Reducing the drag of a two element wing through stall

[kalinka]

Really great simulations. Slimjim ' I just want to tell you that I am with this idea eversince 18th feb. on MP4-25 forum :

"
This is my first post. I was following this forum since the start of the 2010 testings. I am following F1 since 1989. Some explanation I found for blown flaps :

http://www.aerolab.com/Display_Pages/Flow_Vis.html

At the bottom of the page there is an example of not blown/blown pair of wings, and visible airflow around them. It's clearly visible that a blowned wing has a great advantage in retaining the flow much longer around the wing.
"

I also think since than that stalling the wing is not an option here. Your results just seems to underline that.

[slimjim8201]

I think its quite plausible that the air exiting the gap is at or significantly above the velocity of the free stream. While I don't know the cross section of the incoming supply air to this fluid path, it is safe to say that the exit cross section is minimal and most likely the bottleneck, resulting in air acceleration.

Increasing the air temperature will certainly increase the energy but I'm not sure McLaren would place a highly restrictive oil cooler in an already highly restricted flow path. This would result in the need for a larger cooler.

I'm using CFdesign. This all took me about 30 minutes...

[slimjim8201]

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...

With the steep angle of attack on the top wing section, how much DF is produced by drag (wing frontal area?) as opposed to (downward) lift?
Surely DF is a combination of the two, unlike an aircraft wing, where drag is a negative force.
This being the case, smoothing airflow over a steep aerofoil section and bleeding air into the rear low pressure area will reduce drag and therefore the DF produced from that drag a double gain in top speed.

Drag and downforce vectors never change direction. Drag is always in the opposite direction of an objects motion and lift is always perpendicular.

The numbers I have reported indicate the correct forces in those two directions.

[horse]

I'm using CFdesign. This all took me about 30 minutes...

Ah, well, now you have to validate your results...

In all seriousness though, I'm still a little confused as to why so many people keep insisting there is a stall effect going on, but not even that, some sort of lossless stall. This demonstrates you can get more down force for equal amounts of drag, but not that you can reduce both on demand.

EDIT: But hang on, you do lose some drag at 1.5 free stream injecting, which seems to cause a partial stall, so is it a case of making sure the wing is partially stalled without completely reattaching the flow?

[autogyro]

I still think that drag as well as lift is converted into DF.
So the L/D is not the important thing but the over all DF resultant is.
Reducing drag by smoothing the flow and blowing air into the rear area of the wing will reduce drag which will also reduce downforce.
The reverse of any stall attempt.

[slimjim8201]

I still think that drag as well as lift is converted into DF.
So the L/D is not the important thing but the over all DF resultant is.
Reducing drag by smoothing the flow and blowing air into the rear area of the wing will reduce drag which will also reduce downforce.
The reverse of any stall attempt.

I'm not sure I follow. Don't see how drag can be "converted" into downforce. One is a force in one direction, the other is a force in another direction. They are exclusive.

Smoothing the air flow and allowing it to stay attached will reduce drag, by reducing or eliminating flow separation which causes stall. This does not necessarily reduce or increase downforce. Depends on the application... though with most airfoils, stall results in a reduction in lift (or downforce).

[horse]

I still think that drag as well as lift is converted into DF.
So the L/D is not the important thing but the over all DF resultant is./quote]

I am assuming that the results provide this information. As slimjim said:

Drag is always in the opposite direction of an objects motion and lift is always perpendicular.

Therefore these are the resultant forces in those directions. Relative to the chord of the flap, the forces are different, but in the end, like you said the resultant drag and down force is what you're after and I assume the L/D is calculated from those resultants

[slimjim8201]

I think the confusion comes from this diagram, which is labeled incorrectly. Lift is always the force perpendicular to the body in motion, or the free stream in this case. Drag is always the force opposite the body in motion, or in the direction of the free stream here.

The green and red vectors here to not indicate drag and lift.

You are correct, the numbers that I reported indicate the forces in the correct directions and also the correct L/D ratios (based on those forces).

[horse]

Smoothing the air flow and allowing it to stay attached will reduce drag, by reducing or eliminating flow separation which causes stall. This does not necessarily reduce or increase downforce. Depends on the application... though with most airfoils, stall results in a reduction in lift (or downforce).

Would you need to have the flap blowing constantly then in an F1 application?

The green and red vectors here to not indicate drag and lift.

Yeah, that's my diagram being rubbish again, sorry. I was thinking what the foil would be doing relative to the chord line with an attached flow, but lift and drag was confusing wording for those forces. I should redraw it.

slimjim, can you split the results into the outcomes from the main and flap section? I'm interested to see which is being effected more by the blowing.

[autogyro]

I still think that drag as well as lift is converted into DF.
So the L/D is not the important thing but the over all DF resultant is.
Reducing drag by smoothing the flow and blowing air into the rear area of the wing will reduce drag which will also reduce downforce.
The reverse of any stall attempt.

