Wednesday, November 25, 2009

Lewis Hamilton and instability

The Eurofighter-Typhoon jet aircraft is so unstable, that it cannot be controlled by a pilot alone, and requires the intervention of electronic control systems to prevent it from stalling in flight. Hold that thought in your head as you read the description of Lewis Hamilton's raw driving ability, which McLaren director of engineering, Paddy Lowe, gave to Autosport's Mark Hughes in 2008:

"He's tremendously good at controlling a car in oversteer. We saw that from the first moment he got in our car. We saw the data, and on every entry we could see there was a massive correction on the steering, and our normal drivers would have been bitching like hell that the car was undriveable, yet he didn't even pass comment. So with a driver like that, you're better equipped to push the boundaries to new levels. Speaking generically of that characteristic, a lot of the performance limit of a car is set by stability; if you can't hang on to it, you will have to introduce understeer in that zone. But if you have a driver better able to deal with oversteer in those zones that induce it, then you will have a less-understeery car elsewhere and therefore more total grip over the lap. The great drivers over the years - Senna, Schumacher, Mansell - have all had that ability. Like for like compared to other drivers, they want more front end."

There are two particular concepts in Lowe's analysis which need to be distinguished: corner entry oversteer, and entry instability. To understand Hamilton's unique capabilities, we therefore need to briefly introduce some definitions from stability theory.

If a car (or aircraft) is initially in an equilibrium state, and there is a transitory control input (or external disturbance), a stable vehicle will return towards its initial equilibrium state of its own accord, whilst an unstable vehicle, in the absence of any further control inputs, will diverge even further from the initial state. To be precise, the first condition is sometimes called static stability, and the latter condition is called static instability. Whether a vehicle is stable or not can be speed dependent. For example, a bicycle is stable at higher speeds, but is unstable at low speed, requiring continuous corrective inputs from the rider to remain vertical.

In the case of an F1 car, an initial steering input induces an initial slip-angle in the front tyres, which induces an initial direction change (a rotation about the vertical axis, called a yaw motion). If an F1 car is statically stable, the car will then return towards a state of zero yaw. If an F1 car is statically unstable, an initial steering input would not just induce an initial slip-angle and change of direction, but an ever greater change of direction (in the absence of corrective action from the driver), giving the vehicle a tendency to spin on entry to every corner. In particular, if an initial steering input provokes the car into oversteer, then that oversteer will increase the initial direction-change. Hence, the driver must supply opposite-lock steering corrections to reduce the direction-change. Oversteer and instability are therefore related. To be precise, turn-in oversteer is a statically unstable handling characteristic, albeit one which Lewis Hamilton is clearly capable of dealing with.

There is a further nuance here, however, because even statically stable vehicles can be either dynamically stable or dynamically unstable. After an initial input, the attitude of a dynamically stable vehicle will oscillate with simple harmonic motion of decreasing amplitude about the initial attitude. In contrast, in the case of a dynamically unstable vehicle, whilst its attitude will at first return towards the initial state, it will then oscillate with increasing amplitude about that initial attitude, leading to a loss of control (in the absence of corrective inputs). These two behavioural characteristics are also sometimes dubbed positive stability, and relaxed stability, respectively. The Eurofighter Typhoon possesses dynamic instability (relaxed stability).

If an F1 car was statically stable, but dynamically stable on turn-in, an intial steering input would create an initial direction-change, and the direction-change would then oscillate with decreasing amplitude. If, however, an F1 car was dynamically unstable on entry to a corner, then the direction-change would oscillate with increasing amplitude, (in the absence of corrective action), giving the vehicle a tendency to spin.

Rear-end instability on corner entry is reportedly the handling characteristic which Jenson Button struggles most to deal with, but as his Brawn team-mate, Rubens Barrichello, demonstrated this year, it is a characteristic which different drivers can cope with to different degrees. Perhaps, then, the type of instability exhibited at times by the Brawn in the second half of the 2009 season, was merely the dynamic instability of a statically stable car.

Judging from Paddy Lowe's remarks, one can speculate that not only is Lewis Hamilton able to cope with such dynamic instability on corner entry, but to a degree unique amongst his peers, he is able to supply the corrective inputs necessary to prevent a statically unstable car from spinning on corner entry.


Sean said...

Or maybe he sees the future?

Gordon McCabe said...

Yep, those corrective inputs will be anticipatory as much as reactive.

Sunil Dongre said...

Copy that about Kimi Raikkonen.