Symmetric Quasi Steady Maneuver Load Alleviation – a method to predict the control surface efficiency when used as load alleviator

The main purpose of the Load Alleviation system on which this paper is focused is a drop in structural wing loads for improved performances, fatigue life, Operative Empty Weight reduction. The driving idea is to perform the alleviation during a quasi-steady symmetric maneuver by means of a symmetric actuation of dedicated control surfaces. The control effectiveness of the load alleviator is evaluated, in terms of required deflection for a desired bending moment reduction in the wing, but keeping the same maximum vertical load factor attained during the unalleviated maneuver. When high performance is important, internal structural loads (maneuver loads) reduction becomes significant, if the alleviated maneuver can be performed with the vertical load factor kept unvarying. Consequently evaluations on the load alleviator effectiveness in the early design process are appropriate to find the best set of controls. The load alleviator deflection time history is evaluated a priori, without feed-back calculations, simply depending upon elevators time history, system characteristics and flight condition. An application on a business aircraft has been performed (not neglecting structural flexibility) for two kinds of symmetric maneuvers. By adopting the ailerons as load alleviators, the maximum bending reduction at the wing root is about 37 percent, with a maximum aileron deflection less than 12 deg.