Diffuser vs. Rear Wing

[speedsense]

Just a note of interest, but wouldn't relying on the diffuser/groundeffects be more dangerous than relying on the rear wing for downforce. I should think that the wing would provide some safety features which the diffuser could not. Better braking and stability for an unstable chassis perhaps? Would this not be a correct asumption?

Not quite sure what you mean by dangerous. Though IMHO, the diffuser by itself without wings and only has the body surface for downward forces acting on the lower pressure area created by the diffuser. The diffuser would allow what small amount of forces to occur due to the removal of the "lift" forces that get created underneath (greater possibility of lift occuring without a diffuser).
It becomes a matter of balancing that with the setup of the car and any addition elements that could be added to the body, spoilers, air flaps etc.
The same is true of adding wings along with the diffuser, the diffuser adds more leverage of downforce to the package. Balancing the wings for stability becomes the object of focus.
In my opinion it's incorrect to say the diffuser generates or creates any downforce at all but rather increases the leverage of wing's (downforce) or any down ward forces created through spoilers, body panels, flaps etc. All of these have increased efficiency with a diffuser present. Personally I prefer to describe a diffuser as an anti-lift device.
Though some may argue that it's only semantics and that "down force" and "down ward forces" are the same thing, when looking at car balance and setup balance as a whole package, there are some very fundamental differences between the two in when considering stability.
A note about braking ability. A winged car has far greater braking ability than without wings even with a diffuser on either. The air drag (and slightly increased wing angle due to pitch) produced at the initial strike and beyond of the brakes being the difference. Diffusers offer no drag penalty that helps braking.

[F1_eng]

speedsense, I find it very difficult to follow the points you are trying to make in your posts, they seem totally random and all over the place.

What exactly are you trying to say about diffusers?

What do you mean by "diffuser"?

[Scotracer]

Just a note of interest, but wouldn't relying on the diffuser/groundeffects be more dangerous than relying on the rear wing for downforce. I should think that the wing would provide some safety features which the diffuser could not. Better braking and stability for an unstable chassis perhaps? Would this not be a correct asumption?

Not quite sure what you mean by dangerous. Though IMHO, the diffuser by itself without wings and only has the body surface for downward forces acting on the lower pressure area created by the diffuser. The diffuser would allow what small amount of forces to occur due to the removal of the "lift" forces that get created underneath (greater possibility of lift occuring without a diffuser).
It becomes a matter of balancing that with the setup of the car and any addition elements that could be added to the body, spoilers, air flaps etc.
The same is true of adding wings along with the diffuser, the diffuser adds more leverage of downforce to the package. Balancing the wings for stability becomes the object of focus.
In my opinion it's incorrect to say the diffuser generates or creates any downforce at all but rather increases the leverage of wing's (downforce) or any down ward forces created through spoilers, body panels, flaps etc. All of these have increased efficiency with a diffuser present. Personally I prefer to describe a diffuser as an anti-lift device.
Though some may argue that it's only semantics and that "down force" and "down ward forces" are the same thing, when looking at car balance and setup balance as a whole package, there are some very fundamental differences between the two in when considering stability.
A note about braking ability. A winged car has far greater braking ability than without wings even with a diffuser on either. The air drag (and slightly increased wing angle due to pitch) produced at the initial strike and beyond of the brakes being the difference. Diffusers offer no drag penalty that helps braking.

A diffuser does create downforce and improves the efficiency of the underbody of the car too, Speedsense. At the kick of the diffuser you get a low pressure/downforce improvement and also get mass flow increase under the car.

You appear to be all over the place with this post. Did you write it in more than one sitting?

Also, this appears to be the first ever justification for increasing the drag of a car.

[riff_raff]

sknguy,

"wouldn't relying on the diffuser/groundeffects be more dangerous than relying on the rear wing for downforce."

However aero downforce is produced, whether by an underbody/diffuser or by a front/rear mounted airfoil, what's important is how stable the performance of that aero device is at all times while the car is on the track.

