definitely less drag because of the smaller vacuum area at the rear, due to the sleeker silhouette than a saloon.

So, what I'm missing?

Maybe is some sort of "induced drag" caused by negative Cz (downforce)...

I'm not an expert in this field but I remember reading somewhere that Cx depends on the speed. Maybe this is the reason it is so high, those values must have been taken at an incredible speed.
However, I've never seen the speed at which Cx is measured.

I cannot say for sure except to that the goal behind the aero work was NOT around reducing drag, but in generating enough downforce to make the car safe at speed. Wide wheels and tires exact quite a bit of aero drag, as does the high angle diffuser on the back of the car.

The coefficient of drag is meant to be independant of velocity and frontal area. Therefore, despite having a higher Cd figure, this does not necessarily translate into higher drag forces than other cars.


The reason for a coupe having a higher Cd is pretty straightforward - its the longitudinal vortices eminating off the 3/4 panel .

See here on the Ahmed body test case:

[img::]http://www.cfd.tu-berlin.de/PICTURES/ahmed.jpg[/img]

The two longitudinal vortices that are coming off the sides of the sloped section at the rear - that is where alot of the energy goes into.

BreezyRacer wrote:I cannot say for sure except to that the goal behind the aero work was NOT around reducing drag, but in generating enough downforce to make the car safe at speed.

That's true even within "normal" cars. It's more of a rule than exception that one manufacturers lineup has the smallest CD on an economy grocery getter and the sports models have higher CDs.

vis wrote: I found this aero-data for Ferrari P4/5 by Pininfarina:

Cx 0,340
Czf -0,123
Czr -0,025
Frontal Area (m2) 1,906

I was impressed and don't know why such a supersleek car has a Cx that high. It is not the first coupè I found with higher drag figure than common saloon cars. With my rudiments of aerodynamics I would state a coupè to have definitely less drag because of the smaller vacuum area at the rear, due to the sleeker silhouette than a saloon.

So, what I'm missing?

Maybe is some sort of "induced drag" caused by negative Cz (downforce)...

The need to counteract the lift (or even generate downforce) is certainly an aspect, particularly because as other said the car’s stylists, even if sometimes they brag about it being the case, usually don’t really take in account aero requirements on the basic design, so the aero work is more often than not a second stage fix leading to not optimal solutions (Audi TT anyone ?)

Still there’s a more important factor that leads invariably to increment of drag, and that’s the fact that the powerful engines typically fitted in sportcars generate lot of heat. The required cooling causes conspicuous increment of the drag, in particular because, even if these cars have everything needed to go really fast, they are still road cars so the dimensioning has to take in account that most of time it will run at low speed and possibly in traffic.
Also considering the same car model, say a normal saloon car, the version fitted with the most powerful engine often suffers a not negligible increment of drag compared with the base version, less powerful, exactly because of increased cooling requirements.

Shall we add wide tyres to the list

Good post Reca - as ever, thanks

Maybe it's not a case of a coupe having a surprisingly high Cd - maybe you should look at some saloon bodies having a lower Cd than you might expect.

The hatchback shape can be surprisingly low drag - I think it's called the Kamm tail. Where a cutoff back can produce a low drag shape, rather than following the idea that the tail needs to taper over a great length to reduce drag.

I suppose you should also look at the car's plan form not just the side view when considering how "draggy" it appears. Some front wheel drive cars have quite a tapering plan form - while a Ferrari (never looked I am guessing probably gets wider to the rear.

Because it is rain drop going in opposite dirrection - slim and tight nose growing into fat and bulky rear end.

Unusually Manchild you're wrong there.
The most streamilned shape in the world is a raindrop (

http://www.f1technical.net/articles/10

) whcih is why raindrops are that shape, because water is not solid (although, unlike air, cannot be compressed) it forms the most streamlined pattern it can to reach the ground.

Of course with the rules of F1 as they are a complete raindrop shape is hard to create esspecially as it offers almost no downforce, so the cars are made as close to that shape as practical. Ever noticed how the sidepods of cars taper off into the gearbox at the back? according to manchilds theory the engine cover would also get wider towards the rear, which of course it doesn't, the backbone of the car sweeps down as soon as it gets past the engine.
Remember F1 cars are not very aerodynamically efficient, my Citroen has a lower co-efficient of drag, because they have to have plenty of cooling for their engines, they must have a safety cell for the driver to sit in and the size and position of the engine will have a great effect on power produced and therefore the shape of the bodywork to squeeze it all in.

Tom wrote:Unusually Manchild you're wrong there.
The most streamilned shape in the world is a raindrop ...

You've misunderstood my post. Raindrop going in oposite direction has bad aerodynamics. That is what I wanted to say. When aerodynamics of raindrop is analyzed than it is not considered that a car with ideal aerodynamic shape would have pointy front end but pointy rear end. Coupe's are oposite - long slim noses and bulky rear ends reminding on raindrop not falling down but rising up.

[img::]http://img503.imageshack.us/img503/1333/aerooi7.jpg[/img]

Tom wrote:Unusually Manchild you're wrong there.
The most streamilned shape in the world is a raindrop (

http://www.f1technical.net/articles/10

) whcih is why raindrops are that shape, because water is not solid (although, unlike air, cannot be compressed) it forms the most streamlined pattern it can to reach the ground.

I think you misinterpreted Manchild sentence.
He was pointing out that the "raindrop" is in the opposite (wrong) direction,
not the raindrop shape itself.

Sorry, I didn't let time for Manchild to reply

Sorry, my mistake, didn't read the whole sentance.

Should have known MC would be right.

If we are talking about this car:

[img::]http://www.carbodydesign.com/archive/20 ... 5-4-lg.jpg[/img]

then I would think that it has a higher Cd because of the downforce/cooling generating features such as spolier, diver plates, hidden bonnet wing and NACA cooling pods. As we know induced drag is a function of lift (downforce) which is why Cd is slightly higher than you expect

Well that's not a coupe.

a coupe is a car body style with a close-coupled interior offering either two seats or 2+2 seating.
The SAE distinguishes a coupé from a sedan primarily by interior volume; SAE standard J1100 defines a coupé as a fixed-roof automobile with less than 33 ft³ (0.93 m³, 934 L) of rear interior volume. A car with a greater interior volume is technically a two-door sedan, not a coupé, even if it has only two doors. Some car manufacturers may nonetheless choose to use the word coupé to describe such a model, e.g., the Cadillac Coupe de Ville.

it is a coupe