Analysis: DRS Activation

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Since its introduction in 2010 the Drag Reduction System (DRS) has gone through a series of evolutions in how the team actuate the movable rear wing flap. Having replaced the adjustable front flap, teams have all switched to hydraulics to power the opening of the flap, where as the front flap angle system introduced in 2009 was commonly achieved with electric motors and only a few teams employed hydraulics.

Hydraulic and Mechanical set up
For DRS to work the rear wing flap needs to pivot upwards, from a point 5mm from its trailing edge. When opened the flap must open up the slot gap to the rear wing main plane to a maximum of 50mm. The flap pivot arrangement is simple pegs near the flaps trailing edge locate in bearings in the endplate, allowing the flap to pivot open and closed with little friction. This is important the flap must close by air pressure alone, DRS does not employ a powered close. This is in case of a DRS failure the wing should return to its normal (i.e. downforce producing state) and not allow the driver to enter a turn with the flap open (i.e. reduced downforce).
The DRS opening mechanism employs the cars normal powered electro\hydraulic control systems. Hydraulic pressure is routed to a Moog valve, this valve will open by instruction from the cars SECU and pressurise the pipe work leading to an actuator. The actuator is a piston inside a cylinder that will move rapidly when pressure is applied. When the SECU asks for the DRS to close the rear wing flap the Moog valve is closed to allow pressure inside the cylinder to drop and release the pressure on the flap. The pressure of the oncoming airflow is enough to snap the flap back into the closed position.
All of the hydraulic hardware and control strategies are the same as the other hydraulically control systems around the car, so the DRS circuit is simply an extension of the hydraulic circuit managing the gearbox functions (gearshift, clutch, differential and reverse gear).
Drivers will request the DRS to open the flap via a variety of methods, often a button or paddle on the steering wheel or in Ferrari’s case a small foot operated button, by the brake pedal. Its important that the driver closes DRS before braking for the approaching corner, so teams will have fail safes to close the flap, if the throttle is lifted, brake is applied or other deceleration is detected.
DRS actuation types
Push up

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On its introduction, many teams still used a vertical pylon to help support the top rear wing, wit this design it was logical to mount the hydraulic actuator inside the pylon and have the actuator push up and push the flap open from the bottom.

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With this packaging the hydraulics coming up from the gearbox, passed neatly inside the pylon and were largely unaffected by the top rear wing being removed.
This remained a valid means of actuation, but other demands made teams move away from pylon support wings, namely the blockage they have on the rear wing and the surface interference where the pylon meets the underside of the wings surface.

Pods & Rockers

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Even on its debut some teams did not employ the ‘push up’ solution, Red Bull for example placed the hydraulic actuator in a pod mounted above the rear wings top surface. The actuator pulled on a rocker that increased the leverage of the piston, the rocker then being connected to the leading edge of the flap by link.

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For this solution the hydraulic lines need to pass from the gearbox, across the beam up through the rear wing endplate, across the wings main plane and upwards to reach the actuator. This requires the hydraulic dry-break connectors to be disconnected to remove the rear wing, its never ideal to open the hydraulic system if it can be avoided. Also the rocker requires extra links and bearing to make the system work. But this packaging is small handicap for a more effective solution aerodynamically speaking.
Albeit the pod is a relatively large component, it is placed in a more aerodynamically beneficial location. Having the pod mounted above the rear wing removes the interference with the harder working underside of the wing. With the rocker the wing will open fractionally quicker and allows some access to the mechanism for checks and maintenance.

Pod Pull Type

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The evolution of the Pod\rocker set up, has been to simplify the mechanism and reduce the size of the pod. Thus the pull type pod has been created, this is now the most common system. It does way with the rocker, so the piston simply retracts and pulls on the link to the flap in order to open wing. Without the mechanical advantage of the rocker, this system presumably requires more hydraulic effort for the same speed of opening, but as high pressure hydraulics are already in use in the gearbox , this is not much of an issue for the hydraulics and mechanical designers.

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Mercedes Cable
On a tangent completely different to all the other teams Mercedes have used alternative DRS system up until this year. To keep the hydraulic system simple, the valve and actuator are mounted atop the gearbox, then the movement of the actuator is transferred to the rear wing flap via cables running inside the wing. This keeps the entire rear wing free of aerodynamic interference of the DRS. However the cables probably add some friction and thus delay to the wings opening and closing.

DRS Mechanism Failures
Mercedes had a failure of their DRs when the dry break coupling came away from the valve\actuator, this locked the hydraulic pressure in the system preventing the wing closing. Subsequently Mercedes redesigned the valve\actuator package to allow pressure to bleed from the system when either the valve closes or the coupling fails.
Ferrari have adopted the Pull type system and the failure for Alonso in Bahrain, it appears that the mechanical ‘stop’ in the system failed, this allowed the wing to go over centre and with the air pressure now underneath the wing prevented the wing flipping shut. This isn’t a fault of the pull system specifically, rather than a mechanical failure of part of the system. In normal use it shouldn’t be possible for the wing to go over-centre, so when it did this for the Ferrari it subsequently over-stressed and broke the actuator mounting, such that DRS could not be used through tout the race.