Part2 - Steady, Level Forward Flight I . Introductory...

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© Lakshmi Sankar 2002 1 Steady, Level Forward Flight I . Introductory Remarks
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© Lakshmi Sankar 2002 2 The Problems are Many. . Thrust Aeroelastic Response ° = Ψ 0 ° = Ψ 270 ° = Ψ 180 ° = Ψ 90 Dynamic Stall on Retreating Blade Blade-Tip Vortex interactions Unsteady Aerodynamics Transonic Flow on Advancing Blade Main Rotor / Tail Rotor / Fuselage Flow Interference V Noise Shock Waves Tip Vortices
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© Lakshmi Sankar 2002 3 The Dynamic Pressure varies Radially and Azimuthally ° = Ψ 0 ° = Ψ 270 ° = Ψ 180 ° = Ψ 90 V R V tip = R V tip = - = V R V tip + = V R V tip R Reverse Flow Region Advancing Side Retreating Side
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© Lakshmi Sankar 2002 4 Consequences of Forward Flight The dynamic pressure, and hence the air loads have high harmonic content. Above some speed, vibrations can limit safe operations. On the advancing side, high dynamic pressure will cause shock waves, and too high a lift (unbalanced). To counter this, the blade may need to flap up (or pitch down) to reduce the angle of attack. Low dynamic pressure on the retreating side. The blade may need to flap down or pitch up to increase angle of attack on the retreating side. This can cause dynamic stall. Total lift decreases as the forward speed increases as a consequence of these effects, setting a upper limit on forward speed.
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© Lakshmi Sankar 2002 5 Forward Flight Analysis thus requires Performance Analysis – How much power is needed? Blade Dynamics and Control – What is the flapping dynamics? How does the pilot input alters the blade behavior? Is the rotor and the vehicle trimmed? Airload prediction over the entire rotor disk using blade element theory, which feeds into vibration analysis, aeroelastic studies, and acoustic analyses. We will look at some of these elements.
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© Lakshmi Sankar 2002 6 Steady, Level Forward Flight II. Performance
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© Lakshmi Sankar 2002 7 Inflow Model To start this effort, we will need a very simple inflow model. A model proposed by Glauert is used. This model is phenomenological, not mathematically well founded. It gives reasonable estimates of inflow velocity at the rotor disk, and is a good starting point. It also gives the correct results for an elliptically loaded wing.
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© Lakshmi Sankar 2002 8 Force Balance in Hover Thrust Weight In hover, T= W That is all! No net drag, or side forces. The drag forces on the individual blades Cancel each other out, when summed up. Drag Drag Rotor Disk
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© Lakshmi Sankar 2002 9 Force Balance in Forward Flight Flight Direction Weight, W Vehicle Drag, D Thrust, T
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© Lakshmi Sankar 2002 10 Simplified Picture of Force Balance T W D c.g. Flight Direction α TPP Rotor Disk, referred to As Tip Path Plane (Defined later) W T D T TPP TPP = = α cos sin
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© Lakshmi Sankar 2002 11 Recall the Momentum Model V V+v V+2v
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© Lakshmi Sankar 2002 12 Glauert’s Conceptual model Induced velocity, v 2v
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© Lakshmi Sankar 2002 13 Total Velocity at the Rotor Disk Freestream V Induced Velocity, v V cosα TPP V sinα TPP Total velocity ( 29 ( 29 2 2 v sin V cos V Velocity Total + + = TPP TPP α
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Part2 - Steady, Level Forward Flight I . Introductory...

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