Underbody/diffusers can be very hazardous, as shown by the flat-bottomed '99 Merc Le Mans incident:



But wing downforce can also be sketchy. Just ask any open wheel chassis driver (especially Champ Car super speedway oval track racers) about how their car loses front grip and dangerously understeers when they get up close to pass another car. Or even how a trailing car can cause them to lose rear grip and oversteer, when it follows too closely.

Once again, I personally am enjoying this discussion. And I hope it will continue.
Regards,
Terry

[ringo]

I don't think it should be a vs. arguement. They both complement each other, but i would chose to go with having the wings most of the time.
Diffuser performance is dependent on the shape at the trailing edge of the car, they work better when there is flow separation and low pressure behind a vehicle with a big rear end, so the down force isn't really free. Lower pressure behind means bigger pressure differential with the air at the front of the car so more drag.

What I am interested in is the extent to which a diffuser and floor can contribute to down force.

[hydrop]

In the early 2k's, Nasa patented a natural trailing vortex annulation technic by forcing the two trailing vortex to encounter each other; This reduced to distance of wake from 7km to 5km on large planes like 747's.

Musashi, any chance you could please expand on this, or provide me with a link abut this trailing vortex technique?

thanks

[richard_leeds]

sknguy,

"wouldn't relying on the diffuser/groundeffects be more dangerous than relying on the rear wing for downforce."

However aero downforce is produced, whether by an underbody/diffuser or by a front/rear mounted airfoil, what's important is how stable the performance of that aero device is at all times while the car is on the track.

Underbody/diffusers can be very hazardous, as shown by the flat-bottomed '99 Merc Le Mans incident:

But wing downforce can also be sketchy. Just ask any open wheel chassis driver (especially Champ Car super speedway oval track racers) about how their car loses front grip and dangerously understeers when they get up close to pass another car. Or even how a trailing car can cause them to lose rear grip and oversteer, when it follows too closely.

Once again, I personally am enjoying this discussion. And I hope it will continue.
Regards,
Terry

I dont think its about under vs over aero, I think its design specific. People have their opinions about the cause, In my opinion, it is the way they limit aero that pushes everyone towards certain designs that are sensitive. There are some designs of underbody tunnels that would take a whole lot more pitch before the car went into lift.

[ffangio]

In an attempt to drag us out of the quagmire of drivel being posted in this topic (Ogami musashi I'm looking at YOU)...

Cars with no wings and equivalent underbody-generated downforce WILL promote more overtaking than cars with front and rear wings.
Reasoning goes as follows:
- floors with a diffuser at the rear (forming a venturi - there is no distinction between the two) are far more efficient aerodynamic devices than wings outside ground effect (i.e. the rear wing)
- therefore for a given amount of downforce we will have less drag and therefore overall a smaller wake
- also, diffusered floors produce less upwash than a wing out of ground effect so the smaller wake will have a smaller vertical velocity component
- following cars will be less affected because (1) the wake is smaller so there is more available total pressure with which to create downforce and (2)the reduced upwash means the following car is effectively at a greater angle of attack = more downforce
- having more downforce around a corner means higher exit speed and therefore more chance to overtake on the straights

The 'but's...
- Unless you have a full length floor a front wing will still be needed to get any sensible level of aero balance
- Normally regulators baulk at the idea of increasing car top speeds and rear wings are good air brakes
- Having a smaller wake means less drafting and therefore a smaller drag benefit from tailing the car in front

The most successful designs from a pure overtaking point of view have been e.g. ChampCars at superspeedways where they used the Handford lifting rear wing for drag and relied purely on underbody downforce. It was a farce because you could never tell who was going to win until the last lap but some people like that sort of racing.

[Ogami musashi]

I'm very happy that you call the works of bryne, brawn, lowe and many other aerodynamicists of the F1 field producers of "quagmire of drivel"...


1/ Diffusers do produce a wake and it stays longersn (due to ground proximity) AND at ground level (just so that it spoils the following car's front wing and venturi...) Which is clearly not what is wanted.

