Aerodynamics and Aircraft Performance (Marchman)

1.1 Aerodynamics

Aerodynamics is probably the first subject that comes to mind when most people think of Aeronautical or Aerospace Engineering. Aerodynamics is essentially the application of classical theories of “fluid mechanics” to external flows or flows around bodies, and the main application which comes to mind for most aero engineers is flow around wings.

The wing is the most important part of an airplane because without it there would be no lift and no aircraft. Most people have some idea of how a wing works; that is, by making the flow over the top of the wing go faster than the flow over the bottom we get a lower pressure on the top than on the bottom and, as a result, get lift. The aero engineer needs to know something more than this. The aero engineer needs to know how to shape the wing to get the optimum combination of lift and drag and pitching moment for a particular airplane mission. In addition he or she needs to understand how the vehicle’s aerodynamics interacts with other aspects of its design and performance. It would also be nice if the forces on the wing did not exceed the load limit of the wing structure.

If one looks at enough airplanes, past and present, he or she will find a wide variety of wing shapes. Some aircraft have short, stubby wings (small wing span), while others have long, narrow wings. Some wings are swept and others are straight. Wings may have odd shapes at their tips or even attachments and extensions such as winglets. All of these shapes are related to the purpose and design of the aircraft.

In order to look at why wings are shaped like they are we need to start by looking at the terms that are used to define the shape of a wing.

A two dimensional slice of a wing cut parallel to the centerline of the aircraft fuselage or body is called the airfoil section. A straight line from the airfoil section leading edge to its trailing edge is called the chord line. The length of the chord line is referred to as the chord. A line drawn half way between the airfoil section’s upper and lower surfaces is called the camber line. The maximum distance between the camber line and chord line is referred to as the airfoil’s camber and is usually enumerated as a percent of chord. We will see that the amount of airfoil camber and the location of the point of maximum camber are important numbers in defining the shape of an airfoil and predicting its performance. For most airfoils the maximum camber is on the order of zero to five percent and the location of the point of maximum camber is between 25% and 50% of the chord from the airfoil leading edge.

When viewed from above the aircraft the wing shape or planform is defined by other terms.