Do stalled wings or diffusers provide more drag to brake?

[horse]

trinidefender, no offence, but you've just restated the point that Tommy Cookers and hollus have made. I'm not arguing the case for stalling the rear wing flap like the f-duct did, I'm more arguing that increasing drag through stall would not be a bad thing, should it be possible.

[trinidefender]

Now the people who are talking about the Veyron and mp4-12c and whoever else's rear wing that pops up to slow it down. If you look at the concepts I introduced in the last post then it should make sense. By raising the AoA of the wing like that you do two things.

1. You increase in induced drag by virtue of an increased angle of attack.
2. You increase the form drag as a result of the wing being almost a big object in the way of the air creating more drag

[trinidefender]

trinidefender, no offence, but you've just restated the point that Tommy Cookers and hollus have made. I'm not arguing the case for stalling the rear wing flap like the f-duct did, I'm more arguing that increasing drag through stall would not be a bad thing, should it be possible.

Horse I realise this. However many people do not understand why. They spout things yet have no clue why they are true or untrue. All I am simply trying to do is explain WHY the wings behave the way they do in the simplest terminology. Hopefully this will educate people and allow them to be better informed for future posts so that wrong information stops getting passed around.

That has always been my pet peeve with this forum. Many say things as fact without even attempting to back up their statements.

[horse]

OK, fair point trinidefender. I've thought for a long time we should have some educational reference (like a wiki), but I believe it would be hard to organise without argument.

I actually think the OP's question is quite an interesting one and I'm enjoying exploring the concept. I think, in terms of F1, making a wing stall like an aerodynamic brake is pretty hard (blowing the leading edge anyone?) while I've learned that maintaining the DF levels is also important for grip. Nonetheless. I still think that it would be beneficial if you could do it, which I think is what the OP was getting at.

[hollus]

To add context: The 0.66g air braking of the Veyron are not that impressive (until one considers the weight). A current F1 car at terminal speed (where engine power exactly compensates drag) has almost 1g worth of drag. Credit for that number goes to Xpensive, who also calculated it in a more straightforward way:

If you give Cv times cross-section area the value of 1.5, it should not be that far off.
This makes for simple calculations, when aerodynamic drag becomes 0.75 times density times speed squared.
Conclusively, an F1 car at 320 km/h (200 mph), has to overcome a force equal to 6700 N. That force, if acting on a 700 kg object means an acceleration of almost 10 m/s^2.
In popular wording, if an F1 car loses all power at 200 mph, it will accelerate with one g from air-resistance alone. Think about it.

So at max speed the car would slow down with 1g just by lifting the right foot, the other 3-4g coming thanks to the combined effect of tires, brakes and downforce.


As for why stalling the wing reduces drag (in a current F1 car), I make this diagram long ago, which illustrates one simplified way of looking at the induced drag mentioned by trinidefender (all arrows have the same length, representing air speed).

[trinidefender]

OK, fair point trinidefender. I've thought for a long time we should have some educational reference (like a wiki), but I believe it would be hard to organise without argument.

I actually think the OP's question is quite an interesting one and I'm enjoying exploring the concept. I think, in terms of F1, making a wing stall like an aerodynamic brake is pretty hard (blowing the leading edge anyone?) while I've learned that maintaining the DF levels is also important for grip. Nonetheless. I still think that it would be beneficial if you could do it, which I think is what the OP was getting at.

Well that is simply to do with numbers. This would only be beneficial if you can create more drag by the stalled wing (not even sure it is possible without active aero) than the extra braking force provided by the downforce from the wing. Remember that parasitic drag rises with the square of the speed. Therefore on a working wing moving at a very low speed there will be some additional braking force provided by the downforce producing wing. With a stalled wing you may get large amounts of drag helping to slow the car down at higher speeds, the drag will quickly drop off at lower speeds. Remember that parasitic drag vs speed is exponential, not linear.

I think what people should remember is that while they may think F1 cars operate at high speeds, this is simply not the case. The fastest F1 cars probably top out at around 340kph (94.4 m/s) on the fastest tracks. The speed of sound is 1236 kph (~340 m/s).

[horse]

Remember that parasitic drag vs speed is exponential, not linear.

Firstly, drag has a quadratic relationship to speed, i.e.



where







Secondly, exponential growth is when a quantity increases as a power of itself, so you are describing a quantity like this

.

EDIT: Added definitions of terms. For further reading see Drag (physics).

