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Skin friction and fiber-optics-based surface pressure measurements for aircraft applications

Periodic Reporting for period 2 - SKOPA (Skin friction and fiber-optics-based surface pressure measurements for aircraft applications)

Reporting period: 2019-10-01 to 2021-03-31

The technology that is currently developed in this project is a response to the need to integrate Ultra High Bypass Ratio (UHBR) engines on the wings of transport aircrafts. Within the Clean Sky 2 Technical Programme, this need is addressed in the WP 1.5 of the Large Passenger Aicraft IADP (“Applied Technologies for Enhanced Aircraft Performance”). Such UHBR engines, which have a higher propulsion efficiency than current models and thus contribute to a more ecologic and economic aircraft-engine platform, are associated with relatively large nacelles. Therefore, in order to provide sufficient clearance between the nacelle and the runway without introducing the penalty of longer landing-gear struts, the nacelles must be integrated closer to the wing. This, in turn, increases the risk of flow separation in the region of the wing/pylon junction, especially in the take-off and landing phases when the aircraft is close to the critical stall condition. Actual flow separation would be particularly detrimental from an aerodynamic point of view since it would limit both the maximum lift coefficient and the lift-to-drag ratio of the aircraft, two aerodynamic quantities that are critical for landing and take-off, respectively. The Clean Sky 2 Work Plan tackles this issue by introducing novel, integrated, active flow-control techniques (AFC) applied at the wing-pylon interface in order to reduce or eliminate possible separated-flow zones (Technical Programme, WP 1.5.3).

In order to validate the effectiveness of the AFC system proposed in WP 1.5.3 the overall objective of the SKOPA project is to deliver a set of sensors that allows to perform in-flight, time-resolved measurements of skin friction and surface pressure in near-stall conditions. To reach this goal, two sensor technologies were suggested: “Delta Surface Hot Film” (DHSF) were selected to measure skin friction while “Fiber Optic Pressure” (FOP) sensors were selected for pressure measurements. Furthermore, the following sub-objectives of SKOPA were identified:

1) Design and fabricate a complete, flight-ready, measurement system based on DSHF sensors for skin-friction measurements and FOP for pressure measurements
2) Validate the measurement system in preliminary wind-tunnel tests.
3) Demonstrate the flight readiness of the measurement system in relevant environments by performing time-resolved skin-friction and surface-pressure measurements
a. on a industrially relevant wind-tunnel model of an outer wing with active flow control (AFC);
b. on the wing of an ultra-light aircraft.

Within the Cleansky 2 JTI, the results obtained in this project will be used to demonstrate and quantify the effect of active flow control in the region of the upper-surface wing near the wing-pylon interface. This will not only directly impact the goals of the Work Plan (Section 1.2) but also contribute to achieve the main objectives of Europe’s Strategic Research & Innovation Agenda (SRIA-ACARE), namely serving society’s transportation needs and maintaining global leadership in the aviation field.
The SKOPA Project was divided into 3 technical work packages:

In WP1 (“Hardware design and maturation”), design requirements for each sensor technology (DSHF and FOP) were worked out in cooperation with the project’s topic manager. This enabled the consortium to work with clear boundary conditions within the maturation phase of the sensors and the necessary calibration devices and contributed to reaching the first sub-objective. Furthermore, basic robustness test of both sensor types were performed in hot/cold and humid/dry environment.

In WP2 (“Proof of concept”), preliminary wind-tunnel tests were conducted that helped to improve the sensors for different operating conditions. Specifically, both sensors were tested in both a low-speed and a high-speed wind tunnel at TUB. In the two wind tunnel test campaigns the ability of the FOP sensors to measure static pressure was massively improved. The measured static pressures and pressure fluctuations were validated by Kulite reference pressure sensors. Furthermore, the post-processing routine for the DSHF sensor measurements was optimized during the small-scale wind tunnel tests. Furthermore, it was extended by a procedure to approximate the measurement error of the devices. The results of the shear stress data were compared to empirical formulas from the literature in low-speed testing. In the high-speed testing the flow angle indication of the sensors were evaluated in terms of oil-paint visualization. In both tests the capability of the sensors to measure unsteady flow fields was proved. As an example, the figure “DSHF sensors compared to oil flow visualization.png” compares the direction of the shear stress measured by DSHF sensors and oil-flow visualization in the TUB high-speed wind tunnel. Furthermore the figure “diffusorLS.png” provides results of shear-stress measurements in the TUB low-speed wind tunnel.

In WP3 the two sensor technologies were validated:
- In wind tunnel tests at TU Berlin. Here, an industrially relevant outer wing model with AFC was used to test the response of the sensors (see figure “WP3_WTT.png”). The results indicated that the DSHF sensors were capable of measuring the increased wall shear stress that was generated by the AFC actuators. Furthermore, comparison with reference measurements of the local pressure distribution validated the results of the FOP sensors.
- In flight tests on an ultra-light aircraft. Here, the LASER ultra-light aircraft of TUB was used to validate the flight readiness of both sensor technologies (figure WP3_FT.png”). Specifically, both DSHF and FOP sensors favorably compared to reference measurement in cruise conditions and near separation. As an example, pressure fluctuations obtained on the flap of the flight test aircraft are shown in figure “WP3_FT_pressure.png”.

Within the course of SKOPA, 5 dissemination activities were started:

- Two conference papers were submitted and presented at the Aerospace Europe Conference 2020:
o Kienitz S., Kreft S., Schmid M., Staats M., and Koch A.W. „Miniature Airworthy Fiber-Optic Pressure Sensor for Measuring Static Pressure and Acoustics”, Paper AEC2020_045
o Staats M. and Weiss J. „Measuring Wall Shear Stress in Magnitude and Direction”, Paper AEC2020_122

- One video presenting SKOPA was produced and released on YouTube:

- Two peer-reviewed journal articles are currently in process
o Kienitz S., Lohr, L., Schmid, M., and Koch, A.W. “ Static and Dynamic Pressure Measurement in Flight Test Application with Optical Fabry-Pérot Sensors”, accepted for publication in IEEE Instrumentation and Measurement.
o Kienitz S., Staats, M., Irsperger, J., Schmid, M., Koch, A.W. Weiss, J. “Fiber Optic Pressure Measurement of Active Flow Control on a Complex Outer Winglet Model”, in preparation.
Within SKOPA, innovative aerodynamic measurement technology was developed and validated. In particular, the capacity of DSHF sensors to measure the time-resolved wall shear-stress downstream of AFC system was demonstrated. Furthermore, the capacity of FOP sensors to measure both steady and unsteady pressure was validated. With this technology, engineers can now validate the effectiveness of Active Flow Control Systems, further optimize the aerodynamics of new transport aircraft, and eventually, help reduce CO2 emissions.
The figure compares the direction of the shear stressn and the oil-flow visualization
WP3 Flight Test Aircraft
low-speed test setup and key results of FOP and DSHF sensor measurements
Pressure fluctuations measured in flight
WP3 Wind Tunnel Model