Automatische Operationelle Modalanalyse im Flugschwingungsversuch

Die Entwicklung neuer Flugzeugkonstruktionen erfordert eine Untersuchung der aeroelastischen Stabilität, um das Phänomen des Flatterns – eine selbsterregte Schwingung der Flugzeugstruktur bei bestimmten Flugzustandsbedingungen – zu vermeiden. Da die rein numerische Analyse eine gekoppelte Simulation zwischen strukturdynamischen Modellen und instationären aerodynamischen Kräften einschließt, ist deren Durchführung sehr komplex, so dass für eine zielorientierte Untersuchung Vereinfachungen vorgenommen werden müssen. Daher müssen die zugrundeliegenden Modellierungen bezüglich ihrer Gültigkeit durch den Vergleich von numerischen und experimentellen Ergebnissen validiert werden. Neben dem sogenannten Standschwingungsversuch, der am Prototypen eines neuen Flugzeugtyps durchgeführt wird, um die modalen Parameter Eigenfrequenzen, Dämpfungsmaße und Schwingungsformen zu messen, sind auch vergleichende Schwingungsmessungen im Flug während der Flugerprobung wünschenswert, um die reale Antwort des aeroelastischen Systems mit den vorhergesagten numerischen Ergebnissen vergleichen zu können und somit deren Gültigkeit zu demonstrieren. Hierzu eignen sich insbesondere Methoden der operationellen Modalanalyse (OMA), die fortlaufend das Schwingungsverhalten des Flugzeugs während der Erprobungsflüge analysieren können. Im vorliegenden Beitrag wird ein automatisches OMA-Verfahren auf Basis des Stochastic-Subspace-Identification Algorithmus SSI vorgestellt, das kontinuierlich die modalen Parameter in Abhängigkeit des Flugzustands identifizieren kann. Als Beispiel werden Messdaten aus mehreren Testflügen mit dem Segelflugzeug SB10 der Akaflieg Braunschweig verwendet, das mit Beschleunigungsaufnehmern instrumentiert wurde. Auch wenn hier der Identifikationsprozess nachträglich durchgeführt wurde, ist ein Einsatz des Verfahrens im Flug in Echtzeit möglich, weil neben der Datenerfassung lediglich ein leistungsfähiger tragbarer PC oder Tablet benötigt wird. Die Analysezeit beträgt je nach Anzahl der gemessenen Signale und des betrachteten Frequenzbereichs nur wenige Sekunden

Aeroelasticity in Sailplane Design

One of many problems in the field of sailplane design is that of aeroelastic stability. Practicing designers of gliders and light aircraft seldom have the opportunity to spend a great deal of time studying the difficult background of flutter. Therefore, the flutter engineers at the Institute of Aeroelasticity are traditionally consulted to test and to certify light aircrafts and gliders for flutter stability. In the following the basics for the mechanism of flutter are presented and the certification process is explained for a representative modern sailplane of the 18m-class starting with the requirements of the Joint Aviation Authorities (JAA). The objectives and methods of the ground vibration test are introduced to measure the vibration properties of the aircraft structure. These data are used to perform the flutter analysis in order to find possible flutter instabilities in the flight envelope. As the maximum flight speed of modern high performance sailplanes is further increased, costly modifications of the prototype are often necessary to satisfy a safe design. Therefore, a method is presented to estimate the flutter behavior during the design process

Automatic Operational Modal Analysis for Aeroelastic Applications

The development of new aircraft requires the evaluation of the aeroelastic stability to avoid the phenomenon of flutter, a self-excited oscillation of the airframe. Since the rational analysis of the flutter stability comprises coupled simulations using numerical structural models and unsteady aerodynamic loads, the accomplishment is complex and the implementations must be checked for their validity by comparison of analytical and experimental results. In the so-called Ground Vibration Test (GVT) the natural modes, eigenfrequencies and damping ratios of the prototype aircraft are identified using classical Experimental Modal Analysis (EMA) methods. Depending on the complexity of the new design, conducting such a test requires a time slot of several days shortly before the first flight. Consequently, there is an ongoing need to reduce the testing time to improve the availability of the aircraft prototype. This paper addresses the application of Operational Modal Analysis (OMA) methods during the GVT of an aircraft, which might cut down the efforts in time and labour. An automatically running fast implementation of the Stochastic Subspace Identification method (SSI) is introduced, which analyses the output acceleration response of the airframe randomly excited by modal shakers. The identification process is specified in detail for a glider aircraft, where acceleration time series must be evaluated to generate the stabilization diagram. To isolate the physical mode shapes from the mathematical poles, a pole-weighted Modal Assurance Criteria (MAC) is evaluated for several model orders to clean the stabilization diagram. Since the process needs no further operator interaction, it is suitable for monitoring airframe vibrations of the aircraft in flight, provided that the changes in flight conditions are significantly slower than the duration of the vibration periods considered. For the sucess of the methods, the OMA requirements should be fulfilled, i.e. the excitation of aircraft should be non-deterministic with broad-band spectra. Such conditions are provided by atmospheric turbulence excitation and/or pilot control inputs. The presented autonomous process is applied to a simulated Flight Vibration Test (FVT) of an research aircraft with real-time modal identification where changes of eigenfrequencies and damping ratios are tracked with changes in flight conditions

