Manoeuvre load control
Active load control may also be used to reduce structural loads during manoeuvres. One method of manoeuvre load control (MLC) is to use inboard flaps to increase the load on the inboard portion of the wing when performing manoeuvres that require a high lift. By concentrating the lift inboard, the bending stresses at the wing root are reduced.
Alternatively, by using a large number of individually adjustable trailing edge flaps or flaperons, it is possible to adjust the spanwise loading to give a low-drag elliptical distribution, even during high-load combat manoeuvres. Again, these techniques require the use of reliable automatic control sytems.
Once again, birds have beaten us to it, and have been using complex forms of active load control for millions of years.
As we have seen, aeroelastic effects occur as a consequence of insufficient structural stiffness, rather than a lack of strength. Problems of aeroelastic failure are
Fig. 14.5 The development of torsionally rigid wing sections
(a) Early fabric-covered aircraft used spars which provided some bending stiffness but very little torsional rigidity (b) Later a torsion box was introduced (c) Supersonic and transonic aircraft often have very thin wing sections. A thick skin is used, often with integrally machined stiffeners. The spars and skin form a number of torsion cells
as old as aviation itself, and a number of early attempts at flight are thought to have failed due to structural divergence.
The biplane arrangement of struts and wires initially provided an acceptable solution. By suitable criss-crossing of the wires, this arrangement could produce a surprisingly stiff structure. In contrast, many early monoplanes suffered aeroelastic failures due to a lack of torsional stiffness.
Early aircraft wings were constructed using a number of spars which, though capable of withstanding large bending moments, produced little torsional resistance (Fig. 14.5(a)). The torsional stiffness was initially improved by adding stiffening webs between the spars, but later, it was found that by placing two spars close together and closing them to form a ‘torsion box’, as shown in Fig. 14.5(b), the torsional rigidity could be greatly increased. Closed tubes offer considerably better torsional stiffness than open sections. Try twisting a cardboard tube such as an empty toilet roll tube, and you will find it almost impossible. Now slit the tube from one end to the other, and you will find that it will twist easily.
With the adoption of metal skins for wings, instead of doped canvas, the torsional rigidity increased considerably. In Fig. 14.5(b), it will be seen that the leading and trailing-edge sections themselves form closed tubes, in addition to the central box. Figure 14.5(b) thus illustrates a torsion box construction with two additional closed cells.
For transonic and supersonic aircraft it is advantageous to use thin wing sections. This in turn requires the use of very thick skins in order to provide the necessary stiffness. Consequently it has become practical to machine the skin out of solid plates of metal. Stiffening elements and details can be machined integrally with the skin, eliminating the need for rivets. By this method, it is possible to produce the smooth surface and precise contours required for low – drag aerofoil shapes.
When thin sections with thick skins are employed, it is normal to use the skin to form the top and bottom of a number of closed cells, as shown in Fig. 14.5(c). No separate specific torsion box is then required.