Fig. 1. (A) Double canister flapping foil actuator design with the foil and the inline AMTI six-axis force sensor assembly between the pitch canister and the foil. The distance from center of roll axis to root of foil is indicated by r₀; S is the foil span. (B) Foil motion coordinate system. The freestream flow is in the Y_c direction and Z_c is oriented in the opposite direction from gravity. Roll, (t), is constrained to the X_c–Z_c plane and pitch, (t), is an angular motion about the foil shaft.