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

Periodic Reporting for period 1 - SKOPA (Skin friction and fiber-optics-based surface pressure measurements for aircraft applications)

Reporting period: 2018-10-01 to 2019-09-30

The technology that is currently developped 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 particluarly 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). The SKOPA project aims at developing and maturing the technology that is required to validate the performance of the active flow-control system implemented at the wing-pylon interface. Specifically, the proposed skin-friction and surface-pressure measurement system will be mounted on Platform 1 (Advanced Engine and Aircraft Configurations) of the Large Passenger Aircraft IADP in order to measure the time-resolved skin friction and surface pressure in the complex 3D unsteady flow field occurring near the wing-pylon interface when the aircraft nears stall conditions. The experimental database that will be acquired with the proposed system will then 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. As such, the project directly addresses the requirements highlighted in the Work Plan within the Large Passenger Aircraft IADP. The main objective of this proposal is to deliver a set of sensors designed for in-flight, time-resolved measurements of skin friction and surface pressure near the wing-pylon interface of a transport aircraft approaching stall. 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.
Within the SKOPA project two types of sensors were matured for in-flight application. A fibre-optic pressure sensor is utilized to measure static pressures and fluctuations. Hot-film technique is used for skin friction measurements. Last was matured to measure static values, as well as the direction of the shear stress and also the fluctuations. Basic robustness test of both sensor types were performed in hot/cold and humid/dry environment. Thereby in-flight ambient conditions were simulated. Those were controlled by a climate chamber. During the tests and after it the sensor characteristics were unchanged. Furthermore, the sensors were tested in small-scale wind tunnel tests with active flow control. In a first wind tunnel test the TUB owned low-speed wind tunnel was utilized. Both types of sensors were installed on a 2D diffusor ramp that was equipped with a pulsed blowing active flow control system upstream the sensors. The static pressures fluctuations, measured by the fibre-optic sensors, were verified by Kulite reference sensors. Both data were in good agreement. Based on the skin-friction sensors the local shear stress could be measured and the state of flow could be monitored (separated or attached). For the experiments with pulsed blowing actuation, the data could be evaluated w.r.t. the working phase of the actuator. Thereby, the footprint of the induced shear-layer instabilities were visualized and tracked.
Methods to accurately measure surface flow-quantities like pressure and skin friction are now well established in laboratory environments. For example, piezo-resistive pressure transducers are routinely used in laboratory wind tunnels to measure the fluctuating pressure on test surfaces. Similarly, average skin friction can be measured reasonably well in laboratory environments using pressure-based sensors like Preston tubes. For unsteady measurement of skin friction, surface hot wires or hot films are typically preferred. Performing the same type of measurements in a flight-test environment, however, is radically more complicated and, at this point, no commercial technology can be readily used for unsteady surface pressure and skin-friction measurements. Compared to sensors used in laboratory wind tunnels, flight-ready sensors must be robust enough to widthstand environmental challenges like rain and variable air temperature. They must also be resistant to electro-magnetic interference, allow the use of long lead cables, and be able to integrate seamlessly into the existing flight-test equipment. These requirements make it necessary to develop and mature a new measurement system for unsteady surface pressure and skin friction, which is the objective of the present proposal. Within the SKOPA project, the TRL of time-resolved surface pressure and skin-friction measurement systems will be increased from 4 (laboratory environment) to 6 (flight-test environment). We plan to integrate the DSHF and FOP technologies into one, flight ready, measurement system. The developed technology has a far reaching innovation potential that goes beyong the current project. In addition to providing a tool to demonstrate the effectiveness of AFC technology, the measurement system could be used in the future to mature other aerodynamic technologies like natural/hybrid laminar flow airfoils or morphing surfaces. As such, the actions in the SKOPA project are a required step towards in-flight validation of innovative aerodynamic technologies. The results will contribute to a better undestanding of the meachanism of active flow control in general.
The figure compares the direction of the shear stressn and the oil-flow visualization
low-speed test setup and key results of FOP and DSHF sensor measurements
High-speed test section with FOP and DSHF sensors installed.