Downforce in wet condition

[Ciro Pabón]

hi all, 1st post, might as well get stuck in

they are both fluids for the purposes of physics and the movement of them. To take this to the extreme - if the cars drove through water would they generate more downforce? Of course they would.

It makes sense.

With regards to engines producing less power in the wet - I see no reson why this would be...

Welcome, CMSMJ1! Good reasoning. You remind me (this is some kind of joke) of the 2005 Chemistry IG Nobel prize that settled "the longstanding scientific question: can people swim faster in syrup or in water?"

About the engine: does not the water change the fuel stoichiometry? Besides, if I remember well (help me here!) water has an enormous latent heat when compared with air.

I do decrease tyre pressure, thinking ahead: when rain starts to stop, the wet tyres heat really quickly if you don't.

You have to balance that carefully: if you are able to "drive around the dry spots in the track", you better change tyres quickly: a set of dry tyres on a humid track can outperform wet tyres by several seconds. Remember Schumacher in the 2003 US GP?

Besides, wet tyres are more porous (seriously, you can feel it by touching them), they are designed to work at lower temperatures (that's logical) and they degrade quickly on a drying track. You can scrub the tyres beforehand to reduce this effect.

When you drive in the rain you can also feel clearly how downforce increases by "itself". I'm totally sure about that: if you use a gurney flap on a stock car and do not change anything, you get understeer (the tail gets heavier). Rainy air is denser.

I think you can "feel" the mere presence of a cloud over the track when you drive an open wheeler (that makes the air over the track cooler and thus changes the front wing conditions) because my daughter, who has been tested on Pan Am formula told me so, and the girl knows her trade, so when water starts to evaporate F1 cars must know it's happening.

"Driving in the wet" has been compared to "going through the house blinded" and we may have experienced how much fear inspires following another car in heavy rain, but I think it is hard to understand how fast F1 cars travel. In 1976 Niki Lauda was beaten by James Hunt because he understood. I love to drive in the rain... when I'm alone in the track.

[BreezyRacer]

I have it from a very good source that actual downforce is reduced as much as 40% for wet weather due to rain in the air and on the wings, etc.

You might contemplate that the next time you fly thru a storm ...

[checkered]

When you drive in the rain you can also feel clearly how downforce increases by "itself". I'm totally sure about that: if you use a gurney flap on a stock car and do not change anything, you get understeer (the tail gets heavier). Rainy air is denser.

I have it from a very good source that actual downforce is reduced as much as 40% for wet weather due to rain in the air and on the wings, etc.

In a way I

want to believe that your statements aren't mutually exclusive. If I've understood it correctly flow separation isn't much dependent on density, but the matter is of course much more complicated when the airfoil is affected (and indeed changed) by media in different phases. I certainly have experienced something similar to what Ciro is describing, but I have to speculate that the sensation could at least partly be because of sharper (and mostly, but perhaps not singularly, unfavourable) dynamic changes in the L/D ratio. (Edit: Or perhaps even slower dynamic changes in certain conditions, rather. As I've understood it, in dynamic downforce there are hysteresis effects resulting in time lag in the separation of flow of a certain energy compared to steady state flow. The film of water on the airfoil is a "constantly" dynamic system, so perhaps there could be unexpected side effects to this.) An overall average reduction of 40% in downforce may well be accurate, but it can't accurately reflect the dynamics in various separate instances.

There is some research on the subject of heavy rain. Two mechanisms are identified as contributing to a decrease in L/D ratio and increased incidence/earlier onset of stall:

1) Splashback, droplets accelerated at an angle from the airfoil robbing energy from the boundary layer (momentum sink).
2) Uneven rivulets (on the water film) roughening the airfoil surface.

Airfoils seem to perform the worst in high lift configurations in very rainy conditions. This would perhaps partly explain why those (Renault, for example) who had opted for dry setups fared surprisingly well in Fuji Motorpark's conditions. Had the rain been constant but lighter, it might've been a different story. I also encountered an abstract that seemed (to my best understanding) to suggest that rivulets increase such frequencies in the wake that lead to increased drag.

I don't know how much F1 teams can take heavy rain into account in their designs. I guess not much, since in very bad conditions it's unlikely that a race will be run. Most of the research seemed to center on airplane design and understanding the risks especially in approach (microbursts, rain, a lot of flap). Interestingly there were also studies on windmill blade design, suggesting that contamination (pollutants and insects) can reduce efficiency by as much as 50%. I guess one could compare that effect to "rain roughening".

