Blown Rear wings: seperating and stalling

NOTE: Update on McLarens Snorkel\Rear wing here http://wp.me/sNdA9-235 

Renaults CFD shows how the flow passes around a multiple element rear wings

For an F1 car the rear wing creates around a third of the cars downforce.  But running at high speed the drag from the rear wing is tremendous.  Anything that reduces the drag of the rear wing will aid top speed.  If this can be done in a non linear way, that is; high downforce\drag at lower speeds increasing towards top speed and then less drag only at speeds where car is in a straight line and doesn’t need downforce, then laptimes will show an improvement.

A single element wing sees the flow separate (circle) at steep angles

As airflows over the surface of a wing it has a tendency to slow down and separate from the wing.  Particularly underneath the wing which runs at a lower pressure than the top surface.  This separation initially reduces efficiency by adding drag to the wing, before the airflow totally breaks up and the wing stalls.  When a wing stalls the wing loses most of its downforce and drag. 

A single element wing will then stall, as the flow breaks up under the wing

The steeper a wings angle, the greater chance of separation.  To combat this aerodynamicists need to speed up the flow near the wings surface, to do this they split the wing into separate elements, this creates a slot.  Which sends high pressure air from above the wing through the slot, which then speeds the local flow underneath the wing.  The more slots the steeper the wing can run. 

With a two element wing, flow passes through the slot to prevent seperation

In the nineties teams were unlimited in the number of elements they could use.  Slowly the rulemakers sought to reduce the wings potential for downforce and reduced the number of elements (defined as ‘closed sections’ within the rules), initially to four then three and currently two.  Modern rear wings are made up to two elements, a main plane (the forward section of wing) and a flap (which sits behind it).  Thus the wing is intended only to have a single slot and hence only one place to speed up the flow under the wing.  However the rules are typically vague, thus a small 15cm section in the middle of the wing is exempt from this rule, teams have been adding a slot in this area for several years now.  This slot is the same dimension on the front as it is on the back of the wing, so there has been no issues of legality within the rules, most team run a wing of this configuration.

Last year BMW Sauber and McLaren ran wings with the narrow 15cm opening on the front of the wing, but this inlet diverged to make a slot the full width of the rear wing (normally within the main plane).  This slot was aligned to send its airflow at an acute angle, roughly inline with the general flow over the wing.  Again this was deemed legal as the slot made the wing profile an ‘open section’ only in the middle of the wing, where as the outers spans remained a ‘closed section’ albeit one with a “U” shape.  With this design the slot could allow the entire wing to be steeper and not just the geometry in the middle 15cm of the wing.  This year Williams have joined the group running these sorts of wings.

With a blown wing, the extra inlet\outlet creates a legal second slot

Again previously teams have sought to use the wing stalling to gain top speed (from the reduced drag).  By flexing the wings at higher speed, the wings move to create smaller slot gaps and this leads to the wings stalling.  The FIA has acted with both load tests and in the past few year slot gap separators to prevent this practice.  Slot gap separators are now mandated for the rear wing, and appear a plate fitted around the profile of the two wing elements to prevent them moving.

The McLaren 2010 wing uses a slot in the flap (not the main plane), this time fed by the shark fin and an opening above the drivers head.  If the teams’ protests about its legality are true, then the issue is that McLaren are using the slot to stall the wing. 

A slot in the flap could break up the airflow and allow the wing to stall

This could be possible in several ways; one could be having the slot orientated differently to the airflow over the wing, if it were at nearer right angles to the flow it could blow hard enough to disrupt the airflow enough to stall the wing.  Another solution might be that the slot blows at lower speed maintaining a clean airflow over the wing, then at higher speed the slot chokes with the greater airflow trying to pass through it, the slot no longer blowing stalls the wing.

These approaches would have to be tuned to have no effect at speeds lower than the top speed on the straight, thus the wing would provide normal downforce until near top speed.  Then near top speed the flow through the slot would start disrupt the wings flow and stall the wing.  The difficulty in getting this tuning to work is what’s given rise to the rumour about the driver operated snorkel duct on the McLaren.

47 thoughts on “Blown Rear wings: seperating and stalling

  1. Pingback: The F1 Technical Developments Thread - Page 25 - FinalGear.com Forums

  2. I bearly understood a word of what are describing, any chance a of a picture to go with it, as i would like to understand it better.

  3. I hope you can have a picture that has what it looks like when you cut the rear wing in half and see the slot, because i’m still partially confused

  4. Sounds pretty ingenious to me. I hope the FIA don’t ban it. Especially the snorkel idea feeding the slot by way of the drivers knee!

