Determining Aerodynamic Loads Based on Optical Deformation Measurements

2018

The knowledge of the movements and the deformation of rotor blades is of great importance for the performance and the safe operation of a rotor system, especially for blades with a high aspect ratio like on helicopters or wind turbines. To measure these parameters in the rotating system is not easy, because the number of sensors is limited due to their impact on the aerodynamics and the modification of the structure. Strain gauge measurements can furthermore directly be affected e.g. by the choice of the sensor location and temperature effects. To avoid those problems and in addition enable direct shape measurements contactless optical methods have still been applied to rotor and propeller deformation measurements in the past. These attempts have been done out of the rotating frame observing either only small rotors or the blade passing within the field of view. DLR and Hardsoft developed a rotating 3D image acquisition system for helicopter rotors and performed first tests on the w...

2008

Descriptor(s) optical measurement techniques flight tests wing deflection Image pattern correlation technique High Accuracy In-Flight Wing Deformation Measurements based on Optical Correlation Technique Problem area Deformations of wings are difficult to assess. By applying additional instrumentation to the wing, the wing properties would be changed. Therefore optical non-intrusive techniques are very suited to measure these deformations. In order to further develop these optical measurement techniques for project application in flight testing the EC 6 th Frame work project AIM (Advanced In-flight Measurement techniques) was initiated. This paper that was presented at the 19 th SFTE EC Symposium, Manching EADS, 22-24 September 2008, describes the measurement objectives, the preparations, flight trial activities and finally the obtained results. Description of work Adequate equipment was selected and partly procured. After laboratory tests components were installed into NLR's Fairchild Metro II research aircraft. Images of a speckled wing area were collected during various maneuvers, providing a wide range of wing loads. Images of the deflected wing were cross correlated with a reference situation using commercially available and inhouse developed software. Results and conclusions Measured wing deflections were described according to a wing deflection model in terms of wing heave, torsion, twist and wing local behavior. Measurements proved to be feasible up to 0.2 mm accuracy. Applicability Apart from wing deformation measurements the technique is applicable for wing modes determination.

2014

The Image Pattern Correlation Technique (IPCT) is a powerful optical tool to measure the surface shape and deformation of aircraft structures, e.g. wings, propeller or rotor blades, non-intrusively in flight. The latest development of this technique is its application to 360° propeller deformation measurements. Therefore a special stereo camera system rotating with the propeller at its full rotational speed was designed, whereby the system was exposed to extreme vibration and centrifugal forces. By beginning of 2014 the whole system had been successfully tested on an Evektor VUT 100 COBRA airplane. In the paper, this highly sophisticated rotary camera system and the measurement technique IPCT is described. Furthermore some results of the first flight tests are presented.

2014

Airbus Defense and Space (Airbus DS) is working on the definition of a new methodology for the validation of the theoretic static loads model by implementing an innovative wing deformation measurement system based on photogrammetry on a medium transport aircraft (C295MW). The measured deformations are compared to those calculated by means of a high fidelity structural model subject to the loads derived from the theoretic loads model. This technique is expected to be as accurate as current strain gauges methods being at the same time far more flexible and economical.

Mechanical Systems and Signal Processing, 2015

Health monitoring of rotating structures (e.g. wind turbines and helicopter blades) has historically been a challenge due to sensing and data transmission problems. Unfortunately mechanical failure in many structures initiates at components on or inside the structure where there is no sensor located to predict the failure. In this paper, a wind turbine was mounted with a semi-built-in configuration and was excited using a mechanical shaker. A series of optical targets was distributed along the blades and the fixture and the displacement of those targets during excitation was measured using a pair of high speed cameras. Measured displacements with three dimensional point tracking were transformed to all finite element degrees of freedom using a modal expansion algorithm. The expanded displacements were applied to the finite element model to predict the full-field dynamic strain on the surface of the structure as well as within the interior points. To validate the methodology of dynamic strain prediction, the predicted strain was compared to measured strain by using six mounted strain-gages. To verify if a simpler model of the turbine can be used for the expansion, the expansion process was performed both by using the modes of the entire turbine and modes of a single cantilever blade. The results indicate that the expansion approach can accurately predict the strain throughout the turbine blades from displacements measured by using stereophotogrammetry.