[CMSMJ1]

I agree with the reasoning on both camps - That denser/wetter air caused there to be more downforce but only in a perfect world.

The disruption o fthe surfaces with the "rain roughening" will certainly lessen available downforce but I would think 40% is a very large figure to lose.

Perhaps they meet in the middle? We gain 20% addtional downforce from the denser air adn lose it by the rivulets on the carefully designed and managed surfaces??

Engine power - My motorbike loves the cold and damp air and so that is the only expereince I have with this.

tyre pressures - I would go with std pressures to ensure the tread blocks are not allowed excessive movement causing the edges to degrade and also addiotnal, unwanted heat buildup. The blocks are already designed to move and generate the heat via hysteresis (sp?)

[mep]

Forget about this engine thing.

One is clear colder air gives more power because the engine geths more
oxigen.

But I must admit that I don't know the effect of the water.

On the one side could it give more power because it gets steam and rises it's volume drastically.
On the other side those big water trobs maybe take some space needed
for oxigen or even block the burning process.
Maybe it only depends on the amount of water.
Little is positive and to much is bad.

[Ciro Pabón]

Wel, I don't know if rivulets disrupt the flow. I'm talking about a gurney, where I would say I haven't seen any rivulets (or drops!) in the full two or three inches chord the thing have. But it's clearly understeer what you get or I throw my racing gloves into the trashcan.

Besides, when it rains, air cools and thus is denser. This you can feel in endurance races at night. Of course, feeling is not the same as measuring.

Good point about the "steam potential" of water, mep.

[mini696]

You shouldn't be discussing whether colder air increases downforce or engine power, because that is proven to be true.

What you should be discussing is "Does rain affect downforce levels"?

If you were able to control the conditions, humidity, temperature etc, would a car have more or less downforce due to the rain?

[checkered]

Wel, I don't know if rivulets disrupt the flow. I'm talking about a gurney, where I would say I haven't seen any rivulets (or drops!) in the full two or three inches chord the thing have. But it's clearly understeer what you get or I throw my racing gloves into the trashcan.

Oh, sorry Ciro, I kinda

lost focus there and went back to considering mainly F1, wings and all, especially with regard to the hard rain in Japan. I forgot about the gurneys and stock cars in the process.

One could speculate that in lower speeds especially a gurney would experience a lot of "splashback", disrupting the intended flow separation and de-energising the shedding vortices. Once there's enough speed, though, it's harder to imagine what happens. Would the upstream vortex be energetic enough to deflect those drops that haven't yet met a surface from the gurney and to what effect? Press a thin film of water advancing along the previous surface against the flap, jettisoning it at a high angle from the trailing edge as a sort of a micro-spray? And it's no use thinking about the gurney separate from the system it's attached to. I couldn't find any applicable information about gurneys and rain in the time frame I can look into this.

About stock cars, one could of course speculate that understeer in rain is mainly because when the front wheels experience yaw, perhaps tyres don't function at their most effective with regard to displacing water from the contact patch. (What are the physics of viscosity etc. under the tyre on a wet surface when force vectors are dynamic and part of the rolling friction is replaced with kinetic friction?) But I'm sure you're more familiar with tyres than I am. On the other hand, I find it logical that when air is denser (in "dry" conditions), the gurney will be more effective, perhaps resulting in a discernible change in a stock car's aero balance towards the rear.

What you should be discussing is "Does rain affect downforce levels"? If you were able to control the conditions, humidity, temperature etc, would a car have more or less downforce due to the rain?

I attempted to discuss that in my previous post. I hope it wasn't completely unintelligible gibberish ... from what little I have read on the subject I understand that (reasonably heavy) rain both reduces downforce and also reduces the L/D ratio. The volume that the airfoil penetrates is of course denser as a whole, but since H2O is at its liquid phase, the interactions (the types of its energy transfer) with the airfoil negate any possible advantages from the increased overall density.

[checkered]

Well, I dug up

more, a little something on the subject of air temperature, density and rain. First, an old Autosport aricle by Will Gray - Technical Focus: The Rain-Men (link). Mostly not a very technical feature, but there's something there anyway:

Rain is acknowledged as a great leveller. A car has a limit - it's downforce and mechanical grip mean it has an ultimate, achievable, on-the-limit lap time (that is, if the driver can drive it on its limit). But the limit of adhesion is given by the grip levels offered by the car, and the adhesion of the track. In the dry, the level of adhesion offered by the track is such that the limiting factor is the car, so the cars with more downforce and mechanical grip will go faster.

