The paper describes and discusses the design and testing of an efficient and high-sensitivity calorimetricthermal sensor developed for bi-directional wall shear stress measurements in aerodynamic flows. Themain technical application targeted is flow separation detection. The measurement principle is basedon the forced convective heat transfer from a heater element. The sensor structure is composed of threeparallel substrate-free wires presenting a high aspect ratio and supported by periodic perpendicular SiO2micro-bridges. This hybrid structure takes advantages from both conventional hot-films and hot-wires,ensuring near-wall and non-intrusive measurement, mechanical toughness and thermal insulation tothe bulk substrate, and it allowed to add the calorimetric sensor functionality to detect simultaneouslythe wall shear stress amplitude and direction. The central wire is made of a multilayer structure com-posed of a heater element (Au/Ti) and a thermistor (Ni/Pt/Ni/Pt/Ni) enabling measurement of the heatertemperature and a layer of SiO2 between them for electrical insulation. The upstream and downstreamwires are thermistors enabling operation in the calorimetric mode. This design provides a high tempera-ture gradient and a homogeneous temperature distribution along the wires. The sensor operates in bothconstant current and constant temperature modes, with a feedback on current enabled by uncouplingheating and measurement. Welded on a flexible printed circuit, the sensor was flush mounted on the wallof a turbulent boundary layer wind tunnel. The experiments, conducted in both attached and separatedflow configurations, quantify the sensor response to a bi-directional wall shear stress up to 2.4 Pa anddemonstrate the sensor ability to detect flow separation.