Mercedes W02 preseason side pod & exhaust update

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After a slow start to the 2011 campaign Mercedes GP brought along the long expected changes to the W02 at the last Barcelona test. We have already covered the front wing (http://scarbsf1.wordpress.com/2011/03/11/mercedes-w02-new-front-wing-analysis/). But more crucially was the revised sidepod and exhaust package. Mercedes have gone their own way with the design of the W02, with its short wheelbase set up and the resultingly bulbous mid section. Contrary to my expectations the new sidepodexhaust package was not as unconventional as expected. Which still leaves some questions over some design choices on the car or the permanence of the solution shown in Barcelona.

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Firstly the new sidepods are formed of a completely new moulding, common to several other teams the sidepod bodywork is one piece and is not formed by add-on sections to the monocoque. Even though the general shape appears the same as the launch format, the overhead view shows the sidepod inlets are angled inboard slightly. Although the bigger visual change is the exhaust and cooling arrangement. Uniquely the exhausts are sited halfway along the sidepods, exiting where the sidepod is nearly at its widest and starts to taper in to the coke bottle shape. Unlike Red Bull and Ferrari Mercedes have not extended the exhaust towards the diffuser, instead the exhaust blows over a long length of open floor. A small vane redirects the flow inboard of the rear wheels and into a coved section that sends the exhaust flow under the diffuser to be more effective at creating downforce. To keep the bodywork safe from its close proximity the exhaust pipes numerous grilles are moulded into the sidepod. The rearmost of these are outside the exclusion zone for cooling outlets, but the larger removable grille appears to be at odds with the bodywork rules. Perhaps the low exhaust position (below the 100mm above the reference plane) allows the grille to be regarded as the opening for the exhaust. Equally these could have been precautionary fitments for overheating (which blighted the cars earlier tests) and might removed for the Australian race.
Having the exhaust so far forward does not make the exhaust act like Renaults Front-Exit-Exhaust, nor like Red Bulls ducted set up. The exhaust gas will lose energy as its merges with the freestream airflow before it reaches the diffuser. Its exactly this energy that teams want to exploit to drive more flow through the diffuser for more downforce. So why is the set up a less efficient solution? Potentially there are several reasons, last year Mercedes struggled with overheating bodywork, unable to get enough supply of the permitted Glass Ceramic Composite (GCC) material used to protect the phenolic composite of the cars floor and bodywork. When they ran their blown floor, the heat, simply melted and warped the bodywork. Its unlikely supply of the material is still an issue, but keeping the bodywork cool and the nature of the exhausts might be the problem.

All three Mercedes teams (McLaren, Mercedes GP and Force India) all had issues with sensitivity of the car when run with EBDs in 2010. McLaren found the cars balance changed significantly on and off throttle, while Mercedes found that the exhaust plume would touch differing parts of the bodywork in different sessions and even differed between cars. This suggests that the exhaust plume was less than predictable. Where-as CFD and wind tunnel tests use a simulation of the exhaust blowing, perhaps the knowledge of what the exhaust flow is actually like is missing. Strangely this seems to be a very Mercedes engine specific problem. Being too aggressive with the exhaust blowing and too specific with the heat shielding makes the car throttle-sensitive and prone to overheating bodywork. McLaren have more problems with their EBDs in pre-season testing and Force India have yet to truly shine, with an otherwise good looking design. If this is the case, then the teams either have to lose potential downforce by having to use a less aggressive EBD solution or suffer the sensitivity problem. Its hard to be clear how easy an unpredictable exhaust plume might be to solve, its not likely to be a solution teams and engine suppliers have had to look at before.

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Elsewhere on the sidepods the cars pod vanes have been enlarged from the truncated versions seen in the cars early tests. Why the team would be run stunted versions of long standing designs is again part of the confusion around the W02 debut. The pod vane features an unusual outwards bulged lower section. This mimics the shape of the short launch spec vane. I presume this is mated to the sidepods undercut to feed more flow around the sidepod and over the diffuser. Along with the new undercut the car sports new serrated bargeboards and the complex shaped under nose vanes from late last year have been revised with the more common nose cone mounted vanes.

One last unsolved conundrum is the side impact protection on the sidepods. Normally teams pass the side impact tests with two pairs of crash beams, one upper pair above the sidepod inlet and a lower pair in line with the floor. Each of these pairs are formed of one larger carbon beam and a smaller one to spread the load over a wider area of the chassis. Rules demand these parts are not exposed to the exterior airflow and must be covered by bodywork. These structures are quite heavy and unavoidably raise the cars Centre of Gravity (CofG). This years car sports something appearing very much like a side impact structure passing horizontally across the middle of the sidepod inlet. This would be beneficial as the weight is that much lower down and better for a low CofG, a high CofG was a problem that afflicted the 2010 W02. Meanwhile at floor level the structure is unusually slim, which is better for aerodynamics.
But this mid placed structure appears to be in contravention of the rules as its exposed to the airflow. The FIA have started to be stricter with teams interpretation of these structures, so its hard to understand why this set up has been accepted. Possibly the structure is covered by vestigial bodywork to bypass the rules, but this detail did again promote some of my ideas that the sidepods were to be more unconventional. If allowed this year, we can expect the FIA to stamp out this set up for future years. Of course teams cannot copy this, as crash structures are homologated for the year, and cannot be changed.



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