Aerodynamics Focus: The Undertray

While the majority of the team was away competing with M15-R in Europe over the last 3 months, a smaller crew, made up mostly of junior team members, back at the workshop have been busy starting manufacture for M16 before FSAE-A this December. In the aerodynamics section, we’ve been focusing on the preparation of one of the most important – yet mostly hidden – features of our downforce package: the undertray.

Overview
The undertray is not only the largest component we look after, it’s also the most aerodynamically efficient, producing nearly 9 times more downforce per unit of drag than our rear wing. Since the major rule changes in 2015 which cut down heavily on wing sizes, a focus on optimisation of the undertray has made it a critical part of our package as a component which now produces a significant portion of our total downforce on its own.

As the name suggests, the undertray bolts onto the bottom of the chassis to form the floor of the vehicle. This location is important because being in such close proximity to the ground means we are able to take advantage of the phenomena of ground effect, which essentially magnifies the suction force produced by airflow beneath the undertray surface. Bringing these surfaces closer together accelerates the flow, resulting in lower pressure and a greater amount of undertray downforce.

Design
Our undertray assembly consists of four major elements: the central flat section in the middle, outboards and tunnels to the sides, and behind it, the diffuser. The tunnels and outboards are both features which help confine air to the centre of the undertray, ensuring that suction pressure isn’t lost to the outer environment. Peak downforce is achieved towards the rear of the centre section, where the pressure is lowest. Our development of the undertray concept has also introduced strakes, which condition the flow to achieve optimal diffuser performance.

The diffuser, at the rear of the car, acts to slow the flow back down again. It’s important to re-equalise the large pressure difference that the undertray creates between it and the ambient pressure which surrounds the car: without a diffusion system in place to gradually slow the air down, significant drag would be induced by the forced, sudden mixture of high and low pressure airflow. Additionally, the expanding air exiting our diffuser is able to interact with the rear wing improving their combined performance.

Analysis
Development in aerodynamics is driven almost entirely by Computational Fluid Dynamic (CFD) analysis, with licenses for ANSYS CFX generously provided to us by Leap Australia. This allows us to simulate small changes to the car with a turnaround time of around 24 hours from design to evaluation, saving countless hours of manufacturing which would otherwise be required to examine every concept. The undertray is certainly no exception to this rule, as manufacturing it from scratch is a process that takes months of careful effort.

Manufacture
The last few weeks have been a part of that journey, with the Melbourne-based team beginning the construction of M16’s undertray mould. A smooth mould surface directly influences the quality of our final part finish, and significantly reduces the amount of work that needs to be done post-layup, so we have been working to perfect the mould in preparation for layups happening in the next couple of weeks.

Uniquely among our aerodynamic components, the undertray layup includes both carbon fibre and Kevlar, ensuring the floor can withstand the abrasion and wear that inevitably affects the car out on track. Once the layup process is complete, an assembly of individual undertray components is undertaken before it’s mounted onto the chassis, ready for on-track validation.

We’re excited by our progress on M16’s manufacture in the aerodynamic department and the new undertray will hopefully help us deliver even better balance, control and most importantly speed out on track in December.