Reducing the drag of a two element wing through stall

[simoncm]

Simoncm, I'm with you up to this point...

Everything cancels nicely, therefore the ratio of the drags is
Cl/Cd

A reasonable Cl is 1.5, an angled plate in an air flow has a Cd of 0.5 to 0.8

Therefore the "drag" due to the wing generating downforce is greater (ratio > 1) than the "drag" due to the wing being stalled. In this case by a factor of 2.

What assumptions are you making on the angle of the wing? i.e., where are you getting your numbers for Cl and Cd?

The main assumption I am making about the angle of the wing is that it has a "good" angle to the incident airflow. I'm using the assumption that the airflow has already been bent upwards by the first wing element (by an arbitrary amount) and therefore the angle of the second wing is such that it has an optimal AoA to this airflow.

Under this assumption I pulled a 'reasonable' Cl figure off the wiki page.

I should stress that the AoA of the rear wing to the incident airflow is not the angle of the wing (which many have noted is in the region of 50 degrees or so).

The Cd number is an approximation, again from wiki, for an angled square block. I would have absolutely no idea what the Cd of the wing should be - shall we say its non-trivial. You would have to take into account the incident air flow, and how the stall is created (prob by removal of the blown slot effect discussed in detail here).

I think it is reasonable to conclude that the stalled wing would have a Cd of less than one.

Consequently, if Cd < 1 and Cl > 1 the stalled wing has less drag.

So, to a first approximation, I think it is reasonable based on the theoretical analysis above to conclude that a stalled rearward wing can have less drag.

I was thinking about this a bit more last night, and I think the big issue that classic aerodynamicists are having is that they are not allowing for the fact that the incident airflow over the rearward wing is not parallel to the ground due to the forward wing. Therefore the lift is not classic lift (perpendicular to the 'overall' airflow, that is perpendicular to the ground) and the stall is not a classic stall in aeroplane terms.

[horse]

I was thinking about this a bit more last night, and I think the big issue that classic aerodynamicists are having is that they are not allowing for the fact that the incident airflow over the rearward wing is not parallel to the ground due to the forward wing. Therefore the lift is not classic lift (perpendicular to the 'overall' airflow, that is perpendicular to the ground) and the stall is not a classic stall in aeroplane terms.

I see what you're getting at here. If the incident flow vector to the flap is always such that it has a relatively shallow AoA, then the impact of a stall would be quite small as the presented forward area is small. Ahhh! That main element must be doing a good job of "curving the airflow" to achieve this, mind. I also suppose the velocity must not return to free steam for a little way off the top of the wing, also, or the apparent shallow stall would get overwhelmed by circulation from this flow (maybe? - Not sure about that...).

Am I right, simoncm?

EDIT: Like this (with the flap being artificially stalled):



My feeling is that the upwash from lower down may have to be pretty strong in order to stop the free stream flow across the top the wing turning the corner, as it were, and making the stall worse.

Is this what everyone has been getting at for a long time? If so, sorry for being thick.

@ringo, how well does just putting a ridge on the back of the wing perform to stall it? Don't worry if you can't answer this, I know the grid will need to be quite fine to see the impact.

[outer_bongolia]

This is good news. If Ferrari won't protest, there's a good chance it'll stick as legal.

"Ferrari sources have confirmed to AUTOSPORT that the team has no intention of taking the matter further, and have ruled out the possibility of it lodging a protest."

Maybe the FIA gave them a very quick clarification: "Hello Mr di Montezemolo, Mr Todt says 'pffff!' "

I think the wording was "asked for clarification", not protest. Sometimes they use this approach to ask if something is legal or not

The comment of RBR about them working on their own version was interesting, though. What do they know that we don't?

[raymondu999]

Can someone give me a link to where Horner said RBR was going to develop their own version? I can't seem to find it anywhere

[simoncm]

I see what you're getting at here. If the incident flow vector to the flap is always such that it has a relatively shallow AoA, then the impact of a stall would be quite small as the presented forward area is small. Ahhh! That main element must be doing a good job of "curving the airflow" to achieve this, mind. I also suppose the velocity must not return to free steam for a little way off the top of the wing, also, or the apparent shallow stall would get overwhelmed by circulation from this flow (maybe? - Not sure about that...).

That's what I think.

I've generated this graphic:



The top left image shows the rearward wing (flap) working normally and the lift vector. Note the extra flow from the middle of the rearward wing (representing a blowing slot).

The bottom left image shows the rearward wing when stalled (a bit), due to the blowing being stopped. The lift vector has moved forward and around since the lift from the stalled portion of the wing is lost. The drag has increased. The grey arrow shows the old force vector, the red arrow shows the resultant force (lift + drag).

Less lift (downforce), more "drag", but less backward force on the car.

The images on the right are the same as the ones on the left, except they have been rotated and inverted to look like an aeroplane wing. Also the forward wing (main) has been removed.

Bottom right is bad in plane terms (less lift and more drag) but, as mentioned, we are not talking about a plane.

[Just_a_fan]

This is good news. If Ferrari won't protest, there's a good chance it'll stick as legal.

"Ferrari sources have confirmed to AUTOSPORT that the team has no intention of taking the matter further, and have ruled out the possibility of it lodging a protest."

Maybe the FIA gave them a very quick clarification: "Hello Mr di Montezemolo, Mr Todt says 'pffff!' "

I think the wording was "asked for clarification", not protest. Sometimes they use this approach to ask if something is legal or not

Hence me using the word "clarification" in my little joke

[Pup]

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.

SLC, I think your terminology might confuse some people who are using the wikipedia articles on drag to follow along. What you're calling 'pressure drag' is in fact referred to on wikipedia as 'induced drag': induced drag; and I think what you call 'induced drag' is the same as what they call 'form drag': form drag

I think you might want to re-read that Wiki article. My usage of the drag terms is correct.

