Reducing the drag of a two element wing through stall

[Richard]

Scarbs got it right from here. Autosport forums also stole it from here too. This was the first forum with it as far as I know.

It didn't start here, it came from the Atlas forum ...

Posted: Sun Jan 31, 2010 10:04 pm

Hmmm. What to make of this? The 'dead zone' comes full circle?

via Atlas...

I've heard a rumor from "a friend of a friend" that the McLaren extended engine cover is a little sneakier than people perhaps think. We know there is a "cooling" duct in there somewhere - and the rumor I've heard is that they are ducting air into the rear wing (and yes, I mean into - their rear wing main plane was hollow last year, which is how they ended up with an effective three element top rear wing) and due to clever positioning of exit ducts they are able to make the wing assembly stall at high speed.

I love rumors like this. So far fetched, yet so intriguing. They just won't die.

So is there room in that fin for a wee puff of air?

See page 40 ... viewtopic.php?p=142999#p142999

[slimjim8201]

I've been running some tests in 2D to determine the effectiveness of this air injection system and the results are quite optimistic. I'll be putting together some images and data later today, but the air injection most certainly reduces drag and increases downforce.

Stay tuned...

[slimjim8201]

This is an actual rear wing profile from Racecar Engineering. I've adjusted the secondary flap from it's basline orientation of 30 degrees to both 40 and 50 degrees inclination. Note the separation that occurs at the higher angle of attack.



The image below illustrates the effectiveness of adding an air injection stream to the low pressure side of the secondary flap. The first image is the basline 50 degree run, the second utilizes a gap with an air speed 1.5x that of the free stream, the third 2x that of the free stream.

[slimjim8201]

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

[slimjim8201]

Some raw data:

[SoftBatch]

slimjim8201 can you run that again with suction instead of blowing just to see?

[slimjim8201]

Softbatch,

Not sure it makes much sense to run with "suction". It's pretty obvious that McLaren are using the hollow secondary flap as a manifold and creating an tangential airblade on the low pressure side. I imagine they inject fowvis somewhere upstream to determine how well the air travels both in between the profiles and through the internal manifold of the flap. I don't really see the point of directing the flow in any other way or creating a "suction" in this zone.

Perhaps you could elaborate?

James

[SoftBatch]

Basically going back to the argument that it's possible this could be used to stall the wing to reduce the total drag (induced+pressure) to a lower level at highspeed.

What velocity was the free stream flow at?

[horse]

This is an actual rear wing profile from Racecar Engineering. I've adjusted the secondary flap from it's basline orientation of 30 degrees to both 40 and 50 degrees inclination. Note the separation that occurs at the higher angle of attack.

=D>

Beautiful bit of work, slimjim8201, very nice. Would you mind posting the grid? Would be nice to see the what sort of refinement you needed around the profiles.

Just one point though. This is a blown flap, right? The outcome of this process is increased DF at no drag cost. Thing is, many have been saying that the system is for DF + drag reduction on straights like SoftBatch just described. Do you think this system meets those targets?

[slimjim8201]

This is an actual rear wing profile from Racecar Engineering. I've adjusted the secondary flap from it's basline orientation of 30 degrees to both 40 and 50 degrees inclination. Note the separation that occurs at the higher angle of attack.

=D>

Beautiful bit of work, slimjim8201, very nice. Would you mind posting the grid? Would be nice to see the what sort of refinement you needed around the profiles.

Just one point though. This is a blown flap, right? The outcome of this process is increased DF at no drag cost. Thing is, many have been saying that the system is for DF + drag reduction on straights like SoftBatch just described. Do you think this system meets those targets?

Here is a shot of the mesh. Certainly could have gone finer, but it's not necessary to show the trends.

You are correct, this is a blown flap. If you look at the "non-blown" 50 degree run, you will notice that the wing is stalling: the drag is rising while the downforce is dropping. By "blowing" the wing, downforce is noticeably increased while drag is noticeable decreased.

[Pup]

Great stuff, slimjim. Can you tell if speed makes any difference in the effectiveness of the blown slot? That is, how does the wing work at 200mph vs 100mph. I'm wondering if there is a point at which everything goes wonky, so that McLaren could somehow tune the system to stall on it's own at a particular speed, vs having to mechanically switch it on and off.

Also, what are your units for downforce/drag?

[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...

[ringo]

Well done slimjim, i could never come close to that mesh size with solidworks.
Glad someone else took on the task.
I should have went with an existing wing profile to mitigate chances of my home made ones stalling at the first element.

One favour i want to ask you is to do the test again with wind speed at free stream.
What i also did was put the air temerature a little lower than the oil temperature, to simulate the air having heat exchange with the oil coolers.
A higher temperature should add a little more energy to the flow.

I suppose Mclaren will not have the means to accelerate the air to 1.5 or 2 times, this is why i mentioned free stream velocity or a little less to account for any friction losses in the fin.
My prediction is it should work nearly just as well.

Which program are you using?

[autogyro]

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.

[ringo]

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...

You make a good point about not wanting it to stall. I was of the same opinion until irealized how restricted the teams are with the regulations concerning the wing. They are force to settle with a 2 element wing that is clearly less advantageous than 3 at those kinds of camber.
If you take a look at my failed wing , even my first element is less cambered than what they use. The second one is almost verical too, maybe about > 45* ?
It's possible stalling becomes a benefit at those kinds of angles. My tests gave a reduction in drag, but a poorer L/D. It would be interesting if you could also obtain any wing profiles as close as possible to what is currently used in F1.
That one that you have tested is pretty mild, and wont have to resort to stalling as an improvement.
What is your take on wake control? I am seeing variations in you wake as well.