Reducing the drag of a two element wing through stall

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tok-tokkie
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Joined: 08 Jun 2009, 16:21
Location: Cape Town

Re: Reducing the drag of a two element wing through stall

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slimjim8201 wrote:Some raw data:

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Image

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Many thanks for the simulations. I have been completely unable to follow the argument saying stalling the wing is going to be a benefit.

I am very surprised by the LD ratios. 30° without air injection is the best of the 5 shown. So downforce is what you really need; even if it does spoil the LD ratio.

I have posted this image previously. It clearly shows streamlined flow. A stalled wing has absolute turbulence. I could not follow how stalling could be discussed in the face of this evidence.
Image

I tried to calculate the stagnation pressure of the freestream airflow. At 200kph I got 0.02 bar. Do you know the pressure and quantity required to obtain 1.5x & 2x the free stream velocity?

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ringo
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Joined: 29 Mar 2009, 10:57

Re: Reducing the drag of a two element wing through stall

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Keep in mind the wings are completely different.
For Sure!!

SoftBatch
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Joined: 29 Jun 2006, 21:53
Location: Madison, AL, USA

Re: Reducing the drag of a two element wing through stall

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I see the only benefit for the stalled wing would be to reduce the total drag of the wing at top speeds.

Basically at the speeds of 250 kph to 320 kph (~150-200mph) would the induced drag of the wing be so high that causing the wing to stall and transitioning to almost purely pressure drag actually reduce the total drag of the wing allowing a higher top speed because the down force would not be needed while at speeds in that realm (on the straights).

Devils advocate - The cars still need massive amounts of down force at the end of the straight when under braking.

CMSMJ1
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Joined: 25 Sep 2007, 10:51
Location: Chesterfield, United Kingdom

Re: Reducing the drag of a two element wing through stall

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If this is a theory to be looked at..a couple of questions.

1: Is the inlet to this wing is created in the 150mm (or is it 75mm) zone that allowed piggy back wings on the crash structure?

2 If so, then this small inlet has to then divert the flow into the hollow wing to then be diffused out of the "magic slot" (tm)

3: The turning of the flow and the bottleneck into the wing is going to cause problems isn't it?

Anyone care to speculate on the internal structure of the wing and how it keeps structural integrity with a large slot out of the back plane?
IMPERATOR REX ANGLORUM

cornermarker
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Joined: 18 Feb 2010, 03:05

Re: Reducing the drag of a two element wing through stall

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CMSMJ1 wrote:If this is a theory to be looked at..a couple of questions.

1: Is the inlet to this wing is created in the 150mm (or is it 75mm) zone that allowed piggy back wings on the crash structure?

2 If so, then this small inlet has to then divert the flow into the hollow wing to then be diffused out of the "magic slot" (tm)

3: The turning of the flow and the bottleneck into the wing is going to cause problems isn't it?

Anyone care to speculate on the internal structure of the wing and how it keeps structural integrity with a large slot out of the back plane?
I like your line of questioning. This shot gives us a good idea of the inlet, the bulge where wing and fin meet.

Image

I'd suggest that the loss in integrity created by the slot is more than made up for by the fin. Note how it flattens out toward the endplates. There really isn't much space in there, which is why I have trouble with the theory of a tangential jet. In that amount of space, how to you forcefully direct air upward along a surface that has so little curvature? And this mechanism has to curved, just as the slot is.
ringo wrote:Keep in mind the wings are completely different.
Thanks to slimjim, we have a great idea of one of the possible phenomena. What we now need is a more accurate representation of the 25's wing, illustrating each of the possible uses at their respective speeds. By all possible uses, I mean tangential slot jet in crossflow, transverse slot jet in crossflow, and vacuum. At this time, it seems to me that any one of these has potential to be useful in a racing environment.

So who's up for that?

Kelpster

Pup
Pup
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Joined: 08 May 2008, 17:45

Re: Reducing the drag of a two element wing through stall

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CMSMJ1 wrote:1: Is the inlet to this wing is created in the 150mm (or is it 75mm) zone that allowed piggy back wings on the crash structure?
I don't know the numbers, but it doesn't look like they are using the entire allowed width. Compare it to last year's slot, or to the ones on some other cars and you'll see.
CMSMJ1 wrote:3: The turning of the flow and the bottleneck into the wing is going to cause problems isn't it?
Possibly, but at this point, it's a wild guess if it does. And I think there's room for them to smooth out the transition if they need to.
CMSMJ1 wrote:Anyone care to speculate on the internal structure of the wing and how it keeps structural integrity with a large slot out of the back plane?
A curved piece of carbon fiber is pretty darn strong. The thickness in the wings isn't so much structural as it is for creating the airfoil section they want. Plus they've got the fin supporting the top element, a strut supporting the lower, and a stiffening flange connecting it all.

