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Aeroelastic Concepts for Flexible Aircraft Structures
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In this thesis, aeroelastic concepts for increased aircraft performance are developed and evaluated. Active aeroelastic concepts are in focus as well as robust analysis concepts aiming at efficient analysis using numerical models with uncertain or varying model parameters.

The thesis presents different approaches for exploitation of fluid-structure interaction of active aeroelastic structures. First, a high aspect ratio wing in wind tunnel testing conditions is considered. The wing was developed within the European research project \textit{Active Aeroelastic Aircraft Structures} and used to demonstrate how structural flexibility can be exploited by using multiple control surfaces such that the deformed wing shape gives minimum drag for different flight conditions. Two different drag minimization studies are presented, one aiming at reduced induced drag based on numerical optimization techniques, another one aiming at reduced measured total drag using real-time optimization in the wind tunnel experiment. The same wing is also used for demonstration of an active concept for gust load alleviation using a piezoelectric tab. In all studies on the high aspect ratio wing, it is demonstrated that structural flexibility can be exploited to increase aircraft performance.

Other studies in this thesis investigate the applicability of robust control tools for flutter analysis considering model uncertainty and variation. First, different techniques for taking large structural variations into account are evaluated. Next, a high-fidelity numerical model of an aircraft with a variable amount of fuel is considered, and robust analysis is applied to find the worst-case fuel configuration. Finally, a study investigating the influence of uncertain external stores aerodynamics is presented. Overall, the robust approach is shown to be capable of treating large structural variations as well as modeling uncertainties to compute worst-case configurations and flutter boundaries.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , 31, 24, 16, 16, 16, 23 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2007:29
Keyword [en]
aeroelasticity, active aeroelastic aircraft structures, aeroelastic control, wind-tunnel testing, flutter analysis, robust flutter analysis, worst-case configuration
National Category
Vehicle Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4419ISBN: 978-91-7178-706-4 (print)OAI: oai:DiVA.org:kth-4419DiVA: diva2:12233
Public defence
2007-06-15, F3, Lindstedtsvägen 26, Stockholm, 13:15
Opponent
Supervisors
Note
QC 20100713Available from: 2007-06-01 Created: 2007-06-01 Last updated: 2010-07-13Bibliographically approved
List of papers
1. Approach to Induced Drag Reduction with Experimental Evaluation
Open this publication in new window or tab >>Approach to Induced Drag Reduction with Experimental Evaluation
2005 (English)In: Journal of Aircraft, ISSN 0021-8669, Vol. 42, no 6, 1478-1485 p.Article in journal (Refereed) Published
Abstract [en]

An approach to minimize the induced drag of an aeroelastic configuration by means of multiple leading- and trailing-edge control surfaces is investigated. A computational model based on a boundary-element method is constructed and drag-reducing flap settings are found by means of numerical optimization. Further, experiments with an elastic wind-tunnel model are performed in order to evaluate the numerically obtained results. Induced-drag results are obtained by analyzing lift distributions computed from optically measured local angles of attack because standard techniques proved insufficient. Results show that significant reductions of induced drag of flexible wings can be achieved by using optimal control surface settings.

Keyword
Aeroelastic configuration; Drag reduction; Flap settings; Lift distributions
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7268 (URN)10.2514/1.11713 (DOI)000233846200012 ()2-s2.0-29544442463 (Scopus ID)
Note
QC 20100531. Konferens: AIAA/ASME/ASCE/AHS/ASC 45th Structures, Structural Dynamics and Materials Conference Palm Springs, CA, APR 19-22, 2004Available from: 2007-06-01 Created: 2007-06-01 Last updated: 2011-12-08Bibliographically approved
2. Drag Minimization of an Active Flexible Wing with Multiple Control Surfaces
Open this publication in new window or tab >>Drag Minimization of an Active Flexible Wing with Multiple Control Surfaces
(English)Article in journal (Other academic) Submitted
Abstract [en]

In this study, the performance of a highly flexible wing with multiplecontrol surfaces is optimized for different flight conditions. A windtunnel model with 20 independent control surfaces is used as the testobject. The problem is posed as a nonlinear optimization problem withthe measured drag as the objective. Constraints include keeping the liftconstant, as well as having bounds on the control surface deflections. Nocomputational estimation of the drag is performed. Instead, experimentalmeasurements of the drag are used to perform the optimization in realtime using a generating set search method (GSS). The algorithm makesthe wing adapt to the current flight condition to improve performanceand the experimental evaluation of the method shows that the drag issignificantly reduced within a wide range of flight conditions. A study onthe number of control surfaces needed is also included, and the resultsshow that, if chosen properly, only three control surfaces distributed overthe wing are needed to reduce drag significantly.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7269 (URN)
Note
QS 20120327Available from: 2007-06-01 Created: 2007-06-01 Last updated: 2012-03-27Bibliographically approved
3. Analysis and Wind Tunnel Testing of a Piezoelectric Tab for Aeroelastic Control Applications
Open this publication in new window or tab >>Analysis and Wind Tunnel Testing of a Piezoelectric Tab for Aeroelastic Control Applications
2006 (English)In: Journal of Aircraft, ISSN 0021-8669, Vol. 43, no 6, 1799-1804 p.Article in journal (Refereed) Published
Abstract [en]

