Rear wing, how does it work?

[arisen]

Rear wing picture

On the rear wing in the picture, the first element seems to be a “normal” wing upside down, generating “negative lift”. Fine...

But looking the last element (the one at the back), it’s probably inclined at about 70-80 degrees. As I remember, if you increase the angle of attack of a wing profile the lift increases also, but up to about 20 degrees. After that if you continue to increase the angle of attack there’s a sudden loss in lift (I think it’s the same thing as the stall angle in aviation).

So what it the function of that last element apart from generating huge amounts of drag?

[arisen]

I guess what I want to ask is: Is this on purpose, so that the drag force acts as a momentum force trying to rotate the car around the rear wheels (see drawing), working against the weight of the car to generate extra pressure on the wheels or this is just the way the wing generates more downforce and the extra drag is just a penalty?

[Mini Bernie]

sorry Arisen, your aerodynamic theory is letting you down a little,
while you're right that a wing will experience a loss in lift after about 15 degrees, normally known as the point of stall.
If you continue to increase the angle of attack, the lift will come back, although at a much higher drag penalty.

Also it should be noted that the three elments of a rear wing work together, often acting as slats for the other elements, to energise the flow over the other wings, and increase the angle of stall, hence reducing the drag.

As for a moment rotating the car about the rear wheels? Its an interesting idea, but the speed of an F1 car is mostly dependent upon the grip it can generate, and as such anything that reduces the presure being applied between the front tires and the track, will reduce the grip available, and slow the car down, this is why downforce is so important in the first place.

Have a good one
Bernie

[arisen]

I see what you mean, thanks for clarifying that.

I was just looking at the completely different approach between Ferrari and McLaren in terms of rear wing design (same race at A1, side by side), and I was thinking there might be another reason behind it except pure wing efficiency. Helping the diffuser or the “momentum force” were the things that crossed my mind…

[Mini Bernie]

I can see where your coming from, McLaren and Ferrari do have very different wings on first sight, however there are some common elements,
for example both have been designed with the different flows the wing experiences in mind;
Ferrari have chosen to add a small lip in the centre to help deal with the disturbed air that comes off the engine cover, while McLaren have changed the camber of their wing, making it look much bigger in the centre.
Also Ferrari have the small cuts in the ends of their wings, because the ends experience a much cleaner flow,and hence you don't need as much surface area to generate as much downforce, so you can reduce the drag, and balance the car,
while McLaren have the reduce camber at this point, I think they also have a cut in the endplate to bring more air onto the final element.

These are two different solutions to the same problems,

Bernie

[Monstrobolaxa]

Hummm....I'll have to disagree with you Mini-Bernie...about stalling! Yesteday after reading your post I talked to a friend of mine (both of us are studying aeronautical engineering but I'm here because os cars)...and you can stall anything at any speed.....it really depends on how fast you're going....if you stall at 15º....eu you increase the angle of attack without increasing your speed you'll still be stalling. This is really simple to prove...just rent an airplane and stall it....if you increase the angle of attack.....you'll keep on falling...

That's why even with a high downforce configuration you'll only get some (noticable) downforce at about 40km/h.....because bellow this speed you're "stalling"! And like you know downforce is an exponencial equation!

[Guest]

Hello everybody,

I've also a question, but concerning the technical description of the rear wing aerodynamics on the f1technical-site, more precise, about the diffuser:

The volume of the diffuser increases towards to the end of the car. Where a certain amount of molecules filled for example 1dm³ under the car, these now fill 2dm³. This drop of pressure causes a car to be sucked towards the ground.

I thought the pressure rises at the diffuser, since the diffuser decreases the velocity of the air accelerated under the floor of the car. The law of Bernoulli states that if the velocity decreases the pressure will increase.

The diffuser converts the high velocity air to the velocity of the normal air stream, so the velocity under the floor increases, and thus the downforce increases.

Am I right?

[Alex M3]

Yeah, doesn't the pressure rise when the diffuser opens up the volume?

