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In this VIDEO at 5:40 we can see how stall decrease pressure at upper wing surface.

If stall/flow separation reduce static pressure how in this VIDEO they reduce drag when stalling the wing?

Here is description how works: https://formula1techandart.wordpress.com/2010/12/14/mc-laren%E2%80%99s-innovative-rear-wing-system-f-duct/

https://www.formula1-dictionary.net/f_duct.html

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    $\begingroup$ I don't know about pressure, but stalling always increases drag. $\endgroup$
    – HiddenWindshield
    Commented Nov 11, 2022 at 19:03
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    $\begingroup$ This is not about aviation, voting to close. $\endgroup$
    – Michael Hall
    Commented Nov 11, 2022 at 22:05
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    $\begingroup$ @MichaelHall I see your point and respect your verdict, however, I find this question interesting as an aerodynamic optimization task. Also, I wonder whether a car site will be able to answer it well. $\endgroup$
    – Peter Kämpf
    Commented Nov 11, 2022 at 23:58
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    $\begingroup$ @PeterKämpf, and I agree with what you just said... But, I believe Jurgen has a responsibility to establish a connection with aviation in order to make it relevant enough to keep open. $\endgroup$
    – Michael Hall
    Commented Nov 12, 2022 at 1:52
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    $\begingroup$ @JurgenM Aerodynamics is at the basics of aviation, but that does not mean that every aerodynamics question is fit for this site. Since this question is about a stalling airfoil, the aerodynamics could fit into the domain of aviation and be of interest to our community, but it is borderline. $\endgroup$
    – DeltaLima ♦
    Commented Nov 12, 2022 at 8:46

2 Answers 2

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As the linked site explains, this is not about aerodynamic drag of the flap but the whole drag of the vehicle.

F1 cars use rear wings to increase downforce so they can turn faster. The downforce of those wings increases the load on the tires and, consequently, the amount of side force they can tolerate before they skid. This allows to drive faster through turns.

This tire load, however, increases rolling resistance, so on straights it would be better to do without this downforce. In airplanes we use moveable flaps but the F1 cars have decided to vary flap lift by blowing air into the low-pressure side. This raises local pressure, reduces downforce and, consequently, rolling resistance so the car can drive faster.

In effect, by switching the air flow on in straights and blocking it in turns, the car can be optimized to both run faster on straights and run faster in turns without skidding.

Since the low-pressure side of the wing and flap is pointing backwards, increasing local pressure also should reduce the amount of aerodynamic drag there. But the main effect is to vary the lift this wing and flap produce and thus to vary downforce such that the car can reach higher speeds overall.

Stall in aeroplanes means drag goes up because lift needs to be produced and stalling makes this less efficient. Here, however, stall will greatly reduce lift - in cars there is no need to keep lift equal to weight! And reducing lift will also reduce drag, even though the L/D ratio might go down in the process.

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  • $\begingroup$ If you close the slot between main wing and "flap wing",all rear wing will stall. Will you then increase drag and will you increase lift of rear wing compare when slot is open and flow is attached? $\endgroup$
    – 22flower
    Commented Nov 12, 2022 at 7:12
  • $\begingroup$ @JurgenM closing the slat will greatly increase drag and decrease downforce. While blowing into the low suction area reduces downforce, too, it causes drag to increase less because the additional air volume will raise pressure in the separated flow. Closing the slot is less helpful than blowing. $\endgroup$
    – Peter Kämpf
    Commented Nov 12, 2022 at 8:11
  • $\begingroup$ " While blowing into the low suction area reduces downforce, too, it causes drag to increase less " You write in your answer that blowing air in low pressure side reduce aero drag of wing, not increase. ? $\endgroup$
    – 22flower
    Commented Nov 12, 2022 at 8:22
  • $\begingroup$ @JurgenM … relative to separation without blowing. $\endgroup$
    – Peter Kämpf
    Commented Nov 12, 2022 at 16:02
  • $\begingroup$ So when f duct is ON(blowing) aero drag of wing is lower then when f duct is OFF(attached flow)? $\endgroup$
    – 22flower
    Commented Nov 12, 2022 at 16:50
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This is more race car - technical, but in this reference they explain that only the rear slotted portion of the airfoil is stalled, not the entire (inverted) wing.

Apparently, the reduction in downforce in straightaway results in an increase in speed. This may be due to a variety of factors, such as the rear tires being "squished" or compressed less, reducing their friction and energy loss from deformation.

The claim is that the lift to drag ratio is reduced, but lift is reduced so much more that there is a net loss in drag. This explanation may be somewhat dubious but apparently the device works.

A more suitable reason may be that the flap is angled in such a way that it's "lift" not only produces downward force, but also a significant amount of rearward force. When the "bleed slots" are activated by the driver, they simply reduce the flap lift without stalling it.

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  • $\begingroup$ ,some teams blown air in main wing, some in second wing $\endgroup$
    – 22flower
    Commented Nov 11, 2022 at 22:04

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