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Aeroelasticity : Complexities and Challenges in Rotary–Wing Vehicles C. Venkatesan IIT Kanpur.

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Presentation on theme: "Aeroelasticity : Complexities and Challenges in Rotary–Wing Vehicles C. Venkatesan IIT Kanpur."— Presentation transcript:

1 Aeroelasticity : Complexities and Challenges in Rotary–Wing Vehicles C. Venkatesan IIT Kanpur

2 AEROELASTICITY Study of fluid and structure interaction Applicable for Civil Structures Ships, Offshore Structures Aero Structures More specifically used to address issues related to flying vehicles

3 CIVIL STRUCTURES Tall chimney/Buildings Bridges Overhead cables Flow through pipes (head exchanger)

4 Aircraft (Wings, control surface) Rockets (Panels, control surface) Helicopters (Rotor blades, rotor/ fuselage system) Gas Turbines (Blades) AEROSPACE STRUCTURES

5 BASIC INGREDIENTS A-E Static Aeroelasticity A-I Flight Mechanics E-I Mechanical Vibrations /Structural Dynamics E A I C Aerodynamics Elasticity Inertia Control A-E-I Dynamic Aeroelasticity A-E-I-C Aero-Servo-Elasticity

6 AEROELASTIC PROBLEMS Static aeroelasticity –Divergence –Control effectiveness / reversal –Wing deformation Dynamic aeroelasticity –Dynamic response (Gust, landing) –Flutter

7 MATHEMATICAL FORM LINEAR/ NONLINEAR/ TIME INVARIANT/ TIME VARIANT COMPLEXITIES IN - STRUCTURAL MODELING - AERODYNAMIC MODELING FORM OF BASIC EQUATION

8 STRUCTURAL COMPLEXITY DISTRIBUTED PARAMETER FUSELAGE (INFINITE DOF) FE DISCRETISATION (FEW THOUSAND DOF) MODEL TRANSFORMATION WITH TRUNCATED NUMBER OF MODES DYNAMIC ANALYSIS IN MODAL SPACE GEOMETRIC NONLINEARITY: LARGE DEFORMATION MATERIAL NONLINEARITY: ELASTOMERS

9 FUSELAGE STRUCTURAL DYNAMIC MODEL HIGH MODAL DENSITY: CLOSELY PLACED MODAL FREQUENCIES (20 MODES WITHIN 3Hz – 30Hz) Mode 1: 3.51Hz Mode 2: 4.15Hz Mode 3: 5.35Hz Mode 4: 12.05Hz

10 AERODYNAMIC COMPLEXITY UNSTEADY AERODYNAMICS - SUBSONIC, TRANSONIC, SUPERSONIC - 3-DIMENSIONAL EFFECTS ATTACHED FLOW/ SEPARATED FLOW

11 INTRODUCTION Since the First Successful Flight of Truly Operational, Mechanically Simple and Controllable Helicopter by Sikorsky ( ) - Continued R&D Efforts to Improve Helicopter By Incorporating New Technological Developments As and When Matured and Available Composites Automatic Flight Control Systems Noise and Vibration Control Advances in Fundamental Understanding of Rotor/ Fuselage Dynamics, and Aerodynamics

12 HELICOPTER: AEROELASTICIAN’S VIEW AERODYNAMICS - COMPLEX WAKE - BVI - ROTOR/FUSELAGE DYNAMICS - BLADE MODES - FUSELAGE MODES - STRUCTURAL COUPLING - HIGH MODAL DENSITY

13 R&D EFFORTS INTENSELY PURSUED BY ACADEMIA AND INDUSTRY CONSIDERABLE PROGRESS IN THE PAST 40 YEARS STILL SEVERAL DISCREPANCIES EXIST BETWEEN THEORY AND EXPERIMENT MODEL TESTS AND FLIGHT MEASUREMENTS PROVIDE DATA FOR CORRELATION IMPROVE UNDERSTANDING OF THE PHYSICS OF THE PROBLEM MODIFY, DEVELOP SUITABLE MATHEMATICAL MODELS

14 HELICOPTER DYNAMICS CLASSIFICATION OF PROBLEMS - ISOLATED ROTOR BLADE AEROELASTICITY (COUPLED FLAP-LAG-TORSION-AXIAL MODES) - COUPLED ROTOR-FUSELAGE DYNAMICS

15 ROTOR BLADE MODEL LONG-SLENDER-TWISTED BEAMS UNDERGOING IN-PLANE BENDING (LAG), OUT-OF-PLANE BENDING (FLAP), TORSION AND AXIAL DEFORMATIONS

16 ROTOR BLADE MODELING FIRST MODEL 1958 (Houbolts&Brooks) SUBSTANTIAL WORK AFTER 1970 FINITE DEFORMATION MODEL

17 Aerodynamics in Forward Flight Advancing side : High velocity  Low angle of attack Retreating side : Low velocity  High angle of attack Blade stall occurs in the retreating region. Advancing Side i.e., Retreating side i.e.,

