Figure 3.10. Aerofoil section and definitions – NACA family

that is, in the vertical direction, lift = weight, and in the horizontal direction, thrust = drag.

The aircraft weight is exactly balanced by the lift produced by the wing (the fuselage and other bodies could share a part of the lift – discussed later). Thrust provided by the engine is required to overcome drag.

Moments arising from various aircraft components are summed to zero to maintain a straight flight (i.e., in steady level flight, Moment = 0).

Any force/moment imbalance would show up in the aircraft flight profile. This is how an aircraft is maneuvered – through force and/or moment imbalance – even for the simple actions of climb and descent.

3.7 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.