Roque Corral,1,2 Nelida Cerezal, 2 and Carlos Vasco 1

1Industria de Turbopropulsores SA

Parque Empresarial San Fernando, 28830 Madrid


roque. corral@itp. es


School of Aeronautics, UPM Plaza Cardenal Cisneros 3, 28040 Madrid Spain

Abstract The aerodynamic damping of a modern LPT airfoil is compared to the one ob­tained when pairs of blades are forced to vibrate as a rigid body to mimic the dynamics of welded-pair assemblies. The stabilizing effect of this configuration is shown by means of two-dimensional simulations.

The modal characteristics of three bladed-disk models that differ just in the boundary conditions of the shroud are compared. These models are representa­tive of cantilever, interlock and welded-pair designs of rotating parts. The differ­ences in terms of frequency and mode-shape of the three models are sketched. Finally their relative merits from a flitter point of view are discussed using the 2D aerodynamic damping characteristics.

Keywords: Flutter, Low Pressure Turbine, Stability Map


Flutter has been a problem traditionally associated to compressor and fan blades. However the steady trend during the last decades to design high-lift, highly-loaded low pressure turbines (LPTs), with the final aim of reducing their cost and weight, while keeping the same efficiency, has lead to a reduction of the blade and disk thickness and an increase of the blade aspect ratio. Both factors tend to lower the stiffness of the bladed-disk assembly and therefore its natural frequencies.

As a result of the afore mentioned evolution vanes and rotor blades of the latter stages of modern LPTs of large commercial turbofan engines, which may


K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 3-16. © 2006 Springer. Printed in the Netherlands.

be designed with aspect ratios of up to six, may potentially flitter and undergo alternate stresses similar to the ones encountered in fans and compressors.

Vibration control of shrouded LPT blades may be accomplished using either cantilever, inter-lock or welded-pair configurations. Flat sided shrouds may vi­brate freely even for very small clearances specially for low inter-blade phase angles (IBFA) and provide little control over the vibration characteristics of the bladed-disk. To remedy this deficiency z-shaped shrouds (interlocks) were de­signed with the aim of remaining tight during the whole fight envelope. This type of designs significantly modify the vibration characteristics of cantilever blades, however, the mode-shapes of a given family may significantly vary with the nodal diameter and induce bending-torsion coupling. Finally, pairs of blades welded in the tip-shroud, were devised as a practical alternative to control the vibration characteristics of LPT bladed-disks and may be seen in some turbofan engines. This latter configuration substantially modifies as well the mode-shapes and frequencies of the baseline (cantilever) and interlock so­lutions.

It is well known that flitter boundaries are very sensitive to blade mode – shapes and that the reduced frequency plays a secondary role. A comprehen­sive numerical study of the infbence of both parameters for LPT airfoils was performed by Kielb & Panowski (2000) and supported by experimental work (Nowinski and Panovski, 2000). The aim of this work is to investigate the infbence of pairing the blades in the aerodynamic damping of a typical LPT section and apply the results to a realistic configuration to elucidate the po­tential benefits of such configurations in the modal behaviour of bladed-disk assemblies.