Not sure am I doing something wrong, but it seems I can't find any lift while going in reverse.
First I've tried to go forward (max wings...) and force from wheels to ground increases by like 3 times compared to car in stationary.
But while going in reverse at same speed forces stay same as in stationary. Car should be flying with all that lift
That's not how it works. If you turned the wings upside down then you would get lift. If you go in reverse you will get a negligible amount of downforce, and definitely no lift. The lower pressure is still on the underside of the wing whichever direction you travel in.
The angle of attack has little to do with it (more to do with drag). The profile of the wing is what causes lift (downforce), and that will work properly only in the forwards direction. Backwards won't cause lift (although air under the car itself might)
Yes Tristan is right - even in reverse the top surface is curved compared to the bottom which means you're definitely not going to get any downward force (lift in the case of the car). The reason you get very little lift (when you reverse it) is because your trailing edge is curved. You need a sharp trailing edge to produce any decent amount of lift.
EDIT: And in your first picture you're going to get a very large amount of turbulence produced and the wing would quite probably have stalled at that angle of attack.
You will never lift wheels from ground going backwards too fast by the vehicle's only power. When the wheels lose enough grip, you will spin, in a inpossible case of not spinning, the wheels won't accelerate the car anymore (no grip, no traction), and the car will go back to ground...
I agree profile of wings depends a lot of lift, but still every wing should provide some lift in oposide direction no mater how it looks. Lift force is few thousand times lower in backwards than forward at same speed. IMO even with the best wing profile there shouldn't be so much difference,
thats why I put this
but imagine if you got force pushing your car independent of wheels than it would
Because in that case, the entire car is acting like a wing - if you raise the car off the ground, the car looks exactly like a wing (relatively flat bottom, curved top) and so produces lots of lift. See Mark Webber @ Le Mans, that one at Road Atlanta and so forth.
As you can see it's pretty damn similar to the cross-section of the aeroplane wing above, but the other way around (as would be expected). Hence we can use the same analysis of this F1 wing as we can an aeroplane wing
The equation used to work out the amount of lift is L = V * density * circulation. If you have a 2D cylinder travelling at a certain speed, it will produce no lift. However if you add a small trailing edge (like a little notch in the bottom right hand corner) then the system will start to produce some lift. This is because a theorem states that when there is a sharp trailing edge in a system, the system itself will induce a certain amount of circulation (kind of like air moving in a circular motion) to keep a stagnation point at the trailing edge. That's why you get lift. If you extend the trailing edge to more of a wing-type shape then you get more lift produced.
If we flip it around and instead put the trailing edge at the front (akin to going backwards) then our actual trailing edge (which is the round bit) is no longer a sharp point, and our theorem above no longer applies. Hence not much (if any - I'm not 100% sure exactly) circulation is produced by the system, and by reference to the above equation it can be seen that very little if any lift will be produced at any speed.
To flip the whole system upside down, it means you'll get downforce going forwards but practically none, if any, going backwards.
Wind speed has little to do with producing the downforce, it's the air itself flowing around the wing as the car goes at a certain speed. Of course wind can change the downforce load, depending on its direction, but the main "source" of downforce is the speed of air relative to the car. I hope I got it right, don't trust me, I'm not an engineer :P
Yes, However there was contact, but most other liftoffs, such as Nascar, contact has always been made otherwise it was blown tires etc, but It's quite common for Formula Cars also to lift off.
There are some cases of Formula/open wheel and Sportscar/Prototypes lifting off the ground whilst travelling backwards/sideways. However, the likely cause for the lift is air rushing underneath the car (either under the diffuser/floor when travelling backwards or the side skirts when travelling sideways), rather than travelling over the wings 'backwards' (as Tristan referenced earlier). Perhaps the clearest example of this behaviour unfortunately comes in the form of a fatal crash for Toru Takahashi at Fuji in 1983. It's the clearest example (that I know of anyway) because there are no other cars, punctures or kerbs involved that could cloud the issue.
I didn't say AoA has no influence. But the profile is the major factor (to do it efficiently anyway) - you don't see F1 cars running planks of 2x4 as wings.
have you ever actually seen an f1 car? they run their wings at a massively high aoa to generate their downforce
just the profile alone does hardly anything as evident from the fairly typical graph that clearly shows that a wing at 0 aoa doesnt produce even a third of the forces its capable of
for this admitedly not very f1 like airfoil the point where the drag gradient overtakes lift making it seriously inefficient to raise the aoa would be at around a massive 15° http://en.wikipedia.org/wiki/File:Lift_drag_graph.JPG
unfortunately i cant seem to find any curves like these for airfoils going though the air the wrong way round but considering even a flat profile can create enough lift to keep something flying they will undoubtedly produce a significant amount of lift
Look at F1 wings. The first element has a very low AoA, and the air coming goes upwards. The second element has a low AoA to that airflow, but larger against the ground.. The third element is even more steeply angled, but it's AoA to the air that's hitting it is surprisingly low.
Look at planes - they always fly with their wings at 15° AoA. Don't they??
Also, I didn't say that zero AoA is 'best' in any way, shape or form. You don't run a flat wing at high angles of attack, cars and planes do run profiles at lots of angles - the profile is the more important part.
Exactly. The air hits the first element, which turns the air so that it meets the second element with quite a low AoA. The effect of air 'hitting' the steeply inclined section is negligible.
If AoA was all important, then why do they bother with the 'flat' first element at all? Get rid of it, save the weight, and just have a wing at high AoA to make it work....
And cars and planes wings work in the same way - just different compromises required.
To exptand on the "optimal flow" part, I think this discussion misses something:
Downforce is not just profile of wing or AoA.
It is not as simple as putting a certain amount of planks onto cars.
Well, for some time it was:
Such construct might really generate some amount of lift in reverse.
Now also important are laminar VS turbulent air stream and vortexes.
Those are more difficult than just "high AoA = high downforce", otherwise it would not need complex sensors as these:
For example vortexes:
This car has little "wings" next to the mirror, but they purpose is not to generate downforce:
Ferrari did not think "Hey, we found an unused place to put some more small downforce planks."
The idea is to create air vortexes shape the air flow in whatever way is needed. Other parts of areo are designed to avoid vortexes or direct air towards (or away) from other elements.
Then these short vertical flap on end of wings:
(silver/metallic color)
F1 areo is more complex than in these pictures nowadays, but principle of these trailing edge things is still used:
Think one can see which areo designed in such ways only generates vertical force -whether it be up or down- when driven in correct direction.
