School of Aviation Safety

Rotor SystemsChapter 4

LCDR Frank ‘MOTO’ CollinsHelicopter Aerodynamics

(850) 452-5217franklin.collins@navy.mil

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SAFEGUARDED IAW OPNAVINST 3750 SeriesSAFEGUARDED IAW OPNAVINST 3750 Series

1. List the three degrees of freedom for a rotor blade, and their purpose.

2. Draw or explain how flapping equalizes lift distribution over the rotor, using a blade element diagram.

3. Describe what rotor “blowback” is, and how the rotor’s thrust axis separates from the control axis as airspeed increases.

4. List the solutions to obtain a more ideal lift distribution over the rotor disk.

5. Describe the forces that are responsible for determining a rotor blade’s coning angle.

6. Explain how the mechanisms and design of the rotor blade affect flight.

7. Discuss the differences of the rotor axes in hover and in forward flight.

Learning Objectives – Ch 4

Pilot in Rotor

Juan de la Cierva Autogyro

Pitch Control by Direct Rotor Tilt

1. Feathering Increases/decreases the pitch (AOA) of the rotor blades collectively (all blades same pitch change) or cyclically (independently) , depending on blade azimuth position.

2. Flapping Solution to dissymmetry of lift between advancing and retreating blades. Relieves hub stresses.

3. Lead-lag Relieves dissymmetry of drag forces. Relieves hub stresses due to conservation of angular momentum.

Rotor Blade Degrees of Freedom

p.59

• Precession takes into account Phase Lag and the applied aerodynamic forces

• Function of rotating system

• Maximum displacement occurs 90 degrees after force introduction

Precession

p.60

• Collective and cyclic feathering are the only means available to the pilot in “adjusting” the rotor system. (baring the effects of pedal adjustments through the mixing unit if applicable)

• The pilot does not literally “move the head”. Pitch changes are made by the pilot and the head moves as an aerodynamic reaction.

Feathering

Fwd Flt - Velocity Distribution

R-Vf

R+Vf

R

R

Advancing Tip Speed

Retreating Tip Speed Reverse flow

region

Nose

Tail L2SCV

2

1L

p.56

Fwd Flt - Lift Distribution

Advancing BladeRetreating Blade

Without Flapping

With Flapping

View: Looking Forwardp.56

Equalizes lift moment on opposite sides of the rotor disk (Dissymmetry of lift solution)

1. Longitudinal flapping equalizes lift laterally (dissymmetry of lift). Causes - Blowback

2. Lateral flapping equalizes lift longitudinally (transverse flow effect) Causes - Roll towards Advancing Blade

Flapping

p.55

Blowback

Control Axis

• Blowback is the separation of the Virtual Axis (Tip Path Plane) from the Control Axis (Swashplate). • The Virtual Axis blows back.

V

Virtual Axis

Virtual Axis

Control AxisShaft Axis

Virtual Axis

p.61

Longitudinal Flapping

Tip path plane = VIRTUAL AXIS

Up flap velocity

Plane perpendicular to SHAFT AXIS

Advancing Blade(flaps up)

Down flap velocity

Retreating Blade(flaps down)

Swashplate

Fwd Aft

p.61

“Transverse Flow” Effect

Topic covered in Ch-9: Forward Flight. This is Lateral Flapping.

Lateral Flapping

V

VUpward component of Inflow

Downward component of Inflow

Transverse Flow effect

Ch 9

• Flapping results from a change in aerodynamic forces as blades ‘fly’ to tilt the tip path plane

1. May be due to change in aerodynamic forces from differential linear velocity (V2 component)

- Longitudinal Flapping (Blowback)

2. May be due to change in aerodynamic forces from differential induced velocity component (AOA – CL component). Causes a roll towards the advancing blade.

- Lateral Flapping

3. Also, Will be due to changes in aerodynamic forces due to cyclic feathering (AOA – CL component)

- Cyclic Flapping

Flapping

Flapping

• Precession takes into account Phase Lag and the applied aerodynamic forces

• Function of rotating system• Maximum displacement occurs 90 degrees

after force introduction

• But why aren’t servos 90 degrees from their intended action Effect of hinge location (e) and servo arm size Point of pitch control rod and blade attachment.

Precession

p.60

Lead-Lag

Compensates for:

Conservation of Angular Momentum

Dissymmetry of Drag

Advancing Blade (Flaps Up)Moves Forward on lead-lag hinge

Retreating Blade (Flaps Down)Moves Aft on lead-lag Hinge

p.58

Lead-Lag Diagram

Vforward

Front Rear

Velocity Distribution

R-Vf

R+Vf

R

R

Advancing Tip Speed

Retreating Tip Speed Reverse flow

region

Nose

TailD

2SCV2

1D

Dissymmetry of Drag

vi

vi

DiDi

Non-uniform induced velocity

Feathering, Flapping, Lead-Lag

A Few Problemsto Lift Distribution

Spanwise Lift Distribution?

This is Ideal, but not Achievable.

p.62

Spanwise Lift Distribution

L2SCV

2

1L

p.62This is Hover Lift Distribution if there is no blade twist.

Solutions for Spanwise Lift Distribution Problem

Blade Twist (Washout) Used to even out induced flow across the disk. Optimum condition is uniform induced velocity

over disk.

1. Geometric Twist• Change angle of twist

2. Aerodynamic Twist• Change Shape of Airfoil

p.63

Geometric Twist

Aerodynamic Twist

• Change the airfoil shape Root

• Thicker at root

• Higher Cl values

Tip• Thinner Tip• Smaller chord > less surface area

Airfoil and Taper yield Aerodynamic Twist

Spanwise Lift Distribution

Ideally Twisted(In Hover)

Twist + Flapping

Taper

Taper

Coning

A balance of lift and Centrifugal Force

Per blade the Centrifugal Force is approximately 10x that of the Lift Force

p.64

Control Moment Teetering Rotor Head

Control moment produced as Thrust vector is moved, without the production of lift there is no moment.

Control Moment Fully Articulated Rotor Head

If the flapping hinge is displaced from the center of rotation then cyclic inputs will incur a control moment coupling even without the production of lift.

p.66, See Fig 12

Questions

1. Describe 2 types of flapping.

Longitudinal, Lateral.

Questions

2. What is Blowback.

Blowback: Longitudinal Flapping:

The separation of the virtual axis from the control axis in forward flight; tends to make the nose pitch up.

FYI – Lateral Flapping• To be covered in CH 9.

Transverse Flow Effect: Also know as Lateral Flapping:

Non-uniform induced velocities in forward flight; tends to make the helicopter roll toward the advancing blade.

(CCW rotation --> right roll)

Questions

3. State 2 solutions to the spanwise lift distribution problem.

Geometric and Aerodynamic Twist

1. List the three degrees of freedom for a rotor blade, and their purpose.

2. Draw or explain how flapping equalizes lift distribution over the rotor, using a blade element diagram.

3. Describe what rotor “blowback” is, and how the rotor’s thrust axis separates from the control axis as airspeed increases.

4. List the solutions to obtain a more ideal lift distribution over the rotor disk.

5. Describe the forces that are responsible for determining a rotor blade’s coning angle.

6. Explain how the mechanisms and design of the rotor blade affect flight.

7. Discuss the differences of the rotor axes in hover and in forward flight.

Learning Objectives – Ch 4

Extras