18 Sources of unsteadiness in Helicopter rotor blade A) B) C) Unsteady Motion of Airfoil

19 Velocity Components Velocity distribution and effective angle of attack : Unsteady motion + High angle of attack  DYNAMIC STALL

20 COUPLED ROTOR-FUSELAGE DYNAMICS VEHICLE DYNAMICS (FLYING AND HANDLING QUALITIES) - FUSELAGE RIGID BODY - BLADE FLAP DYNAMICS (DOMINANT) - FREQUENCY RANGE 0.3Hz – 1.5Hz AEROMECHANICAL INSTABILITIES (GROUND/ AIR RESONANCE) - FUSELAGE RIGID BODY - BLADE LAG DYNAMICS (DOMINANT) - FREQUENCY RANGE 2Hz – 5Hz HELICOPTER VIBRATION - FLEXIBLE FUSELAGE - FLAP-LAG-TORSION MODES - FREQUENCY RANGE (ABOVE 10Hz)

21 GROUND RESONANCE

22 (a) Collective (b) Cosine cyclic (c) Sine cyclic (d) Alternating ROTOR MODES vs BLADE MOTION SHIFT OF ROTOR SYSTEM C.G FROM CENTRE IN CYCLIC MODES AS THE BLADES ROTATE, MOVEMENT OF ROTOR C.G CAUSES CHURNING MOTION TO HELICOPTER

23 GROUND RESONANCE   BLADES: FLAP, LAG FUSELAGE: PITCH, ROLL BLADE MOTION IN ROTATING FRAME FUSELAGE MOTION IN NON-ROTATING FRAME

24 GROUND RESONANCE STABILITY ANALYSIS   LINEARISED STABILITY EQUATIONS INERTIA, STRUCTURAL, AERODYNAMIC EFFECTS INCLUDED IN MASS, DAMPING AND STIFFNESS MATRICES {q} – ROTOR/FUSELAGE/ INFLOW DOF EIGENVALUES S=  i   - MODAL DAMPING (NEGATIVE STABLE; POSITIVE UNSTABLE)  - MODAL FREQUENCY

25 GROUND RESONANCE STABILITY: EXPERIMENT {BOUSMAN, US ARMY RES. & TECH. LAB (1981)} BLADE ATTACHMENT TEST SETUP SEVERAL BLADE CONFIGURATIONS TESTED CONF-1: NON-ROTATING NATURAL FREQ:  F0 =3.13Hz  L0 =6.70Hz CONF-4: NON-ROTATING NATURAL FREQ:  F0 =6.63Hz  L0 =6.73Hz

26 _____ Uniform Inflow Δ  o Experiment , Hz , RPM MODAL FREQUENCY CORRELATION (CONF.-1) {UNIFORM INFLOW MODEL} ROLL PITCH

27 , RPM , Hz ______ Uniform Inflow Δ  o  Experiment MODAL FREQUENCY CORRELATION (CONF.-4) {UNIFORM INFLOW MODEL} ROLL PITCH-FLAP

28 ______ Perturbation Inflow Dynamic Inflow Δ  o  Experiment , RPM , Hz MODAL FREQUENCY CORRELATION (CONF.-4) {TIME VARYING INFLOW MODEL}

29 REMARKS CORRELATION STUDY TAUGHT THE LESSON: A GOOD (OR ADEQUATE) ANALYTICAL MODEL FOR ONE ROTOR CONFIGURATION MAY NOT BE ADEQUATE FOR OTHER ROTOR CONFIGURATIONS REMINDS THE PROVERB WHAT IS GOOD FOR THE GOOSE, IS NOT GOOD FOR THE GANDER

30 FLIGHT DATA Hz.736E+3 NM E E E E E E E E E+3 PWR SPECTRUM Ch A moment Time signal Freq. contents

31 Lift coefficient Moment coefficient Drag coefficient DYNAMIC STALL Courtesy: Principles of Helicopter Aerodynamics G.J.Leishmann

32 Unsteady Aerodynamic Coefficients k=0.03 k=0.05 k=0.1 Reduced freq.

33 Response of 2-D airfoil undergoing pitching and heaving in a pulsating flow is analysed The pitching motion and oncoming flow velocity are taken as 2-D Airfoil response simulating cross-section of a rotor blade RESPONSE STUDY

34 HEAVE RESPONSE C.G location Response Frequency content Phase plane plots Effect of initial condition Liaponov Exponent 0% 3% 5%

35 TORSIONAL RESPONSE C.G. Location Response Frequency content Phase plane plots Effect of initial condition Liaponov Exponent 0% 3% 5%

36 CONCLUDING REMARKS SEVERAL ISSUES STILL NOT UNDERSTOOD FULLY CONTINUED RESEARCH TO IMPROVE HELICOPTER PERFORMANCE VERY FERTILE FIELD FOR CHALLENGING RESEARCH THANK YOU


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