  5. the key is the driver input in all this mess. I understand why the rear wing stalls at high speed (3 element wing vs 2 element wing) but fail to see the influence of the snorkel and driver’s knee position. Unless they made room for a left foot rest…

    In any case wouldn’t the rear wing need to be an integral part of the sharkfin and viceversa for this to work, thus making the rear wing exceed legal size?

    • The snorkel\knee thing is unproven and potentially the wing could still be tuned to stall at the right speed.

      The wing is dimensionally legal as the shark fin joins in the middle 15cm of the wing

  6. I have a slight question about this stalling issue: I am just wondering how stalling the wing will help to reduce drag ??? Is there anyone that can explain it physically…
    The drag becomes even greater post-stall for a wing, and that fact is shown in all the report on wings at angle of attack post-stall (critical for wind turbines for instance)…
    Maybe I have missunderstood something …

    • When air flows over a wing it creates drag in two ways; form drag – is the effect of the air flowing over the skin of the bodywork, then – induced drag which is due the interuption the wing creates in the surrounding air, such as wing tip vortices. The effect of induced drag is far greater, especially for an aggressive narrow F1 wing (as compared to say an aircraft wing). Form drag may increase with seperation, but when a wing stalls the wing stops working. Thus downforce dramatically reduces as the air isnt flowing as it should. Therefore the induced drag is also reduced as the wing is no longer working as hard.

      • well, that sounds almost like a physical explanation.
        When there is a stall, no doubt on this, downforce is reduced. And so is the induced drag. However, due to endplates, the induced drag is already significantly limited without stall.
        Then comes the pressure drag. Post-stall, the increase of the pressure drag is much much larger than the max induced drag you will get.
        Let’s put some numbers. I have the airfoil NACA4415 (AR=6 – aspect ratio) in front of my eyes.
        Max C_Lift: 1.4 (Re=1 Million) for a angle of attack of 18 degree.
        C_Drag for max C_lift: 0.2
        If I take the base formula for evaluating the induced drag, it is Cdi=Cl^2*k/(AR*pi). This gives approx. 0.11, about half of the drag (the rest I reckon is dominated by friction).
        If I continue further on the curve, say, 40 degree, I find a lift of 0.8 but a drag coefficient of 0.8 too.
        The induced drag still exits but is smaller (0.04) and everything is dominated by pressure drag.

        So, I still beleive that the claim to “stall a wing to colapse the drag” is a myth …

      • Tristan,
        Its clear your theoretical aero knowledge is in advance of mine!
        I understood it was the endplates that add to the induced drag, despite detail design, they still create powerful tip vortices.
        Even if I cannot describe it accurately enough, every F1 technical director tells me a stalled F1 wing creates far less drag. which is why they have been playing with bending wings and slot gaps for years.

      • I am an aerodynamicist :o)
        But I am not working in F1.

        Don’t get it wrong, endplates reduces the induced drag. Induced drag are due to the tip vortices as you mentionned earlier. Basically, high pressure below the wing is attracted by the low pressure on the top of the wing so, on the wing side, some air tends to go up towards the upper surface, creating hence vortices, and then, drag.
        I read that people claim that “stall wings” create less drag … but it seems that either they are confusing terms or they have some wrong ideas … that’s why I want to understand where this idea comes from.

        A stall wing reduce significantly the downforce but not the drag. I agree that thin wings are very sensitive to the stall, the loss of negative lift is often brutal.

      • As an aerodynamicist, I was very puzzled when I read about the “stalling” issue. Actually, I realized that it comes all from some confusions with the terminology.
        In aerodynamics, the term “stalling” refers to a drop of lift (or downforce) due to highly separated flow at the upper (or lower for cars) surface of a wing. This will produce a significant increase of drag no matter the wing you refer to (included the “highly cambered and high lift generator wing”). So any post-stall situation should be prohibited whatsoever.

        HOWEVER, you can improve the performance of any wings before stall by blowing some air on the upper (or lower for cars) surface on the wing. You increase slightly the boundary layer speed and in turns, you can increase significantly the lift (or downforce) generated – almost twice more – but you also increase a bit the drag. A solution to blow some air in the upper (or lower for cars) surface can be done with a small slot on this surface. This is a known theory, for instance, this has been tested for wings in 1929, see the book from I. Abbot, Thoery of wind section, 1959.
        Now, imagine that you are on a straight line and that you remove the action of the slot, you will switch from one wing situation (high lift, higher drag) to another (normal lift, normal drag). You have lowered the drag and lift, but please, don’t call that “stalling”!

