Can we use multi element wing for slow flight planes, like F1 multi element rear wing?

**If we know that pressure act prependicular to the surface why this two upper elements are "almost" vertical?**

**It seems to me that they produce "tons" of drag instead lift.Resultant force point to far back,instead down!**

[![enter image description here](https://i.stack.imgur.com/3ooHs.png)](https://i.stack.imgur.com/3ooHs.png)

[![enter image description here](https://i.stack.imgur.com/6D1T2.jpg)](https://i.stack.imgur.com/6D1T2.jpg)

[![enter image description here](https://i.stack.imgur.com/YSUpm.png)](https://i.stack.imgur.com/YSUpm.png)
['https://aviation.stackexchange.com/questions/80120', 'https://aviation.stackexchange.com', 'https://aviation.stackexchange.com/users/-1/']
Excellent application inverted on the drag racer, which brings home the point perfectly. If you have enough excess thrust, devices like these can dramaticly increase Coefficient of Lift, but at the cost if loss of lift efficiency, or lower lift to drag ratio.

In the case of the dragster, more propulsion efficiency is lost if tires break loose from the pavement, so the drag of the "inverted Fowler flap" is an acceptable trade-off to the **downlift** increasing traction.

As mentioned in comments, this is why the Fiesler Storch requires 240 HP to achieve its legendary STOL performance.

And just as flaps 10 is better for lift than flaps 30 for taking off, one might question the need for the second and third flap on a lifting wing, but heavy airliners love them when coming in to land as Fowler flaps.
No, it is not if you are not limited in wing area and / or wingspan.

The other answers already do a splendid job explaining how this multi-element airfoil works and that it creates a lot of drag.

Now look at [slow flight airplanes](https://aviation.stackexchange.com/questions/23929/why-would-maximum-fly-time-endurance-not-coincide-with-max-l-d-operating-point/23944#23944) – they all use high aspect ratio wings without complex flap systems. Doing this gives them better efficiency. Yes, they need more wing area and wing mass to achieve enough lift, but at low speed the friction drag increase caused by the larger surface is small. On the other hand, their induced drag, which is dominant at low speed, is kept small by the high wing span.

The induced drag of the multi-element airfoil, on the other hand, is prohibitive, as is the pressure drag caused by the incidence of the rear elements (which actually **is** induced drag) and possible flow separation on the upper surface. This makes the multi-element airfoil less efficient: It needs much more power for the same lift. With race cars that makes sense: The wing cannot be wider than the car itself, and in order to develop the maximum possible downforce, an inefficient way of producing it has to be selected. With airplanes, no such restrictions exist and the glider-like wing is the better choice.