Page 562 - Aircraft Stuctures for Engineering Student

13.1 Load distribution and divergence 543

Equation (13.2) shows that divergence occurs (Le. 6 becomes infinite) when
1 2 acL
K = -pV Sec-
2 da
The divergence speed vd is then

(13.3)

We see from Eq. (13.3) that vd may be increased either by stiffening the wing (increas-
ing K) or by reducing the distance ec between the aerodynamic and flexural centres.
The former approach involves weight and cost penalties so that designers usually
prefer to design a wing structure with the flexural centre as far forward as possible.
If the aerodynamic centre coincides with or is aft of the flexural centre then the
wing is stable at all speeds.

-- Wing __U.r_ll.__IIm.-"-__U-~--~~ (finite wing) ..__*.I____
divergence
torsional
13.1.2
1.-
We shall consider the simple case of a straight wing having its flexural axis nearly
perpendicular to the aircraft's plane of symmetry (Fig. 13.3(a)). We shall also
assume that wing cross-sections remain undistorted under the loading. Applying
strip theory in the usual manner, that is we regard a small element of chord c and
spanwise width 6z as acting independently of the remainder of the wing and consider
its equilibrium, we have from Fig. 13.3(b), neglecting wing weight
-
T
(T +g&) + ALec + AM, = 0 (13.4)

AY

Line of
ACs

z
Flexural axis

dz
(bl
Fig. 13.3 Determination of wing divergence speed (three-dimensional case).