<p>This paper presents the results of a wind tunnel investigation into the stall of tiltrotor blades in propeller mode. Blades were instrumented with fully bridged axial and shear strain gauges, so that the flap bending and torsional strains could be monitored at two radial locations each. The power demand of the propeller was analysed by measuring the motor torque. The propeller was tested in a range of conditions, varying blade pitch angles, advance ratios, and rotational frequencies to clearly assess the blade stall boundary. Stall was identified by analysing the torque data, and both the steady and unsteady content of the strain gauge data. As the pitch angle was increased, a more sudden development of stall was observed when varying the rotational frequency at constant advance ratio than when increasing the advance ratio at constant rotational speed.</p>