>Workpackages>Surface Flow Measurements
Monday, 10. August 2020

WP 4 Surface Flow Measurements

To measure the surface pressure distribution in-flight on a complete area without disturbing the flow often is big challenge and sometimes it is even not possible, because the sensors can often not be applied in the desired density or without modifying the structure. The Fibre Bragg Grating Technique (FBG) has the potential to measure a complete area using only a very thin and lightweight layer of sensors and thus preserving the original shape of the investigated surface.

Beside the surface pressure also the laminar-turbulent transition of the boundary layer has a significant influence on the aerodynamic forces of wings, rudders and the fuselage of an aircraft. The Infrared Thermography (IRT) is a proven technique to measure these effects. Although the infrared thermography has been applied successfully to aerodynamic investigations of the boundary layer in wind tunnel and flight tests and a new generation of infrared cameras with enhanced sensors and electronics is now available there remains the need of additional developments for future application.

Both the progressive FBG and the mature IRT is improved in this Work package in a step by step approach. First the main challenges is identified and solutions for a suitable in-flight setup are found and tested in the laboratory and the wind tunnel. Secondly research flight testing will be performed with the improved measurement techniques to test their viability for industrial flight testing.

Task 4.1. Development of FBG and IRT towards industrial application

Current FBG techniques have been primarily developed in a laboratory environment with a number of applications to industrial problems. While limited strain measurement by FBG techniques has been applied to flight tests, no such tests have been performed for pressure measurement. To date, only limited static pressure measurements have been obtained from an FBG system from a 2D wing in a wind tunnel. A previous wind tunnel test on a subscale wing had demonstrated dynamic strain measurement using embedded FBG sensors. Therefore development work must be completed to reduce the risks before FBG application to flight test. In particular a number of questions must be answered including finding a suitable optics package for the aircraft flight test environment, finding acceptable solutions for packaging and mounting the fibre systems onto the aircraft aerodynamic surfaces and further development of data processing of FBG signals given potential contamination of the signal from sources such structural vibration.

The infrared thermography (IRT), based on the measurement of the infrared radiation from surfaces, allows a global determination and visualisation of the surface temperature distribution with high accuracy. In aerodynamic research (in wind tunnel and flight tests) the thermography is used for the investigations of the boundary layer. Due to the jump in the wall stress coefficient and therefore in the heat transfer coefficient at the laminar-turbulent transition it allows the detection and visualisation of the transition from laminar to turbulent flow. Due to the temperature effects in vortices and shocks also laminar separations bubbles, vortices and shocks are detectable as well as flow separations. In most projects of the past the steady investigation of the boundary layer has been the point of interest. For example the determination of the laminar-turbulent transition has been done point to point as a function of the angle of attack. But in the mean time many requirements for an investigation of the unsteady behaviour of the boundary layer has been come up. The enhanced infrared technique of today allows the measurement of the temperature distribution on a surface with very high precision (20 mK) and at very high sampling rates (up to some 100 Hz). In this task a suitable setup for unsteady infrared thermography is developed. Furthermore the application of mirrors to improve the accuracy of the IRT is thinkable.

The main topics of this task are:

  • Development of laboratory based FBG technique into robust flight test technique
  • Development of unsteady IRT for flight tests
  • Feasibility study of application of mirror techniques in infrared thermography

Subtask 4.1.1 Development of FBG for Flight Test

In this subtask, the work completed does concentrate on development of a robust FBG technique which can be subsequently packaged and mounted for flight test in the Cranfield University Bulldog light aircraft. In particular the most suitable fibre optic FBG optical configuration is investigated which will provide high signal to noise under the Bulldog cockpit conditions to output static pressure and surface strain. These include increased levels of vibration and increased normal ‘g’. In addition, given the cockpit conditions will ultimately degrade the FBG signal, further work is completed to analyse the potential contamination of the FBG signal under these conditions and then how this signal degradation can be removed at the data processing stage. This work does include an element of FBG signal modelling and data processing.

Furthermore, the fibre optic packaging which will be suitable for mounting the FBG fibres onto the aircraft aerodynamic surfaces is investigated. Previous wind tunnel work with the FBG systems has found that the calibration and signal output of the fibres to some degree depends on the on the fibre packaging and mounting. Therefore work will be completed to find the best form of the fibre packaging, which will allow simple mounting and removal of the fibre on the aerodynamic aircraft surfaces without major calibration issues. This work is completed in part in the laboratory and the most promising fibre package solution are to be wind tunnel tested on a simple aerodynamic configuration to validate the behaviour of the fibre, the FBG optics and the data processing techniques. This validation process will run in parallel in the last six months of the initial FBG 24 month development program. Thus in summary the following tasks are completed within subtask 4.1.1.

