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About this book

The book presents a synopsis of the main results achieved during the 3 year EU-project "Advanced Inflight Measurement Techniques (AIM)" which applied advanced image based measurement techniques to industrial flight testing. The book is intended to be not only an overview on the AIM activities but also a guide on the application of advanced optical measurement techniques for future flight testing. Furthermore it is a useful guide for engineers in the field of experimental methods and flight testing who face the challenge of a future requirement for the development of highly accurate non-intrusive in-flight measurement techniques.

Table of Contents




Chapter 1. Foreword to the Proceedings of the Final Workshop

Flight testing is the final proof of maturity for a technology under the most representative levels of acceleration, aerodynamic load on structures and in the most representative flight environment (e.g. Reynolds number). Therefore, measurements performed during flight tests are particularly valuable. They are also scarce because a flight is costly, requires a long preparation time for the instrumentation while satisfying certification requirements to ensure safety.
R. Dénos

Chapter 2. AIM: An Introduction to the Project

The European Specific Targeted Research Project (STReP) AIM—Advanced In-Flight Measurement Techniques was launched on the \(1\mathrm{st }\) of November 2006 and was finished by the \(30\mathrm{th }\) of April 2010. This project intended to make advanced, non-intrusive measurement techniques applicable for time and cost effective industrial flight testing as well as in-flight testing for research. In the AIM consortium, eleven Partners from aircraft industries, airport services and research organisations coming from seven countries were working closely together. AIM was coordinated by DLR in Göttingen. The purpose of the AIM program was to further develop measurement techniques in such a way that they can be routinely applied to flight tests, hence providing comprehensive planar information on various important parameters such as wing and propeller deformation, thermal loads on the structures of helicopters, the surface pressure distribution on a wing, density gradients of strong vortices generated by airplanes and helicopters and velocity flow fields near airplanes and helicopters. The chapter will provide a brief overview on the activities within AIM and the project itself.
Fritz Boden

Wing deformation studies


Chapter 3. Highly Accurate Aircraft In-Flight Wing Deformation Measurements Based on Image Correlation

NLR developed a system for in-flight wing deformations measurement based on the Image Pattern Correlation Technique (IPCT) as part of the AIM project (Advanced In-flight Measurement Techniques) funded by EC under FP6. The IPCT system was first tested in a laboratory environment and subsequently integrated into NLR’s Swearingen Metro II research aircraft. Aircraft integration tests were performed at the hangar, while also verification measurements of the aircraft-integrated IPCT system versus micrometer were made. The ground-based verification demonstrated the inherently high accuracy of the method. The IPCT system was used successfully in-flight for wing deformation measurements. The aircraft wing deflection was measured under various load conditions ranging from 0 to 2.5 g. Optical displacements of a randomly speckled part of the wing relative to a reference frame were determined using cross correlation techniques. These optical displacements were converted to geometrical wing deformations in a reference frame relative to the wing in reference condition. With respect to these geometrical wing deformation results a wing deflection model could be fitted. From this model various wing deflection parameters were determined and presented as function of the wing load. Parameters included e.g. change in wing heave, dihedral, torsion. Also the dynamic behaviour of the wing, e.g. during landing, can be investigated with high accuracy using IPCT. The flight trial demonstrated the usefulness of the IPCT technique for high accuracy, static and dynamic in-flight wing deflection measurement.
H. P. J. Veerman, H. Kannemans, H. W. Jentink

Chapter 4. IPCT Ground Vibration Measurements on a Small Aircraft

One of the AIM subtasks required the measurement of structural vibration using IPCT on a small aircraft. This has been done in an industrial environment on a Piaggio P 180, without interfering with production testing and other flight test activity. A ground test, simulating flight conditions and measuring wing vibration was initially done and, due to budget restrictions, only a limited IPCT flight test could be completed using the ground set-up. For the ground tests, a set of accelerometers was also installed to compare the IPCT results with those obtained with traditional means. For these tests, an exciter, driven by a PC, was placed at the wing tip to generate a signal at the desired frequency. With this system, a frequency sweep was performed around the known natural frequencies, exciting the first wing bending mode. Furthermore, accelerometers were installed on the camera support in order to evaluate whether the camera itself was vibrating. During ground testing, the IPCT method showed promising results for in-flight testing, although the resolution was insufficient for the higher frequency, lower amplitude results. These limitations would require supplementing any IPCT system with traditional accelerometer measurements. In what follows, a brief description of the test set-up, the test instrumentation, the data analysis and the results of the ground vibration measurements will be presented.
Fritz Boden, Thomas Wolf, Claudio Lanari, Anwar Torres

