Williams and Caterham have created a stir during winter testing as both the Williams FW35 and the earlier launched Caterham CT03 feature elements within the exhaust channels. In both cases, the exhaust tailpipe exits into a channel that is part of the sidepod's aerodynamic shell. The channel is designed in a manner that exhaust gases are curved down towards the car's floor as much as possible. Such exhausts, named Coanda according to the identically named principle that a fluid (in this case the exhaust gas) tends to be attracted to a nearby surface (in this case the downward sloping, U-shaped exhaust channel. It would of course be much easier to point the exhaust pipes downward and extend them further towards the diffuser, but that was banned when the FIA tried to eliminate the use of exhaust gases for aerodynamic purposes.
The issue now is that there is discussion about whether these two particular designs comply with article 5.8.4 of the FIA technical regulations. This defines a cone behind the exhaust tailpipes where it is not allowed to have any kind of bodywork.
a) Shares a common axis with that of the last 100mm of the tailpipe.
b) Has a forward diameter equal to that of each exhaust exit.
c) Starts at the exit of the tailpipe and extends rearwards as far as the rear wheel centre line.
d) Has a half-cone angle of 3° such that the cone has its larger diameter at the rear wheel centre line.
Furthermore, there must be a view from above, the side, or any intermediate angle perpendicular to the car centre line, from which the truncated cone is not obscured by any bodywork lying more than 50mm forward of the rear wheel centre line.
In the case of Caterham's design, their exhaust channel with internal vane can only possibly comply with this rule if the turning vane is underneath the 'exhaust cone' as defined above. It seems however that this is not the case, as the FIA has apparently contacted Caterham to note that it believes the design is not allowed.
The Williams solution is even more interesting, as they have an 'open bridge' over the exhaust channel which attempts to bend exhaust gases down. While the small spacer in the middle of the bridge may possibly make this design legal, again, the FIA thinks otherwise as Williams was approached as well with the exact same message.
Both teams however stand firm that their designs are legal, but it will be interesting to see how long they will hold on to the solutions, or perhaps immediately not take the risk and test with a more conventional channel.