  • Laboratory development of suitable FBG technology for the measurement of surface pressure and surface strain during flight test:

    1) Study of suitable optical configuration for flight test
    2) Development of suitable fibre mounting and packaging techniques onto aircraft surfaces
    3) Study of the characteristics of suitable adhesive materials for bonding FBG sensors to the surface of the aircraft.
    4) Study of FBG signal contamination sources from flight test
    5) Development of suitable processing software for contaminated FBG signals
    6) Development of suitable calibration method for FBG strain and pressure measurement
  • Calibration and validation of the FBG technology for flight test

    1) Validation of FBG strain measurement through laboratory strain rig
    2) Validation of FBG pressure measurement through wind tunnel tests
    3) Assessment of the influence of temperature to the strain and pressure measurements and devise compensation techniques

Subtask 4.1.2 Development of unsteady IRT for flight tests

The actual developments of infrared cameras enabling IRT measurements with a high frame rate (some 100Hz) and with a high precision. But the response time of the former wind tunnel models and wings due to their high heat capacity are too low to take advantage of the features of the new infrared cameras. Therefore to achieve higher response times of the investigated surfaces a modification of the surface with the aim of a very low heat capacity and therefore a fast response is necessary and is investigated in this subtask. At former flight tests with a glider and wind tunnel tests with a first approach of such a modified surface on the wing respectively the model the principal feasibility of unsteady infrared thermography has been shown. But there remain some problems concerning the applicability of these modified surfaces. This subtask is addressed to further development of suited modification of the surfaces of models respectively wings and corresponding CFD unsteady calculations and wind tunnel for the validation.

Subtask 4.1.3 Feasibility study of application of mirror techniques in infrared thermography

At wind tunnel and especially at flight tests the position of the infrared camera and therefore the distance between the camera and the region of interest on the investigated surfaces is more or less predetermined. In addition to this only a limited number of (expensive) lenses for infrared cameras are available. In summary the choice of a suitable lens is therefore always a more or less good compromise and only in a few cases the region of interest could be mapped to the infrared image with a sufficient number of involved pixels (sensor elements). A solution of this problem could be found in the usage of infrared mirrors. Another problem which arises especially at flight test is the very low angle of view when the infrared camera is mounted on the fuselage or behind a window in the cabin to measure the temperature of an outer part of the wing. The strong varying distance between camera and surface causes a strong distortion and a high reduction of the spatial resolution in the outer part. In these cases a specially designed infrared mirror with corresponding variation the focal length along the span wise direction should overcome this problem because the spatial resolution remains span wise constant and sufficient.

For an efficient designing of the special infrared mirrors and the exact determination of the field of view or region of interest (previewing) the methods of so called ray tracing which are well known from computer graphics should be made available to the infrared thermography system.

This subtask is addressed to develop ray tracing routines for mirror techniques and previewing and to design and manufacture specially designed infrared mirrors for flight tests with the help of a subcontractor.

Task 4.2. Research flight test with FBG for strain and surface pressure measurements on a small aircraft

In this task, the initial development work of task 4.1. completed for a flight test capable FBG system is extended to produce a boxed FBG system suitable for mounting inside the Bulldog aircraft. Once this system is complete, the actual Bulldog flight test program will follow.

To package the FBG for flight test, the advice of a suitable sub-contactor will be sought and this subcontractor will then be responsible for the preparation of the aircraft for flight test. Given the solid state nature and low power output of the FBG optics and light emitting diode (LED) and given the system will be packaged in a light tight box with the exception of the fibre output, it is expected no major LED safety issues will be encountered. The FBG system and mounting on the Bulldog, however, will still need to be approved to the appropriate airworthiness standards. These mountings and approval work will be completed by the sub-contractor.

The positioning of the fibre on the Bulldog will ensure an appropriate set of static pressure data can be recorded for direct comparison with a parallel wind tunnel study. Strain data will also be recorded and compared to suitable published data. The two possible positions for the fibre mounting are either a chordwise section of the Bulldog wing or a chordwise section of the aircraft tail depending on the airworthiness requirements. In either case, scaled wind tunnel data will be recorded for comparison with the FBG data. Each of these two stages of Task 4.2 is expected to take 6 man months with the flight test completed in the last 6 month of the program. In summary, the tasks to be completed in this subtask are:

  • FBG flight test preparation and commissioning:

    1) Manufacturing and commissioning of FBG sensor pack for flight test
    2) Installation and certification of FBG sensor into Bulldog aircraft
  • FBG flight test and validation studies:

    1) FBG flight test
    2) Acquisition of wind tunnel validation data
    3) Comparisons between wind tunnel and flight test data

Task 4.3. In-flight application of IRT for surface flow measurements in an industrial environment

The measurement IRT system, developed in task 4.1., is to be adopted and installed on the new motor glider AOS71. The motor glider is designed as a special test-bed for multi-purpose research and development activities commonly by Warsaw and Rzeszow Universities of Technology. The aim of this task is to measure the surface flow during flight tests with unsteady IRT. The measurement setup designed by DLR in subtask 4.1.2 and if suitable also the mirror setup from subtask 4.1.3 will be installed on the AOS71. The work to obtain the permit to fly is done by RUT, who afterwards will perform the flight testing together with DLR.

AIM² Advanced In-Flight Measurement Techniques, c/o German Aerospace Center (DLR), Bunsenstrasse 10, 37075 Goettingen, Germany, Tel: +49 551 709 2252, Fax: +49 551 709 2830, Email: