Reducing the drag of a two element wing through stall

[Callum]

Manchild, I agree with DaveW that to somehow fan more air to the rear wing would require an input of energy. You won't get anything for free unless you are scavenging the energy from some kind of source where it would otherwise be lost.

And in general I don't like the idea, surely the energy you'd put into the air from the small 'flapping' of the fin would be minuscule.



I just imagined Kimi waving his hands on the rear wing "flapping of the fin"

[wesley123]

though i beleive it would be illegal either, i mean it will be a flexing object, though in higher frequencys it will be possible to do, it wouldnt be noticed it will be moving.

[horse]

Ok, so this is been very heavily discussed in the McLaren MP4/25 thread, but I wanted to continue the discussion of the premise that the new wing configuration can reduce the resultant drag through the means of stalling the second element, possibly from a jet or ridge at the back of it.

The challenging part is that stall normally induces more drag by causing eddys and circulation where clean airflow once existed, so how do you reduce both the lift and drag vectors of the second element?

It could, obviously, be a question of relative magnitudes, but I don't have the experience myself to calculate this.

I also though this might give the MP4/25 thread a bit more room to discuss the actual car rather than this theoretical debate.

Anyway, this is what I thought the setup might be like. Please feel free to correct me, as aerodynamics is far from my first subject. I'm assuming an attached flow for the second element.

[Pup]

Obviously, this is a bit of a contentious subject. There have been a few threads on it before...

viewtopic.php?f=6&t=5230
viewtopic.php?f=6&t=2860
viewtopic.php?f=6&t=2863

...but I've yet to see the concept explained fully. At least not in language that a talking dog can understand. It's certainly counterintuitive, since the common understanding is that stalling=immense drag. Nonetheless, here is my purely intuitive understanding of why stalling on a straight might be a good thing, but it would be good to hear from someone who's aero repertoire is greater than my own Physics 101.

Taken from the MP4-25 thread...

EDITx: This is my feeling for it, if you disrupt the normal airflow on the left by injecting airflow on the right, you actually make the drag worse by redirecting the vector more in the plain of motion of the car.



Ok, you lose some of the DF vector acting in the wrong direction too, but I can't see this system being all gain and no loss. I guess the multi-element wing is a tad more complicated than this too. I think the assumption is that injecting into the suction side, would not disrupt the airflow about the wing, and that's where I would contest it unless you're energising the boundary layer to avoid separation.

I think this diagram is more like it, since the angle of attack is more accurate for the upper element. Like I said, I accept that stalling the wing is a good thing, even though I don't really understand it. But I'll give you my take on what I think is happening, and then maybe SLC can educate us both. There are two types of drag, form and induced (and friction, but who cares about that), and what you've shown as the downforce vector is what creates the induced drag, which happens when the resulting vector isn't perpendicular to the motion of the wing. So you can resolve your DF vector into a horizontal and perpendicular, and the horizontal is the induced drag. OK, you probably know that. My suspicion as to why stalling the wing works is that the induced drag is actually much greater than you've shown and the form drag is much less. Therefore, when you stall the wing, the decreased induced drag more than makes up for the increased form drag. How much more? 10-20kph more, according to SLC.

At least, that's the only way it all makes sense to me.

The reason I think this is the case, is that F1 cars are downforce monsters; and while their designers I'm sure do consider drag, I think they're more than willing to accept most any amount of induced drag even if it gives them only a whiff of downforce. Which is why the AoA of the rear wing upper elements is nearly vertical, and why the Cd of an F1 car is greater than a Hummer. F1 cars are made to just power through whatever drag they have.

So it wouldn't surprise me at all then if the induced drag of the rear wing is off the chart.

[ringo]

@horse about my last post, It's zero lift as you say.
This is at an AOA where there is no lift.
The use of the word is the problem, but you can't deny that the pressures are equal on both sides of the wing.

AoA changes is not possible with an F1 wing. To reduce lift the wing has to be "stalled" artificially, ie by equalizing pressure.

[ringo]

You are thinking changing angle of attack and single element wings.
I am going to try and draw up something more like an F1 multi element wing. Gimme 3 hours!

[horse]

Obviously, this is a bit of a contentious subject. There have been a few threads on it before...

viewtopic.php?f=6&t=5230
viewtopic.php?f=6&t=2860
viewtopic.php?f=6&t=2863

Thanks, Pup, I should have guessed this sort of thing has been discussed a few times before. Perhaps a wiki wouldn't be a bad idea, get all the best theories in one place?

AoA changes is not possible with an F1 wing. To reduce lift the wing has to be "stalled" artificially, ie by equalizing pressure.

