Patent ID: 8521493

Claim:
A method of measuring the position of the blade stagger angle axis for one or more blades of a row of blades attached to a rotor of a gas turbine engine, the method comprising the steps of: during operation of the gas turbine engine, obtaining forward and rearward blade tip timing datasets for successive rotations of the blades from two axially spaced blade tip timing probes, a forward one of the blade tip timing probes being forward of a rearward one of the probes along the axial direction of the rotor, the forward and rearward blade tip timing datasets allowing times of arrival of the blades at the respective probes to be measured; during operation of the gas turbine engine, obtaining a once per revolution dataset for said successive rotations of the blades, the once per revolution dataset allowing angular velocity of the blades to be measured; determining, from the forward and rearward blade tip timing datasets, measured blade tip times of arrival at the forward and rearward probes of a blade for a reference revolution of the blades; determining, from the once per revolution dataset, predicted blade tip times of arrival at the forward and rearward probes of the blade for a further revolution of the blades assuming that there is no change in shape or relative position of the blade; determining, from the forward and rearward blade tip timing datasets, measured blade tip times of arrival at the forward and rearward probes of the blade for said further revolution of the blades; calculating the position of the blade stagger angle axis of the blade at said further revolution from the expression: D.sub.FA=D.sub.FR (.DELTA.. sub.F /(.DELTA. sub.F+ .DELTA. sup.R )) or from the expression: D.sub.RA=D.sub.FR (.DELTA. sub.R /(.DELTA. sub.F+ .DELTA. sub.R )) where D.sub.FA is the distance along the axial direction of the rotor between the forward probe and the position of the blade stagger angle axis, D.sub.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.sub.FR is the distance along the axial direction of the rotor between the forward probe and the rearward probe, .DELTA.sub.F is the difference between the measured blade tip time of arrival of the blade at the forward probe and the predicted blade tip time of arrival of the blade at the forward probe for said further revolution, and .DELTA..sub.R is the difference between the measured blade tip time of arrival of the blade at the rearward probe and the predicted blade tip time of arrival of the blade at the rearward probe for said further revolution.