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LMP 1 REAR WINGS


tutored to look for this. And this was the answer to why the swan neck rear wing mounts came into being. With the use of higher camber rear wing mainplanes and higher angles of attack, the conventional method of mounting the rear wing proved to be a source of flow separation. And so, it was with much


Seen here in 2009, the ORECA customer Peugeot 908 (with orange livery) features the old rear wing uprights, while the two factory cars sport the new swan neck uprights


We started with a 2008 wing


profile that had been used by a named LMP effort who will remain anonymous. The first case tested was the 2008 profile to the 2008 full width (2000mm) span and 300mm chord, mounted to a conventional bottom rear wing mount. In isolation, this case generated 1739lb of downforce and 226lb of drag, 7.69:1 L/D.


REALITY CHECK Okay, so we shouldn’t get too fixated on the absolutes. But, for a reality check, the numbers were put in front of someone with knowledge of what a contemporary LMP car rear wing should generate, and their response was: ‘The absolute forces you calculated seem reliable to me.’ And throughout this process we had people with relevant knowledge looking over our work, making sure we didn’t lose the plot too much. Next, we lopped 400mm


off the wing span and scaled the ‘2008’ profile to the 2009 regulated wing chord (250mm). Naturally, this wasn’t a bespoke wing shape, given the ACO legality box, but we were simply looking to see what the downforce loss was if we took the old wing and made it fit the new regulations. The results was a 593lb loss in downforce (1146lb total) for a 70lb loss in drag (156lb total). Interestingly, efficiency stayed about the same at 7.36:1. This was a 34 per cent loss in rear wing downforce for a 31 per cent reduction in drag,


but only a four per cent loss in efficiency. On this, Dome’s Hiroshi Yucchi commented: ‘34 per cent, just by wing change, is almost the same as our wind tunnel results. It sounds quite accurate to me.’ Playing the part of a design team within a major LMP programme, and with the ‘encouragement’ of a nearly 600lb downforce loss, it’s pretty evident development would immediately commence to gain back much as much as possible of what we lost. Obviously, teams wouldn’t merely scale down their old 2008 rear wings, they would look to optimise the wing to the new regulation box. And this meant getting into the wing development business. This wasn’t for the faint of heart and the project could easily fall off


copacetic, CFD runs showed it nearly 720lb down (1019lb total) over the benchmark 2008 wing. What was going on? This should have been the ticket. A clue was in the drag figure (214lb) as it was gaining even over the previous scaled 2008 wing case. So we were losing even more downforce and gaining drag when at very least we expected increases in downforce.


FLOW SEPARATION We suspected the culprit was flow separation. And indeed, flow visualisations showed a large disturbance in the area of the rear wing mounts. Testing our theory, two additional runs were tested that backed the mainplane angle out, first 1.5 degrees and then three degrees, rotating around the trailing edge, all the


“we were able to claw back 16 per cent of the initial 34 per cent loss”


the rails here. At this point our experts were brought back into the fold to get an idea of what manipulations would produce the best ‘bespoke’ wing for the 2009 regulations. In discussions, we came to understand that the mainplane angle of attack and camber were two basic methods used to modify the rear wing to gain back the lost downforce. So with our wing modified as directed, and everything looking


76 www.racecar-engineering.com • January 2012


while keeping the flap angle and all other parameters constant. The 1.5-degree reduction showed little better than a repeat: +3lb downforce, -8lb drag (1022lb and 206lb respectively). But most interestingly, with the three- degree reduction, suddenly the bespoke 2009 wing came alive, with downforce increasing by 205lb and drag dropping 27lb (1227lb and 179lb). But in reality we had been


anticipation that we tested the swan neck wing mount case. Things immediately began to get even better: 1299lb of downforce for 186lb of drag. The flow separation went away and, at this point, we were ‘merely’ 440lb down on the 2008 full- span case. In terms of efficiency, we weren’t that badly off, only eight per cent down on wing L/D. And matching drag through an increase in flap angle (+8 degrees) saw downforce further increase to 1413lb. At that point, we were within four per cent of our 2008 rear wing drag level, so there was a tad more to be gained, downforce-wise (perhaps, L/D was now down 15 per cent compared to the ‘08 case, suggesting we were coming to the end of this set up’s potential), but we moved on to other areas of development. We also tried a number of


rear wing end plate iterations, but saw little benefit. This isn’t to say this couldn’t be an area of successful development but, in our limited running (all straight line), we saw nothing promising. We also tested a reverse swan neck, one that came up over the trailing edge of the wing. Overlooking the practicalities of locating such a mount on a contemporary LMP gearbox, given rear overhang maximums and desired rear wing position, it essentially didn’t perform any worse than the standard swan neck. The results were 1288lb downforce, for 186lb drag.


THE RESULTS


In the end, with only our limited number of runs, we were able to claw back 16 per cent of the initial 34 per cent loss when matching for drag. Certainly, with further development on the wing (we only contemplated extruded 2D sections, after all), as well as entertaining other areas of the car, gains well beyond what


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