Downforce and ground effect in Motorsport
Aerodynamics: Lesson 7
The aerodynamic forces: Drag, Lift and more.
The picture below shows the main forces acting on an aircraft: lift (L “Lift”), resistance (D “Drag”), weight (W “Weight”) and thrust (T “Thrust”). V indicates the apparent wind direction.
The procedure by means the plane flies and the race car, despite having a high speed, remains attached to the ground is not so simple; many forces act on the wings of the plane and on the various components of the car: the lift and the force-weight act vertically, therefore their resultant allows the vehicle to be raised or lowered while the resistance and the thrust of the engine act horizontally, therefore their resultant allows the vehicle to go faster or slower.
We Need Downforce.
Lift is not always opposite to force-weight, but it can follow any direction. In fact, in the cruising motion of airplanes generally the lift has the same direction of the weight and in the opposite direction, but when the plane rises or falls in altitude, or when it makes a turn, the lift will tilt with respect to the vertical. In some acrobatic maneuvers, lift can take the same direction as force-weight. In this case the term downforce is used. This force is exploited above all in the automotive sector, when the cars reach high speeds, in order to guarantee the adhesion of the tires to the ground.
The idea therefore is to increase this normal force (called aerodynamic component) by adding it to the static component, given by the weight of the car.
To obtain this effect, special ailerons are applied (both in front of the front and rear on the rear), which exploit the same principle as the wings of the aircraft, but in the opposite direction. When the track engineers change the incidence of the ailerons, they do so to change the downforce according to the characteristics of the track (twisty or not), the required trim or the environmental conditions of the ground (dry or wet).
In addition to changing the impact of the ailerons to change the contribution of the downforce, another method of increasing downforce is to use a flat bottom of the car together with the use of the so-called miniskirts and a rear extractor (the known rear diffuser), so as to create a depression under the car by accelerating the fluid between the ground and the bottom of the car with respect to that above it, always according to the Bernoulli principle.
The Ground Effect.
This stratagem exploits the ground effect, which consists in a modification of the aerodynamic field around the body when it moves close to the ground, and in the corresponding variation of the aerodynamic forces applied to it.
Approaching this body to the ground the aerodynamic field is no longer perfectly symmetrical: a fraction gradually increases overhead with a lower speed (therefore higher pressure) while a smaller percentage passes below, with a higher speed (and lower pressure). In racing cars, however, it is necessary to create downforce and to obtain it from the ground effect, we need to change the shapes of the appendixes so that an air flow passes under the car with a speed greater than that which passes over the car.
Downforce and ground effect in Motorsport. Bottom flat and the take-off risk.
In general, the aerodynamic advantage of downforce occurs mainly in curves, where the inertia forces that cause the centrifugal forces tend to make both the trajectory and the grip unstable; here therefore an increase in the total weight of the car manages to keep it more adherent or flattened on the ground. It also gains stability on the straight: without a rear wing the car would risk taking off. However, the most common disadvantage is that the more the incidence of the ailerons increases, and in general therefore the aerodynamic load and the adherence, the more the aerodynamic drag force increases with a decrease in the top speeds in the straight stretches.
The high risk of “take-offs” due to the design of flat-bottomed cars, however, has suddenly “brought flying races closer …”. We have thus moved on to stepped funds, safer and more efficient.