>>A few weeks ago we posted a really interesting image, taken at the Le Mans Test Day, of Oak Racing's rear wing flap. We had a think, talked to a bunch of people, and have the answer as to what's going on there. This was the process:

Since the 2004 LMP1 regulation changes (and even before that with the LMP900 regulations), the ACO have mandated a gurney on the rear wing flap that is 90 degrees to the plane connecting the top of the wing to the trailing edge (see FIA Drawing 258-8 in Article 3.6.3 of the ACO regulations). Initially the mandated height was 15 mm, then in 2009 this was raised to 20 mm. 2011 saw the introduction of engine capacity reductions for LMP1 resulting in the loss of about 100 horsepower for the top diesel LMPs. This has led to a trend shift in aerodynamics development from pure downforce generation to drag reduction and efficiency first, downforce second.

While it is only a small detail when looking at the entire car, the mandatory 20 mm gurney is even more of a liability than in years past. But there's precious little that can be done about it...or is there?

First seen at the 2011 Le Mans test day, Oak Racing have hit upon a clever design execution that maneuvers around the gurney regulation. Ultimately the concept is a regulatory work around and nothing more, though more on the specifics in a moment. But getting to this conclusion was fraught at first. Initially it was suggested that the protuberances were designed with pure aerodynamics in mind; discussions were rampant about the bumps acting as vortex generators. There was even mention the bumps were utilizing an aerodynamic concept seen in nature and on of all things, whales. Yes, the giant mammal kind.

Apparently scientists have always been puzzled about whale’s agility given their size. And when researched, they took note of bumps on the leading edge of whale fins. They came to the conclusion the bumps help reduce span wise leading edge flow migration and locally better organized the water flow leading to a much more efficient fin (or wing). Hey, don’t shoot the messenger, I’m just repeating what I read.

I must admit I wasn’t convinced that was the cue Oak Racing had taken, though admittedly I had nothing else. Apparently the research is three years old, and lacking direct access to their numbers, one has to utilize the school of observation. And the best place to do that is to observe what’s going on in F1, the “bleeding” edge of motorsports aerodynamics. No one has, to my knowledge, shown up on a F1 grid recently with a wing that had bumps or similar on the leading edge.

At about this time I received an email from a former colleague who is now with an F1 effort, asking me, of all people, what I knew about the Oak Racing wing flap. Lacking anything else, I repeated the rather weak-sauce story about the whales. Yeah, I did. But the fortunate thing for me was that in the email ping-pong the answer shook out.

Ultimately it's pretty simple. But first you have to think about the wing in cross section. The mandatory 20 mm gurney has to be perpendicular to the line connecting the top of the wing to the trailing edge. But Oak Racing has increased the height of the wing opposite the “conventional” cross section in an alternating pattern (the bumps). And when you connect the top of the bump to the trailing edge, the perpendicular gurney is leaned back relative to the conventional section (approximately 30 degrees in our example). And as the top of the wing is relatively insensitive (note that the important low pressure side is untouched), any loss from the bumps is more than made up for by the alternating gurney angle reduction.

Throughout all of this I was finally able to reach Oak Racing's Technical Director Christophe Chapelain. Chapelain immediately confirmed the concept was indeed a rules work around driven by the desire to reduce drag. Says Chapelain, “The gurney is one of the most important piece for the drag. It (the idea) came from a sort of brain storming.” Team aerodynamicist Nicolas Clemencon utilized CFD to flesh out the concept and followed that up with wind tunnel testing at RUAG in Switzerland to validate. The result, according to Chapelain, is the same level of downforce for slightly less drag. Chapelain also indicates that there is some negative interaction from the bumps (flow separations), but not enough to take away from the positives. The wing is also just on the edge of improved L/D; the flap wasn't used at Spa because at higher downforce levels it's more efficient to use a standard flap without any bumps.

And Oak Racing's concept hasn't been dismissed outright and rival outfits are already mulling the possibilities. “Well, it depends on how big the negative effect of the flap top surface modification...I will know more after trying it, possibly in a nicer way,” this according to one aerodynamicist for a current LMP project. One wonders if there's been enough time between the Le Man test and the race...we'll see soon enough!