The need for even more speed: drag reduction of a Formula One rear wing
In present Formula One (F1) racing, the continuous need for speed drives engineers to push both driver and car to the limit. Competing teams constantly introduce new features and technologies to gain a split second per lap, hoping to beat their rivals. In their quest for the ultimate advantage, several teams introduced flexible wings, which soon became banned by the leading authority of F1 motorsports, the Fédération Internationale de l'Automobile (FIA). The question, however, of how the ultimate flexible wing should look like, still remains.
by: Ir. Glenn Thuwis, PhD-student, Aerospace Structures
The Theory behind Flexible Wings
In order to understand why flexible wings are a hot topic in F1 racing, it is necessary to understand the theoretical motivation behind the application. Although several methods of applying these flexible wings are rumoured to have been used in the past, the main objective remains the same. By applying a flexible wing, teams try to reduce the downforce, which is the downward lift produced by an inverted airfoil, of the wing at high velocities. Since induced drag is related to the amount of downforce created, the drag is reduced as well. Thus applying a flexible wing to the car has two advantages. First, the downward force acting on the tyres is reduced, thereby reducing the friction of the tyres, and secondly, the overall drag is reduced. Both effects allow the car to have a higher acceleration, creating the difference between the winner and the runner up.
But why not simply reduce the angle of attack of the wing? Wouldn’t this make life easier instead of coming up with a fancy flexible wing? The answer to this is simple, the reason the flexible wing is applied, is because at some stages on the circuit, the higher angle of attack of the wing is necessary. More specific during cornering, the driver would like to go as fast as possible, thus requiring a large friction from the tyres. The higher the downward force on these tyres, the larger the friction becomes, thus the faster the pilot can manoeuvre the car through the turn. Modern F1 cars are capable of creating a lateral cornering force of 3.5g. Only when accelerating the car down a high speed straight line, the driver would like to accelerate as fast as possible, so at this stage, the flexible wing becomes important. It can potentially make the difference between a world champion and some lower ranked driver!
The History of Flexible Wings
The idea of using variable angle of attack settingson the wings is not new. The first sportscar to use a rear wing to create downforcewas the Chaparral 2C in 1965. It has to benoted that the principle of using an invertedwing to create downforce is already knownfor about 80 years, ever since engineers atOpel mounted inverted wings on the OpelRAK 1, a rocket propelled car, in 1928. However,what made the Chaparral 2C so specialwas that it allowed the driver to vary theangle attack of the rear wing during a race.The wing was mounted between hinges, asindicated in figure 1, and via an actuator anda special pedal, the driver could easilychange the angle of attack.
It still took until 1968 before the fixed rearwing found its way into F1 racing, when theengineers at Lotus introduced the Lotus 49B, depicted in figure 2. Soon, rear wings becamea familiar sight in F1 racing. And it wasnot until long when teams started playingaround with moveable rear wings like on theChaparral 2C. These experiments lead tosome spectacular accidents, so by 1970, theFIA decided to ban all movable aerodynamicdevices. Ever since, actively changing theangle of attack of the rear wing has been forbidden.
Figure 1: Graham Hill, racing towards victory in his Lotus 49B at Brands Hatch, England 1969.
This Lotus 49B is the first F1 car using wings to create downforce.
The Passive Approach
Since active movable wings have beenbanned from the F1, the passive wing, or theso called flexible wing, became a hot topic in1999. The idea was simple, create a wingwhich is flexible, and allow it to deformunder the airload. It is said that severalingenious designs have been used: for examplea flexible upper most rearward elementon a two-element rear wing. At higher velocity,the upper element would close the gapbetween both elements, causing the wing tostall. The result is a sudden reduction ofdownforce and drag. However, since F1 is acompetitive sport, most teams tend to shieldthe information about their systems fromother teams, and as such also from the world.Therefore, no actual data is available onthese flexible wings.
So the goal of this Master Thesis was todesign a passive flexible rear wing, whichcan significantly increase the car’s overallperformance by reducing the drag at highvelocities. At the same time, however, thelow speed turning performance has to bemaintained. In order to facilitate such aneffect, so called bending-torsion coupling is used. The idea is to tailor the composite structure of the rear wing, such that when the wing bends under the higher airload, the wing will passively twist to reduce its angleof attack.
To simulate this interaction between the structure and the surrounding air, a fluid structureinteraction routine is set up. The modelling of the structural wing model is done in Nastran, while the aerodynamicpressure forces on the wing are evaluatedusing VSAERO, a 3D aerodynamic solver. Tocreate the coupling between the structuraland aerodynamic model, ModelCenter isused. This program allows to control severalsubroutines and the data transfer betweenthese subroutines in a user-friendly environment.This fluid structure routine is used tooptimise the composite structure of thewing, minimising the drag at high velocities.
Figure 2: Movable rear wing on a Chaparral 2c.
Top: the wing is at low angle of attack
Bottom: the wing is at a high angle of attack
Optimised Rear Wing
Using this optimisation routine, an optimalflexible wing is created. The resulting wingdesign is capable of reducing the drag coefficientup to 15% at a velocity of 300km/hwhen compared to a rigid benchmark wing,as shown in figure 3.
As part of my internship at the DLR’sInstitute of Aeroelasticity at Göttingen, theoptimised wing was coupled to a Multi BodyModel of a F1 car, making it possible to checkthe performance increase of the entire car asa result of the modified rear wing. It wasfound that when accelerating along astraight part, after merely 700 meters, thecar, using the optimised wing, already has adistance advantage of 0,4m. This might seeminsignificant, but during an entire race, thedifference can become crucial. At the sametime, it was seen that the performance atlower velocity turns remained nearly equalto a benchmark car with a rigid wing.
Whether this novel design finds its way intothe competing world of F1 racing is dependingon several aspects. Allowing moveableaerodynamic devices would be the first step,and as it seems, this step might be taken bythe FIA in the near future. However, onlytime can tell whether or not one day thiswing will be racing down the track, givingthe driver a higher chance to victory.
Figure3: Comparison of induced drag versus velocity for both an optimised and benchmark wing.
The drag reductino becomes larger at higher velocities, due to the decrease in angle of attack.