A concept for exploitation of a piezoelectric actuator by using aeroelastic amplification is presented. The approach is to use the actuator for excitation of a tab that occupies the rear 25 % of a free-floating trailing edge flap. A flexible high aspect ratio wing wind tunnel model is used as a test case. Wind tunnel experiments were performed to determine frequency response functions for validation of the numerical model used for the control law design. The aeroservoelastic model is based on state-space equations of motion that accept piezoelectric voltage commands. Control laws are derived for gust alleviation with flutter and control authority constraints, and numerical results that demonstrate a significant reduction in the structural response are presented. Possible applications and feasibility of the concept are discussed.

Keyword
Actuators; Aspect ratio; Electric potential; Frequency response; Numerical methods; Piezoelectricity; Wind tunnels
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-7270 (URN)10.2514/1.20060 (DOI)000242989800023 ()2-s2.0-33846068455 (Scopus ID)
Note
QC 20100713Available from: 2007-06-01 Created: 2007-06-01 Last updated: 2010-07-13Bibliographically approved
4. Robust Flutter Analysis Considering Mode Shape Variations
Open this publication in new window or tab >>Robust Flutter Analysis Considering Mode Shape Variations
2008 (English)In: Journal of Aircraft, ISSN 0021-8669, Vol. 45, no 3, 1070-1074 p.Article in journal (Refereed) Published
Abstract [en]

A study was conducted to perform robust flutter analysis, considering mode shape variation problems. The study demonstrated that the assumption of a fixed modal base can lead to incorrect flutter results. It was demonstrated the nominal model can act as the basis for performing robust flutter analysis. The study assumed that the critical flutter mode shape can be represented by a linear combination of eigenmodes. The study also demonstrated that the modal base method can be used, to solve the problem of mode shape variations. It was also demonstrated that results of mode shape variation problems can be improved significantly, by adding structural eigenvectors.

Keyword
Lead; Modal analysis; Zinc; Eigen-modes; flutter analysis; Linear combination (LC); Mode shapes
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-7271 (URN)10.2514/1.28728 (DOI)000256688600032 ()2-s2.0-46449130257 (Scopus ID)
Note
QC 20100713. Uppdaterad från Submitted till Published 20100713.Available from: 2007-06-01 Created: 2007-06-01 Last updated: 2010-07-13Bibliographically approved
5. Assessment of Critical Fuel Configurations Using Robust Flutter Analysis
Open this publication in new window or tab >>Assessment of Critical Fuel Configurations Using Robust Flutter Analysis
2007 (English)In: Journal of Aircraft, ISSN 0021-8669, Vol. 44, no 6, 2034-2039 p.Article in journal (Refereed) Published
Abstract [en]

An approach to assess critical fuel configurations using robust flutter analysis is presented. A realistic aircraft model is considered to demonstrate how an available finite element model can be adapted to easily apply robust flutter analysis with respect to structural variations such as fuel-level variations. The study shows that standard analysis tools can be used to efficiently generate the system data that are required to perform robust flutter analysis. The μ-k method is used to compute the worst-case flutter speed, and the corresponding worst-case fuel configuration is found. The main advantage of the proposed approach is that μ. analysis guarantees robustness with respect to all possible fuel configurations represented by the tank model.

Keyword
Aircraft; Finite element method; Robust control; Statistical methods; Fuel configurations; Standard analysis tools; Tank models; Flutter (aerodynamics)
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-7272 (URN)10.2514/1.30500 (DOI)000251703900028 ()2-s2.0-38149030900 (Scopus ID)
Note
QC 20100713. Uppdaterad från Submitted till Published 20100713. Konferens: AIAA/ASME/ASCE/AHS/ASC 47th Structures, Structural Dynamics, and Materials Conference, Newport, RI, MAY 01-04, 2006.Available from: 2007-06-01 Created: 2007-06-01 Last updated: 2010-07-13Bibliographically approved
6. On the Uncertainty Modeling for External Stores Aerodynamics
Open this publication in new window or tab >>On the Uncertainty Modeling for External Stores Aerodynamics
(English)Manuscript (Other academic)
Abstract [en]

A wind tunnel model representing a generic delta wing configurationwith external stores is considered for flutter investigations. Complexeigenvalues are estimated for the wind tunnel model for airspeeds up tothe flutter limit, and compared to eigenvalues predicted by a numericalmodel. The impact of external stores mounted to the wing tip is investigatedboth experimentally and numerically, and besides the impact of thestores as such, the capability of the numerical model to account for increasingmodel complexity is investigated. An uncertainty approach basedon robust flutter analysis is demonstrated to account for modeling imperfections.Uncertainty modeling issues and the reliability of uncertaintymodels are discussed. Provided that the present uncertainty mechanismcan be determined, it is found that available uncertainty tools may beused to efficiently compute robust flutter boundaries.

National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-7273 (URN)
Note
QC 20100713Available from: 2007-06-01 Created: 2007-06-01 Last updated: 2010-07-13Bibliographically approved

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