Pages

Airfoils

If you are like me, you’re still amazed when you see an airplane such as the Boeing 747 take off. Just imagine roughly 340,000 kg being lifted off the ground. It is almost like magic. However, the more interesting part yet is that the means by which this happens is not even close to be magic but it is all explained by fluid mechanics. I would like to be able to explain in great detail the many hydrodynamic features that make such an aircraft fly, but I can’t. Fluid mechanics is such a complex topic that computer simulations, experimental data and models are used to simplify the engineering analysis required to understand this topic. One thing we all know for sure is that the wings of an aircraft play a very important role. Airfoils, or wings as we commonly called them, have been design in many different shapes and sizes according to their purpose and even though the hydrodynamics of an airfoil are also very complex, a very simple explanation can be drawn to understand the purpose of the wings. Let’s look at the results of a numerical study to help illustrate the idea.

Imagine you have a symmetric airfoil as the one seen on the picture above. Air approaches the airfoil at 100m/s. Since the airfoil is symmetric and it is oriented completely parallel to the incoming flow, the flow will symmetrically distribute itself around the airfoil. If one look at the pressure contour around the airfoil for such a case (picture below), it is clearly seen the symmetry of the pressure field around the airfoil.

For this case, the airfoil does not experience any lift force. Now, let’s do the same thing, but this time let’s orient the airfoil 20° from the horizontal (20° = angle of attack). Looking again at the pressure contour for the 20° angle of attack case (picture below), it is clear that now we have a low pressure zone above the wing, while maintaining a high pressure zone below the wing.

This normal pressure difference multiplied by the area of the wing gives the aircraft the lift force required to keep it off the ground. The low pressure zone above the wing arises because due to the orientation of the wing, the air must accelerate above the wing in order to conserve mass. This can be seen in the velocity vector plot below.

The acceleration of the flow causes a drop in the pressure which as mentioned before causes the lift force. This is a simplified explanation but it retains the fundamental concept being airplane wings.