The rear spoiler / GT Wing is used to increase the amount of downforce to the rear tires. Increasing this downforce increases the normal force between the tire and track surface, which theoretically should increase the potential to use the engine power to accelerate the car. If there is little normal force then there is potential for the car to experience wheel slip and thus not have traction to accelerate.
Sometimes when designing a rear spoiler / GT Wing it is desired to produce more downforce. Different methods used to obtain more downforce are increasing the wing area, increasing the camber of the airfoil, and delaying flow separation by slotted flap design or multi-elements [Katz, 1995].
Typical rear spoiler / GT Wing wings have an aspect ratio of around 2.4. The aspect ratio of a wing is defined by the equation as being the ratio of the span of the rear spoiler / GT Wing squared to the area of the rear spoiler / GT Wing.
AR(aspect ration) = b2 / S
In an attempt to increase the performance by decreasing this loss added to the rear spoiler / GT Wing called an endplate can be used. The endplate provided by [Katz, 1995], maintains a pressure difference between the upper and lower portion of the rear spoiler / GT Wing that not only improves the performance of the rear spoiler / GT Wing at the tip and thus improves the overall rear spoiler / GT Wing performance. Therefore there is an effective aspect ratio calculation that can be made that is presented by [Raymer, 1992] and is shown in the equation.
AR effective = AR [ 1+1.9(h/b)]
Thus by increasing the height of the endplate to span ratio of the endplate a greater effective aspect ratio can be obtained and a larger new endplate controls the flow better by reducing this spillover effect. The wings with the endplates have a more uniformly distributed suction pressure strip along the entire length.
This improvement to the wing design was beneficial to the rear spoiler’s / GT Wing’s downforce. Because of the wing’s thin trailing edge, you need a material that is strong so that it would not break off. Carbon fiber could be used to make this trailing edge strong enough.
For a feel of the angle of attack on how much it affects the performance of the rear spoiler / GT Wing, it is necessary to look at the downforce and drag of the rear spoiler / GT Wing. For example, an increase in the angle of attack to α = 5° increases the downforce by 8.9%. The drag also increased but increased by 20.4%. This is greater than the increase in downforce and therefore it decreases the downforce to drag ratio by 9.7%.
In rear-wheel cars, this is significantly vital and the rear spoiler / GT Wing will not only add acceleration and braking abilities but also cornering grip. A greater rear spoiler / GT Wing angle increases the downforce and produces more drag, thus reducing the car’s top speed. So, when racing on tracks with long straights and few turns it is better to design the rear spoiler to have a small angle of attack. Opposite to that, when the car is racing on tracks with many turns and few straights, more downforce is required thus it is better to design the GT Wing to have a greater angle of attack.
The rear spoiler / GT Wing should be optimized between the downforce needed for grip whilst braking and the drag they produce.
At high angles of attack, the air is unable to follow the contour of the lower rear spoiler / GT Wing surface and can detach (stall), lowering the efficiency of the rear spoiler / GT Wing and adding drag. Adding a small lip on the trailing edge causes a lower pressure just behind it which sucks the lower flow back up to the rear spoiler / GT Wing surface.
The Gurney Flap is a small tab projecting from the trailing edge of a rear spoiler / GT Wing. Typically it is set at higher angles of attack on the high-pressure side surface of the airfoil, and its height must be of the order of local boundary layer thickness or 1% – 4% of the wing chord length in order to be effective.
This trailing edge device can improve the performance of a simple airfoil to nearly the same level as a complex high-performance design. The device basically operates by increasing pressure on the pressure side of the rear spoiler / GT Wing, decreasing pressure on the suction side, and helping the boundary layer flow stay attached all the way to the trailing edge on the suction side of the airfoil.
At the same time, a long wake downstream of the flap containing a pair of counter-rotating vortices can delay or eliminate the flow separation near the trailing edge on the lower surface. Correspondingly, the total suction on the airfoil is increased. The designer has to get all the downforce possible out of the rear spoiler / GT Wing surfaces allowed by the rules.
The Gurney flap surely causes some extra drag but can generate more downforce from the allowable rear spoiler / GT Wing because of the higher angles of attack. Gurney flaps are also used as a quick way to fine-tune the force a wing generates in order to adjust the way a car handles.
Varying the height of the Gurney flap adjusts downforce (and drag, of course) and so most teams have devised ways of changing the Gurney quickly.
In an Acceleration event, for example, many teams often choose to set their rear spoiler in low or even zero angles of attack in order to reduce the drag of the elements in minimum while until the car reaches at least 40km/h the rear wing doesn’t produce enough downforce to give the required grip to the tires to avoid spinning. On the other hand in Endurance and Skidpad events where the downforce is vital for the ongoing turns, teams set their GT wings at the angle of attack that they have designed them to operate and be most efficient.
Some of the most advanced FSAE teams are also using a drag reduction system(DRS) operating with electronic actuators which give the ability to the driver with just the press of a button to level off the angle of attack, of the rear wing elements while driving, to reduce drag and downforce, thus increasing top speed.