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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Schematic of performance map with stall flutter boundary. Changes in the thermodynamic conditions can move the boundary, as shown.
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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).
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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.
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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.
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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, ×.
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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.
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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*.
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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.
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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).
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Schematic of flutter clearance rule on K*−g/ρ* map
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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.
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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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