Reducing the drag of a two element wing through stall

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Post Sun Jun 18, 2006 8:19 am

I recently read the following in an in an autosport.com article:
There are also suggestions that above a certain speed some teams have managed to get their aero package to stall - which leaves them with zero drag.

:? Can someone explain to me the phenomenon of stalling and how that can be used to achieve zero drag. Is this at all possible?
Timstr
 
Joined: 25 Jan 2004

Post Sun Jun 18, 2006 9:20 am

stalling in aeronautics = no lift (upforce), stalling in racing = no downforce.

For F1 car it would be ideal if on straights they'd have no downforce (that's why the flexing wings are used). Drag decreases as attack angle of wing reduces but achieveing zero drag is more like theoretical even on wing itself because that would require absolutely neutral shape (like on-board TV cam case).

:arrow: http://en.wikipedia.org/wiki/Stalling
manchild
 
Joined: 3 Jun 2005

Post Sun Jun 18, 2006 11:31 am

But that article says that a stall is a loss in aerodynamic effect (downforce or lift) and a dramatic increase in drag....
Diesel
 
Joined: 11 Mar 2006
Location: ...

Post Sun Jun 18, 2006 2:22 pm

Diesel wrote:But that article says that a stall is a loss in aerodynamic effect (downforce or lift) and a dramatic increase in drag....


Stall will occur if attack angle becomes abnormally high or with F1 and flexi wings if wing flexes so that it changes shape and stops generating downforce (theoretically). For first mentioned extreme position is vertical and for second (F1) extreme situation is horizontal. In both cases you get stalling - wing under to great attack angle or wing flexed leading to deformation that kills its downforce generating ability. Only difference would be that wing flexing as shown on animated gif would create no drag when flexed completely.

If F1 wings would have to great angle of attack they’d also stall and generate nothing but drag. Since their flexing has effect similar to reduction of attack angle, possibility to have stall because of exceeded attack angle is out of the question.

Image

From left to right

1. Good attack angle non-flexing wing generating downforce.

2. Too big attack angle causing stall and huge drag.

3. Good attack angle flexing wing - flexing causing stall (theoretically).

========================================

Another possibility for stalling in F1 is to stall upper wing (in two wing assembly) in a way that it flexes closing the gap between itself and lower wing but in that case it actually becomes part of the lower wing.

Image

So it is not really stalled completely since it generates downforce as part of the lower wing but only changes its function without air flow below front end of its own bottom.

FIA thinks that flexing wings used now flex only this way so from Canadian GP separators are imposed to be placed between upper and lower wing to prevent closing of gap.
manchild
 
Joined: 3 Jun 2005

Post Sun Jun 18, 2006 10:50 pm

(nb. I will refer to the air flowing over the car, rather than the car passing through the air)

This flexing wing issue has always had be a bit perplexed. I understand that by reducing the angle of attack, the drag is reduced. So if the team were to manually reduce the angle, then drag is reduced. (along with downforce clearly) However, considering the method of flexing wings: if the wing requires a certain velocity of air flowing over it in order to flex, surely the energy of the air is being used to bend the wing? And I would expect the air being used to bend the wing to create a thick boundary layer, and cause lots of drag for the air above the wing?

Obviously it's been wind tunneled and CFD'd, and it works, but it confuses me a bit.

Here's a pic of what I mean:

http://img117.imageshack.us/my.php?imag ... ils5jd.jpg
wowf1
 
Joined: 5 Jan 2004
Location: Brunel University, England

Post Sun Jun 18, 2006 10:57 pm

wowf1 wrote:Here's a pic of what I mean:

http://img117.imageshack.us/my.php?imag ... ils5jd.jpg


In that picture, if you look at your manually reduced wing and flexed wing, the angle of attack is the same so the depiction of air flowing over the wing is probably incorrect or misleading.
zac510
 
Joined: 24 Jan 2006
Location: London

Post Mon Jun 19, 2006 6:52 am

There's alot more complexity to aero than can be easily analyzed with 2D pictures of wing profiles. Obviously, the 'stalling diffuser' took decades of development in the wind tunnel and with CFD to even be created.

If you think about a high-lift rear wing, you know that it causes a great deal of momentum change on the incoming airstream by diverting it upwards. A stalled wing (or diffuser) would certainly reduced it's effectiveness because the air is no longer following the profile of the wing, therefore reducing the amount of momentum change imparted on the incoming airstream. Because less air is being diverted, it actually creates less drag.

Technically, you do not want stalled wings, as they are bad for downforce, but of course, you don't really need downforce in a straight line, so flexy wings rule on the straights. I really wish they'd just legalize them so we could see some real technology be developed.
I love to love Senna.
ginsu
 
Joined: 17 Jan 2006

Post Mon Jun 19, 2006 8:08 am

2D pics are there just to make what I wrote more understandable. I know that it is very complex but this was like basic explanation. :wink:
manchild
 
Joined: 3 Jun 2005

Post Mon Jun 19, 2006 9:01 am

Timstr wrote:I recently read the following in an in an autosport.com article:
There are also suggestions that above a certain speed some teams have managed to get their aero package to stall - which leaves them with zero drag.

:? Can someone explain to me the phenomenon of stalling and how that can be used to achieve zero drag. Is this at all possible?



Stalling cannot and will not ever acheive zero drag - its a misprint or a mistake.
kilcoo316
 
Joined: 9 Mar 2005
Location: Kilcoo, Ireland

Post Mon Jun 19, 2006 11:28 am

Honestly I’m always a bit (actually, a lot) perplexed by the suggestion that the aim of gap reduction is stall of the flap because stall reduces drag. (as anybody with a bit of knowledge of aero knows that usually effect of stall isn’t exactly reduction of drag, quite the opposite)
IMO it’s possible that aero guys from teams describe it saying that closing the gap and “stalling” the flap there’s drag reduction, just to make it short, even if it’s not a totally correct explanation. Give then the term to journalists and there you are with a complete misunderstanding of the phenomenon.

