Structural influences

Although this book is primarily concerned with aerodynamics and flight mechanics, we must consider some of the important interactions between the structure and its aerodynamic characteristics.

The final shape of the aircraft often results from some form of compromise between conflicting aerodynamic and structural requirements. We have already cited the case of the elliptical planform. Another example is in design of aerofoil sections, where it is important to take consideration of the fact that a structure has to be fitted within the contour. Much recent research effort has concentrated on designing low-drag wing sections that are also relatively thick.

The materials and methods of construction can also affect the degree of aerodynamic optimisation that can reasonably be achieved.

Aeroelastics

Apart from the obvious fact that the structural strength imposes limits on the aerodynamic loads that can be tolerated, the flexibility of an aircraft can have a profound effect on its aerodynamic behaviour. The aerodynamic effects due to structural flexibility are grouped under the heading of aeroelastics. Aero – elastic problems can be subdivided into static cases, where the inertia of the structure has little effect, and dynamic cases, where the inertia is significant.

Static cases

Divergence

If an upward load is applied to the leading edge of a wing near the tip, then it will try to twist (leading edge up) as well as bend (Fig. 14.1). Similarly, if we

apply a load near the trailing edge, it will twist in the other direction. There is, however, one intermediate position at which the applied load will produce just bending, with no twisting. This is known as the flexural centre.

If the line of action of the resultant aerodynamic lift is in front of the flexural centre, then the wing will twist, with the angle of attack increasing towards the tips, as shown in Fig. 14.1(a). As the angle of attack increases, the lift force will rise, causing the wing to twist even more. If the wing torsional (twisting) stiffness is too low, the twist can continue to increase or ‘diverge’, until either a stall occurs, or the wing breaks.

The solutions to this problem of torsional divergence include making the wing sufficiently stiff torsionally, and trying to ensure that the flexural centre is reasonably well forward. We shall describe the means of improving torsional stiffness later.

On forward swept wings, in addition to the torsional divergence described above, it is possible to encounter divergence due to bending, as illustrated in Fig. 14.2. As the wing tip bends upwards, the angle of attack increases, pro­ducing a greater lift, with more bending. If the bending stiffness is insufficient, the wing bending may diverge, until failure occurs. Bending divergence can also occur on unswept wings during violent sideslip.

Just as forward sweep increases the likelihood of divergence, rearward sweep decreases it, and on highly (rearward) swept wings, divergence is unlikely to occur. The problem of divergence, and other aeroelastic effects on forward-swept wings, is one reason why they were initially rejected in favour of the rearward-swept alternative.

Because aerodynamic loading increases with speed, the risk of divergence increases as the aircraft flies faster. The minimum speed at which divergence occurs on any particular aircraft is known as its critical divergence speed, and the maximum operating speed must be less than this critical speed.