The slots between the elements help the flow stay attached, allowing more aggressive angle of attack and design than a single-element wing. So they can obtain more downforce.
I'd be interested to see data on whether the L/D ratio improves. I've added a second element to my rear wing, but whilst I'm sure it improves downforce, I don't know how much I've added to the drag.
If multiple elements added lift without increasing L/D then gliders would use them all the time. They don't. There's a hundred papers out there on performance of multi element wings, including a whole book by Katz which ANYBODY who pretends to be discussing racecar aerodynamics should own, read and understand.
Correct answer, aspect ratio is the all important factor for a glider. L/D for aircraft is typically extremely high, nevertheless. There are also significant structural/weight implications to running a glider with such long multi element wings that also affect it's glide path performance. A glider isn't lifting huge loads, it has no need for so much lift, it only really needs to decrease drag once the lift generated gets close to the weight. On another note, multi element wings are WIDELY used in aircraft design competitions for carrying loads (bricks, weights, whatever). There, range is not so much the issue
As it has been stated with the glider case, the ideal number of elements / the require Cl and L/D ratio and the Aspect ratio all depend on the given implementation.
For cars the L/D ratio changes with respect to the track design. Monaco with its short straights and low grip puts a premium on downforce to enable the power transmission to the road, therefore a wing with a high angle of attack, more camber and consequently higher CL is chosen and drag is negligible as they are not power limited but traction limited.
For Spa, the balance of downforce to drag is changed. As the straights are very long and the speeds extremely high, a reduction in drag will result in a higher top speed for a given power output from the engine, so the angle of attack and camber of the wing is reduced to provide a more efficient wing section in terms of Lift to drag ratio. Downforce is still important for the 2nd and 3rd sectors, but there is a greater drag penalty.
Therefore in motorsports where the usage of wings is to provide better contact with the track surface more drag is acceptable if it allows more power to be converted into speed and it is that crossover point to which Adrian Newey will optimise his car for at every race track.
"I continuously go further and further learning about my own limitations, my body limitations, psychological limitations. It's a way of life for me." - Ayrton Senna
wings work by having camber and angle of attack, beyond some limit of either/both they stop working (well)
a single element has lower limits than the 2 element (aka slotted flap)
within those limits the single element will work better than the 2 (thats what defines the limits)
beyond those (single element) limits the 2 element will work better (thats what they are for, the slot flow does this)
car designers don't choose the wing span, but choose the wing type according to the downforce benefit relative to cost in drag
(a low powered car can be served better by a single element, others not)
unless they want to vary the (otherwise natural) flow in a race, eg by changing the slot/gap flow
the aviator varies the wing type, area, camber and/or AoA throughout flight, but flight is basically single element
(2nd etc elements (flaps) are always retracted for most of the flight, 99.9% of planes use natural slot flow only)
Wont adding more elements decrease the cross section of the aero device and in effect reduce the darg relative to one big element? isnt that basic aero law? smaller cross section decreased drag?