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EU-Projekt SKOPA

Within the SKOPA project two measurement techniques to measure surface static pressures and wall-shear stresses are developed/matured to be applied for real aircraft testing. 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. Compared to sensors used in laboratory wind tunnels, flight-ready sensors must be robust enough to withstand 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. The work in the project is sub-divided into the three following parts:

A)    Adaption of the sensor designs and calibration needs to the environmental conditions for real aircraft testing;

In order to ensure highest calibration accuracy the shear-stress sensors (DSHF sensors) are calibrated on-site, after installation on the surface. The calibration nozzle is shown in the Figure 1.


Figure 1: portable calibration device for DSHF sensors

Figure 2 shows the calibration surfaces, as obtained for one DSHF sensor. Each sensor element a unique characteristic w.r.t. the local value of the shear stress and the flow angle. When the sensor measurement data are evaluated, iso-voltage lines are projected on each calibration surface at the measured sensor voltage. The combined calibration cures are shown in the lower right-hand side plot. The three curves intersect at one point. That point indicates the measured flow angle and skin friction. In the example the local shear stress was converted into the skin friction velocity.


A)    Proof of concept in low speed/high-speed wind tunnel experiments at TUB facilities;

The test environment for the low speed testing is shown below. Furthermore, key results are plotted. The measured shear stress field without AFC features low values of shear stress, due to the detached flow regime. When AFC is used, the reattached flow increases the local shear stress values. An animation of the phase-dependent shear stresses is shown in Figure 4.

Figure 4: gif animation of the measured shear stress.

C)  Testing the sensors on an industrially relevant wind tunnel model and on the LASeR ultra-light aircraft.

It is foreseen to test the sensors on a large wind tunnel model in the GroWiKa wind tunnel at ISTA at TU Berlin. Furthermore real aircraft testing will be performed on the LASeR ultra-light aircraft. The test aircraft is shown in Figure 5.

Figure 5: LASeR ultra light aircraft, used for sensor in-flight testing

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