Frontal Area Calculation

[hardingfv32]

When calculating the frontal area of a open wheel car... do you include the frontal area of the rear wheels whether or not they are in inline with the front wheels? Is there some kind of convention or standard related to this issue?

Brian

[Tim.Wright]

When calculating the frontal area of a open wheel car... do you include the frontal area of the rear wheels whether or not they are in inline with the front wheels? Is there some kind of convention or standard related to this issue?

Brian

Only the part you see from the front. Basically you want the area of the shadow cast by a light behind the car to a wall in front of the car.

[Vyssion]

In motorsport it kind of seems a little unintuitive that the frontal area is used for both drag and downforce. Given that it is basically a pressure over an area which gives a force, you would think that for downforce, using the vehicles planform area (area from above) would be the way to go. Whilst some CFD software does this (XFoil and a couple of other 3D basic packages), in general, downforce is given in terms of the frontal area.

But yeah, as was said by Tim.Wright, frontal area is just what you see from the front. If the rear wheels are wider and therefore poke out a bit, then you include that of course. But essentially what is done, is the view from the front is projected from a parallel field of vision onto a plane. The shape it makes then has its area calculated and that is what is used.

Most aerospace simulations use planform area for lift calculations. Hence, it is important to check, especially when looking at isolated aerofoil performance, which method was used; i.e. frontal or planform because you may find the value looks way too high or too low when you test it with a motorsport design.

[Tim.Wright]

Frontal area is used because thats the direction from which the air is coming from.

[livinglikethathuh]

In many calculations, drag is calculated together with downforce, you cannot think one without the other. (Disclaimer: I may use the word "lift", I'm trying to say "downforce")

In airfoils and other aerodynamic devices around the car, Cl (lift coefficient) is variable, as is Cd. The ratio of these two(L/D) however, tends to change less.

In aeronautics, Cd is generally calculated with respect to frontal area, whereas planform area (wing's area when looked from above or below) is used for Cl. But we'll be comparing apples to oranges if we try to calculate Cl/Cd with these values, so we'll calculate Cl again in terms of frontal area. Which leads us to:
CL=Cl . FA / PA
CL: new lift coeff.
Cl: old lift coeff.
FA: frontal area
PA: planform area or whichever area you used when calculating/considering Cl.

So my answer is; depends on where you got your Cl or Cd.

[DiogoBrand]

Possibly dumb question here: Does the frontal area matter that much? From my little aerodynamic knowledge I believe you can have two completely different drag and downforce values with the same frontal area. All would depend on how tapered is the shape, how the airflow is managed, etc.

[Vyssion]

Possibly dumb question here: Does the frontal area matter that much? From my little aerodynamic knowledge I believe you can have two completely different drag and downforce values with the same frontal area. All would depend on how tapered is the shape, how the airflow is managed, etc.

You will find that the way in which most CFD software packages calculate the downforce is by summing up all of the vertical components of the force that each of the relevant (not all...) mesh cells converge to. From this, you get your downforce as a "force" in Newtons. The force is obtained first through means independent of the coefficient.

From this point, it is purely an "in general" type of convention that when working out an objects coefficient of lift/downforce (within motorsport applications) that the coefficient is calculated using the frontal area. The comparison of coefficients can then be made between two objects if and only if both of them followed the same convention. If one used planform and the other used frontal, then there is no information to be had by comparison.

But yes you are right that you can have the same frontal areas of two objects and have drastically different coefficients. But that arises because of the individual force vector summations over the mesh cells being different; and not (necessarily!) because of a different planform area.

[trinidefender]

Just to add some information in here (predominantly relevant to aircraft). When a plane changes it's pitch angle to change its Cl (coefficient of lift) (assuming no speed loss) then the the frontal area will also change. The fuselage will be at an angle to incoming airflow instead of the "perfect" fuselage angle = 0 degrees.the frontal area of the wings and tail plane will also change. Therefore for drag calculations for a plane, changing its Cl through the use of a pitch change (no change in design of plane or speed change) then drag calculations will change through an increase in frontal area as well as any other variables.

[Tim.Wright]

Possibly dumb question here: Does the frontal area matter that much? From my little aerodynamic knowledge I believe you can have two completely different drag and downforce values with the same frontal area. All would depend on how tapered is the shape, how the airflow is managed, etc.

The frontal area is used to normalise the Cl/Cd values for the the physical size of the object.

Therefore, 2 objects the same shape but different size will have more or less the same Cl/Cd but a different Cl.A due to their size difference.

Essentially, the frontal area tells you the size of the object and the Cl/Cd tells you the shape.

[Greg Locock]

When I get my results for the 6 coefficients vs yaw angle they are in terms of Cd*A etc, that is, I get to choose what if any reference area I bother with. So does anybody else get raw windtunnel or CFD results in this format or is it just a local fad?