Following visits to two unusual circuits in Monaco and Montreal, Silverstone marks the return to representative tracks, allowing Formula 1 teams to judge car developments properly.
Unfortunately, rain blighted yesterday's running and prevented teams running as many parts as they would have liked. But it did not stop Lotus from trying a heavily-updated aerodynamic package and reintroducing its passive drag reduction device.
On Friday, Romain Grosjean's car was fitted with the new aero package. This primarily comprised new sidepods and engine cover, which follow the Williams 'figure-hugging' trend.
Rather than the engine cover forming a rounded cooling outlet at its tail, it is slimmed down to wrap around the engine and gearbox, with the area around the gearbox being notably low and tightly-packaged.
As a result, the spine of the engine cover is reduced to a flat shark fin to meet the rules on minimum bodywork dimensions.
Other modifications included detail changes to the suspension profiles and the front wing.
A close look at the 'device' on Raikkonen's car © XPB
On Kimi Raikkonen's car, the as-yet unraced passive drag reduction device, which was first tested at Silverstone a year ago and cropped up during Friday practice on an irregular basis during the rest of the season, reappeared.
Unlike the F-duct in 2010 or the double DRS used by Red Bull last year, this drag reduction device is a legal way of stalling the rear wing for increased top speed. The system is closer to the F-duct in operation as it uses a fluid switch to alter the airflow through the ductwork.
The two inlets on the roll hoop have been opened up and feed an X-shaped duct above the engine. This duct is the fluid switch. It has no moving parts but above certain speeds (approx 220km/h) the flow switches to send the airflow from the inlets up under the rear wing, rather than exiting neutrally under the monkey seat wing.
By blowing up under the rear wing, the airflow is interrupted and stalls. This reduces both downforce and drag to boost top speed.
The beauty of this system is that, unlike DRS, it works every time the car goes above the switching speed rather than only in the DRS zones. Providing an 8km/h boost, this allows the team to run more wing for faster cornering without compromising straightline speed.
Unrelated to the DRD, Raikkonen's car also has new brake ducts, which encourage more air to flow through the wheel to improve the aerodynamic wake of the front wheels.
Mercedes ran a major aero update, a significant part of which was the new floor and diffuser. Allied to this was a revised front wing flap.
The drivers reported the car felt unsettled over Silverstone's bumps and breaks in its surface, so the car was then returned to its standard specification.
The front wing is interesting as it is the first time the Mercedes has run with a full width flap. Normally, the flaps in the front wing join the centre section with a simple coved joint.
Now, the flap extends all the way to the neutral centre wing section. This is not likely to be a downforce producing part, but a flow-control solution.
Mercedes has been quick but has been unnerving its drivers © XPB
The inner edge of flap forms a spiralling vortex, which is a key part of the airflow structure leading to the front of the floor. This airflow path is known as the Y250 vortex, as its aligned 250mm off the car's centreline.
Aerodynamic parts are not allowed outboard of this zone so these parts, which include the under nose turning vanes and the splitter all work to increase flow under the floor for more downforce from the diffuser.
Mercedes also arrived with a multitude of revised aero details, including front brake ducts. As with Lotus, these duct encourage airflow to pass through the wheel without having a cooling effect on the brakes to alter the airflow passing out of the wheel and prevent turbulence forming behind the front tyre.
Red bull's changes were purely ones of detail, with subtle revisions to the sidepods, exhausts and diffuser.
Red Bull did some rear-end tweaks © XPB
On Friday morning, the car ran with both flow-visualisation paint on the sidepods and an aero sensor array ahead of the rear tyres. This will be to assess the impact of the slightly revised sidepod and exhaust set-up.
In repackaging the exhausts, Red Bull has resorted to heat reflective coating on the exhausts to protect the bodywork. The white coating is a solid ceramic material, which melts as it is applied by plasma spray, resulting in it being literally welded to the exhaust.
As part of the change in airflow along the flanks of the sidepods/floor, the outer diffuser sections have been curved differently to help seal the diffuser with the exhaust flow.
Surprisingly the updates tried in Canada were not brought to Britain. Instead, the F138 practiced with a revised rear and front wing, as well as continuing tests with the longer Coanda sidepod.
The rear wing had a new endplate and revised beam wing. The endplate follows the popular solution with a vertical slot in its leading edge.
Ferrari seemed to drop its Canada updates for new ideas © XPB
This encourages more airflow to pass through the rear wing from the edges. As with the slatted trailing edge to the Ferrari rear wing, this increase in airflow reduces pressure to increase downforce. Also helping in this process is the new beam wing, which forms a curvier more twisted profile.
At the front, the wing gained new R-shaped vanes to the inside of the cascades. These vanes create a vortex that is aimed towards the upper\inner corner of the front tyre to offset the turbulence forming that disturbs the airflow further back along the car.
Following Red Bull's idea from the last race, Force India introduced a series of shark teeth to its front wing. Four triangular fins were added to the wing's main plane aligned with the four fences that sit beneath the wing.
Their purpose has been described as either as fenders to prevent marbles clogging the wing or aerodynamic devices. My opinion is they are there for aerodynamic purposes. As they are aligned with the under wing fences, their function to prevent clogging would be achieved by the fence any way.
The Force India now has teeth © AUTOSPORT
I understand that they are vortex generators. As we have already described with other teams, sending the right airflow up and over the front tyre has a huge effect on its wake and the airflow passing back along the car.
Looking closely, you can see the fins are not aligned to the airflow as the fences are. It is this angle that creates the vortex. These narrow strings of spiralling airflow pass up over the wing and over the top of the tyre, altering the turbulence created by the rotating tyre.
With Marussia's aerodynamic development proving muted so far this year because of the need to devote resources to 2014, the appearance of the blown front hubs initially seemed surprising.
It was Red bull who first forced airflow through the hollow front axle last year, but after its design was protested, it was left to Williams to develop a legal version this year.
Marussia is looking at blown hubs © XPB
The front brake ducts collect airflow and pass it through the hollow axle holding the wheel on. This airflow is not for cooling, but as always for a beneficial aerodynamic effect on the front tyres wake.
To make the system legal, the air must pass inside a non rotating duct inside the hub and exit no further outboard than the wheel nut. This gives the axle its distinctive large diameter hole inside the wheel nut.
Marussia is probably able to justify the investment in the new parts, as the development will still be relevant next year despite the other regulations changes.