        Considering the McLaren rear wing, since I am not the designer of this car and do not have access to the data, I can’t tell if this is what they want to achieve.

        Finally, I guess this is what C. Horner and most of people from F1 who are aware of such practice but don’t really understand the physics, call “stalling the wing”.

        Now, I reckon that it is all about “trade off” because such solution, if this is actually implemented in some way, would be difficult to control accurately and might lead to situations where some downforce will be missing for the braking moments. But that’s another debate.

      • Have you actually worked out how much downforce your NACA 4-digit airfoil gives you as 80m/s? It gives you the equivalent of about 50kg. Now F1 manufacturers are quite tight-lipped about how much downforce they actually get, but the indications are that total downforce is greater than the weight of the car and the rear wing supplies about a third of that. This would put your CL estimate out by a factor of 5 and therefore your CDi estimate out by a factor of 25 (by your brutally crude lifting-line approximation). Suddenly that induced drag doesn’t look quite so insignificant.

        The lifting-line approximation is derived for a finite wing with an elliptical lift distribution where the tip vortex is shed from a nice point right at the wingtip: not a very good description of an F1 rear wing. This means that what your claim that “stalling the wing could never reduce the induced drag more than increase in pressure drag based on a back of an envelope calculation using lifting line theory and a NACA 4-digit aerofoil” doesn’t really stand up to scrutiny.

        F1 wings have a very low aspect ratio and operate at huge lift coefficients. This means that the induced drag will be large. Saying that stalling the wing will reduce the total drag may or may not be correct, but it is certainly plausible.

      • I guess high pressure is not below but above the wing in F1 – as the air flows at lower velocity there.

        High velocity air has lower pressure in the lower streamline thus creating downforce there.

        Aircrafts have different aerofoil – the pressure and velocity of upper and lower streamline are opposite.

  7. See this image. Attached top rear wing is on the left, stalled is to the right.

    People need to stop thinking about this as a classical aerofoil example that you saw in Aero 101. This is a heavily loaded, highly cambered two element wing. The resultant load vector has a significant horizontal component which is acts as the main contributor to the drag.

    This whole issue is not related to induced drag or skin friction type drag (which is my way of saying it’s not related to anything particularly fancy aero wise). 90% of a Top Rear Wing’s drag is pressure drag – or just the horizontal component of the wing’s load vector.

    Stalling the wing leads to a large reduction in the resultant load vector – and this also means a large reduction in the vector’s horizontal component. Drag is reduced.

    Scarbs: just wanted to say that this is an excellent, factually correct article covering all the major bases, so well done.

    Also, if teams do complain about this to the FIA it won’t really be regarding the fact that they are stalling the wing (that, in itself, is perfectly legal – or rather, there are no rules outlawing this). What they will be questioning is the driver’s involvement in the activation of the pressure switch (almost certainly located along the left hand side of the cockpit and operated by the driver’s knee) required to control the wing’s state.

    • I see your point, I am getting a F1 aero contact to clarify this point in more technical detail, such as you and Tristan have raised.

      PS thanks for the compliments…🙂

    • “Stalling the wing leads to a large reduction in the resultant load vector ” —> can you describe the physics behind this statement?
      or better, what do you mean by “stall” then?
      As soon as you get separation (what I know in the classical deifintion of stall), you will get a low pressure area and therefore an increase of drag. I would be very interested to know how to reduce the aero resultant…

      • I’ve talked about this in depth on the f1 technical forums (I’m assuming you are the same Tristan as on there).

  8. I am not educated in aerodynamics but I will try to explain what I think is going on: From what I understand the blown section stalls the wing right?

    I think what people are not getting is that when the wing is operating normally, there is effectively suction bubble created behind the wing as the car goes faster. This suction slows the car down a bit, but is 99% offset be all good things the down-force lets the car do to get up to high speed, especially in the corners.

    What teams are trying to do is get the best of both worlds, which is get rid of the suction bubble (possibly at the expense the wings down-force creating abilities) once the car has already achieved 99% of top speed.

    However the team activates the blown section, they must do it near the top speed, in mostly a straight line. The sudden rush of “extra” blown air basically pops the suction bubble that would normally be created behind the rear wing. The car speeds up, but possibly with a loss of some down-force.

    My guess is that the diffuser would provide enough suction force to keep the car on the track, but the FIA could rule that it is not safe to “turn off” the rear wing right when the car is going it’s fastest.

    As the car gets to the corner, the team needs it’s down-force back, and thus stops the “extra air” from popping the suction bubble, and letting the wings normal airflow get back to providing down-force.