I'm not sure I follow. Don't see how drag can be "converted" into downforce. One is a force in one direction, the other is a force in another direction. They are exclusive.

Smoothing the air flow and allowing it to stay attached will reduce drag, by reducing or eliminating flow separation which causes stall. This does not necessarily reduce or increase downforce. Depends on the application... though with most airfoils, stall results in a reduction in lift (or downforce).

Drag works in the reverse direction to vehicle motion. The wing mount is ahead of the wing and the drag force and the lift force work through this moment to produce downforce.

[ringo]

I think its quite plausible that the air exiting the gap is at or significantly above the velocity of the free stream. While I don't know the cross section of the incoming supply air to this fluid path, it is safe to say that the exit cross section is minimal and most likely the bottleneck, resulting in air acceleration.

Increasing the air temperature will certainly increase the energy but I'm not sure McLaren would place a highly restrictive oil cooler in an already highly restricted flow path. This would result in the need for a larger cooler.

I'm using CFdesign. This all took me about 30 minutes...

It can be like that if there is a nozzle in the fin as you say, but this can create some drag in it self. I am thinking the fin distributes the air across the whole slit, so i suspected if it's a nozzle it may not accelerate the air so much that it has a drag penalty.

A little of topic, my idea of an oil cooler was not the typical plate cooler. I have a feeling they attached cooling surfaces on top the oil cooler that project into the fin, where the air freely exchanges heat energy. A heat sink if you will.
I think one of the old F1 cars of the 80s or 70s experimented with cooling surfaces, it's possible this was employed in the fin to release extra heat, without adding too much drag. They could be gold sheets.

[ringo]

Slim jim, another little favour, try to stall the wing with the jet. Set the jet at 30* to horizontal, or any angle not tangential to the elements surface. I want to see the results on drag and down-force.
Thanks,

[Pup]

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...

I think that is really the point of this thread - to figure out why exactly it might be advantageous to stall the wing, given that we hear this so often from so many different, seemingly authoritative, sources. Like you, I find it counter-intuitive. But then, so much in aerodynamics is just that.

The only hypothesis that I can come up with as to why a stall might be worthwhile is if the induced drag on the wing is so great that it more than makes up for the added form drag that occurs in a stall. Regardless of how horse's diagram is labeled, the wing vector, perpendicular to the chord, is relatively correct, and it does indeed have a drag component as well as lift. So it's this drag that I'm curious about.

Now, what stands out to me the most in your calculations is that the drag force really doesn't change that much from a normal to a stalled state. 39.3 vs 40.4 - I'll call them "drags" since I don't know what your units are. 1.1 drags*. This surprises me, and I'd guess it would you also, given the common understanding of stalls and drag. The difference should be much more, right? So, I think there is a trade-off happening there between induced and form drag; and I suspect that if you cranked the AoA of the whole assembly just a tad, you might actually see the total drag fall when the wing stalls.

Just a theory, of course - but to me it's the only one that fits the stall=good paradigm.

Oh, yes; paradigm**.

And the second question, of course, is how one would get the wing to stall. But I'd be happy enough if I could find an answer to the first.



*approx. equal to one pink boa and a blonde wig, for those following along at home.

**sorry - sidecars - I'll send you the recipe

[ringo]

Your comment about the increase is what i was saying earlier. Maybe in the case of these almost vertical wings, drag due to stalling is not a sharp increase. Different wings have different stall characteristics, some are gentle and some stall erratically.
Sometimes the stall only means the lift coefficient drops with increased AoA but it does not necessarily indicate a drastic increase in drag. There is a diagram called a drag bucket that can illustrate this very well. I don't know of any for multi element wings though.

[cornermarker]

I like Softbatch's early idea, to investigate the effects of suction in a model. What he was referring to is demonstrated here: http://www.aerolab.com/Display_Pages/Flow_Vis.html and suction was tested on the F16-XL to reduce drag by inducing laminar flow.

Suction does the same thing acceleration does, keeps the boundary layer attached for longer that it otherwise would be=more lift.

I'm with him in thinking we should keep an open mind. All we really know at this point is that it's a rocket down the straight, and that there's a slot in the wing, and we have a flow vis shot showing they seem to be using the slot to keep the boundary layer connected. This can be done with blowing tangentially, blowing transversly and creating a laminar bubble, or by suction.

Any of these could be happening, some more likely than others. My wildest idea? Atomized water droplets being injected into the flow to increase its mass. Not likely, but unless illegal, I'm not ruling it out just yet.

Also, it would be nice to see a model closely resembling the 25's shallow airfoil, high angle of attack, small gap, and perhaps a transverse jet? I'm not convinced yet that the jet is tangential, though the flow vis certainly makes you want to say so. Wouldn't it be difficult to manufacture with the cross section so shallow. Why not just a straight line? Are the curves there to create large vortices from a separated boundary layer? Questions, questions.

Kelpster