Chapter 5. In-Flight IPCT Wing Deformation Measurements on a Small Aircraft

After the successful application of the Image Pattern Correlation Technique (IPCT) on the P 180 experimental test aircraft for wing vibration measurements on ground, the complete camera installation has been certified for flight testing to perform a feasibility test with in-flight IPCT for wing deformation measurements. Finally, several successful flight tests have been executed at the Piaggio Aero Industries plant in Genova (Italy). As the measurement technique and the measurement installation have been described in a detailed way in Chap. 4, this paper mainly contains a brief description of the applied installation, the performed flight tests and a presentation of the measurement results.
Fritz Boden, Claudio Lanari, Anwar Torres, Thorsten Weikert

Chapter 6. Assessment of IPCT for Wing Deformation Measurements on Small Aircrafts

After the performance of the IPCT wing deformation measurements on the Fairchild Metro II and on the Piaggio P 180, the aircraft manufacturer Evektor performed an assessment of the image based measurement techniques for wing deformation and flutter testing. For this assessment the papers and reports of the AIM project, as well as the data obtained by ground and flight tests of DLR [1], NLR [2] and Piaggio [3] were used. The image based measurement techniques used in AIM are compared to classical deformation measurement sensors like strain gauges and accelerometers. Furthermore, an assessment is done from the side of view of the effectiveness of this method. Finally, a summarized conclusion and some recommendations for further development are given.
Pavel Ružička, Jan Rýdel, Miroslav Josefik, Fritz Boden

Chapter 7. IPCT Wing Deformation Measurements on a Large Transport Aircraft

Aircraft wings are subject to deflection. Designers must therefore take into account in-flight wing bending and torsion, so that structural loads are well taken care of and that the wing has optimum aerodynamic performance. Models are used to calculate wing torsion and bending. These models need to be validated in flight and therefore non-intrusive measurement methods are preferred in order to avoid aerodynamic interferences. A photogrammetry method is already applied for these measurements, although this method has some constraints. Only static measurements are provided and accuracy depends on careful and time-consuming calibrations. Within the AIM (Advanced In-flight Measurement techniques) project various novel advanced non-intrusive measurement techniques are introduced for usage in flight. One of these novel techniques is the IPCT (Image Pattern Correlation Technique) applying advanced image correlation techniques as used in Particle Image Velocimetry (PIV) on surfaces. Applying the IPCT can improve the accuracy, reduce installation time, provide local surface deformations and give dynamic results. IPCT uses image correlation to calculate the deformation of a wing with a speckled pattern. In this chapter the measurement technique, the installation on a large transport aircraft i.e. the A380 and the test performed in June 2009 will be described. At the end some results of the test will be presented.
Fritz Boden, Henk Jentink, Christian Petit

Propeller deformation studies


Chapter 8. Propeller Deformation Study on an P 180 Push Propeller Aircraft

Within AIM one of the most challenging tasks was to measure the propeller blade deformation with QVT (Quantitative Video Technique) and IPCT (Image Pattern Correlation Technique) in flight. Beside this measurement task, it should be tried to assess the installed propeller performance. The idea was to relate the deformation to the thrust provided by the blade. First the load distribution on the blade was determined by means of CFD and in the following a representative load was applied to the blade during a static test in order to validate the FEM model of the blade which was then used to relate the deformation to the thrust. One further step was also planned consisting of the assessment of the possibility to investigate the effects of the exhaust impinging the blade in terms of performance loss. Due to some difficulties with the triggering system during the flight test it was not possible to compare the simulation and ground test results with the flight test results. Nevertheless a valuable work has been done to perform a preliminary evaluation of the in-flight propeller blade deformation, simulate the load on the blades and to develop and validate a finite element model suitable for the test.
Claudio Lanari, Fritz Boden

