TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

A Framework for Flutter Clearance of Aeroengine Blades

[+] Author and Article Information
A. Khalak

Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139e-mail: akhalak@alum.mit.edu

J. Eng. Gas Turbines Power 124(4), 1003-1010 (Sep 24, 2002) (8 pages) doi:10.1115/1.1492832 History: Received December 01, 2000; Revised March 01, 2001; Online September 24, 2002
Copyright © 2002 by ASME
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Grahic Jump Location
Schematic of performance map with stall flutter boundary. Changes in the thermodynamic conditions can move the boundary, as shown.
Grahic Jump Location
Flight envelopes for typical supersonic aircraft of flight Mach number versus altitude (adapted from McCormick 18). The corresponding plot of (K*,g/ρ*) is shown, assuming a front stage with an ideal inlet and constant modal parameters. The (K*,g/ρ*) plot is normalized such that sea level static conditions are at point D, location (1,1).
Grahic Jump Location
Dual view of performance map and K*−g/ρ* map. Two simultaneous views depict a point (denoted by the ×) in relation to the flutter boundary in the four parameter space, (ṁc,Nc,K*,g/ρ*). Movement of the × one one set of axes affects the flutter boundary location on the other set of axes. A full description of the operating point requires an × on both axes.
Grahic Jump Location
Boundaries on K*−g/ρ* map using linearized-unsteady compressible potential model (Hall 20) in 10th Standard Configuration. The effect of increasing Mach number at constant (K*,g/ρ*) is destabilizing.
Grahic Jump Location
Fan data on K*−g/ρ* map, for early multimission aircraft. A ○ indicates a flutter point, while  *  and × are stable points. The dashed envelope is estimated from the “generic” aircraft shown in Fig. 2, anchored on the sea level static (SLS) rig tests on the original design,  * . The original design (and several minor design variations) were unstable at flight conditions other than SLS, shown in the cluster of ○’s. The eventual redesign, corresponding to the solid envelope, was tested to be stable at all relevant flight conditions, ×.
Grahic Jump Location
Example of stability boundary, for (0.68<K*<0.69 and 0.74<g/ρ*<0.75), with ○ in flutter, and  *  stable. Data outside this (K*,g/ρ*) analysis box, + points at lower K* and g/ρ*, and ▹ at higher K* and g/ρ*, are used to generate upper and lower bounds for the stability curve. The dashed lines indicate the uncertainty in the boundary estimation process.
Grahic Jump Location
Flutter boundaries (a) with constant K*=0.705, and varying g/ρ*, and (b) with constant g/ρ*≈1, and varying K*. Each boundary corresponds to one of the analysis boxes of the inset plots. The trend for increasing g/ρ*, is stabilizing, as is the trend for increasing K*.
Grahic Jump Location
Effect of temperature at a constant pressure. A series of flutter boundaries are shown corresponding the same pressure (within 2%) and varying temperature. Increasing in temperature, at constant pressure, destabilizes the flutter boundary, showing that inlet pressure is not the only relevant flight condition.
Grahic Jump Location
Family of flutter boundaries on K*−g/ρ* map, for 74% corrected speed, and various critical pressure ratios, πcr. The boundary resolution is limited by sampling of (K*,g/ρ*) points (represented by ▪’s).
Grahic Jump Location
Schematic of flutter clearance rule on K*−g/ρ* map
Grahic Jump Location
Flight requirements for example in terms of Mach number versus altitude (a) and region on K*−g/ρ* map (b). The sea level static condition is assumed to be at (K*,g/ρ*)=(0.8,1). The critical points, 2 and 2 are labeled on both plots.
Grahic Jump Location
Flutter boundaries for clearance example. The tests are performed at four points: first, at sea level static (point 1), then at point 2, where a flutter occurs on the operating line, then at points 2 and 2, which establish that the minimally acceptable envelope is clear, but is close to a flutter event on the operating line at point 2.



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