[Monstrobolaxa]

Pressure drops when the diffuser opens up to volume...how can I explain.....try filling a glass of water half way....then cover it and take all of the air out of it.....water (inspite the fact that it's an incompressible fluid, not a good example)....will tray to ocupy the space left by the air that hes been taken out of the glass. This means that the pressure drops.....another example when you fill up a ball with air....its pressure rises...if you take the air out the pressure drops and the ball crumples....this because pressure tends to stay at the outside pressure.

Another example....if the diffuser has 2dm^2 .....and the under tray has 1dm^2.......imagine the air circulating.....in the under tray you have a normal pressure of 1bar (example only)....if the volume increases and you only have 1dm^2 of air.......how are you going to ocuppy the other 1 dm^2?....this means that air will tend to expand.

If you use the formulae

PV=(rho)T

P= pressure
V= volume
rho=density
T=temp

Imagina that temp. remains constante.....and so does rho....in order to calculate the pressure you change the formulae to:

P= [(rho)/t]/V

since rho and t are constantes .....[(rho)/t]=cte.

So.... P=cte/V

and this means if you divide a constante by a number that is getting bigger....P will lower....drop.

About the guests question, yes the air tends to slow down in the diffuser....but since it creates a low pressure system, and air travels from high pressures to low pressures, the high pressure in the under tray will be sucked....which will make air travel slightly faster in the undertray section.

Another thing is that the air traveling around the car is circulating much faster then in the diffuser.....so the diffuser air is also sucked from its placing underneth the car.......

[akbar21881]

Monstrobolaxa,Do you know the area ratio of the diffuser??

[Monstrobolaxa]

no I don't....it isn't one of does number that you get hands on that often....but I can try getting hold of it....

[akbar21881]

Wah very quick reply.I better chat with you rather than passing msg in chatroom like this.

[Monstrobolaxa]

you can say that again.....LOL

Akbar do youhave a better example to show that pressure drops when volume increasses?....I think that the perfect fluid equation.....says it all....

[Guest]

Once more, as in the MP4/19 topic, someone has to go back to the books and study some more before posting some theories. Monstrobolaxa, the ideal gas law that you state is wrong. It is not p*V=rho*T but p*V=R*T where R is the gas-constant. Furthermore V is not volume but specific volume (this is 1/rho). So unfortunately the ideal gas law doesn't help you with pressures in moving situations and you'll have to go back to conservation of mass and Bernoulli. And once again: if the area increases, the velocity lowers and hence the pressure rises. And before you say that the density changes... please do the following little exercise: use the ideal gas law to calculate the density at say 15 deg C, 101325 Pa (R=287.15). Now lower the pressure by 5000 Pa (roughly Cp of -2 at say 250 pkh, which would already be quite a challenge to achieve over a large part of the car) and calculate the density again (T and R haven't changed after all). Low and behold the density variation is small (~5%). That's why for aero-problems at these speeds density is always treated a constant (if the air temperature isn't changed). This means that the area increase in the diffuser is not creating downforce as such. It is merely helping to create downforce in a different way. So please start using the brain again and think how a diffuser can help the car create downforce. (And just as a final helping hand: fluids are not only flowing from high to low pressures. It is more willing to do so, but unfortunately for the aero guys it also has to flow back from the low pressure to a higher pressure again... and that's where the design problems start)

[akbar21881]

yeah...sound logical to me...the bigger the area...velocity will decrease..to increase it we need a nozzle.but again..if the area increase,flow will expand and hence lowering its pressure coz the molecul will need to move abt more and hit the surface less,thats common sense.

so how come the diffuser help car's aerodynamic then? is it just a passage to aid the flow so that it won't turn back and disturb the flow upstream?this is because for a low speed aerodynamic like f1 car the flow downstream can influence the one upstream.so guest,can you explain what is your view about diffuser?

yeah just in case someone is wondering,F1 car is really low speed in flow classification.For a reference:

M=0 until M=0.8 =subsonic flow..low speed
M=0.8 until M=1.2=transonic region=most law is useless in this region
M=1.2 and above..high speed (supersonic)

M=mach number

f1 car does 320kmh has a long straight.at sea level this is M=0.26

Monstro,can you write down perfect fluid equation? does it has anything to do with continuity,euler,navier-stokes etc...