Chapter 9. Image Based Propeller Deformation Measurements on the Piaggio P 180

One of the most challenging tasks of the AIM project was to measure the deformation of a fast revolving propeller blade during flight by using the QVT (Quantitative Video Technique) and the IPCT (Image Pattern Correlation Technique). In order to be able to perform such a measurement, Piaggio and DLR worked together closely to solve the major problems to record images of the blade in a high quality to be processed with IPCT. Where to install the camera(s), how to illuminate the blade and how to trigger the camera and consequently the illumination device, were important questions to be answered before the test. The blade deformation measurement by itself was already a demanding target, but in order to give more value to such an activity one more step forward was also planned consisting in the evaluation of the possibility to investigate the effects of the exhaust impinging on the blade in terms of performance lost. Therefore, the recording system was designed so that it was possible to take recordings with a freely adjustable phase angle. Due to present problems with the triggering system it was not possible to make this evaluation. Nevertheless, in-flight propeller blade deformation measurements have been performed for one phase angle and for different flight conditions. In this contribution, the measurement task and the measurement setup are described briefly followed by the results of the performed ground tests and the executed flight tests.
Claudio Lanari, Boleslaw Stasicki, Fritz Boden, Anwar Torres

Chapter 10. Assessment of Propeller Deformation Measurement Techniques for Industrial Application

After the performance of the IPCT flight tests on the Piaggio P 180, the aircraft manufacturer Evektor performed an assessment of the image based measurement techniques for propeller deformation measurements. As background the papers and reports, as well as the data obtained by ground and flight tests of DLR and Piaggio [1, 2] were applied. The image based measurement technique IPCT is compared to the commonly applied strain gauge method. An assessment is done more from the practical side of view and effectiveness of the compared methods. Advantages and disadvantages of the assessed methods are analysed and emphasised. Finally, a conclusion and some recommendations for further development are given.
Pavel Ružička, Jan Rýdel, Miroslav Josefik, Fritz Boden, Claudio Lanari

Helicopter investigations


Chapter 11. Towards In-Flight Measurements of Helicopter Blade Tip Vortices

In the framework of the AIM project the near field of the blade tip vortex of a full-scale helicopter in simulated hover flight was investigated by combining three-component Particle Image Velocimetry and Background Oriented Schlieren measurements. The velocity field measurements in the range of wake ages of \({\uppsi _{\mathrm{{v}}}={1}^\circ }\) to \({30}^\circ \) in azimuth provided a reference for a quantitative analysis of the Schlieren results yielding vortex core density estimates. Ongoing vortex roll-up was observed at \({\uppsi _{\mathrm{{v}}}={1}^\circ }\) while considerable aperiodicity was persistent thereafter. The vortex parameters for \({\uppsi _{\mathrm{{v}}}>{1}^\circ }\) were consistent with the Scully vortex model. The particular challenges of full-scale, outdoor testing, especially the limited spatial resolution and aperiodicity effects, resulted in elevated measurement uncertainty as compared to sub-scale experiments.
Kolja Kindler, Karen Mulleners, Markus Raffel

Chapter 12. $$1.5\,\upmu \mathrm{{m}}$$ 1.5 μ m LIDAR for Helicopter Blade Tip Vortex Detection

The objective of the ONERA study in the AIM project “Advanced In-flight Measurement Techniques” is to assess the capability of on board LIDAR technique to investigate in-flight tip vortices behaviour. This paper presents the design of as \(1.5\,\upmu \mathrm{{m}}\) LIDAR sensor dedicated to tip vortex characterization and tests on ground during trials on a DLR helicopter in hover flight. The relevant information resulting from these trials is the tip vortex velocity field as well as the time evolution of the vortex. The technical challenge here is to characterize a very small phenomenon at short range: the core radius varies from typically 10 to 30 mm as the vortex ages. The study results show that LIDAR technique is promising for onboard measurement during flight. The velocity measurement is direct and absolute (no calibration needed) and its accuracy can be up to 0.25 m/s and commonly 1 m/s. However, seeding is necessary to realize a compact and reliable LIDAR system with components ‘off the shelf’: in the framework of flight trials, clouds could provide efficient seeding enabling the use of LIDAR as a powerful technique for tip vortex characterization.
Béatrice Augere, Claudine Besson, Agnès Dolfi, Didier Fleury, Didier Goular, Matthieu Valla