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outer_bongolia
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Joined: 13 Feb 2009, 19:17

Re: Reducing the drag of a two element wing through stall

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I would like to add another part to the discussion: the internal structure of the upper element.

Now that the simulations have established that the direction of the injected flow is very important, I assume McLaren has some kind of a system adjusting that. I guess (hope) that they don't just hope the fairies will ensure a nice direction and distribution. :twisted:

Would you have any insights to that?
Skeptical scrutiny is the means, in both science and religion, by which deep thoughts can be winnowed from deep nonsense.
Carl Sagan

Raptor22
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Joined: 07 Apr 2009, 22:48

Re: Reducing the drag of a two element wing through stall

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ringo wrote:I drew a 2 element wing and ran it at 90m/s; over 200mph. This wing is my creation, it may as well be rubbish :lol:

Image
^^Standard wing, no blown flap. this is velocity not pressure, (do not use this to visually determine where the high pressure is, there are density changes as well.)
Image
^^tangential blowing.
observations, blowing tangential to the 2nd element surface, 90m/s 1atm.

1) no effect on top of main element, though slot is less choked. The separation point is move slightly backward beneath mean element.

2) no major difference to boundary layer, maybe reduces it's thickness which is very minsicule.

3) Slight difference in velocity gradient near trailing edge, and has a pull on the air on the front side of 2nd element, changing velocity gradient there as well.
4) wake is unchange.

What I observed is that, blowing at the second element does little to change the wing performance. Reason being the velocity is already high on the elements surface,and there is no danger of separation because of the strong up-wash coming through the slot.
I also observed that the boundary layer seemed to be quite small, which is good. No major differences though.

From observing the existing flap blown planes, flap blowing is best done in the slot. Having it at the middle of the second element is just in the wrong place. It would do well in the slot or on the bottom of the first element. It is also meant for blunt thick trailing edges where separation occur, such as in the pocket of a wing where the flap retracts in.

So with the understanding that blowing tangentially to the elements surface is redundant because the boundary layer is already highly energized; I angled the blowing away from the surface.
Image
^^blowing at an angle
This of course "stalled"?? the element leaving a detached boundary layer on the back side of the element.

observations for blowing at and angle at 90m/s, 1atm. and 373k :

1) Though the air jet was at free stream pressure, it quickly diffused to the local pressure immediately behind the wing.

2) The off shot jet was diverting the flow from off the wing and out into the wake. leaving a stalled area on the element surface downstream of the jet.

3)The wake was being noticeably distorted by the jet.

What i realized from this is the fact that the element stalled has little to do with the total outcome on the drag. The stalling takes place in the wake because of the almost vertical element. It does not increase the size or height of the wake.
This is slightly different than an aircraft, where the wake is slightly behind and stalling of the top surface can increase the wake size.

The whole point is not realy stalling the wing. Stalling is part of it. I think the bigger picture is passive wake control. The angle of upwash from the wing is virtually an extention of it's angle of attack. The up wash is also connected to the wake. If the up wash is pulled in, it will virtually reduce the AOA and the drag. Varying jet speeds will pull on the wake with varying strength.

This slot more than likely jets air outward from the element into the wake and distorting it, maybe that is why we saw that gap in the flow vis, the flow shot off into the wake.

I got some results, which could be completely off because of the size of the computational domain that i use. My PC crashed a couple times looking for the right level of detail.
The reductions in down-force were very small, and reduction in drag was also small.The made up wing has a L/D ratio of 0.9148 .

from normal flap to blowing tangentially: Lift/Drag ratio 0.91566 ,.32% drag reduction, .229% DF reduction

from normal flap to blowing into the wake: L/D 0.9128, .5% drag reduction, 0.71% DF reduction

And remember my wing profiles are made up!, this is not a true reprsentation! :mrgreen:
These numbers and opinions are also to be taken with a grain of salt!!! I jut put them there to give an idea of the scale of the improvements.

Nice work, but you do have to take the Diffusor and lower rear wing element into account as well.

The flow under the main element seems to detach to early.

