7 min read •
11 de setembro de 2014
Wings - How They Work (Part 3)
With the introduction of the side fins, as illustrated in figure 4b, the airflow that crosses the same wing became bi-dimensional, creating bigger differentials of pressures between the two surfaces, and avoiding the formation of the vortexes in the side extremes, which, together, allowed a notable increase of the wing’s efficiency.
In some wings the interest is not only to create a negative lift, but also to direct the airflow in certain directions. This effect is of major importance in the case of vehicles of categories of Formulas, since the air flowing through the front wing will influence the aerodynamic behavior of all rear of the vehicle.
This effect can be accomplished, as we can check in the inferior face of the front wing of the Ferrari F2005, illustrated in the figure 5, not only with the vortexes induction at the level of the side fins and in the edges of the trailing edge of the "flaps" (1), but also with the flow conditioning at its passage between the superior face of the wing and the inferior face of the front cone that supports it (2), or even by the use of flow deviators (3), attached in the inferior face of these wings.
As we referred about image 2, the bigger the relation is between the span of a wing towards its cord, the bigger its efficiency will be, in theory.
On the other hand we know that, by the 2005 technical regulations, it is allowed up to a 25cms distance from each side of the central line of the vehicle, so that any aerodynamic appendix could be put as low as the reference plan (RP), and that the front wing should raise to 15cms above that plan, from each side of those limits.
It was too much obvious that some teams would use this space in order, not only to increase the span of its front wing, but also to try to use the central 50cms to obtain more aerodynamic load, while putting that part of the wing working in “ground effect”.
From the observation of the Nature we easily realize that the gentle curves are more efficient with the air interaction, considering that the edges should be especially well used for obtaining specific results.
So, while comparing the front wing of the Renault R25 (image 6) with the Ferrari’s F2005 (image 7), we easily realize that the biggest simplicity of the first one reveals an easier obtaining of the aerodynamic load wanted, only obtained by the Ferrari with a much more complex solution.
We can think that the aerodynamic load of the Renault is less dependent of the downforce obtained in the extremities, by the front and rear wings, than what happens in the Ferrari, or that this team needs a bigger aerodynamic load in order to make the tires work in good conditions.
While analyzing the Renault’s wing, in illustration 8a and b we can see that it’s central part, very much curved, almost reaches the reference plan (RP) level, working in “ ground effect”, and that the interaction of the trailing edge of the "flap" with the inferior face of the front cone, is going to accelerate the air flow in that thin passage and help to maintain the airflow adherent to the inferior face of the wing, in spite the inclination, or it’s angle of attack.
We can also check the importance of the existent slot between the two wing profiles, and that it allows the airflow that crosses it, accelerated in its passage by the Venturi shape of the slot, to increase the flow energy of air that passes under the wing, allowing it to remain not only adherent to the wing, as also, while accelerating that flow, to increase the pressures differential and, consequently, the downforce generated by the wing.
In the extreme side the wing, in drawing 8c, we realize that the supplementary wing is placed more than half cord above the main profile, and that is the reason why it does not interfere with main profile’s airflow, and it is not even affected significantly by the air that is moved by the main profile’s wing.
Still, in this same drawing, compared with the 8d, we can see that, in an attempt of obtaining a bigger aerodynamic load, while increasing the attack of angle (α angle formed by the inclination of the wing’s cord with the direction of the free airflow at its front), from α1 to α2, the airflow that passes under the wing, and that was adherent with α1, is not adherent anymore, with the biggest inclination of α2, because, as we mentioned before, and due to a friction phenomena next to the wing’s surface, the airflow has a bigger difficulty in keeping itself adherent during the slowing down, with consequent increase of the air static pressure in that area.
Due to this phenomenon, this wing is in stall, with reduction of the aerodynamic produced load, which, associated to the biggest drag provoked by the turbulent airflow, has a obvious reduction of the wing’s efficiency, and that is the reason why this situation must be avoided to the all costs.
The front wing of the Ferrari (figure 5) is more complex than that of the Renault, and it uses all the elements panoply to increase its efficiency.
It is composed by three wing profiles, so it must produce a bigger aerodynamic load than Renault’s that uses only two profiles, since we know that the positive influence of the slots placed between the wing profiles leads to the creation of a bigger downforce, by the energy that is added to the airflow that passes under the wing.
It is composed by three wing profiles, so it must produce a bigger aerodynamic load than Renault’s that uses only two profiles, since we know that the positive influence of the slots placed between the wing profiles leads to the creation of a biggerdownforce, by the energy that is added to the airflow that passes under the wing.
In its central part, limited to 25cms from each side of the medium line of the vehicle, in spite of being bent down, the wing presents an aerodynamic device suspended from the main profile, composed by two wings with different inclinations and distinct functions (image 9a).
As we can see in the drawing 9b, the central superior wing of this device works like a "slat", i.e. an aerodynamic appendix which function is of, while channeling more airflow for the inferior face of the principal wing, allows this wing to work with abigger angle of attack without entering in stall, allowing it to obtain, this way, higher aerodynamic loads.
In the same drawing, we can see that the central inferior wing of this device works with opposite inclination to that of the principal wing. Its function is to orientate down the air flow that is going to be crossed by the principal wing, increasing, in this way, its angle of attack, regarding the airflow that crosses it, also allowing a profit of aerodynamic load.
In the last drawing, 9d, we can admire the excellent behavior of the airflow around the triple wing profile, in the extremities of the principal wing, common to the front wing of the McLaren MP4/20 of 2005, also very efficient.
A situation that can be seen when the front wing is put to work in extreme "ground effect", very close to the pavement, is the sudden blockage of air passing between the lower surface of the wing and the track, by the turbulent flow created by a vehicle that precedes it. This blockage causes an also sudden loss of the aerodynamic loads generated by the wing, with consequent up wash displacement of the vehicle’s front wing, that, will allow, once again, the passage of air under the wing, that will be able to create a more negative aerodynamic load with consequent downwash movement of the front of the car, near, the track.
This effect is checked, and so evidently, in the late '80s and early '90s, when designers began to explore the maximum limits of the technical regulation, and the front wings were put to work in "ground effect" and very close to the pavement, coming often in stall when the drivers approached the vehicle that preceded, in contention braking for the next curve.
The phenomenon, profoundly destabilizing of the behavior of the vehicle and totally undesirable, is called "pitch sensitivity" and is, at present, strongly limited by the considerable distance between the bodywork and the track, that the legislation imposes, for security reasons, to the teams, in the different categories of motor sport.