Adding Aero, Justifying Aero

Author’s note:

I was terrible at the design event as a student.
My team worked hard, thought hard, and showed occasional talent in our designs.
But what I presented was rarely more than a packaging exercise.
The scores we got, in hindsight, accurately reflected our gap to experienced engineers in a professional environment.
There are no grade curves for beginners, or practice problem simplifications with simple answers.
Collegiate Design teams are where you learn to consider EVERYTHING when you make an engineering decision.
As a judge, I am impressed that any team manages show up with a car, and it ramps up with the level of understanding.
I am going to push you to grow your perspective and look at the whole context from every angle.
If every generation doesn’t learn faster than the previous, we’ll get stuck in one place.

1. Why aerodynamic elements do not guarantee performance gains in FSAE:

Race car aerodynamic theory is mostly derived from road course racing.
By comparison, most FSAE corners are very low speed.
Many FSAE cars go to very high yaw.
If the total Coefficient of Lift * Reference Area (ClA) is too low, the corners could be so slow that there is no detectable change, with the straights still fast enough to see a drag penalty.

2. How to show aerodynamic elements increase performance in FSAE:

Focus your aero program on the specific needs of FSAE from the very beginning.

2a.
Research and do some quick calculations about how much downforce is possible.
Learn rules limits on element size, theoretical performance coefficients, and what other teams are claiming.

2b.
Calculate the change in performance in a corner and over a straight.
Teams can stitch together multiple corner and straightline segments into a lap.
Or, if using a lap simulation program, show the change in performance for each braking, midcorner, exit, and full throttle section.
Your team needs to understand and quantify the performance effects at every point on track, not just the change in laptime.
Not good enough: “Will wings make the car faster? We put wings on, and it went faster.”

2c.
Analyze previous year FSAE circuits, calculate the effects of theoretical aerodynamic elements, and be ready to show your work.
Note circuit characteristics that give additional weight to downforce or drag.
Monaco and Le Mans do not have the same downforce or drag compromises.
Define the speeds where downforce is the most important, and design and test around those speeds.

3. For first year aero, start simple:

With the coefficient and speed targets from step 2 in mind, design elements that:

• are VERY simple

• are VERY cheap

• are VERY fast to make

• are sturdy

• Make a significant amount of downforce

• Have adjustments that will make measurable and noticeable aerodynamic impacts

Use the simple simulations from step 2 to choose if there is more than one option.
Have a plan to keep plenty of airflow going to the radiators.
Radiator flow and analysis is important even for teams without aerodynamic elements.
Finally, calculate whether bump rubbers will be needed to stop the suspension from bottoming out.

Deep dives that can wait for future iterations include:

• Aerodynamic efficiency

• Airfoil selection

• CFD fine tuning

• Front/Rear element interaction

• Lightweight construction matched to load and stiffness targets

• Suspension compromises between mechanical grip, aero loading, and platform control

Teams don’t have to bring early test parts to competition.
But if nothing else is ready, and they’re proven to make the car faster, why wouldn’t you?
If a non-aero team adds flat plywood wings, because they have improved the L/D from zero or negative, the car is most likely faster in high speed corners.

Warning:

It is difficult to estimate how many aero judges would be furious over flat wings and no deep dives.
I feel the same way about aero subteams working in silos with no vehicle dynamics integration.

4. Go out in the parking lot and test.

• Make sure the radiators are still getting enough airflow first.

  • Keep the powertrain cool, there is a lot of testing to do.

• Simple tracks: Skidpads with different radii, egg-shaped or figure-8 with one slow corner and one fast corner.

  • Slaloms are the next level of mathematical complexity.

• Test with no elements

• Each element individually

• All elements together

• Make adjustments that should measurably and noticeably change the balance.

• This may be good training for driving on the limit, understanding front or rear limits, and giving useful driver feedback.

I do not recommend tuning the suspension differently for each test.
Simply finding the combination that gives the fastest laptime is missing the point.
The point is to understand the size of each effect, and how to engineer each effect to achieve the fastest laptime.
Then you can start making engineering decisions about suspension tuning and future aero developments.

As with any test session, make multiple runs, and record for every run:

• Laptime

• Course measurements

• Tire temperature (all 4)

• Ground temperature

• Ambient temperature

• Driver feedback

• Plus any and everything from your sensors

Compare results to predictions for each of these tests.
Did the drivers reach the targets? If not, why?

Record keeping is even more important at competition events.
Data will help your team much more than weight savings from wiring and sensors.

5. Make multiple efforts to measure downforce and drag.

Check the simulations from multiple angles.
Traditionally, teams know to look for changes in suspension load and/or compression.

• Zip ties marking maximum travel on the dampers are enough to get started.

• Bump stops make travel tests more complicated.

Downforce will also show up in changes to the traction limit.

• Change in traction limited acceleration

• Change in lateral acceleration

• Balance changes will affect the lateral and longitudinal limit of either the front or rear axle

Drag will show up in power-limited acceleration, top speed, and coasting.
Braking is increased by both downforce and drag.
All effects are variable with speed.
Back-calculate the forces and aero coefficients from your test data in every different way.
They should all be close.

6. ITERATE

Your team now has aerodynamic models and lap simulations with some validation.
Talk to the judges about your theoretical performance, what you achieved in the parking lot, and what you predict for the next day at competition.
Talk about all the corrections the models needed.
Do mathematical studies on different adjustments of your elements for acceleration, skidpad, autocross, and endurance.
Follow up on the promising results in testing and at competition.

There is a good chance the car is already faster, especially compared to spending all year in CFD and never building anything.
By first understanding and validating how much impact simple aerodynamic elements have, your team can set smart priorities for the next iterations.
Recognizing diminishing returns in each of the deep dives will help your team efficiently make another big jump.

Show all of your new members why and how aerodynamic elements make a difference.
Keep improving the models.
Don’t let anyone get stuck in one place.