> Okay, when I said "let's talk about aerodynamics..." it was a JOKE!
> Ha ha, it is to laugh! Enough already! Mercy! Look, I'm the poor
> programmer who gets to implement all this stuff. My head hurts! ;<)
> Todd
> >> Oh no, formulas, please no formulas :o). Lets wait with the
> formulas until we have to
> >> calculate something and instead concentrate on how to understand
> this phenomena, okay :o)?
> >I'd advise starting with basic formulas, actually.
> >You can do an awful lot of vehicle dynamics/performance
> >analysis with them. (This is only as deep as I go working
> >in dynamics and performance, and I have a MS in aerodynamics.
> >In order to go deeper, one hires a specialist in the field ;)
> >Really "knowing why" some shape drags or lifts
> >is complicated because you have to deal with the
> >"boundary layer" between the car surface and the
> >pretty-much-smoothly-flowing air. If you don't
> >include these complicated effects (or cheat by throwing
> >in a few experimental observations) you can't even
> >do a theory that predicts lift or drag. ("Inviscid Flow")
> >The air splits and flows around the vehicle. At the car
> >surface, the velocity of the air is equal to the velocity
> >of the surface - it's drug along with the vehicle. Even more
> >air is drug along not-quite-so near the surface to some extent,
> >just not fast enough to be as fast as the vehicle.
> >Speeding up all this air decelerates the car somewhat
> >(Newton's equal and opposite reactions, and all) - call
> >it friction drag.
> >The "static pressure" of the airflow
> >(what would be measured by a pressure port at the surface
> >and what creates the force that presses onto the surface)
> >drops as the air molecules flow faster. They have to
> >flow faster or slower to make changes in direction.
> >Going up the windshield the molecules slow and the
> >static pressure is higher. Going down the back of the car
> >they speed up and the pressure drops. When it drops
> >far enough, the air very near the surface actually is
> >forced back forward by the static pressure gradient
> >(rate of change). The boundary layer "Separates" and you
> >get a lot of air roiling around behind the car.
> >The pressure here is relatively low. So we have
> >high pressures near the front of the car, and lower pressures
> >at the back. Pressure times area is force,
> >so the summary is we have a net force aft. Call
> >it "Pressure Drag" or "Form Drag".
> >When you use Lift to get more downforce
> >what you are doing is forcing a lot of air upwards
> >to get that downforce. The average force to
> >change the direction of all that air slightly
> >upwards has a component aft as well - call it
> >"Induced Drag", for drag induced by lifting.
> >Finally, if you get near supersonic speeds,
> >you will add an important drag effect from shockwaves.
> >"Wave Drag".
> >> At the front of a moving car there is something trying to push the
> car backwards - air
> >> resistance. In front of the car there is of course air and to place
> the car where the air is,
> >> we have to move the air, and as we all know air has mass, so this
> will need some force :o). I
> >> guess we can do this in two way, either push the air in front of us
> (a flat front) or push it
> >> aside (a front formed as some kind of projectile). I wont try to
> understand why pushing the
> >> air is a bad idea, but we dont want a parachute effect, I guess. So
> we push the air aside;
> >> over, under and to the sides of the car.
> >Actually, if you don't include the effect of the complicated friction
> >forces you are doing "Inviscid Flow" above and your theory will
> >eventually
> >find the air will part around the vehicle, the pressure will be
> >different around different parts of the vehicle, but the overall net
> >effect of all those differing pressures is zero lift, and zero drag.
> >> The amount of air pushed aside must be approximately the front area
> of the car (take a picture
> >> of the car from the front, and any part of the picture covered by
> the car is the front area)
> >> mulitplied with the length we have travelled. This mass of air
> being moved is of course moved
> >> faster the faster we go. The faster we have to move the air the
> greater force needed to do so,
> >> hence this force increase with speed. To calculate the force needed
> could be tricky, because
> >> the air doesn't only get moved it also compresses and the
> compression increases with speed.
> >This is why I suggest starting with the basic equations useful for
> >vehicle dynamics. You have just made a justification for why
> >drag is commonly estimated as proportional to the area
> >and also proportional to the "dynamic pressure".
> >Density times velocity times velocity divided by 2 (just for fun.)
> >(Unless you get near sonic speeds, the air can be considered
> >"incompressible" (just like water) so the air really
> >doesn't compress noticibly.)
> >Hehe, I just glanced at "Fundamentals of Vehicle Dynamics,"
> >by Gillespie, S.A.E., Warrendale, PA, 1991, ISBN 1-56091-199-9
> >and find these are the FIRST and SECOND equations in 30 pages
> >on aerodynamics ;)
> >In practice, you use the equation:
> > drag = drag_coefficient times area times dynamic_pressure
> >for everything, including understanding that if you make the
> >car twice as big, and go twice as fast then you will need
> >six*** times the engine power portion alloted to fighting drag.
> >If you want to know why the car gets less drag with the
> >new wing airfoil in place of the old one
> >("what is the drag_coefficient")
> >you ask your aerodynamicist ;) (Or learn enough
> >aerodynamics to guess yourself)
> >> What's turbulence?
> >See Gilespie's aerodynamics chapter ;)
> >- Matt
