RC Helicopter Aerodynamics Sylvain Marron
1: Flight theory • Blades • Swash Plate • Collective Pitch • Cyclic Pitch • Tail Rotor • Flybar
Blades • Blade sections are symmetrical • Lift is due to blade rotation speed and angle of incidence (pitch) relative wind Low pitch = low lift low pressure High pitch = high lift high pressure
Swash Plate • The swash plate transmits order to main rotor • It is around the main shaft • It is made with 2 plates : • an upper rotary plate • a lower fixed plate • → Both plates are linked together by a bearing • The upper plate is linked to blades and it rotates with main rotor • The lower plate is linked to cyclic and collective servos upper rotary plate lower fixed plate
Swash plate operating • 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 • The collective pitch works on all blades uniformly. • This command is used to move up or move down the helicopter. • 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
Collective pitch effect Lift is increased on the whole rotor surface. Regular lift on the whole aerofoil.Blades have hover pitch. Lift force Weight of heli Order to move up Hovering
Cyclic Pitch • 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 • The cyclic pitch gives a different pitch depending the rotary blade position. • This command is used to tilt the helicopter.
Cyclic pitch effect Increased lift Regular lift on the whole aerofoil. Decreased lift No order Order to tilt forward
Tail Rotor • Newton’s third law: for every actionforce there is an equal (in size) and opposite (in direction) reaction force. => 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.
Tail rotor effect 2- Tail rotor force decreases, the fuselage turn to the left. 1- Tail rotor force 3- Tail rotor force increases, the fuselage turn to the right. Note:the red arrow shows the torque which affects the fuselage.
Flybar • The flybar is spinning out of a plane parallel to the main rotor • It has streamlined and ballast paddles. Flybar • 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 operating • 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 • Gyroscopic precession • Cyclic command analysis • Dissymmetry of lift
Gyroscopic precession 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. Resultant force Applied force Rotating rotor
Cyclic command analysis Example: from hovering we apply a command to move forward. • the flybar receives the order → its pitch is modified (+6°, -6°) The rotor turns clockwise • gyroscopic precession → flybar tilts 90 degrees later
Cyclic command analysis • 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 • 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 lift • 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. 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