3. Forces acting on a helicopter
The three main forces acting on a rotor blade are the lift force, the drag force, and the centrifugal
force. Lift is the upward force caused by the interaction between the air flow and the airfoil.
Drag is the force of the air resisting the movement of the airfoil. The centrifugal force represents
the tendency of the rotor blade to fly away from the center. Because of the circular motion, the
air velocity is a lot higher at the tip of the blade than at the base. Because of the quadratic
relation between lift and speed, and drag and speed, the lift/drag increases quadratically with
respect to the distance to the center.
Because of the forces on the rotor blades, the blades have a tendency to cone. This means they
tend to tilt upward during flight. This is caused by the combination of lift and the centrifugal
force. The centrifugal force tries to make the blade as horizontal as possible, while the lift tries to
move the blade up. The combination of these forces mean that the helicopter blade is rotated
slightly upwards. When the pitch of the blade is changed, the lift generated by that blade changes
too, this means that there's a relation between the angle of attack and the coning angle. A tilted
swashplate thus results in an asymmetric cone.
The motor of the helicopter makes the blades spin relative to the helicopter fuselage. However,
because of action and reaction, this means that the fuselage has a tendency to spin in the opposite
direction: this is called anti-torque. To alleviate this, another force needs to be generated outside
of the central axis of the helicopter. In most helicopters, this force is the thrust from the tail rotor.
While in forward flight, the thrust generated by the blades depends on their position. Some areas
will generate more lift than others, which means that the net result isn't a vertical force anymore.
This net result can be split up in lift and forward thrust. The sum of the lift and the weight
determine if the helicopter goes up, down, or hovers. The sum of the forward thrust and the total
drag determine at what speed the helicopter moves forward.
The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of
gyroscopic action, one of which is precession. Gyroscopic precession is the resultant action or
deflection of a spinning object when a force is applied to this object. This action occurs
approximately 90° in the direction of rotation from the point where the force is applied (fig. 13).
Through the use of this principle, the tip-path plane of the main rotor may be tilted from the
The movement of the cyclic pitch control in a two-bladed rotor system increases the angle of
attack of one rotor blade with the result that a greater lifting force is applied at this point in the
plane of rotation. This same control movement simultaneously decreases the angle of attack of
the other blade a like amount, thus decreasing the lifting force applied at this point in the plane of
rotation. The blade with the increased angle of attack tends to rise; the blade with the decreased
angle of attack tends to lower. However, because of the gyroscopic precession property, the
blades do not rise or lower to maximum deflection until a point approximately 90° later in the
plane of rotation. In the illustration (fig. 14), the retreating blade angle of attack is increased and
the advancing blade angle of attack is decreased resulting in a tipping forward of the tip-path
plane, since maximum deflection takes place 90° later when the blades are at the rear and front
Figure 13 - Gyroscopic Precession Principle: When a force is applied to a spinning gyro, the
maximum reaction occurs 90° later in the direction of rotation.
When a rotor blade of a three-bladed rotor system flaps upward, the center of mass of that blade
moves closer to the axis of rotation and blade acceleration takes place. Conversely, when that
blade flaps downward, its center of mass moves further from the axis of rotation and blade
deceleration takes place (fig. 19). (Keep in mind, that due to coning, the rotor blade will not flap
below a plane passing through the rotor hub and perpendicular to the axis of rotation.) The
acceleration and deceleration actions (often referred to as leading, lagging, or hunting) of the
rotor blades are absorbed by either dampers or the blade structure itself, depending upon the
design of the rotor system.
Two-bladed rotor systems are normally subject to CORIOLIS EFFECT to a much lesser degree
than are three-bladed systems since the blades are generally "underslung" with respect to the
rotor hub, and the change in the distance of the center of mass from the axis of rotation is small.
The hunting action is absorbed by the blades through bending. If a two-bladed rotor system is not
"underslung," it will be subject to CORIOLIS EFFECT comparable to that of a fully articulated
Figure 18 - The axis of rotation is the imaginary line about which the rotor rotates and is
perpendicular to the tip-path plane.
Figure 19 - Coriolis effect is the change in blade velocity to compensate for the change in
distance of the center of mass from the axis of rotation as the blades flap.
CORIOLIS EFFECT might be compared to spinning skaters. When they extend their arms, their
rotation slows down because their center of mass moves farther from their axis of rotation. When
their arms are retracted, their rotation speeds up because their center of mass moves closer to
their axis of rotation.
The tendency of a rotor blade to increase or decrease its velocity in its plane of rotation due to
mass movement is known as CORIOLIS EFFECT, named for the mathematician who made
studies of forces generated by radial movements of mass on a rotating disc.