Chapter 13. Blade Deformation Measurements with IPCT on an EC 135 Helicopter Rotor

For the design of a helicopter rotor blade, it is crucial to predict the dynamic behavior and occurring forces and moments. For the validation of prediction tools, reliable flight test data is required. Furthermore, flight test measurements are necessary for the calculation of fatigue loads. Today, strain gauges are applied to a helicopter rotor blade to perform measurements in flight test. Locally, they allow a precise strain measurement for the complete rotor revolution. This instrumentation implies a high effort. Furthermore, wiring can imply difficulties due to its weight and its modification of the aerodynamic shape. An optical measurement technique may overcome some of the limitations of strain gauges. A precise measurement of the deformation of the complete surface of the rotor blade allows to locate high strains and to identify oscillatory modes. The exact blade position can be identified optically. One of these advanced optical measurement techniques is the Image Pattern Correlation Technique (IPCT). Today, IPCT is a state of the art measurement technique for static and dynamic deformations. In AIM, DLR and Eurocopter explore the feasibility of Quantitative Video Technique (QVT) together with the Image Pattern Correlation Technique (IPCT) on the rotating main rotor blades of a flying helicopter. Ground tests of the measurement system on a whirl tower and a tied down helicopter are performed to verify the feasibility and performance of the measurement system previous to flight testing.
Christoph Maucher, Fritz Boden

Chapter 14. Applications of Infrared Technology to Helicopter Flight Testing

This chapter deals with some applications of the infrared technology for industrial flight tests of helicopters. The main application concerns the measurement of structural temperature. Flight tests are performed on a prototype helicopter equipped with suitable infrared cameras in order to measure the surface temperature of structures located in the vicinity of the engines exhaust nozzles. These trials show that the infrared technology is fully usable for helicopter flight testing purposes and the results inferred underline that this instrumentation offers clear advantages compared with the methods generally used.
Laurent Girard

Flow measurements


Chapter 15. In-Flight Application of Pressure Sensitive Paint

A feasibility test for in-flight pressure measurement using Pressure-Sensitive Paint (PSP) techniques has been performed in the frame work of the research project Advanced In-flight Measurement Techniques (AIM). PSP was applied to a pylon surface of the VFW 614 ATTAS aircraft. In the test, three PSP measurement methods: “intensity method with LED-array”, “intensity method with Electro Luminescence (EL) foil”, and “image based lifetime method” were employed to measure pressure distributions and the results were compared. The results showed the good feasibility of PSP measurements at in-flight testing. All three methods could represent the pressure distribution on the pylon. Especially the intensity method with LED-array and EL foil could provide results in good image quality. An EL foil which was applied to the pylon surface outside of the cabin and firstly employed in flight tests worked very well even at high altitudes. From the obtained results, one can conclude that the PSP technique successfully passed the first feasibility tests. The lessons learned from the experiences in the flight test are presented in the following chapter.
Yasuhiro Egami, Christian Klein, Ulrich Henne, Klaus de Groot, Jörg B. Meyer, Claus-Peter Krückeberg, Fritz Boden

Chapter 16. Development of Particle Image Velocimetry for In-Flight Flow Measurement

Wind tunnel testing is widely used for the development and testing of numerous aerodynamic systems. Mature laser based optical methods such as particle image velocimetry (PIV) also now offer detailed flow diagnostics in the wind tunnel environment. The majority of wind tunnels, however, cannot correctly scale both Reynolds and Mach number simultaneously. In-flight testing from a full scale aircraft offers the potential to take correctly scaled data to validate both wind tunnel and computational models. Furthermore, PIV has the potential to take detailed, non-intrusive velocity data from a full scale flight test. The following chapter reports on what the authors believe is the seminal application of a modified PIV setup inside a DLR Dornier DO 228-101 research aircraft and the subsequent PIV flight tests. These tests were completed as part of a major European Framework 6 research program called Advanced In-flight Measurements (AIM). In particular, the chapter reports on the major challenges that were overcome by the German Aerospace Center (DLR) together with Cranfield University to complete the flight tests. These challenges included the laser safety considerations for the test, the certification of the aircraft and the requirement for suitable seeding during the tests. The latter issue was overcome by flight testing inside a suitable cloud formation and successful PIV data was recorded over a series of three flight tests in a region adjacent to the outside of the cabin. The PIV images obtained yield high data quality inside the turbulent boundary layer.
Christina Politz, Nicholas J. Lawson, Robert Konrath, Janos Agocs, Andreas Schröder

