1.3.1 Wing geometry
The planform of a wing is the shape of the wing seen on a plan view of the aircraft. Figure 1.4 illustrates this and includes the names of symbols of the various parameters of the planform geometry. Note that the root ends of the leading and trailing edges have been connected across the fuselage by straight lines. An alternative to this convention is that the leading and trailing edges, if straight, are produced to the aircraft centre-line.
The wing span is the dimension b, the distance between the extreme wingtips. The distance, s, from each tip to the centre-line, is the wing semi-span.
The two lengths cT and c0 are the tip and root chords respectively; with the alternative convention, the root chord is the distance between the intersections with the fuselage centre-line of the leading and trailing edges produced. The ratio ct/c0 is the taper ratio Л. Sometimes the reciprocal of this, namely c0/ct, is taken as the taper ratio. For most wings cj/co < 1.
The plan-area of the wing including the continuation within the fuselage is the gross wing area, SG – The unqualified term wing area S is usually intended to mean this gross wing area. The plan-area of the exposed wing, i. e. excluding the continuation within the fuselage, is the net wing area, SN.
A useful parameter, the standard mean chord or the geometric mean chord, is denoted by c, defined by c = Sq/Ь or Ss/b. It should be stated whether SG or SN is used. This definition may also be written as
where у is distance measured from the centre-line towards the starboard (right-hand to the pilot) tip. This standard mean chord is often abbreviated to SMC.
Another mean chord is the aerodynamic mean chord (AMC), denoted by ca or 5, and is defined by
The aspect ratio is a measure of the narrowness of the wing planform. It is denoted by A, or sometimes by (АЛ), and is given by
д _ span _ b
If both top and bottom of this expression are multiplied by the wing span, becomes:
A _ b2 _. (span)2 be area
a form which is often more convenient.
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 Л or ф. 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 (Л1/4 or фщ), or on the trailing edge (Ate or Фте)-
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 2Г 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.