INVESTIGATION OF STALL FLUTTER OF FAN BLADES BY BLADE TIP-TIMING SYSTEM

Flutter is a system mode that all blades vibrate at the
same frequency but with a different phase angle, which can
form a nodal diameter (ND) pattern. One of the advantages
of blade tip-timing (BTT) technique is to measure the
vibrational displacements of all blades, which makes BTT
a potential method to study flutter. This paper investigates
the stall flutter of fan blades during an acceleration process
through a BTT system with eight casing-mounted optical
probes and one OPR (once per revolution) sensor. By
applying the traveling wave plot, the non-integral vibration
with three different frequencies is observed. Results
indicate that it is a subsonic stall flutter at engine order
(EO) 1.5 approximately with three nodal diameters (2, 3,
4) calculated by a cross-power spectral analysis based on
the data of two probes. For all three nodal diameters, ND 2
is dominant. In comparison with Campbell diagram from
modal analysis, EO1.5 crossing M1 is consistent with
experimental results at around 92% design speed. As the
rotor speed increases, the amplitudes of all blades
experience a rapid growth at first. They then level off into
a steady oscillating condition known as the limit amplitude
region. Further increase in the rotor speed results in the
growth of the blade amplitude into another limit amplitude
region where the maximum blade displacement is nearly
doubled. The development of nodal diameter as a function
of rotor speed explains the formation of flutter.
Furthermore, the change of phase difference between two
blades shows a particular “rhombus” shape before flutter,
which provides a new perspective to predict it.