2/The upwash is not the problem...it is the angle of the upwash..and clearly the steeper the better, just simple geometry there...
And in addition the problem with the upwash is not the loss of the total pressure but the velocity components...hence why a steep upwash is better because it affect the velocity component only in close proximity of the leading car.

As for you "but's" the draft doesn't come from the wake of the wings (they localy produce MORE drag on the following cars) but from the recirculation from the body....


In contrary to you i'm not telling that out of my hat, those were the conclusions of both OWG and several other research studies done with both wind tunnel and CFD analysis.

[DaveW]

Musashi, any chance you could please expand on this, or provide me with a link abut this trailing vortex technique?

This is not the one, I think, but it may be useful.

[ffangio]

I'm very happy that you call the works of bryne, brawn, lowe and many other aerodynamicists of the F1 field producers of "quagmire of drivel"...

Of course not, I just call your explanation of their work total drivel.
"Angle of upwash" is a meaningless term - more upwash = steeper angle of flow.
Also, just because a wake is closer to the ground doesn't mean it "stays longer" - you've made a nonsense assumption there somewhere.

However, neither of those display as much lack of understanding as the statement "the draft doesn't come from the wake of the wings (they localy produce MORE drag on the following cars) but from the recirculation from the body"...!

A wake is a recirculating region of flow with a velocity component in the direction of the car that created it. If a car is running in that wake it will generate less lift as a result of (1) UPWASH giving a more nose-up incidence and (2) the reduced onset velocity.
Downforce is a function of v squared so if your v goes down, you produce less downforce...
Anyone with a moderate grasp of physics will at this point be thinking "but drag is a function of v squared too!" and they would be right so the drag goes down as well.

If you had read and (more importantly) understood the OWG report I suspect you would have known that already.

[Ogami musashi]

Of course not, I just call your explanation of their work total drivel.

Straight from paddy lowe circa December 2008

"You would think that upwash from the rear wing is bad," Lowe said. "The upwash is strong, but a very strong inwash at ground level is also driven by the rear wing. That inwash brings new high-energy air in at ground level. If you took the rear wing off altogether you would lose that effect and the wake would be a lot worse."

If at your next message you don't go on to more polite form..you'll be the first on my ignore list.

[gixxer_drew]

...

If you dont mind, can I ask a couple more questions about this? This is an interesting topic for me. Was that conclusion solely in the context of wing or diffuser? Or wing plus diffuser, all of the above. Because at first thought I wondered... if the rear of the car was a blunt face it would also be pretty "dirty" air ... how dirty the air would be on a diffuser would be more design dependent no? You can compromise a design toward downforce or for wake fill and if the wing is drawing the primary vortices behind the car upward , wouldn't that still apply with a diffuser? Especially one designed to minimize turbulence in the wake.

[richard_leeds]

I dont think its about under vs over aero, I think its design specific. People have their opinions about the cause, In my opinion, it is the way they limit aero that pushes everyone towards certain designs that are sensitive. There are some designs of underbody tunnels that would take a whole lot more pitch before the car went into lift.

There are also other way to avoid losing downforce and going into lift –particularly vehicle-launching lift. For the latter, wings could be mounted to avoid lift with a forward pivot and downforce stops that would allow the wing to change its AoA if it generates lift. Tunnels such as diffusers are a bit more difficult but blowout panels could do much the same should positive pressure develop undercar. Attitude-sensitive spoilers are another means to kill lift.
A little thought about unusual attitudes will bring to mind a number of means to kill unwanted lift. This would avoid much of the danger of aero devices of any ki

[richard_leeds]

There are also other way to avoid losing downforce and going into lift –particularly vehicle-launching lift. For the latter, wings could be mounted to avoid lift with a forward pivot and downforce stops that would allow the wing to change its AoA if it generates lift. Tunnels such as diffusers are a bit more difficult but blowout panels could do much the same should positive pressure develop undercar. Attitude-sensitive spoilers are another means to kill lift.
A little thought about unusual attitudes will bring to mind a number of means to kill unwanted lift. This would avoid much of the danger of aero devices of any ki

I tend to agree. Plenty of clever and elegant devices have been used. Rules are the limiting factor, IMO