Basic Aerodynamics: Part One – What is Aerodynamics?

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fxmd aero
 The FXMD NSX driven by our editor Billy Johnson sports aerodynamics by Andrew Brilliant.  Andrew uses a CFD or Computational Fluid Dynamics program and digital surface data from the NSX to design the car’s aero, it must work well, it took about 4 seconds a lap off the car’s previous time with a full aero package.

If you study basic fluid dynamics, one of the first things you learn about is Bernoulli’s Principal.  Simply boiled down, Mr. Bernoulli states that a high flow stream velocity over a body means a lower surface pressure.  Conversely, a lower velocity over a moving body means a higher surface pressure.  It is by this principle that wings work and airplanes fly.  What does this have to do with automotive aero you might ask?  Let us explain.

 This shows how Bernoulli’s principal works in creating lift. 

An airplane wing is rounded sort of like a half of a teardrop on the top surface and flatter or even concave across the bottom. When an object shaped like a wing profile moves through the air, (Don’t argue with me about Newton’s third law of motion when describing lift, I am trying to keep things simple) the air over the top part of the wing has to take a longer path over the top of the wing due to the curve while the air on the bottom of the wing can just take a straight path across the bottom.

Since the whole wing is moving through the air at the same speed, the air over the longer path, top part of the wing accelerates and goes faster while the air at the bottom of the wing can take a short path in the same time frame and slows down.  This creates high pressure under the wing and low pressure on top of the wing.  The pressure differential created between the top and bottom of the wing is the lift.  Enough speed and the lifting force can pick the plane up off the ground.

Ok so what does this have to do with cars?  Well go outside and look at your car.  The bottom is flat and the top has a hood and high point, the greenhouse where you and maybe your passengers sit.  So there is a short flow path on the bottom and a longer one on top.  So the basic shape of all cars by design is one that creates lift.  Why is this not good?  Well if you cars body creates lift, at high speeds this lift takes the load off the tires reducing mechanical grip.  Less vertical load equals less grip, less grip means more scary, less control, lower cornering speeds and longer braking distances.  In extreme cases a car can become unstable and in the worst case even take off like a plane.

Basic Aerodynamics Part One: What is Aerodynamics?
 This is a CFD pressure model of a wing shape.  Green means high pressure, yellow means low pressure.

 

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