Quote:
Originally Posted by Vman455
The pressure coefficient at the rear of the roof does not matter, in and of itself. Lift is determined by the effect of the pressure over the entire surface area. This figure clearly shows that the notchback achieves higher peak pressure than the fastback, at the base of the rear windshield, while the figures comparing lift and drag show that the total pressure on the rear of the cars is higher on the notchback than on the fastback.
Pressure can be thought of as a scalar, while the surface area is represented by an area vector A. Think of the surface area of the car body as a bunch of small area "cells," each a vector dA normal to the surface at that point, with a certain discrete pressure p acting on each cell (directly proportional to the pressure coefficients in the figure). Multiplying each surface area "cell" (vector) by the pressure acting on it will give you a force, with a direction also normal to that bit of surface (since dF = pdA, the resulting force points in the same direction as the area). Sum all these force vectors together (integrate pdA over the whole surface) and you'll get one force vector representing the force acting on whatever part of the car body you chose as the area--the rear part, in this case. Break that force vector into its components along the x, y, and z axes and you have drag (or thrust) in x, side force in y, and lift (or downforce) in z acting on that part of the car (along whatever arbitrary axes you choose). Do this over the entire surface area of the car and you will get the total drag, total side force, and total lift. The aerodynamic forces result from the local pressures which result from the local velocities which result from moving the car.
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Excellent explanation.
Force = pressure x area, and so you can have a low pressure over a high area, or a high pressure over a small area, and the forces are the same.
That's why measuring overall lift / downforce is better than trying to integrate all the pressures x areas. (However, measuring pressures can be very useful when tuning modifications, as it's a bit easier than measuring downforce / lift.)