Aero Devices

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Being a motorsport enthusiast as an engineer I was always fascinated with F1 cars or for that matter even Germany’s DTM and Japan’s Super GT cars. Apart from all being insanely fast, aerodynamics is a common thread between them.

Additional surfaces of varying size positioned at various different places, do you know what are their names and what are they meant for? In this part, we’ll talk about how several common aero aids work to make a car slick and stick. People tend to bolt them on, to feel the downforce placebo effect.

Components like multi-element wings, diffusers, super-low side skirts, air dams, and splitters, canards and vortex generators combine to produce huge amounts of downforce, however, there is a price for this amount of downforce “Drag”. These components redirect airflow to hold the car down, this creates more resistance for the car to push against. If not optimized properly, the addition of aero aids can be futile or at worst can affect the performance of your car adversely.

The fundamental idea for optimizing should be to produce just enough downforce to maximize the average speed around the track.

In addition aero aids also help in cooling and stability of the car, which if done properly maximizes your car’s performance.

Let’s take a look at some common aero component

1. Multi-Element Front Wing:

It is often said that a racing car’s performance is 90% dependent on its aerodynamics. If that’s the case 60 – 70% of that is governed by the front wing. Why? because it hits the incoming air first and dictates how the air flows around the car. In a modern open-wheel race car, a relatively small part of the wing is used to generate down-force, rest of the wing is used for creating vortices and conditioning the air streams to be used downstream like shaping the stream around the front wheel, directing the air towards side-pod openings etc.

2. Rear Wing:

A crucial aerodynamic component contributes to approximately a third of the car’s downforce. Most often you will see two sets of aerofoils connected to each other using end plates.

The upper aerofoil provides the most downforce, while the lower one is smaller and provides some downforce. However, the lower aerofoil creates a low-pressure region just below the wing to aid diffuser generating more suction.

Higher wing angle increases the downforce but produces more drag, thus reducing the cars top speed. Therefore rear wing similar to the front wing is varied from track to track, to keep the trade-off between downforce and drag in check.

The lift coefficient increases and lift/drag ratio decreases when increasing the number of aerofoils. The relative position of the wings is important. If they are too close together, the resultant forces will be in opposite directions and thus cancel each other.

Rear wing endplates structure provides a convenient and sturdy way of mounting wings. Aerodynamically the endplates prevent air spillage around the wing tips, thus delaying the development of strong trailing vortices. An additional function of the rear endplates is to help reduce the influence of up-flow from the wheels.

3. Diffuser:

It creates nearly 50% of the car’s downforce. It is located behind the rear axle at bottom of the car, air flowing below the car exits through it. The diffuser creates a suction effect, pulling the air from the underside of the car and ejecting it at the back. This pulling action increases the velocity and lowers the pressure of air below the car, so that relatively slow-moving air above the car pushes it downwards creating downforce. Bernoulli’s theorem governs this suction effect.

The diffuser in itself doesn’t lower the pressure. The role of the diffuser is to expand the flow from underneath the car to the rear, decrease the flow’s velocity from inlet of the diffuser to outlet (so that at the outlet the flow velocity is similar to the free stream velocity), in turn produce a pressure potential, which will accelerate the flow underneath the car resulting in reduced pressure and as such, a desired increased downforce generation. This pressure difference is a function of the ratio of the areas at the inlet and the outlet of the diffuser, where this area ratio is set by the diffuser angle and the vehicle ride height. 

4. Side Skirt:

They are used to reduce the amount of high-pressure air from the side of the car to go under the car. If the high-pressure air from the side moves under the car, into the low-pressure region, it diminishes the desired ground effect and downforce. If placed properly, the canard and vortex generator yield the same outcome.

5. Air Dam and Splitter:

They are used to split the incoming air into top and bottom. Air Dam is basically a milder version of Splitter but basically both do the same job. Front end splitter in race car produces aerodynamic downforce by creating a difference in the air pressure on the upper and lower side of the splitter when the car moves.

To understand how a splitter creates downforce you should be aware of the distinction between static and dynamic pressure. An understanding of Bernoulli’s Equation and Coanda Effect is required to fully understand the generation of lift (in our case down-force).

6. Canards:

They are small triangular wings attached to the body of the car for the purpose of modifying the aerodynamic characteristic of the car in a modest way. They generate downforce by redirecting oncoming air’s momentum upward, thus causing a downward force on itself.

The generated downforce is small due to two reasons:

a. Velocity near the skin of the car is significantly less than free stream because of the boundary layer effect.

b. Being a small component, they cannot generate high down-force.

They are mainly used to fine tune the handling of the car before a race.

7. Vortex Generator:

These are smaller triangle winglets fitted few centimetres higher of the bigger canards. They help to guide the air flow on cars with very steep drop-off angles. Airflow has a tendency to become turbulent as it separates from the surface of the car in this region. This turbulent air causes drag and reduce the effectiveness of a rear wing. They help to reduce drag and improve the effectiveness of the rear wing by delaying the air flow separation and reducing turbulence.

In addition, canards, together with vortex generators, generate strong vortices that travel down the sides of the car and act as a barrier. If the canards are positioned correctly, these strong vortices act in a way to keep high-pressure air around the car from entering the low-pressure under-body region, thus maintaining more downforce.

These strong vortices act as a virtual curtain or dam, restricting higher-pressure air around the car’s sides from entering the under-body region.

8. Gurney Flap:

From an aerodynamic perspective, the flap is a device fixed or hinged at the trailing edge of the wing to increase downforce. It is a simple length of aluminium or carbon fibre right-angle rigidly bolted, riveted or glued to a wing’s trailing edge.

The device basically operates by increasing pressure on the pressure side of the 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, along 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 upper surface (aircraft wing) or lower surface (racing car wing). Correspondingly, the total suction on the airfoil is increased. Thus the wing can be operated at a high angle of attacks.

For the Gurney flap to be effective, it should be mounted at the trailing edge perpendicular to the chord line of airfoil or wing. The flap height must be of the order of local boundary layer thickness or typically 1% to 4% of the wing chord length.

9. Flip-ups:

Exposed wheels are the anti-thesis of optimal aerodynamics in that they contribute significant drag and lift. On an F1 car, This positive lift may reduce downforce by approximately 10%. Exposed wheels also disturb the air stream and shed highly turbulent air in their wake. This disturbed air stream is useless as a feeder for other aero devices on the car.

To resolve this problem, engineers design flip-ups on the rear section of the sidepods, in front of the rear tires. Flip-ups guide air over the rear wheels while creating some downforce and shielding rear wing from the influence of dirty air coming from the front and rear wheels. In F1, flip-ups are not allowed any more after rules changes for the year 2009 and after.

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