An airfoil optimised to work at low AoA hence low Cl (like the midwing) is way more efficient than an airfoil with a very curved camberline designed to obtain very high CL (like in the rear wing). So if you are able to generate even a small amount of the total downforce of the car from the midwing (I wouldn’t be surprised if it was in the order of 1-2%), you can reduce by the same amount the downforce from the rear wing, increasing its efficiency and the overall efficiency of the car. Then there are further details to analyse, like the downforce distribution Asphodel was referring to and the pitching moment, clearly lower for airfoils with a small curvature of the camberline.
Apparently Ferrari is using the midwing to improve the overall efficiency also on low downforce configuration, this week in Monza Badoer was testing with it and the overall package was undoubtedly for low downforce.
B.A.R. and Williams on the contrary weren’t using it so probably for them it’s mainly an additional device to reach an higher level of downforce, that could be confirmed also from the fact that the one on the B.A.R. has a span apparently as wide as rules allow in that area (max 60 cm).
I think it is strange that the mid mounted wing on the Ferrari doesn't have endplates. Wouldn't it make the wing more efficient by stopping the air spilling around the end of the wing. I am open to any ideas of why they don't do it. Is there something in the rules?
Maybe the gain would be minimal and probably the increased surface would cause bigger drag and maybe more disturbance to the flow downstream. Consider also that the effect of the endplate is a redistribution of the lift on the span, basically a virtual increment of the aspect ratio, hence of the span. As I’ve said above F1 rules actually impose a maximum width in that area but only BAR is apparently using the full span, others could easily increase it if required. Ferrari did the opposite, reducing the span in Imola compared with the previous model and keeping the same short span also in Monaco. That suggests that the increment of span would carry little benefit if any. Then I’ve a little voice in my head saying that it’s no coincidence that the width is about the same of the camera housing just above it
that is why most airliners now have winglets on the end of the wing...
The usual motivations to use the winglets at the tip on airlines are :
1. A new model of the same airplane with increased maximum take off weight and payload (usually thanks to a longer fuselage). The wing is one of the most complex things to design and it’s cheaper to use a bit more AoA in cruise limiting the increment of drag thanks to the winglets instead of a complete redesign of the wing aerodynamics and structure, many airplanes of the same manufacturer share the same wing.
2. Airports requires to pay a tax that is related with the size of the airplane and based usually on the span of the wing. The winglets allows to mimic an higher aspect ratio (larger span) without actually increasing it.
In absence of these limitations (or similar) it’s usually preferred not to use the winglets at the tip but to increase the aspect ratio instead.