Reca wrote: ↑06 Feb 2006, 16:45
First of all a premise.
It’s not the Cd per se that matters, it’s the SCd.
So to say that a car has a Cd of 0.19 is absolutely useless if you don’t know the reference surface used to calculate it, and notice, I don’t want to know “it’s the frontal surface” because there are several ways to define the frontal surface, I want to know the number, X m2.
The Cd is just an arbitrary number, you measure the SCd, divide it by an arbitrary, reference surface and you have a Cd. Historically that reference surface for cars is the frontal surface (although as I said there are different way to define it), but it could equally be the plan area of the roof, of the bonnet, whatever, it doesn’t matter because at the end of the day you still need to know the SCd, hence both the Cd and the reference surface.
So, what’s that Cd useful for ? Besides to make up a suitable number for advertising fooling ignorant (in the good meaning, ie people who don’t know) people pretending to have established a worldwide record, it’s useful only to know that, if you make a scale model, say at 50%, of the same car, exactly the same shape, exactly in proportion, then the SCd of that model will be 50% of the SCd of the real car, hence it’s useful for example, for wind tunnels. Alternatively the Cd is useful for wing sections, but that’s again because for these shapes, the reference surface is well defined.
But while comparing two very different shapes, for example two different cars like that Mercedes and a F1, the Cd per se is absolutely useless, you need to know the SCd, as a unit, because it’s that number that matters.
To not understand that leads to one of the biggest misconceptions in aero, you hear/read it everywhere... do you want to reduce the drag ? Reduce frontal surface, like if the frontal surface per se had something to do with drag... I even heard someone saying that the Tyrrell P34 had the same drag of the other 4 wheels cars because the frontal surface, due to the rear tyres, was the same...
Generally, while talking about similar shapes it could make sense, but not necessarily because working on the reduction of the frontal surface you are also changing the shape, and if you do it in the wrong way that could easily lead to an increment of the SCd, even if the plan area of the frontal surface is greatly reduced.
An example is the dimension of sidepods of a F1 car, how often you read that bigger sidepods isn’t good because of drag ? Well, it’s false, just look at the Williams, for years they had the smallest sidepods and likely the smallest frontal surface of the cars on the grid, does it mean that they had the car with the lowest drag ? Not necessarily, and in fact with the FW28, in an year when, with drastic power reduction the drag limitation is fundamental, they adopted a different approach.
Now, end of the, long, premise and back more specifically to the topic, to calculate the SCd of a F1 car, at least in first approximation, is quite easy, just take the formula for the drag (force), multiply it for the velocity and you have the power required.
Let’s use now 2005 data just because we don’t have idea yet about the peak speeds they’ll reach with the V8.
Assume the power, spent only for aero (hence removing transmission losses and tyre rolling resistance), was something in the order of 730-750 hp = 530-550 kW, use standard air density (1.225 kg/m3) and assume we are talking about Monza where peak speed was about 360 km/h = 100 m/s, and the SCd of a F1 car, in Monza trim, is :
SCd = (2 * Power) / (density * v^3) = (1100 * 10 ^ 3 [W]) / (1.225 [kg/m3] * 100 [m/s] ^3 ) = 1.1/1.25 = 0.88 [m2] => I said 530-550 so let’s say 0.85-0.88.
(what’s the Cd ? it depends by the reference surface I use, but we don’t care about it)
As for that Mercedes, I don’t know the reference surface but it’s probably between 2 and 2.5 m2, that would mean SCd = 0.38-0.475 m2.
Assuming that the right numbers are 0.85 m2 and 0.4 m2 then that means that, at a given speed, the Mercedes needs roughly 0.38/0.85 = 47 % of the power required (for aero only) by the F1. Nevertheless we should remember that F1 is a open wheel car designed first of all to generate downforce, the Mercedes is a car designed solely to reduce drag, so, at the end of the day, the F1 isn’t that bad.
Obviously with the right numbers you would have better accuracy, that’s just an example to show the method and the method doesn’t change.
At the end just a note. I assumed that the SCd of the car is constant with speed. But... it really is ? Well, better if we avoid to open that can of worms