Totally agree, this would be the ideal, but I'm not sure if injecting flow into the suction side could achieve it. Perhaps in steady state, but the time dependant dynamics of the flow (I think) would be very like a wing in stall. I would say accelerating the airflow on the pressure side would be more likely to achieve the desired effect. Perhaps...

You are thinking changing angle of attack and single element wings.
I am going to try and draw up something more like an F1 multi element wing. Gimme 3 hours!

My diagram up top is multi-element. I agree it doesn't look very F1 and it would be great to see your interpretation. Thanks.

[Shrek]

I was thinking back to a book read on stock car chassis and aero setups and i came across a saying that said if you have to get off the throttle for a turn, go for all out downforce, does that kind of work in open wheel, because if you think about it, if you give up 2km/h on the end of a straight for 1km/h in the turn, then that speed carries for a farther distance than losing that 2 km/h at the end of a straight.

[ringo]

Sounds like it is pretty reasonable, though the off throttle moments are pretty short in F1. Mclaren had some down force issues in the 4 apex turn at turkey. Lewis and kovi had to back off the throttle to go through because of understeer. The car was very lacking in down force.
Who knows if it could have been set up for more DF with a little more drag.

[tok-tokkie]

The evidence we have does not bear out the stalling hypothesis.


What you are hypothesising sounds just like an air brake. That Wiki article includes pictures of the Blackburn Buccaneer & the Space Shuttle. As a sidenote:Wiki is wrong in what it says about the Shuttle; the airbrake is applied very early in the descent glidepath. The Space Shuttle was originally going to have an engine for control during landing until the X-15 team advised them to use their system using an air brake. Set the landing flightpath for 50% air brake applied; if undershooting the runway the pilot reduces air brake &, conversely, if he is overshooting then increase the air brake. Much less weight for the same effect. You can simulate this in a car - put it in neutral as you approach a stop & you can park on the line exactly simply using the brakes.

The vector diagrams show how much induced drag there is associated with the rear wing and the gains to be made if that can be reduced while retaining the same downforce. Blowing the wing seems to do exactly that by allowing the AoA to be reduced for the same downforce.

[autogyro]

So they are not stalling the wing, simply increasing the amount of air behind it to reduce induced drag.
It will depend on how this is controlled and what effect a side flow in cornering has. Could pose stability problems.

[Pup]

What you are hypothesising sounds just like an air brake.

I suppose. I mean, any stalled wing is essentially a fixed air brake. And if you could control the degree of stall, I guess you would indeed have something similar. It's worth thinking about, because the part of the stalled wing theory that's never made sense to me is that the cars are highly dependent on their downforce for braking - for both the traction and the drag. So if you stall the wing, you'd lose braking at the end of the straight. Though I suppose you could explain this away if the car still held enough drag to slow it quickly to the non-stalled state. (Thinking here about the Ferrari from a few years back, not the McLaren.) Which it probably does.

Regardless, if the wing could be used in this way, I think it would be purely a secondary benefit. The more I run it through in my mind, the more of a "stalling" adherent I become.

The vector diagrams show how much induced drag there is associated with the rear wing and the gains to be made if that can be reduced while retaining the same downforce. Blowing the wing seems to do exactly that by allowing the AoA to be reduced for the same downforce.

Yes, though typically, you'd see this used to increase the AoA for more lift. This is why I think the true use of the McLaren wing will remain a mystery for some time, since there are two ways of using this setup to their advantage. Without knowing the numbers, we're just guessing which direction they've gone.

[Pup]

Speaking of the Ferrari, it would help to keep that car in mind when discussing this, since any theory about stalling the wing would need to fit both that case and the McLaren.

In Ferrari's case, they were using a flexible top element to close the gap between it and the lower element at high speed. Like in manchild's diagram here...



You see that it is doing exactly what I suspect that the McLaren is doing; i.e., starving the upper element of the flow of air needed to keep the BL attached to prevent stall. So I think this knocks a hole, so to speak, in the pressure-equalization theory of McLaren's "stall". I do indeed think that it is a true stall they're after, just like Ferrari.

[hecti]

I think that it works during changing yaw angles (ie when the car is turning the air hits the car at an angle slightly of center)
i think it helps air clean the flow to the central part of the wing where i presume that there could be an increase in turbulence there during cornering

[autogyro]

I think that it works during changing yaw angles (ie when the car is turning the air hits the car at an angle slightly of center)
i think it helps air clean the flow to the central part of the wing where i presume that there could be an increase in turbulence there during cornering

If it is it will be the first time they have taken airflow in yaw into account.
Most designers still think the cars run strait all the time.