GROUNDED BY WINGS

It's common knowledge that by placing an inverted airfoil on your car, you can increase the normal load your car experiences while in motion, but the question every team needs to answer before you decide to put it is:

IS IT WORTH IT ?

"If you think about it, it’s neither rocket science nor elementary math,it’s somewhere between, as a race-car engineer your job is to take 250-300 kgs of mass from Point A to Point B as fast as possible."  -Pat Clarke

So every decision you make should help you achieve the above goal, be it vehicle dynamics or powertrain decisions.

With regards to aerodynamics, for analogy if you think designing a race-car is like plucking fruits from a tree then aerodynamics is the fruit at the top of the tree.

 

Simulations help you Decide

 

 

You first need to decide if it’s worth getting that fruit. For this, we need an accurate model of the car and a good lap-simulation to understand if the car needs aerodynamics and whether or not the gain in laptime is worth the money and efforts you’re going to put in.

In our lap-sim we can enter the Coefficient of lift (C_L) and drag (C_D) and the frontal area. It assumes that the downforce is directly proportional the square of the velocity, and one cannot just enter whatever coefficients of lift and drag one presumes. In Macbeath, we found a trend which basically tells us what coefficient of lift is achievable for a race-car given a particular coefficient of drag. We use points from that graph in our lap-sim.

Note-Please try to approximate how much additional weight your aero package will be adding in the lap sim.

 

Time is of the essence, don't waste it

 

We used the FSG Auto-cross circuit as our base lap-sim, simply because it’s worth more points. Once you have an idea about the time you gain by using aerodynamics, analyse if you have the people, time and most importantly the money to translate that into reality. This is especially important when you’re trying aerodynamics for the first time. Taking all the above parameters into account you decide your target C_L and C_D values.

 

Pressure on the wings

 

 

Once you’ve decided the target C_L and C_D values for your race-car, the next step is to split these forces between the rear and front wings. This will decide the aerodynamic stability of your car. In other words, you need to decide where you want the centre of pressure (COP) of your car to be with respect to the centre of gravity (CG). This decides in what direction aerodynamics is going to aid your car. In simple terms if the COP is in front of CG, aero will aid oversteer and if it’s behind vice-versa. This doesn’t mean if you have COP in front of CG the car will oversteer, we’re talking about the proportional grip gain in the front and rear wheels due to aerodynamics. For a neutral car, we place the COP at CG and use simple moment and force balance equations to split the C_L and C_D of our car. During this phase, the aerodynamics sub-system needs to closely work with the vehicle dynamics sub-system to understand the car's behaviour with and without the aero-package.

 

Working on the Undertray

 

 

The size, position and the type of airfoils of a Formula Student aerodynamic package are often rule constrained and manufacturing limited. We begin first with 2D simulations where the first step is to select an airfoil for the main element and the flaps. We look for high lift airfoils which are easier to manufacture. Once the selection of an airfoil is done, we run a 2D simulation to decide it’s optimum operating angle of attack and then do a 3D simulation to co-relate it’s true downforce value. The addition of flap has a few more steps, we need to optimise the X and Y coordinates (the position of the flap with respect to the main element) and the angle of attack of the flaps to gain maximum downforce. This process is repeated until we gain maximum Lift/Drag ratio. The process is similar for the front and rear wing.

An undertray has the most potential in terms of aerodynamics as there is a lot to gain from the undertray in terms of downforce, mainly because of its proximity to the ground. The downforce produced by the undertray unlike the wings does not come with a drag penalty. So a good undertray design is very essential for a formula student car. An efficient undertray ensures sealing of air, optimum diffuser angle and sufficient mass-flow rate under the car as the undertray isn’t a single entity which you’re going to place on the car. It needs to work in harmony with the nose and the front wing of the car. So the design of an undertray is slightly different from the wings, ensuring an integrated undertray is very essential for gaining maximum performance from the floor.

 

Finalizing the Aero Package

 

This is basically part design, but should you make the decision of putting it on your car by just simulating it separately? Absolutely not!

We have a simplified 3D model of our car for CFD, once we’ve designed a part, we place it on our full car model and see if we’re gaining in terms of downforce. Once that is verified we then decide to put the part on our car. This is done basically to ensure an integrated Aero Package. After all this is done a final full car simulation is run to correlate our final downforce and drag values with what we’ve assumed before.

 

On Track Validation 

 

Validation is a very essential part of engineering. Needless to say if you’re going with aerodynamics in your car, the first thing the judge is going to ask you is whether you’ve validated your aero-package. A lot of validation techniques are available out there like wind tunnel testing. To verify our package , we have linear potentiometers on each spring which gives us the load on each tire while the car is in motion. We have the static load data on each potentiometer with different weights on the aerodynamic elements which is used as a database to create a neural network. The data obtained from the on-track run is filtered to eliminate the noise and is then fed into the neural network code which then directly gives us the downforce split up. This is how we can validate the C_L of our car.

Another on-track validation technique is the Coast Down technique where you accelerate the car to a speed and engage the clutch, deceleration of the car is noted and then plotted with respect to speed, this way we validate our C_D value.

The error percentages from simulation and the real life values are around 15 percent

These are the methods which our team uses in designing and testing our car's aero package. If you feel that we have missed something, please do let us know in the comments.


P.S- For knowing more about how our team performs simulations in MATLAB, click here.