Introduction to Aircraft Aeroelasticity and Loads

13

Equilibrium Manoeuvres

Aircraft are controlled by the pilot using the control surfaces (namely aileron/spoiler for roll, rudder for yaw and elevator for pitch) singly or in combination for a range of different manoeuvres. The structure must be designed to withstand these manoeuvres and these load calculations are a critical stage in the aircraft clearance, often involving many thousands of cases. A useful background to meeting most of the loads requirements in the certification specifications (CS-25 and FAR-25) is given in (Howe, 2004, Lomax, 1996).

There is a difference between manoeuvres performed by commercial and military aircraft. Military aircraft (excepting transport and bomber aircraft) are subject to far more severe manoeuvres, involving higher g levels, control angles and rates. However, military combat aircraft are generally stiffer than commercial aircraft, with natural frequencies usually greater than 5 Hz, so the manoeuvre loads calculations are sometimes carried out using a rigid aircraft model, often with corrections made to the aerodynamics for flexible effects, though this is changing for more highly flexible combat aircraft and unmanned air vehicles (UAVs). In contrast, although their manoeuvres are less severe, large commercial aircraft are generally significantly more flexible, some (e.g. Airbus A380) with modes of vibration that are even lower than 1 Hz; thus it is becoming more essential to perform loads calculations using a flexible (or elastic) aircraft ...