Uncertainty Propagation in Flutter Analysis

Since simplifications are introduced in the theoretical formation of the models on one hand, and the measuring accuracy is limited in the experiment on the other, models and their parameters may include uncertainties, making it difficult to determine the dynamic stability boundaries. For the robust flutter analysis, it is necessary to propagate the effects of identified uncertainties towards aeroelastic stability of the aircraft to cover the uncertain-butbounded parameter space. Robust stability is guaranteed when the uncertainties cannot destabilize the system

Robust flutter analysis using interval modal analysis and continuation method

For robust analysis of the flutter stability, it is necessary to consider the uncertainties of the structural dynamics model. These uncertainties can be clarified from estimates of uncertainties of the parameters in finite element models or from statistical evaluations of experiments. As a supplement to the reliable flutter algorithm for solution of the flutter equation, the continuation method is introduced, which allows the specific examination of individual solution branches of the flutter equations that risk being critical for stability. To compute the range of variations of eigenfrequencies and mode shapes caused by possible structural uncertainties, the interval modal analysis is introduced. The modal interval ranges are tracked through the flutter stability analysis. The deterministic computed intervals of critical flutter speeds are compared to the probabilistic results obtained by Monte-Carlosimulation

Interpolation of GVT Results using Volume-Spline Method

For the flutter analysis it is demanding that the deformation of mode shapes is transferred to the aerodynamic grid to analyse unsteady aerodynamic loads induced by the oscillation. If the analysis is based on measurements, the available information of deflections is limited to the applied transducers which will generally measure the acceleration in one translation direction. In general about 100 sensors are applied for small two and four seated aircrafts and gliders and about 200 are used for aircrafts in the CS23 category. For the interpolation of the measured acceleration amplitudes it was assumed that each wing section is rigid in the flow direction. For span-wise interpolation one dimensional cubic splines are applied for each aircraft component to analyze the varying bending, in-plane bending and torsion from root to tip. As the sensors can only measure translational accelerations the twist of the lifting surfaces must be calculated from amplitude differences at leading edge, trailing edge and hinge line, respectively. In addition the deflection angles of control faces with respect to the wing are calculated

Do we need an Update of the Means of Compliance for CS-22 Flutter Paragraph?

The Flutter Phenomen • Aeroelastic instability caused by interaction of structural dynamics and unsteady aerodynamic induced by motion • Coupling of at least two flexible aircraft modes results in self-excited vibration - explosive destruction • Rational flutter analysis for prediction - Monitoring of damping during flight testin

How to perform Flight Vibration Testing based on Operational Modal Analysis?

Flutter is a dynamic instability caused by the interaction of the structural dynamics of the aircraft and unsteady aerodynamic forces. It occurs when one of the elastic modes of the airframe tends to negative damping above a critical speed. Prediction and flutter clearance are important problems in the design, testing and typecertification of sailplanes. From a practical point of view, Flight Vibration Testing (FVT) needs to be performed whenever a new sailplane is built or an existing type is modified. The application of Operational Modal Analysis (OMA) methods may provide improvements to identify modal parameters of multiple modes in one step without knowing the type of external excitation. If the identification is repeated for increasing flight velocities, it is possible to find the aeroelastic damping trends and to extrapolate to the stability boundary. The application of OMA needs a broadband excitation spectrum, which result from impulsive control kicks or continuing random excitation like gusts and turbulence in the atmosphere. To demonstrate the performance of the proposed method measured time histories are post-processed to identify all relevant mode shapes with natural frequencies and damping rates of the flying sailplane. Requirements for test equipment, software and the procedure to perform the FVT are summarized

Aeroelasticity in Sailplane Design

One of many problems in the field of sailplane design is that of aeroelastic stability. Practicing designers of gliders and light aircraft seldom have the opportunity to spend a great deal of time studying the difficult background of flutter. Therefore, the flutter engineers at the Institute of Aeroelasticity are traditionally consulted to test and to certify light aircrafts and gliders for flutter stability. In the following the basics for the mechanism of flutter are presented and the certification process is explained for a representative modern sailplane of the 18m-class starting with the requirements of the Joint Aviation Authorities (JAA). The objectives and methods of the ground vibration test are introduced to measure the vibration properties of the aircraft structure. These data are used to perform the flutter analysis in order to find possible flutter instabilities in the flight envelope. As the maximum flight speed of modern high performance sailplanes is further increased, costly modifications of the prototype are often necessary to satisfy a safe design. Therefore, a method is presented to estimate the flutter behavior during the design process