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Assessment of the interaction of a passive and an active load alleviation scheme

Final Report Summary - PALAST (Assessment of the interaction of a passive and an active load alleviation scheme)

Project context and objectives:

Project PALAST is the result of a successful submission to a call for proposal topic within the CleanSky Joint Technology Initiative. This particular topic is included in the Smart Fixed Wing Aircraft (SFWA) programme and its call identifier is the following: SP1-JTI-CS-2010-05. This research framework has been created with the original purpose of pursuing additional research in terms of alleviation and control of loads acting on the wings of an aircraft.

Project PALAST had a total duration of 18 months (1.5 years), running from June 2011 until December 2012, and involved a group of researchers from different scientific areas, such as lightweight structures, flight dynamics and flight controls, from a single institution - the Technische Universität München. Two different institutes from this university have been in charge of the scientific work: the Institute of Lightweight Structures and the Institute of Flight System Dynamics. After the conclusion of the project, the total effort has been estimated to lie around the originally predicted 26 person-months.

The major challenge addressed throughout project PALAST has been the alleviation of gust loads acting on an aircraft wing by making use of active and passive strategies. Both of these techniques, have proven to be effective for the given aircraft configuration. The passive technique, which is named aeroelastic tailoring, has been successfully applied to the wing-box structure, and depending on the required aileron effectiveness, weight savings up to circa 40 % in the design zone could be achieved. On the other hand, active gust load alleviation has been applied primarily in terms of feed-forward control. Its application in a structural sizing approach generates a potential improvement of circa 30 % in terms of weight saving.

Project results:

Both techniques, active and passive load alleviation, have proven to be effective for the given aircraft configuration (large passenger aircraft in cruise condition). The passive technique, aeroelastic tailoring has been successfully applied to the given wing-box structure. Depending on the required aileron effectiveness, weight savings up to circa 40 % in the design zone have been achieved, when compared to a reference sizing. Active gust load alleviation (GLA) has been applied primarily in terms of feed-forward control. By its application in a structural sizing approach, a potential of circa 30 % weight saving could be identified, considering only gust loads in design. Maneuver load alleviation (MLA), on the other hand, has been identified as less beneficial than gust load alleviation for this aircraft configuration.

The interactions of different structural configurations with active load alleviation have been analysed and evaluated. It has been identified, that aeroelastic tailoring for load reduction is also beneficial for the active load control performance. This is the case, as the tailored design provides higher torsional stiffness of the outboard wing, compared to the reference design. This structural property of the wing increases both effects of static aeroelastic load reduction and active load alleviation by control surface actuation. Further, it has been verified that by maintaining sufficient aileron effectiveness in structural design, active load alleviation is improved.

The good results of gust load alleviation are promising for future aircraft configurations, which shall be highly fuel efficient, but still maintain best ride comfort. For the example aircraft considered in project PALAST, GLA is of limited importance for structural sizing with peak loads, as manoeuvre loads have showed to be more severe. However, the highly dynamic responses of gust incidents could be controlled very well, a fact that improves fatigue life of the wing. Furthermore, ride comfort and flight-dynamic responses are improved by the dynamic peak load reduction, as well as vibration damping.

Potential impact:

Project PALAST main objective was to obtain a better understanding in terms of structural and control interaction whenever load alleviation approaches are applied on transport aircraft. This is considered to be one of the technology enablers in the CleanSky SFWA JU, aiming for higher efficiency of passenger aircraft. Project results obtained in PALAST shall support the SFWA consortium in developing successful load alleviation concepts for the smart wing.

The findings of PALAST can potentially be considered in further aircraft design studies, which address the introduction of airframe load alleviation strategies. The potential impact of active load alleviation in terms of weight saving of the wing box structure has been identified as up to 40 %. With the additional consideration of passive load alleviation in the structural design, the potential of induced drag reduction can be further exploited. Using both techniques together, weight, as well as drag can be pushed to lower levels, allowing for less fuel burn and related carbon dioxide (CO2) emissions. This is completely in line with the proposed goals of the ACARE Vision 2020, one of the main drivers of the SFWA programme.

The development of such smart wings, which take advantage of recently developed control methodologies, like piezo-ceramic actuators, might be one key issue in terms of future aircraft development. Today, the use of composite material, mainly carbon fibre reinforced plastic (CFRP), in primary structural design has been well established. However, the application of this material is considered rather conservative, using well-proven design principles of metallic structures. This somewhat doesn't allow for the full exploitation of the high potential of composite materials. The freedom of controlling stiffness properties is still not fully addressed in current aircraft designs. As seen in the aeroelastic tailoring studies carried out within project PALAST, the unconventional placement of fibre orientations within the skin panels provides passive control of the twisting properties of the wing. These remarkably impact the wing loading, as well as the situation of induced drag.

Analysis methodologies in the field of aerodynamics and aeroelasticity have been continuously improving. High level research focusing on these topics has been carried out worldwide since the level of complexity of the physics behind may be addressed to an arbitrary extent. Tackling the nonlinear and trans-sonic aero-structural effects of transport aircraft in the best possible manner in simulation runs shall be a key issue for the development of intelligent, high performance structures. Rather small scale studies, like PALAST, as well as highly funded L0/L1 research projects, will derive enabling technologies for aero-structural simulation. These will push forward European competitiveness in a global market, which desires more efficient and greener aircraft.

Primary dissemination activities deriving from project PALAST have been contributions to top level scientific conferences within the aero-space community. A conference talk on PALAST related scientific results at the Third Aircraft Structural Design Conference, held in October 2012 in Delft, has been performed. Additionally, a conference paper has been submitted by the consortium, for the AIAA Modeling and Simulation Technologies Conference, which will take place in Boston, Massachusetts, in August 2013: Peter Chudy; Jan Vlk; Miguel Leitao; Felix Stroscher: 'Evolution-driven controller design for an aeroservoelastic aircraft model'. Further dissemination of indirect project results is planned to take place in several lectures, as part of the courses of aerospace structures, adaptive structures, flight dynamics and flight control.