Helicopter Performance The Effect of Blade Twist Travis Ritchie - - PowerPoint PPT Presentation

Travis Ritchie AEM 495 Fall 2014

Helicopter Performance The Effect of Blade Twist

Introduction

Fundamental Helicopter Design Principle:

Blade section at Point A travels faster than the Point B section Velocity correlates with lift Referred to as asymmetry of lift

A B r 0.5r

Circle A Circumference = 2πr Circle B Circumference = 2π(0.5r)

𝑀 = 1 2 𝜍𝑤2𝑇𝑠𝑓𝑔 𝐷𝑚

Blade Twist Concept

Manipulate the lift coefficient by varying the pitch angle along the span of the blade

θ x z

Blade Twist Visual

45⁰ 15⁰

Pitch angle increases from rotor tip to root

Lift Distribution

Constant Blade Angle Ideal Blade Twist Lift Distribution

Blade Twist Approaches

Ideal Blade Twist Linear Blade Twist

𝜄𝑠 = 𝜄0 + 𝜄𝑢𝑥𝑠

𝜄𝑠 = 𝜄𝑢𝑗𝑞 𝑠

Specifications:

• Rotor consisting of four rectangular planform blades
• Hovering
• Neglect tip-loss
• Evaluated for:
• Ideal Blade Twist
• 0⁰ linear blade twist
• -10⁰ linear blade twist
• -20⁰ linear blade twist

Simulation Parameters

Blade Element Momentum Theory

Momentum Theory Incremental Thrust Coefficient Blade Element Incremental Thrust Coefficient BEMT equates above equations, solves for the inflow ratio in hover

𝑒𝐷𝑈 = 4𝜇𝜇𝑗𝑠𝑒𝑠 = 4𝜇 𝜇 − 𝜇𝑑 𝑠𝑒𝑠

𝑒𝐷𝑈 = 1 2 𝜏𝐷𝑚𝛽 𝜄𝑠2 − 𝜇𝑠 𝑒𝑠

𝜇 𝑠 = 𝜏𝐷𝑚𝛽 16 1 + 32 𝜏𝐷𝑚𝛽 𝜄𝑠 − 1

Inflow Ratio

Thrust

Power

Incremental Induced Power Coefficient Incremental Profile Power Coefficient 𝑒𝐷𝑄𝑗 𝑒𝑠 = 4𝜇3𝑠

𝑒𝐷𝑞 𝑒𝑠 = 1 2 𝜏𝐷𝑒𝑠3

Induced Power

Profile Power

Lift Coefficient

Incremental Lift Coefficient 𝑒𝐷𝑚 𝑒𝑠 = 2𝐷𝑈 𝜏𝑠2

Lift Coefficient

Conclusion

Blade Twist Improves:

• Lift Distribution
• Inflow Ratio
• Figure of Merit