RC Helicopter Aerodynamics
description
Transcript of RC Helicopter Aerodynamics
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RC Helicopter Aerodynamics
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Sylvain Marron
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1: Flight theory
• Blades• Swash Plate• Collective Pitch• Cyclic Pitch• Tail Rotor• Flybar
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Blades
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• Blade sections are symmetrical
• Lift is due to blade rotation speed and angle of incidence (pitch)
Low pitch = low lift
High pitch = high lift
low pressure
high pressure
relative wind
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The upper plate is linked to blades and it rotates with main rotor
The lower plate is linked to cyclic and collective servos
Swash Plate
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• The swash plate transmits order to main rotor
• It is around the main shaft
• It is made with 2 plates :o an upper rotary plateo a lower fixed plate
→ Both plates are linked together by a bearing
lower fixed plate
upper rotary plate
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Swash plate operating
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• The main shaft (1), the blades (2) and the upper swash plate (3) are interdependent and rotate
• Linkage rods tilt or move up the lower swash plate (4) which moves the upper plate through bearing (5)
• Then tie rods (6) move up or move down and modify the angle of incidence of each blade.
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Collective Pitch
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• When swash plate moves up
→ ALL blades have a bigger angle of incidence = lift increases
• When swash plate moves down
→ ALL blades have a lower angle of incidence = lift decreases
The collective pitch works on all blades uniformly.
This command is used to move up or move down the helicopter.
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Collective pitch effect
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Hovering
Weight of heli
Lift force
Regular lift on the whole aerofoil.Blades have hover pitch.
Order to move up
Lift is increased
on the whole rotor
surface.
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Cyclic Pitch
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The cyclic pitch gives a different pitch depending the rotary blade position.
This command is used to tilt the helicopter.
• When swash plate tilts forward
→ pitch of the forward blade decreases → pitch of the backward blade increases
• When swash plate tilts backward
→ pitch of the forward blade increases → pitch of the backward blade decreases
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Cyclic pitch effect
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Increased lift
No order Order to tilt forward
Decreased lift
Regular lift on the whole
aerofoil.
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Tail Rotor
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=> The helicopter fuselage tends to rotate in the direction opposite to the rotor blades.
=> to counteract the torque a tail rotor is located on the end of a tail boom extension at the rear of the fuselage.
Newton’s third law: for every action force there is an equal (in size) and opposite (in direction) reaction force.
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Tail rotor effect
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Note: the red arrow shows the torque which affects the fuselage.
1- Tail rotor force
2- Tail rotor force decreases, the fuselage turn to the left.
3- Tail rotor force increases, the fuselage turn to the right.
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Flybar
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Flybar effects:
provide gyroscopic stabilization : tends to keep stable the rotation plane of main blades. → resists to wind
provide force amplification that reduces the cyclic load on the servos :servos control flybar which is lighter than main blades.
Flybar
The flybar is spinning out of a plane parallel to the main rotor
It has streamlined and ballast paddles.
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Flybar operating
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• The flybar is "linked" to main blades.• When flybar tilts, the angle of incidence of main blades changes: one blade increases, the other blade decreases.
=>• When the swash plate tilts, the angle of incidence of paddles changes.• The flybar tilts, then the angle of incidence of main blades changes and the main rotor tilts too.
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2: Physics
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• Gyroscopic precession• Cyclic command analysis• Dissymmetry of lift
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Gyroscopic precession
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This is an effect occurring in rotating bodies:
An applied force is manifested 90 degrees later in the direction of rotation from where the force was applied.
Rotating rotor
Resultant forceApplied force
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Cyclic command analysis
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Example: from hovering we apply a command to move forward.
the flybar receives the order → its pitch is modified (+6°, -6°)
gyroscopic precession → flybar tilts 90 degrees later
The rotor turns clockwise
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Cyclic command analysis
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flybar tilting modifies the angle of incidence of main blades → angle of main blades is modified from (+4°, +4°) to (+9°, -1°)
gyroscopic precession→ the main rotor tilts 90 degrees later
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Dissymmetry of lift
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Directional flight produces a dissymmetry of lift
The aircraft relative wind is added on the advancing blade, and subtracted on the retreating blade => dissymmetrical lift
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Dissymmetry of liftAdvancing blade Retreating blade
Relative speed + -
Lift + -
Motion Up Down
Flap dampers are placed around the feathering shaft in order to add a degree of freedom
So blades are able to slightly “flap” without tilting the helicopter.
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Flap dampers
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Sources
• Okapi
• Article “Débuter en Hélico” by Stéphane Postigo:http://thunderdarkdevil.modelisme.com/debuterenhelico.htm
• Helicopter Aviation:http://www.copters.com/helo_aero.html
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