Since in beamNG we have aerodynamics, I would like to SEE the air (with an option in the debug window for example) with all the turbulence etc.. like this
BeamNG's aerodynamics is simplified. It doesn't simulate turbolence or air flow like that. Vehicles have 'Triangles'. These triangles are used for collisions (nodes can collide with triangles) and also for the aerodynamics. The game simply calculates the speed the vehicle is travelling, and compares the triangle's angle with the travelling direction. If a triangle is 90° compared to the travelling speed (ie. the front fascia when the vehicle is moving forward), that would be maximum resistency. If a triangle is 0°, that would be zero resistency. Triangles aren't influences by other triangles (ie. You have two triangles at 90° one behind the other, both of them will have maximum resistency).
Oh ok! I tought it was "realistic" because of some "experiments" with the wind app. Thanks for the answer
How hard would it be to make only the front of the vehicle contribute to drag? I ask because I have found many cars get less drag by removing grills, bumpers, bonnets and quarter panels.
Too hard... for performance, I can bet devs would find a way to make it work like in 1st post, but then this game wouldn't work on any common PC. They are still working on it and doing lots of research to make it work more realistic. Here's good explaination: http://beamng.com/threads/beamng-science.28426/#post-415585 and 2 posts below.
Would it instead be possible for attachments that reduce drag IRL (bonnets, bumpers, doors ect.) to apply a negative value to the total drag? This would have the same effect while being much less CPU intensive.
Nope, idea ain't look that bad untill you'll think about important details, but hey it's a alpha stage game ( a lot of peoples forgot about this fact ) and a lot will change. Devs are still doing researches to make it more realistic, they try to reach as realistic effect as possible without cheating physics, also it would take a lot of hours of redoing it and then testing it, while it's almost cartain that code will change, making even more work. With current code there isn't even something as "basic" as draft mechanics. Anyway making this parts with negative drag ( I don't know if this is even possible ) would make negative impact on aerodynamics realism, as hoods, fenders, bumpers, windshields etc have to produce drag to make car handle in realistic way, as this parts can produce some downforce, while with your idea without this parts you'd gain downforce effect which would work opposite to real and would made a lot of issues with car handling.
not to mention that with negative drag if the parts were to become separated from the body instead of being slowed down by areodynamics it would be sped up and it would just keep building and building speed the faster it goes the faster it accelerates.