Page 34 - Aerodynamics for Engineering Students
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Basic concepts and definitions 17
If both top and bottom of this expression are multiplied by the wing span, by it
becomes:
a form which is often more convenient.
Sweep-back
The sweep-back angle of a wing is the angle between a line drawn along the span at: a
constant fraction of the chord from the leading edge, and a line perpendicular to the
centre-line. It is usually denoted by either A or 4. Sweep-back is commonly measured
on the leading edge (ALE or $LE), on the quarter-chord line, i.e. the line of the chord
behind the leading edge (A1/4 or $I/& or on the trailing edge (ATE or &E).
Dihedral angle
If an aeroplane is looked at from directly ahead, it is seen that the wings are not, in
general, in a single plane (in the geometric sense), but are instead inclined to each
other at a small angle. Imagine lines drawn on the wings along the locus of the
intersections between the chord lines and the section noses, as in Fig. 1.5. Then the
angle 2r is the dihedral angle of the wings. If the wings are inclined upwards, they are
said to have dihedral, if inclined downwards they have anhedral.
Incidence, twist, wash-out and wash-in
When an aeroplane is in flight the chord lines of the various wing sections are not
normally parallel to the direction of flight. The angle between the chord line of a
given aerofoil section and the direction of flight or of the undisturbed stream is called
the geometric angle of incidence, a.
Carrying this concept of incidence to the twist of a wing, it may be said that, if the
geometric angles of incidence of all sections are not the same, the wing is twisted. If
the incidence increases towards the tip, the wing has wash-in, whereas if the incidence
decreases towards the tip the wing has wash-out.
1.3.2 Aerofoil geometry
If a horizontal wing is cut by a vertical plane parallel to the centre-line, such as X-X
in Fig. 1.4, the shape of the resulting section is usually of a type shown in Fig. 1.6~.
Fig. 1.5 Illustrating the dihedral angle
