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School of Aviation Safety
Rotor SystemsChapter 4
LCDR Frank ‘MOTO’ CollinsHelicopter Aerodynamics
(850) [email protected]
FOR OFFICIAL USE ONLY:FOR OFFICIAL USE ONLY:THIS BRIEF CONTAINS SAFETY PRIVILEGED INFORMATION WHICH MUST BE THIS BRIEF CONTAINS SAFETY PRIVILEGED INFORMATION WHICH MUST BE
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