Aerofoil

The cross-sectional shape of a wing (i. e., the bread-slice-like sections of a wing com­prising the aerofoil) is the crux of aerodynamic considerations. The wing is a 3D surface (i. e., span, chord, and thickness). An aerofoil represents 2D geometry (i. e., chord and thickness). Aerofoil characteristics are over the unit span at midwing to eliminate effects of the finite 3D wing tip effects. The 3D effects of a wing are dis­cussed in Section 3.11. To standardize aerofoil geometry, Figure 3.10 provides the universally accepted definitions that should be well understood [4].

Chord length is the maximum straight-line distance from the LE to the trailing edge. The mean line represents the midlocus between the upper and lower surfaces; the camber represents the aerofoil expressed as the percent deviation of the mean line from the chord line. The mean line is also known as the camber line. Coordi­nates of the upper and lower surfaces are denoted by Yu and YL for the distance X measured from the LE. The thickness (t) of an aerofoil is the distance between the upper and the lower contour lines at the distance along the chord, measured perpendicular to the mean line and expressed in percentage of the full chord length. Conventionally, it is expressed as the thickness to chord (t/c) ratio in percentage. A small radius at the LE is necessary to smooth out the aerofoil contour. It is conve­nient to present aerofoil data with the chord length nondimensionalized to unity so that the data can be applied to any size aerofoil by multiplying its chord length.

Aerofoil pressure distribution is measured in a wind tunnel to establish its char­acteristics, as shown in [4]. Wind-tunnel tests are conducted at midspan of the wing model so that results are as close as possible to 2D characteristics. These tests are conducted at several Re. Higher Re indicates higher velocity; that is, it has more kinetic energy to overcome the skin friction on the surface, thereby increas­ing the pressure difference between the upper and lower surfaces and, hence, more lift.

In earlier days, drawing the full-scale aerofoils of a large wing and their manu­facture was not easy and great effort was required to maintain accuracy to an accept­able level; their manufacture was not easy. Today, CAD/CAM and microprocessor – based numerically controlled lofters have made things simple and very accurate. In December 1996, NASA published a report outlining the theory behind the U. S. National Advisory Committee for Aeronautics (NACA) (predecessor of the present-day NASA) airfoil sections and computer programs to generate NACA aerofoils.