  9. Hello, I completely agree with Tristan. Apparently they use the term “stall” in the wrong way. I think they mean equalising the pressures on both top and lower surfaces and not the classical stall definition, which would increase drag.

  10. Awesome post!
    I found this blog (and your twitter account) just a few weeks ago and it is already one of my favorite!
    Also the comments are really good!

    Keep up your excellent work!

  11. Let us try to solve the mystery of the McLaren blown wing.First,search for ‘Blown Flap’on Wikipedia.In the fourth paragraph under ‘Mechanism’take note of (counter-flow fluid injection for boundary layer enhancement).This seems to be the effect McLaren is after.Stalling a wing cannot be good,stall prevention maybe the goal.A fixed wing is a compromise between downforce and drag.If more wing is used(high downforce setup),more drag sould be expected,bad for maximum speed.McLaren could use more wing for the slow twisty part of a circuit however,this high angle wing would likely stall on the high speed sections of the circuit thus creating more drag and decreasing downforce,again bad for maximum speed.The use of ‘boundary layer enhancement’techniques could ‘delay or eleminate flow separation’,enabling the use of a relatively high downforce wing without suffering much on the high speed low downforce part of the circuit.McLaren may have found a way to have their cake and eat it too.

  12. Lets see if I understand this correctly (and that is not guaranteed;-):
    The rear wing was designed under ‘normal’ conditions to easily stall and reduce its drag level. Now the clever part is that Mclaren has introduced a main bodywork feature which ‘conditions’ the flow so that it improves the flow over the wing, delaying the stall and increasing the gains in downforce and subsequent drag. The flow-conditioning may/may-not be under driver control or perhaps a differential pressure element (i.e. pops open or closed) under different flow conditions?

    Regards, PK

    PS: Are driver-movable aerodynamic elements allowed within the chassis bodywork? For example movable vent louvres above radiators?

  13. Hi

    I’m leaning towards moving having the best of both worlds, a extra slot in the wing allows higher angle before stalling, or lower drag at higher speeds, if you optimise the wing for the slower corner sections you will probably always find seperation at maximum speed along the straight, this slot, while maybe increasing the drag slightly still means higher downforce, while, at the straight sections once the other wings start to seperate this one then has less drag…

    So to cap, maybe a bit more downforce around the corners, a bit more drag as well, a bit more drag on the straight up to some speed, then less drag after that.

    Or maybe not.

  14. This is a comment from an industry expert and ex F1 & Indycar aerodynamicist. In summary an F1 wing does produce a lot of induced drag, stalling can have an effect on drag and this is dones via stalling…..

    “Well I know is that if you stall the flap on an F1-wing (in the wind tunnel)
    then the drag does drop enough to calculate that the top-speed of the car
    could be 3~5kph faster (we did this ten years ago) but the trick is doing it
    in a way that’s legal (well, not illegal). Wind tunnel engineers can do
    this by altering the slot-gap geometry and/or changing parts to simulate
    flexing-on-the-track. It’s very easy to demonstrate in a wind tunnel ~ just
    very difficult to engineer it so that it’s not illegal.

    Also, even a flat plate (e.g. Handford Device) makes less drag than an
    aerofoil of the same height x chord. So, in other words, fully-attached
    highly-loaded multi-element aerofoils on the rear of a racing car produce a
    very large amount of drag (more even than a blunt-rearward-facing block/
    flat-plate)

    And I think the lift coefficient (based on wing-plan-area) of a contemporary
    F1 wing is nearer 3 (well above the 1.4 that you mention) ~ and the
    lift/drag ratio is probably around 3.0 ~ 4.0 too. From these numbers you
    can calculate the approximate contribution to overall downforce and drag
    (quite large forces at 300kph when compared to the weight of the car).

    And I think that the type of stall that is being talked about is due to
    engineering the flow around the flap to stall ~ not to stall the aerofoil as
    on an aeroplane (leading-edge stall).
    Bear in mind that when you stall the flap, whilst the lift/drag ratio
    becomes worse than when unstalled (say lift/drag about 2) the lift
    coefficient of the wing will drop massively (to perhaps 40% of Clmax) and so
    although the lift/drag ratio is worse than before stall, the lift(downforce)
    has dropped-off so much that there is an overall drag reduction.

    Hope this helps ~ I think it’s over-complicating the issue to try to use
    induced-drag numbers lifted-off from the text relating to aircraft (the
    principles are correct but the application is very different and the effect
    of endplates on a car’s rear-wing makes a big difference ~ effectively
    making the wings simulate a much larger aspect-ratio).”

