Lotus: DRS Device Analysis

For three races now, Lotus have had a prototype ‘drag reduction device’ fitted to the cars rear wing. This is a system of ducts and is not linked to the normal DRS that moves the rear wing flap. Mysteriously described by Lotus as the ‘prototype device’, most people in the paddock are still calling it DDRS (Double DRS), as although its not part of the DRS, its aim is to reduce drag on the straight for more top speed.
The Lotus device can be recognized by the cars sporting two roll hoop inlets and ductwork exiting the engine cover between the upper and lower rear wings. This was first tested In Friday free practice in Hungary and again in Germany, albeit only on Raikkonen’s car. For the Belgium GP, the system has been applied to both cars, but the wet Friday practice session means the team have elected not to run the device for Qualifying or the Race.

The device came about from the Lotus request for clarification on Mercedes DDRS, which is linked to the rear wings DRS to stall the front wing. This system was controversial as switchable drag reduction systems were effectively banned after the F-Ducts of 2010. However the rules to counter this were largely worded to reduce the stalling slots in the rear wing and the driver interaction in turning the system on or off. The Mercedes system sidestepped these rules by having the stalling slot in the front wing and the system switch by the DRS opening (an allowable moveable aero device). When in Bahrain the FIA gave clearance for other drag reduction systems, whether linked to DRS or not, Lotus announced they would take advantage of the clarification and develop their own device.
Unlike Mercedes whose system stalls the front wing to balance the aero when the DRS rear wing is used in qualifying (and the race); The Lotus system is passive and not linked to an external switch. Instead the system uses increasing airspeed to send more flow to slots under the wing to stall the airflow and reduce drag (and downforce). Having the passive system means that the Lotus device can be used to stall the wing above a certain speed on every lap, meaning the small c5-8kmh speed advantage is available on every straight and fast corner. With the system being tuned to airspeed, the wing can be designed to stall at speeds high enough to allow fast corners to be taken with the rear wing stalled. At these speeds the diffuser provides enough downforce for cornering and the rear wing in not required for aero load. Typically teams will want this stalling to occur at speeds of over 250kmh.

Construction

lOTUS_ddrs_assy

The system is formed of two roll hoop inlets feeding a fluid switch, and then two ducts tee off, one to exit nuetrally and one “L” shape duct to blow the rear wing. The inlets are clearly visible either side of, and slightly behind the roll hoop inlet, they are reminiscent of the 2010 F-Duct, although they are permanently bonded to the roll hoop structure, so even when the car is running without the device, they are still in place. These inlets form ducts that pass up and over the airbox snorkel to merge into a single duct that then passes down to the fluid switch above airbox. Indeed the pictures from Spa show that part of this duct is bonded to the airbox before a tail section of duct is bolted to it. This is where the other conenctions to the fluid switch is hidden; as we can see the fluid switch splits into two exit above the airbox, with one exit above the other. The outlets are formed by machined metal flanges, to ensure that the connection to the subsequent duct work is air tight.

lOTUS_ddrs_stalled
The neutral duct exits over the centre of the beam wing, a small Y75 winglet (monkey seat) is formed around the exit to reduce pressure at its trailing edge. Visible inside the exit of this duct is a smaller duct exiting within, so the apparently large cross section central duct may be a double walled structure housing two exit ducts.
With the neutral duct’s outlet blowing over a revised beam wing, it’s possible that the effect of the Device when not stalling the rear wing is to aid the upwashed airflow coming up under the centre of the car, to create downforce. Albeit this would be an inefficient way to create downforce, it is probably a way for the system to contribute to laptime when the upper rear wing is not stalled.
Teed off from the from the fluid switch duct is the “L” duct, this is far smaller in cross section than the central duct, and would offer a lot of resistance to airflow, most likely to encourage airflow at lower speeds to pass into the neutral ducts exit, rather than up to the rear wing
The “L” shaped duct has the 90 degree bend, not for aero reasons, but as a workaround to the zone ahead of the rear wing not being allowed to have bodywork. This was part of the 2010 F-Duct ban on bodywork (shark fins and F-Ducts) reaching the rear wing.
Other than the join of the “L” duct to the underside of rear wing’s main plane, the top rear wing and endplates appear to be the same as the non-device set up. The “L” duct meets the wing, but does not blow into it. Unlike the 2010 F-Ducts the stalling slot is not a lateral slot across the wings span, but instead four small vertical slots in the “L” duct, these blow sideways where the “L” duct meets the wings underside. Being part of the “L” duct and in the middle 15cm of wing, they are exempt from the minimum radius rule that was introduced to ban F-Ducts.
It’s clear the system has had to be compromised to fit into the post F-Duct rules, but in every sense is meets the regulations and would be hard to declare illegal without a new clarification of the rules being issued by the FIA.

How does it work?
There are two aero effects being used with this device, the method to ‘switch’ the blown effect on above certain speeds and the effect to stall the rear wing.

When Neutral, flow passes through the switch to the exit by the beam wing

When Neutral, flow passes through the switch to the exit by the beam wing

The ‘Switch’ effect, as previously described is passive, with no moving parts or external interaction. There is clearly something clever going on with the ductwork inside roll hoop inlets to the fluid switch. Its not clear if this fluid switch is somehow linked to the engines airbox and the pressures created within. But I increasingly suspect hat engine airbox pressure provides the “switch effect” to the fluid switch. Perhaps at higher speeds and with full throttle the pressure difference inside the airbox sets off the fluid switch to alter the flow from the neutral to the “L” duct.  Technically this is legal, but other teams might take a different view off this if indeed that is the case.

When the airspeed\airbox pressure change at high speed, the switch 'trips' to send the flow to the "L" duct to stall the wing

When the airspeed\airbox pressure change at high speed, the switch ‘trips’ to send the flow to the “L” duct to stall the wing

As speed or airbox pressure increases the flow passes into the fluid switch, to change the flow from neutral to stalling.  Without seeing the remaining ductwork, this is purely speculation, but the system appears to more complex than simply increasing air pressure in the duct eventually leading to stalling at higher speed.

Sideways blowing slots in the “L” duct create the stalling effect

Then the stalling effect is created by the two pairs of slots in the last section of “L” duct, these are blowing sideways across the wing, the effect sets up a delta shaped pair of vortices that turn the laminar airflow passing under the wing into turbulent flow. This would stall a large section of the wings airflow, reducing downforce and with it the drag induced by the highly loaded wing. We can see evidence of this effect from Hungary when one of the practice runs used flowviz paint the distinctive “V” shaped area of stalled flow emanating from the slots could clearly be seen (these pcitures are posted on the F1Technical.net forum). The stalled flow only appears to cover about half of the wings underside, the limitation of the vertical slots rather than a wider slot as used in 2010 being the restriction.

Summary
We’ve seen Mercedes trial a similar device on their car at Spa, with other teams rumoured to have a system ready to test subject to conditions in Spa and its benefit at Monza. The gain of the system is small, but anything that improves aero efficiency will be useful, whether to allow a higher top speed for a given downforce level, or vice versa, a gain in downforce level for a given top speed. The gain is likely to worth no more than a tenth per lap and limited in use for circuits will shorter straights and lacking fast corners.
One issue facing the FIA and other teams is that the Mercedes DDRS solution will be banned in 2013, via wording to prevent secondary use of the DRS opening. But being passive, the Lotus system does not employ this solution to stall the wing. As it stand the Lotus will be legal for 2013, but the FIA are likely to find some wording to also outlaw this method of drag reduction.

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