Sunday, July 26, 2009

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.


The Dandy Highwayman said...

I'm confused. You say that in 1998 the maximum track was reduced from 220cm to 180cm. Then in 2009 it was widened from 140cm to 180cm. So sometime between 1998 and 2009 it shrank further and now we're back at the 1998 width? Then why are they directing flow outside the wheels now when they directed it inside the wheels in 1998?

Gordon McCabe said...

In 1998, the distance between the wheels decreased to 180cm (1800mm). In 2009, the width of the front wing increased from 140cm (1400mm) to 180cm (1800mm), matching the distance between the wheels.

The Dandy Highwayman said...

Ah, I missed the fact that it was the axle track changing in 1998 and the wing changing in 2009. That makes more sense now, thanks.

Bob said...

So why are te rules being changed all the time? To make the competition fairer? To prevent it from becoming boring and keep the public happy?

Those aerodynamics are quite something. I allways imagine it is a science based more on experience then on theory.

Gordon McCabe said...

Sometimes the rules are changed to slow the cars down, sometimes they're changed to try to improve the quality of the show.

Sean said...

"One rival engineer believes he can see what the McLaren's problem is - but obviously isn't about to spill the beans"

I don't believe that for one minute, sounds to me like someone soliciting business.

So now you have explained why Mclaren are now at the front end of the grid, any thoughts on why Brawn are heading to the back?

Gordon McCabe said...

I'd guess that it's the front-wing endplates Brawn introduced in Turkey. It was probably perfect for the very high temperatures in Turkey, but drops the tyres temperatures below their operating range at most other locations. Just at a guess, mind.

Sean said...

I think it boils down to economics and the old wonga.

Brawn did the aerodynamics last year, and got a great start this year, Mclaren only started last Dec by all accounts.

And no matter what the world or paddock understands the problem to be wind tunnels are expensive, tedious and time consuming work, even if you have one of your own, which both do.

survival of the fittest.