Measurement Science and Technology, 2013

This paper describes the measurement of the deformation of an extremely flexible rotor blade in hover using a novel application of three-dimensional digital image correlation (3D DIC). In this optical method, images of the rotor blade painted with a high-contrast, random speckle pattern are captured using two digital cameras and a strobe light. Photogrammetry is used to calculate a whole-field, three-dimensional map of the rotor blade. Cross-correlation between images captured under two load conditions is used to calculate deformation. The technique was first validated in the rotating frame by correlating DIC measurements on a stiff rotor blade of known geometry with measurements made by two other laser sensors. The technique was then used to measure the deformation of extremely flexible blades on a 46 cm diameter rotor, rotating at 1200 RPM. The blades of this rotor are so flexible that they can be rolled up into a compact volume. The spanwise variation of extension, lead-lag and flap bending, as well as pitch angle, were extracted from DIC measurements. The technique yielded surface heights with a spatial resolution of 0.15 mm and three-dimensional deformation vectors with a spatial resolution of 2.37 mm (1.04% of the rotor radius), at a calculated accuracy of 15 μm. Tip displacement of up to 7.59 mm (3.3% of the rotor radius) and a tip twist of up to 10.8 • were measured. Based on DIC measurements, it is concluded that an accurate aeroelastic model of the rotor blade must include flap and lead-lag bending as well as twist degrees of freedom. In addition, the elastic twist must be considered of the same order of magnitude as the blade root pitch. Finally, the scalability of this technique to the measurement of full-scale rotor blade deformation is discussed.

Journal of Aircraft, 2001

The theory, methods, and applications of the videogrammetric model deformation (VMD) measurement technique used at NASA for wind tunnel testing are presented. The VMD technique, based on non-topographic photogrammetry, can determine static and dynamic aeroelastic deformation and attitude of a wind-tunnel model. Hardware of the system includes a video-rate CCD camera, a computer with an image acquisition frame grabber board, illumination lights, and retroreflective or painted targets on a wind tunnel model. Custom software includes routines for image acquisition, target-tracking/identification, target centroid calculation, camera calibration, and deformation calculations. Applications of the VMD technique at five large NASA wind tunnels are discussed.

Society of Photo Optical Instrumentation Engineers Conference Series, 2008

A combination of high-speed stroboscopic imaging with the Image Pattern Correlation Technique (IPCT) enables for non-intrusive measurement of surface deformation of fast vibrating or rotating objects. In this paper the dedicated instrumentation for the measurement of the deformation of aircraft propellers as well as first results of its application will be described.

Springer eBooks, 2011

Identifying the structural dynamics of rotating components can be difficult. Often times, structural dynamic measurements are obtained while the structure is in a static configuration. There are differences that exist in the structural behavior when comparing these statically performed tests and the dynamic characteristics when in operation. In order to evaluate the actual system while in operation, slip-rings are used during testing with measurements made at only a very few selected points. But this slip-ring configuration can be problematic, suffer from measurement noise and the attached sensors can obscure the true dynamic response due to mass loading and aerodynamic effects. 3D digital image correlation (DIC) has been used to capture the out-of-plane motion on the surface of a small scale rotating fan blade. This work extends prior efforts, by quantifying the performance of the optical measurement on a 46 in (1.17m) diameter, rotating wind turbine. The optical measurements are made using DIC (10,000+ measurement points) and dynamic photogrammetry (providing dozens of effective measurement locations). The motion of the turbine as measured using DIC, photogrammetry and accelerometers is compared at several discrete points. The proposed measuring approaches via DIC and dynamic photogrammetry enable full-field dynamic measurement and monitoring of rotating structures in operation.

28th International Congress on High-Speed Imaging and Photonics, 2008

A combination of high-speed stroboscopic imaging with the Image Pattern Correlation Technique (IPCT) enables for non-intrusive measurement of surface deformation of fast vibrating or rotating objects. In this paper the dedicated instrumentation for the measurement of the deformation of aircraft propellers as well as first results of its application will be described.