Raptor22
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Joined: 07 Apr 2009, 22:48

Re: Reducing the drag of a two element wing through stall

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slimjim8201 wrote:Here is a close-up view of the air under the secondary flap. Note that as the air injection is increased, flow remains better attached to the profile.

Image 1 - No air injection
Image 2 - 1.5x free stream velocity injected
Image 3 - 2x free stream velocity injected

Image

=D> brilliant.

thats the sort of flow diagrams I would expect with a blow a flap.

Raptor22
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Joined: 07 Apr 2009, 22:48

Re: Reducing the drag of a two element wing through stall

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slimjim8201 wrote:It seems that everyone thinks that the teams are trying to stall their wings at high speeds to reduce drag. This goes against everything that I understand about aerodynamics. Please feel free to chime in, but every form of aero stall I've encountered has resulted in a loss of lift and an increase in drag.

I would think that the teams are devising methods to avoid stall, not induce it...

=D> =D>

Now could you please have a telcon with SLC who is a Phd in Ground vehicle Aerodynamics and works for an F1 team and explain this to him very carefully. Not sure were he bought his PhD from but my undergrad led me to the same conclusions you did. You just illustrated it beautifully through using the tools at your disposal extremely effectively.
+1
you are an asset to this forum.

cwatson
1
Joined: 15 Jan 2010, 08:05

Re: Reducing the drag of a two element wing through stall

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It would seem they are trying to get as close to a three-element wing as possible while staying within the letter of the rules. As for whether or not allowing the wing to stall is beneficial for drag - I'll run some 2D cases and project to 3D to estimate the drag when I get a chance (3D cases take a while).

autogyro
53
Joined: 04 Oct 2009, 15:03

Re: Reducing the drag of a two element wing through stall

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Exactly raptor a brilliant drawing.
It shows that DRAG is the main DF component in a nearly vertical rear 'wing' and not lift.
the flow injection is not to stall a conventional 'wing' but to reduce drag from an 'air brake'.
Check out the Hunter 6 rear fuselage airbrakes and before that the Mustang Naca duct on its belly radiator intake.

Raptor22
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Joined: 07 Apr 2009, 22:48

Re: Reducing the drag of a two element wing through stall

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autogyro wrote:Exactly raptor a brilliant drawing.
It shows that DRAG is the main DF component in a nearly vertical rear 'wing' and not lift.
the flow injection is not to stall a conventional 'wing' but to reduce drag from an 'air brake'.
Check out the Hunter 6 rear fuselage airbrakes and before that the Mustang Naca duct on its belly radiator intake.

ah yes i had completely forgotten about the ventral oil cooler on the P-51, a good example of filling in and boosting air flow to reduce drag.

But hold on with the Drag being the main Df component.
Drag is result of the lift no an added force. the two go hand in hand, yin and yang.

Up the lift, y9u almost always up the drag. But through boosting or blowing the wing profile the wing actually "thinks" its workngs at lower angle of Attack hence the lower drag but still high lift (downforce value)

As I mentioned in the MP4/25 thread, stall is great for slowing things down, not speeding them up.

autogyro
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Joined: 04 Oct 2009, 15:03

Re: Reducing the drag of a two element wing through stall

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Raptor22 wrote:
autogyro wrote:Exactly raptor a brilliant drawing.
It shows that DRAG is the main DF component in a nearly vertical rear 'wing' and not lift.
the flow injection is not to stall a conventional 'wing' but to reduce drag from an 'air brake'.
Check out the Hunter 6 rear fuselage airbrakes and before that the Mustang Naca duct on its belly radiator intake.

ah yes i had completely forgotten about the ventral oil cooler on the P-51, a good example of filling in and boosting air flow to reduce drag.

But hold on with the Drag being the main Df component.
Drag is result of the lift no an added force. the two go hand in hand, yin and yang.

Up the lift, y9u almost always up the drag. But through boosting or blowing the wing profile the wing actually "thinks" its workngs at lower angle of Attack hence the lower drag but still high lift (downforce value)

As I mentioned in the MP4/25 thread, stall is great for slowing things down, not speeding them up.
The near vertical wing section in your first drawing is already stalled and it is mainly the drag that is working to produce DF through the moment between the center of 'drag' and the wing mounting point.
When you bleed air to the rear surface, the drag is negated and replaced with a lift force now working perpendicular to the chord of the wing.
The result is a smoother airflow and far less drag for a similar DF.


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