Chapter 17. Ground Based Large Scale Wake Vortex Investigations by Means of Particle Image Velocimetry: A Feasibility Study

The investigation of aircraft wake vortices is a fundamental challenge in aerospace research. The enhanced safety and efficiency given by better prediction and detection of these complex vortex systems is significant. Within AIM several non-intrusive optical methods were considered to visualise and measure the wake vortex of a landing aircraft. The main objective of the herewith presented campaign was the application of the Particle Image Velocimetry (PIV) to the investigation of these vortex flows. This report describes the first steps and approaches for the implementation of a large scale PIV measurement system for the determination of wing wake vortices of a landing aircraft. Many issues have to be addressed before the flyover test could be carried out. During the realisation phase of this experiment several setups were considered and discarded, a prototype of a seeding generator based on helium filled soap bubbles was designed and preliminary studies have been carried out exclusively for the investigation of the laser light sheet behaviour when propagating over large distances. This report presents a summary of all the considerations and pretests for this particular application.
Christina Politz, Reinhard Geisler, Sudesh Ranasinghe

Chapter 18. Investigation of Background Pattern for the Outdoor Application of the BOS Method

The aim of this work was to investigate the influence of such environmental parameters as natural backgrounds, angle of inclination between the background and the registration plane and presence of turbulence on the quality of the outdoor experimental vortex determinations results by the Background Oriented Schlieren Method (BOS) method. In order to do this a collection of artificial and natural backgrounds was created, the modeling of the BOS pictures on these backgrounds basis was carried out and the modeled BOS pictures were processed. According to the results of processing the parameters of the best background were chosen. The influence of the angle of inclination and the turbulence were estimated, recommendations for experimental conditions were worked out.
B. S. Rinkevichyus, N. M. Skornyakova, E. M. Mikhaleva, A. S. Mikhalev, A. Yu. Poroykov, A. V. Udalov

Chapter 19. The Application of Background Oriented Schlieren Method to Aircraft Wake Vortex Investigations

Vortex wakes of conventional aircraft and their propagating nature close to the ground have a great impact on the safety and efficiency of the traffic flow on an airport. The development of a measurement technique which helps detecting e.g. the density gradients in the air due to the wake vortices and understanding the nature of such a flow structure in its natural environment can improve the predictability of this hazardous phenomenon. Therefore, the practical aspects of the implementation of an advanced Schlieren technique on an airport are described in this chapter. Two experiments based on slightly different setups and locations relatively to the runway were conducted in order to assess the feasibility of the Background Oriented Schlieren method (BOS) for the detection and investigation of aircraft wake vortices. The evaluation of the obtained data indicated a sufficient ability of this technique to detect strong density gradient but on the other hand, turned out to be less efficient when dealing with the peculiarities of a large scale outdoor experiment.
Christina Politz, Benedikt Over, Tania Kirmse

Chapter 20. Sensors and Actuators for Laminar Flow Flight Experiments

The present chapter is focussed on different measurement techniques for in-flight experiments. Laminar flow investigation for low and high flight velocities were performed. Several types of surface sensors were used for detection of laminar-turbulent transition. Methods for acquiring information about boundary conditions like angle of attack, flight velocity and pressure distribution are an essential part of this work, too. The method of delaying laminar-turbulent transition by active boundary layer control was transferred from wind tunnel experiments to a glider in real flight. For this purpose, a sensor actuator system was developed and, together with a realtime controller, applied to a glider.
Andreas Pätzold, Inken Peltzer, Wolfgang Nitsche


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