Maneuver Load Controls, Analysis and Design for Flexible Aircraft

This thesis is based on studies about the use of Load Alleviation Systems aimed at controlling the flight maneuver loads. The system has to be able to reduce the wing bending in a specific wing station in the neighborhood of the wing root by means of a symmetrical actuation of the ailerons or other dedicated control surfaces located close to the wing tip. The load alleviator deflection is performed in order to rearrange the aerodynamic loads. The results is the shift of the wing center of pressure inboard and a consequent reduction of the bending moment close to the wing root. As discussed so far, this way to proceed is not new, but the purpose of this work is to offer a practical approach to quantify the Load Alleviation during longitudinal maneuvers and to provide methods and numerical procedures useful for designing and/or analyzing such systems, by giving always particular emphasis to the importance of the structure flexibility, to be taken into account since the early stage of design. The whole work is made of four parts. The first part deals with symmetric balanced maneuvers, providing a method to evaluate the load alleviation effectiveness in an effortless and linear manner. A desired value of the bending moment alleviation in a generic fixed wing station can be obtained by following this method, for which the aerodynamic and load derivatives of the airplane are required. A numerical procedure aimed at determining such derivatives also for an aircraft in an unconventional configuration such as a Joined-Wing one, using a modal approach and taking into account aeroelastic effects, has been presented. A limitation of the method is its inapplicability in high lift conditions, such as those falling between the points VS and VA of the Maneuver Diagram, at the vertical limit load factor, in the case of a positive value of Cmβ (negative swept wing). In such a situation, with the aircraft at its maximum attitude, the activation of the load alleviation system may produce the aircraft stall. Another relevant limitation especially in case of a strongly unconventional configuration consists in the uncontrollability of the internal load far from WCS. Extensive calculations are needed in order to prevent a sudden load increase with consequent unexpected structural failures.In the second part a method to estimate the control surface effectiveness when it is used as load alleviator is provided. An application to an EASA CS-25 Business Aircraft for two different kinds of maneuver and by adopting the ailerons as load alleviators, show that for a generic climb start maneuver, the maximum bending reduction at the wing root is about 37 percent, with a maximum aileron deflection less than 12 degrees. This results are obtained by means of open-loop calculations only and involves methods that permits to take into account the aircraft flexibility together with plunge and pitch rigid-body motions by applying a modal approach. The third part of the work is a conceptual design of a MLC system for longitudinal maneuver. The system, when switched on, is able to minimize the bending moment augmentation in a wing station near the wing root during an unsteady maneuver. The system incorporates a Load Factor Feedback (LFF) to the elevators in order to perform a desired longitudinal maneuver by automatically acting on the elevators through a simple PID controller, whereas the Maneuver Load Control (MLC) is is accomplished by observing the bending on the wing root section and by symmetrically acting on the ailerons by means of a simple P controller. The goal is to minimize the difference between measured bending moment and 1-g bending moment. All numerical analyses aimed at simulating the aircraft behavior during maneuver with MLC-on or MLC-off are performed both by taking into account and by neglecting the flexibility of the aircraft. Indeed the synthesis of the controllers has been made by tuning the gains in either case, i.e. for rigid and elastic aircraft, in order to appreciate the different performances, although gain and phase margins are kept constant. The study demonstrates how much is important to consider the effect of aeroelasticity early in the conceptual design of such a MLC system, hence by providing much more reliable indications about their effectiveness and also about the quality of flight mechanics in general. The fourth part is focused on the estimation of the fatigue life extension of a structural joint (wing lower skin-stringer) located close to the wing root. Analyses are carried-out for a business jet responding to the Part 25 of the EASA Certification Specification for two kinds of mission: short and long range. Estimated fatigue life extensions result well beyond the most optimistic expectations, with life duration improvements up to 67.5 percent of the nominal fatigue life. The better result is obtained for the long range mission for which flight loads are prominent with respect to the ground ones. The benefit to carry a MLC system becomes much more important as regards the fatigue life improvement. Future work will be focused on the load alleviation in a gust environment, for which a correlation of unsteady local accelerations with the load characteristic to be alleviated is the challenging issue. Another relevant effort to be faced with is the introduction of the unsteady aerodynamics instead of the quasi-steady one. The adoption of the modal approach with subsequent Roger approximation of the unsteady generalized aerodynamic forces will introduce in the state-space system further equations related to the modeled aerodynamic delays. A method aimed at observing and controlling them, also from a practical viewpoint, is the main expected difficulty to be overcome.