Williams had a solid pre-season testing and covered a large number of laps, particularly on the last two days, which was critical given each race driver only had one day in the car ahead of the new season. There were some interesting comments in the media about the wind sensitivity of the car, and this topic will be considered in a future post. Looking at the team’s FW43B, there are a number of significant changes to its predecessor.
In the bargeboard area, the main bargeboard has been shifted back considerably, particularly at the leading edge, which is now much straighter. This delays the outwash from the bargeboard, and therefore the mid-lower tyre wake may be further inboard. However, in doing so, the ‘roll-over’ of the upper tyre wake as the lower-mid wake is pushed outboard is delayed, improving the onset flow to the rear of the car. This benefits the rear wing, as well as the diffuser, given that the air that flows over the top of the sidepod’s downwash slope has higher energy.
By pushing the bargeboard rearward, there is more space on the flat area ahead of it to introduce additional turning vanes that manage the lower wheel wake, while not impacting the ‘roll-over’ effect mentioned above. This is not something that Williams has yet taken advantage of, but a number of other teams have a series of devices here, McLaren being one of the more extreme with four sets of mini-bargeboards.
The number of elements parallel to the rear of the main bargeboard has been increased by one to three. It appears as if the outboard one on the old bargeboard has been split into two, which should improve the flow health on its underside. These elements are loaded by the downwash from the Y250 vortex, and generate local load as a result.
The height of the vertical sidepod vanes has been extended, which should reduce drag by increasing the height of surfaces with a forward-facing suction peak. This will be at the expense of some local load thanks to the removal of two upwashing sections, but could be an efficient trade-off overall. Finally, two L-shaped devices have been introduced along the floor edge. Their vertical part should again reduce drag via the same mechanism just mentioned, while the horizontal part will extract more load from the edge of the floor by increasing the mass flow rate on the underside.
Looking at the back of the car, the team has reduced the height of the two upper rear brake duct elements compared to 2020. This will likely reduce the upwash in this area, and potentially reduce drag as there is less stream-wise momentum loss. However, there will be a loss of support to the rearward face of the rear brake duct externals, loading this up and increasing the adverse pressure gradient at the trailing edge, so the team must have had some margin on last year’s car.
Further inboard, the shape of the rear top wishbone has changed, seemingly to more of a lifting profile compared to 2020. This will, of course, result in a load loss locally, but it will also increase the downwash on to the rear wing leading edge, loading this. Again, given this is a forward-facing surface, there should be a drag reduction. If the downforce loss is not too significant, then this may be an efficient development on average over the season. The other factor to consider is the flow health of the rear wing since, by loading the leading edge, the adverse pressure gradients on the mainplane and flap are worsened.
Finally, during testing, Williams tried two different bodywork exits. The first looked very similar (if not the same) to the smallest version from last year, while the second had a lower central trailing edge, but a higher trailing edge outboard. This change in trailing edge height looks to correspond to more/less downwash for the rear wing, with the V2 option loading up the rear wing more centrally and less outboard. This may be due to the rear wing having more flow health margin (quantified using wall shear stress) in the centre than further outboard.
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