Pitching moment and liftoff

[MadMatt]

I am a bit struggling to understand the exact meaning of the pitching moment (and coefficient) regarding liftoff. As I understand, at 0° of angle of attack, we want negative pitching moment coefficient (I will call it Cm) to have what is considered a "stable car". It is usually observed to be just below 0 from the sources I have seen. However, when the car is pitched about its rear axle, the Cm grows.

When Cm is above 0 it means that upon a disturbance at the front, the car will pitch more and more. However, the aero loads have to be counted as well. My question is:

I have a car with let's say a Cm that crosses 0 at 4° of elevation angle (again, about the rear axle). In order to have 0N on the front axle (which means having to lift half the car's weight if the weight distribution is 50/50), we need to first overcome the aero load, and then the car weight. I have seen that this happens at about 18°.

What should I understand with this? I am a bit confused. I don't see how passed 4° the car would pitch more and more without overcoming the front axle load since the wheels are already in the air. I think I am missing something (elementary) there but cannot figure out what.

[bigpat]

Perhaps the aero 'lift' is still increasing, all the way to 18 degrees. When LMP cars were regularly flying in the late 90's/2000, the FIA conducted research into he cars to see how they could stop it happening. This resulted in the full height end plates, shark in on the engine cover, and big louvred in he front wheel arches....

Sports cars took off at only 8-10 degrees from memory. A formula car needs more angle to do so, and 18 degrees sounds about right......

Hope this helps.....

[MadMatt]

Yes the aero lift in increasing with increasing elevation angle. The LMP cars at LeMans had little downforce coefficients so they tended to get lift at quite low elevation angles. My model is a high downforce car hence lift only starts at 14°.

Now the definition of a takeoff. Stop me if I am wrong, but for me it is when the front wheels start to leave the ground (so, no aero load at the front, and enough lift force to lift (!) the car off the ground, which if we have a 1000Kg car with 50/50 repartition is about 5000N of lift), and the pitching moment coefficient is higher than 0. This means that the front wheels will not return to the ground. Because (and correct me again if I am wrong, or someone else), if the pitching moment coefficient is lower than 0, the car will return by itself onto its wheels.

The only thing is that I am not sure about this and the theory I can find (rare I must say) is not helping.

[olefud]

There’s a bit of apples and oranges in comparing LMP and F-1. The heavy aero lifter is ground effect, i.e. getting the nose up a bit and cramming air under the car. The LMP has a large area underside extending to the front of the car. This generates large lifting forces pretty much under the entire car with a central center of pressure. However, since the front is rather lighter than the rear, it lifts more which is a significant positive feedback as the angle of attack increases.

Just a guess, but I suspect that the F-1 center of pressure as the nose comes up is rather rearward with a negative feedback on rotation. From what I’ve observed, F-1 doesn’t fly without a big boost such as going over a wheel. Still, it might not be a bad idea to mount the wings to rotate with a neutral angle of attack when incipient lift is generated

[bigpat]

I'd say liftoff is when the aero load changes from downforce to lift, due to the car stalling aerodynamically. On an LMP car this occurs at 3-4 degrees from memory, and the front wheels are clearly off the deck.

Above 3-4 degrees the whole flat underside acts like a big ramp, raising the car higher until the air can spill out under the rear, at which point it makes it easier for the front to pitch over.....

F1 cars are harder to flip, as firstly the front wing are still effective at a higher angle of attack compared ton LMP front diffuser. Diffuses rely on ground effect, where as wings can irk in free stream air. Secondly, they don't have the huge flat area underneath to generate a pitching force. An LMP would typically have 9 square/mtrs of floor area to catch the air. Thirdly the drag from open wheels on a formula car would help reduce pitching tendencies.

The observation is correct that F1 flips are a result of 'assistance' from a another party....

[riff_raff]

I would say that F1 cars have far less tendency to produce net lift simply due to their limited bodywork surface area. While enclosed LMP cars are worse than f1 cars in this regard, you should also consider the case of NASCAR Sprint Cup cars. These cars weigh almost 1-1/2 tons and have virtually no aero aids (except for a rear spoiler). Yet they can quite easily become airborne when spun sideways. The side profile of the car exposed to oncoming airflow at a slight AoA produces sufficient net lift to propel a 3000lb car 10 or 15 feet off of the track surface.