I would be more inclined to think that the “stall”, more exactly, a bit of separation on the flap, is a collateral, and probably unwanted, effect while the main aim of gap reduction is more likely to modify the characteristic of the mutual interference between the two elements, hence reducing downforce and drag generated by the assembly.

In fact the dimension (and shape) of the “duct” formed by the two airfoils has quite big influence on the characteristics of the assembly; there are a few simple rules of thumb (as for example the area must be decreasing or, for maximum downforce you should have the wake of the main element barely “touching” the edge of the flap b.l. ) but obviously the optimal design depends by many parameters, most important being car speed, and by the objective you are pursuing.

One important thing to always remember if that the point of separation of the boundary layer from the surface depends by the energy of the boundary layer itself (the lower the energy, the sooner the b.l. will separate), and the energy depends by freestream velocity.

Stall is a condition that exists when the separation of the flow from the surface happens so soon that the wing basically doesn’t give lift anymore.
(notice : a symmetrical wing at zero angle of attack doesn’t give lift, but certainly isn’t stalled, the b.l. is attached for the whole chord)
A sometimes used definition is that stall occurs in the moment there’s a change of sign of the derivative of Cl-alfa curve. (ie, increasing the angle of attack you usually get increment of lift up to a point, the stall, after that point you don’t get anymore an increment of lift but a reduction)

Maybe this following diagram helps to understand it qualitatively :
[IMG:152:130]http://img131.imageshack.us/img131/9230/stallwing3qa.th.jpg[/img]

At low speed the main purpose of the gap is to avoid complete stall of the assembly (ie, separation of the b.l. quite soon). In that situation in fact, given the low flow energy, the principal advantage of the main element + flap design is spillage of high energy flow from main element high pressure side to enhance the flow on the flap low pressure side, that increment of energy helps the b.l. on the suction side of the flap to remain attached. Consequently you have to design the gap in the best way for this effect. Closing the gap there would really cause the assembly to stall and generate no downforce and lot of drag.

At high speed on the contrary the energy of the flow is lot higher so it’s very probable that also closing/reducing the gap the b.l. is mostly attached with maybe just a bit of separation near the flap trailing edge, so the advantage of the two elements wing compared with a single one isn’t the spillage, but mainly the mutual interference each element has on the other one. That mutual influence changes the velocity (hence pressure) distribution on the two elements and causes increment of downforce generated. Reducing that mutual interference you have reduction of downforce generated and, if done right, of drag.

For this reason, as I said, I doubt that separation on the flap is the aim of the gap reduction/elimination, actually I’m convinced that ideally they wouldn’t want the flow to separate because drag would be even lower without separation. A bit of separation on the trailing edge is accepted because the configuration allows to cut anyway drag compared with the original configuration (open slot), but that’s not because the flap is stalled, it’s simply because of modification on the mutual interference between the two elements.

This explanation would fit more with my experience/knowledge about multi element wings.
Reca
 
Joined: 21 Dec 2003
Location: Monza, Italy

Post Mon Jun 19, 2006 12:09 pm

Reca, are those just generic aerofoil sections or did you get them from somewhere?

If they are proper sections, is there any chance you could send on the coordinates? :)
kilcoo316
 
Joined: 9 Mar 2005
Location: Kilcoo, Ireland

Post Mon Jun 19, 2006 1:46 pm

Well, thought Id step in ;), you see, the slot size is a different parameter compared to our flexing flap parameter!

Ok, lets take one at a time.

Slot size optimisation generally kinda produces a sin wave of kinda thing for down force and as some one said, one wants to keep the flow attached all over! Little flow separtion is acceptable since the lift to drag ratio remains acceptable but too much screws things up.

Well, the aim of flap flexing is to reduce drag and as a result you lose downforce. Remember we want the flow attached, if the flow separates drag increase and our downforce becomes inconsitent, i.e. transient. So yeah, you speed up the straights, the downforce increases and the flap flexes by reducing its angle of attack reducing the drag and downforce at the same time with out affecting the lift to drag ratio of the component by much.

Hope its clear :? lol
miqi23
 
Joined: 11 Feb 2006

Post Mon Jun 19, 2006 2:49 pm

Miqi, problem is, most are saying this is the leading edge of the 2nd element moving down - so it would be increasing its AoA to the flow.


I think, indeed, its the only logical reason I can think of - stalling the 2nd element reduces the force produced normal to the suction surface [usually seen as the lift component], and since the 2nd element is at such an extreme AoA, a large proportion of the suction surface 'lift' is acting as drag and not downforce.


But even when such thick aerofoils do stall - lift doesn't 'drop-off' in a vertical line, but drag increases :?
kilcoo316
 
Joined: 9 Mar 2005
Location: Kilcoo, Ireland

Post Mon Jun 19, 2006 3:11 pm

Kilcoo, its not the leading edge of the flap, its the trailing edge. How it works is that the trailing edge is made thin and hence its chances of flexing increases.

By closing the slot size, the amount of energy being transfered reduces and stall occurs which results in an enourmous amount of drag! Hence its not acceptable.
miqi23
 
Joined: 11 Feb 2006

Post Mon Jun 19, 2006 4:11 pm

kilcoo316 wrote:Reca, are those just generic aerofoil sections or did you get them from somewhere?

If they are proper sections, is there any chance you could send on the coordinates? :)


kilcoo, don't worry, those B&W aerofoil sections I used for my sketches are from one graph you posted before :lol:
manchild
 
Joined: 3 Jun 2005

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