RC Helicopter Aerodynamics

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Sylvain Marron

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

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

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.

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.

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.

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

Cyclic pitch effect

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Increased lift

No order Order to tilt forward

Decreased lift

Regular lift on the whole

aerofoil.

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.

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.

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.

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.

2: Physics

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• Gyroscopic precession• Cyclic command analysis• Dissymmetry of lift

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

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

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

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

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

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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