[Tommy Cookers]

If you can keep your lift coefficient at the same level when putting the wing into stall, then why wouldn't you [stall the wing]? I found this diagram for the lift coefficient of a NACA-0015 aerofoil and you can see that the lift coefficient at 45 degrees is nearly the same as it is at 15 degrees.

http://www.aerospaceweb.org/question/ai ... 180deg.jpg

@ horse
the above is perverse and nonsensical aka wrong
taken from some prediction for a vertical axis wind turbine (the one that's shaped like a bucket) with various aero sections

[trinidefender]

Remember that parasitic drag vs speed is exponential, not linear.

Firstly, drag has a quadratic relationship to speed, i.e.

.

Secondly, exponential growth is when a quantity increases as a power of itself, so you are describing a quantity like this

.

The first equation you mentioned takes into account induced drag. I was only talking about parasitic drag. Of which in an F1 car the main components are form drag and skin friction. These two forms of parasitic drag both rise with the square of the air velocity.

Also I know what the variables in that equation mean but I am sure many don't. Can you include labelling next time.
Fd = drag force
P = density of fluid
V = the speed of the object relative to the fluid
Cd = the drag coefficient which is a dimensionless number which is worked out by the shape and with the Reynolds number
A = is the cross sectional area

[horse]

the above is perverse and nonsensical aka wrong

Why? I'm sure the L/D ratio at 45 degrees is terrible (like a flat plate), which is the quantity which is important for aerodynamic purposes. For the purposes discussed here, it's great, lots of drag whilst maintaining the original lift.

Here is a similar diagram for a NACA0012 aeorfoil from NACA technical note 3361, available here. Lift coefficient is the lowest graph.



The recovery in lift coefficient is not quite as good for this section, but it's still there.

EDIT: Edited in light of Tommy Cookers editing the post I replied to.

[trinidefender]

It isn't great because while the drag will help a little to slow down, it will hurt everywhere else on the track. Not to mention increasing fuel consumption, which will require more fuel meaning an evens lower lap time

[Tommy Cookers]

the above is perverse and nonsensical aka wrong

Why? Just because the lift coefficient recovers doesn't mean it's a super aerofoil. I'm sure the L/D ratio at 45 degrees is terrible (like a flat plate), which is the quantity which is important for aerodynamic purposes. For the purposes discussed here, it's great, lots of drag whilst maintaining the original lift.
Here is a similar diagram for a NACA0012 aeorfoil from NACA technical note 3361, available here. Lift coefficient is the lowest graph.

http://img833.imageshack.us/img833/787/vxyl.png
The recovery in lift coefficient is not quite as good for this section, but it's still there.

@ horse, with apologies for intemperate wording

this is an aerofoil where the seperation commences but is reluctant to propagate aft, tending to form a 'leading edge bubble'
this behaviour is only found in symmetrical aerofoils, or some of the many categorisable as semi-symmetrical or similar
the section you showed is well suited to use in v/a wind turbines and in boat rudders
many LEB wings are flying today, due to poor L/D at v high AoA when Cl seems ok (as you say) they can literally be a death trap
though this persistence of Cl to v high AoA can be advantageous in aerobatic aircraft
Re has a large effect on this behaviour (failure to allow for this became a design fault in at least one homebuilt design)
low or very low AR of course always acts against seperation anyway

this Cl persistence/LEB is not obtainable from a cambered aerofoil
F1 wings eg the current 2 element wing array use heavily cambered aerofoils
without camber far more than 2 elements would be needed to give the apparent 90 deg velocity component to the air
(this maximal velocity component equates to more work being done on the air than would occur in any 'stall' flow)
so F1 wing arrays of the last 20 years would give less drag when 'stalled'

agreed (as you say) a wing design outside the F1 rules (eg variable incidence) could be more useful outside F1

[Blanchimont]

The interesting thing from the technical note 3361 for me seems to be the hysteresis at 15 degree angle of attack (and 345).
This means that for the same angle of attack there are different numbers for cd and cl available, which is exactly what F1 teams want to achieve when max cl is not needed.

[trinidefender]

this persistence of Cl to v high AoA can be advantageous in aerobatic aircraft

Tommy as you correctly stated, almost all world class acrobatic aircraft run symmetrical wings. Slight deviation from topic but another reason is that when they go inverted the wing will react in the same way so makes the aircraft very predictable. To add to that, symmetrical airfoils produce comparatively little change in centre of lift as angle of attack changes, again making the aircraft easier to fly. Neither really has a direct impact to F1 but it is nice for people to have a better understanding about different types of airfoils either way

[g-force_addict]

both lift coefficient and drag coefficient are reduced
so in F1 such stalling causes less drag, not more drag

Really?
Doesn't stall actually increases drag?