When the surface becomes wet, the grip offered by the track is lower, and this becomes the limit of adhesion. The first thing a team will do is put on a whole heap of downforce - the more force they can put on the tyres, the more grip they will attain - but once they have done this, the limit is determined by the track, and how much grip it can provide - it is therefore more about the driver. For them, once out on the track, it is a question of finding the limit. This is the case in wet or dry, but in the wet the difficulty is that the limit is constantly changing not just every lap, but around the lap.

Has something changed in the past seven years, since from many comments (mainly from drivers with regard to Japan and the possibility of rain in China) it seems highly likely that dry and wet setups are pretty close to each other? No more "whole heaps of downforce". Maybe a better understanding of rain having a detrimental effect on the L/D ratio that gets proportionally worse towards higher DF setups? No such downforce attainable for the loss of speed to drag and earlier stall in corners during rain as to being able to get the most available friction from the track?

Then there's an article in the weather section of USA Today from 2004 that broadly discusses weather vs. motorsport. There's talk of NASCAR and F1, but for me the interesting bit was about ALMS and how local temperature layers affect LMP handling. Here's a quote from Auto racers' winning equations include weather by Jack Williams (link):

... track had warmed to 97 degrees, the air right above the track to 90 degrees, and the air flowing over the wing was 75 degrees. (Air right above a hot road will always be much warmer than the air only three or feet above the road.)

Such an uneven warming of the layers of air flowing around the car complicates the art of setting it up a long race because it changes the car's handling.

When the air that's flowing under the car heats up and becomes less dense, it creates less downforce in the front of the car than in the rear, where cooler air is creating downforce as it flows around the wing. The front tires lose some of their grip and the car begins to "understeer." That is the driver has to turn the wheels more to make the car go around curves.

If the car is set up to do away with much of this understeer during the hot part of the day, the front tires will end up with more grip than the rear when it cools off for the after-dark dash to the finish line. This means rear can swing out on curves as the car "oversteers."

Of course the effect of these layers will vary depending on where and how any particular class of car produces DF, but it's good to remember that these layers exist when it's dry, are accentuated in direct sunshine and are palpable to nonexistent in rain and during late night.

[checkered]

Good point about the "steam potential" of water, mep.

There was a

discussion I read at SpeedTV's forum that reminded me of that remark, it was mainly on how engines get hydrolocked. Avoiding hydrolock in the rain (link, 1st page of 3). Now this is going to take matters somewhat OT, but since it's a tech and rain related matter nonetheless, it can't be that bad. The discussion was interesting (and funny) in itself, someone there calculated that theoretically a mere 24 millilitres surviving in liquid state to full compression in a single cylinder would lock a 2.4L V8 F1 engine. Still, achieving this would propably require a direct and sizable splash entering the airbox and I certainly can't remember hearing about hydrolocked F1 engines, ever. Some other aspects and related topics were dealt with, too.

What that thread didn't discuss was how water relater misfires are produced in F1, and those definitely do exist. Heifeld certainly experienced a few in Japan. The easy guess would be electronics, but hey, get water on those and you've got a rather permanent misfire in your hands. Enough water adhering to the spark plug to bridge the gap and cause a misfire, but still less than the amount required to hydrolock the piston and the valves? Is that even plausible? Also I think that all the misfires were attributed to situations when the field was behind a safety car. Perhaps the harmonics and fluid dynamics are messed up to such a degree in those speeds and conditions that at times the mixture sucked in the cylinder is just all wrong (with the rather long valve overlap and all)?

But hey, I'm just throwing balls in the air here with no intention of catching them.

[kimi]

sorry 2 interupt u.but during d chinese GP i saw during a pit stop that the mechanic turning the front wing nut towards clockwise.so that means they were giving more front wing to the car,rite??if yes then the car previously must be in low downforce configurations..well i am bit confused.can anyone please clear my doubt??

[modbaraban]

sorry 2 interupt u.but during d chinese GP i saw during a pit stop that the mechanic turning the front wing nut towards clockwise.so that means they were giving more front wing to the car,rite??if yes then the car previously must be in low downforce configurations..well i am bit confused.can anyone please clear my doubt??

Both Ferrari drivers were complaining about understeer after the race. That's why they tried to change the aero balance I assume.

[kimi]

well i am still confused and i want to know whether they changed it from higher to lower downforce or from lower to higher downforce.coz the commentators were saying that they are giving more downforce.but how can a car run on lower downforce configurations in a wet track??

[PNSD]

Front wing adjustment is so much about downforce than it is aerobalance....