fig4 by a predicted chord position for the blade the btt datasets however provide the actual measured blade tip times of arrival at the forward and rearward probes of the blade for the further revolution these measured blade tip times of arrival are represented in fig4 by a measured chord position for the blade also shown in fig4 are lines f r respectively which are the paths swept by the positions on the blade tip corresponding to the forward 12 and rearward 13 probes and the line s which is the path swept by the position on the blade tip through which the blade stagger angle axis passes the position of line s and hence the position of the blade stagger angle axis is calculated from the expression d fa u2212 d fr u03b4 f u03b4 f u03b4 r d ra d fr u03b4 r u03b4 f u03b4 r where d fa is the distance along the axial direction x of the rotor between the forward probe and the position of the blade stagger angle axis d ra is the distance along the axial direction of the rotor between the rearward probe and the position of the blade stagger angle axis d fr is the distance along the axial direction of the rotor between the forward probe and the rearward.