Development of two multisensor hot-film measuring techniques for free-flight experiments

Measuring techniques are being developed which can be used in free-flight conditions to detect the transition region and also to measure the skin friction for laminar wings. A description is given of two multisensor hot-film techniques that are being developed and tested in a wind tunnel and on a power glider in free-flight conditions. The techniques are a multisensor technique based on constant temperature anemometry (CTA) and a technique based on constant current anemometry (CCA). The CTA-multisensor probe is composed of commercially available hot films. Fast Fourier transformation analysis of the signals makes it possible to determine laminar, transitional, and turbulent flow in the boundary layer and to detect instabilities such as Tollmien-Schlichting waves. The possibility of measuring the skin friction can be envisioned if a suitable calibrating method can be found. The CCA technique is being developed in order to increase the number of sensors for simultaneous measurements in wind tunnels