Parasitic drag = skin friction drag + pressure drag + interference drag (interference is more aircraft specific, though).

Induced drag = lift dependent drag (due to the trailing vortex system changing the effective incidence onto the wing element - this is NOT really relevant to our current stall/drag discussion).

Total drag = Parasitic drag + Induced drag

Sure. Understand that I'm not disagreeing with you - only lamenting that the definitions of these terms aren't fully explained in wikipedia, which unfortunately is the reference of choice for most. You're referring to presure-induced drag as 'induced drag', while I'm referring to dynamic induced drag.

I'm sure it seems picky, but you see how it can be confusing when I say that this is all about induced drag, and you say it has nothing to do with induced drag when we're both talking about the same thing.

The root of the problem is this...

First, let's consider dynamic induced drag, shown in Figure 4-8. If you hold your hand out of the window of a moving car, with the front edge tipped up at an angle to the relative wind to give it an angle of attack, you will feel a force pushing your hand back, but also slightly upward. In other words, depending on the angle of attack, there will be a force backward (induced drag) and a force upward (lift). The amount of force in each direction will depend on the angle of attack. If the angle of attack is small, the drag and lift are comparatively small. Any increase in angle of attack, up to a certain point, will increase drag and lift. However, at very high angles of attack, approaching the stall point, lift will decrease and the drag will overcome lift and thrust with an accompanying loss of speed and attitude. If you were to hold your hand vertical to the relative wind, the only force would be backward; that is, all dynamic drag and no lift.

Now let's consider pressure-induced drag, which can be divided into the two types. You will remember that the thin layer of air over the upper surface of the wing will break away from the wing at high angles of attack and that the flow will become turbulent as the flow of air breaks away from the wing. This turbulence results in pressure drag and loss of lift. Turbulence and pressure drag also result from the flow of air around the wingtip as the comparatively high-pressure air under the wing flows over the wingtip to the low-pressure area on top of the wing.

Perhaps it would be better to just refer to the 'wing vector' and to 'drag' as the relevant forces?

[Tristan]

Dear F1 fans,

I think you all need to get some explanation about what is going on with the McLaren rear wing. 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 Figure I enclosed (I took them 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.

Sincerely,
Tristan

[thestig84]

Can someone give me a link to where Horner said RBR was going to develop their own version? I can't seem to find it anywhere

Horner stressed that the ramifications this time were nowhere near as serious, and even admitted that his team had begun developing a similar part in case it was declared legal.

from
http://www.telegraph.co.uk/sport/motors ... ation.html

[Pup]

No, the angle of attack is not 50 degrees - you might want to brush up on your definition of that phrase. What is different with an F1 top wing is that its a highly cambered element (dual element, in fact) and as a result its resultant force vector is rather angled.

So what would be your best guess as to the angle of the force vector?

[horse]

What I mean is that it might be possible for the valve that controls the airflow to default to the 'blown' state without driver interaction.

I think this still carries the same dangers, driver controlled or not. Any system where malfunction strips off significant amounts of downforce in case of a failure is still risky.

[Pup]

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”!

That's a good thought, and it may well be that this is how they are using the wing.

But in defense of the stall, and using your chart as a reference, it does appear that the stall point of the unslotted wing is slightly lower than the slotted one - to my eye, 16° vs 18°. So if the wing in your example had an angle of attack of 18°, then it would indeeed be beginning to stall when not blown, and unstalled when blown. Though I do admit that the drop in lift, at least for this wing section, is primarily due to the effect of the slot, not the stall.

Or am I reading this wrong?

What does impress me the most is the relative drop in lift vs the increase in drag from the point of maximum lift to the full stall. I realize that in this example that is all dependent on the angle of attack. It would be fascinating to see the same diagram for an F1 wing, to see if the slopes aren't much more dramatic. That is, if in this example going from blown to unblown we see the wing go into the very beginning of a stall, in an F1 wing, would the difference be so dramatic that you'd go from a normally operating wing into a full stall.

[Pup]

What I mean is that it might be possible for the valve that controls the airflow to default to the 'blown' state without driver interaction.

I think this still carries the same dangers, driver controlled or not. Any system where malfunction strips off significant amounts of downforce in case of a failure is still risky.

Sure, but I'm not convinced that the difference is really that dramatic. I mean, we're only talking about a 10kph increase in speed over the straight, so how dramatic of a change can this be? Maybe I'm wrong, but it seems like an incremental change rather than anything dramatic, like the wing coming off.

Of course, that incremental change could make quite a difference in lap time.

[horse]

Maybe I'm wrong, but it seems like an incremental change rather than anything dramatic, like the wing coming off.

Of course, that incremental change could make quite a difference in lap time.

Yes, I suspect the effect is in no way as dramatic as a full failure as you say.

Off topic again, lap time is very interesting when considering MP4-25, because for all of this extra top-end speed, the car's lap times are not massively better than the rest of the field. Does that imply the rest of the car is not so good?

I have also asked Ciro if I can edit the opening post so I can summarise the alternative theories so far.

[snorri788]

Apologies for my ignorance, but how is the driver supposed to activate this device with his knee? The "snorkel" is quite a bit further forward than the footwell (if you look at the top down shot of the car on the mclaren website you can see it covered off) and wouldn't it be difficult to feed pipes through to the driver.
Furthermore, would it not be really uncomfortable, not to mention distracting, to have to activate a switch pressed with your knee while at high speed (say through eau rouge for instance)?
I just don't see the significance myself. Next thing is they'll use a mechanism to alter the front wing angle by using the drivers left elbow(!)