    • Hi,

      Thanks for this comment! I think that the confusion here started with a miss-understanding on the word “stall”. As your expert says in the end of his comment, we have a different view on “stall”. If we take “stall” as having a separation on the flap, then, I understand the drag reduction (in a similar way that I tried to describe on the post of the 5th March 3:15pm). I was taking the word “stall” as the fully seperated wing (leading-edge separation as your expert says) which is the more academic definition and that explain why many other (not only me) were quite puzzled when you read that stall help to decrease drag.
      Thanks again for this post !

  15. Hello,

    @scarbsf1, I like your blog very much. It (alongside with autosport forum) improved my knowlege of F1’s technical side, which was inconsiderable just copule of months ago.

    Still it’s quite tough to understand all this specialist phrases, especially as English isn’t my native language🙂 Although I hope that I understood your blog entry (good illustrations), then I totally lost in comments – it’s way too much on present English skills🙂

    Anyway, I just wanted to give my appreciation for your blogging. Keep up with all this great work!

    Greetings from Poland!

  16. Hi Guys,

    Nice article and nice comments.
    First for our host and others here is an article about end plates and how they work:
    http://www.mulsannescorner.com/techarticle2.html

    It is about LMP cars but a wing being on a plane or on a car remains a wing.

    One question came up to my mind.
    If you consider the scoop on the front the tub, any air flow channeled from there to the rear wing would have tremendous difficulties to reach the rear wing with significant speed given the tortuous path through the tub.
    To get that relative speed difference you need to compress the air and surfaces just don’t match that criteria.
    So main air supply for the exit on the shark fin must come from something else, ie engine air intake and supposed additional radiator inlet.
    My guess is that as car minimum height is defined by regulation, they use the free space on the top of air intake to channel straight air to the back of the fin.
    The bulbeous shape of the engine cover/shark fin assembly viewed from side seems to confirm this.

    Then the driver “blank actuated” scoop would influence that flow. But how exactly? That driver controlled flow is very small and has a long way to travel to get there.
    I think that getting an idea of how the shark fin exit is supplied and controlled would help us understand what they really want to achieve and how.

    Looking at the actual very small wings used, you see the top part of the flap being near vertical.
    The chord of the airfoils is quite small for so steep angles. So small in fact that flow separation has to happen on the flap.
    The top part of the flap is so cambered that in itself acts like a gurney (the turbulences left behind help to draw the air from bellow the wing).
    This whole concept looks to me as a way to effectively create one slot more to keep the flow more attached to the lower section of the flap and therefore increasing downforce. (as very well explained in previous posts)

    But how does the driver can influence this flow and cutting it (creating the same effect as blanking the slot) with that small airflow from the from of the tub? That’s the main question to me.

  17. Re. Ben’s comment about how the small tub-top scoop can provide much influence, I suspect that all this does is provide an actuating force that in turn flips an air-operated flap within the shark-fin that channels the real “stalling” airstream that originates from the engine air intake. By doing it this way they can just about (I think) stay legal…

    • Re: Jaycb
      That is the whole point!
      There can’t be any flaps in there because then you have a moveable aero device.
      As per the rules, no aerodynamic element can have any degree of freedom except the front wing wich is driver adjustable.
      A flap would definetly come into the moving aero device rule.

  18. Great post – many thanks for explaining what’s going on with the McLaren wing. Was struggling to understand it before but now have a grasp of the forces at play.

  19. “SHAKEDOWN” have used a graphic from your blog in one of their videos, ( if it originated here, which I think it did..)

    At least you know you’re getting more and more readers😉

  20. Pingback: Stalling for time « Kate on F1

  21. Interesting, however I have questions as to how much of the rear wing is actually stalled.

    How wide is the duct onto the wing?

    Is there any information on how the car’s top speed is changed by simply having the “hardware” installed but not working compared to the previous configuration? After all, the “hardware” on the “shark fin” would cause a disturbance anyway.

    I suppose we will never really know given that this information will be closely guarded.

    • Most teams their exit duct on the back of the flap nearly as wide the full span of the wing, leaving just few cm of space towards the endplate.
      I doubt the wider ‘f-duct’ shark fin creates an greater drag than the thin one, is presents little additional frontal or surface to the car. Most teams with the split switched duct (i.e. Red Bull & Ferrari) mentioned a loss of downforce as the switch will always pass some flow the exit duct, adding flow into what needs to be a low pressure region for downforce. Top speed advantage of a stalled wing is said to be 5-10kph.

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