The solution to McLaren's aerodynamic problems

McLaren finally appear to have solved their crippling aerodynamic problems, the main symptom of which was a diffuser which stalled at high speed. Whilst the package of modifications introduced in Germany included a new double-diffuser and front wing, Mark Hughes reports in The Sunday Times that "the most significant part of the car’s upgrade at the last race is believed to have been the redesigned endplates, which feed the airflow to the modified underfloor in a way that keeps the flow much more consistent through a wider range of speeds."

This solves a long-standing mystery. At the first race of the season, Hughes relayed the following intriguing analysis of McLaren's difficulties:

One rival engineer believes he can see what the McLaren's problem is - but obviously isn't about to spill the beans: "There are a couple of cars, and McLaren is one of them, that in their treatment of one fundamental part of the airflow regime show that their aerodynamicists have come to the fore during the era that's just finished, where gains were made in incremental changes. If they had experience of the previous generation of cars they would have known immediately how to treat one particular area that is probably even more important than the diffuser under these regs." It's something he reckons could be cured very quickly once recognised and is not a fundamental part of a car's design. So don't expect the McLaren to be bad indefinitely. (MPH, Autosport, April 2nd, p23).

The most significant prior change of technical regulations was that introduced for the 1998 season, when the maximum track of the car (the lateral distance between the wheels) was decreased from 220cm to 180cm. One aerodynamic consequence of this was that the front wing airflow now interacted with the turbulent and rotating airflow around the front wheels. The response of the teams then was to re-direct the airflow by curving the front wing endplates towards the inside of the front wheels. The 1998 rule-changes were supplemented in 2001 by the raising of the minimum height of the front wing from 40mm to 100mm, thereby reducing the ground effect of the front wing. This combination of raised front wings, with endplates curving to the inside of the front wheels, defined the previous era of incremental aerodynamic development.

For 2009, however, the front wings were widened from 1,400mm to 1,800mm, spanning the width of the car, and were lowered from a height of 150mm to 75mm. The latter has re-introduced ground effect into front wing aerodynamics, and the wider wing-span has caused designers to use endplates which re-direct the airflow around the outside of the front wheels. One presumes that it is these changes in front wing dimensions which McLaren failed to adapt to.

One of the functions of the endplates is to direct airflow around the front wheels to minimise drag. However, the endplates also reputedly generate four or five separate vortices, which can be used to re-direct turbulent air created by the front wheels, and prevent that turbulent air from feeding the underfloor of the car. If such turbulent air were to feed the underfloor, it could cause the diffuser to stall at high speed.