Chapter 3: Rotary Wing Stability and Control

In the previous chapter I have discussed the priciples of helicopter flight. Now I will discuss how it is controlled and some control mechanisms to make it stable. But first I want to show the three dimensional axis of a helicopter. These are:

1) "X" axis or the longitudinal axis
2) "Y" axis or vertical axis
3) "Z" axis or lateral axis

Fig. 12 The Helicopter Three Dimensional Axis of Rotation

The X-axis or longitudinal axis of a helicopter rotates the aircraft by banking left or right by the cyclic controls. This is similar to the rolling motion of a fixed-wing aircraft the only difference is that fixed-wing aircraft uses aileron while a helicopter uses the tilting of the main rotors either to the left or right. The Y-axis or vertical axis rotates the fuselage by means of the tail rotor. The tail rotors' variable pitch blades controls the degree of thrust to compensate the torque or to steer the helicopter. In a fixed-wing aircraft, it uses rudder as means of control. The Z-axis or lateral axis rotates the helicopter by tilting the main rotor cyclic fore or aft. ( see Fig. 9 )

Now we will discuss more about the main rotors' control mechanism. The type we will discuss is the flybar type although there are other types like the hiller system and a fixed-pitch type main rotor blades for simplicity. The flybar type is commonly used my most RC helicopters and trainer types because of stability. As we have encountered before on our last chapter about gyroscopic procession, you can see in Fig. 13 looking at the main rotor on stationary mode or what I mean to say is when the rotor is not yet moving. As the flybar paddle on side 1 goes down, flybar paddle on side 2 goes up. Similar to a seesaw that is why the mechanism that connect the flybar to the rotor head is called a seesaw.

Along with the seesaw effect you see in the flybar is also the change in main rotor blades' pitch or angle of attack. The blades' pitch rotates perpendicular to the flybar. So whenever the flybar on side 1 goes down there is an increase in pitch on blade #1 but a decrease in pitch on blade #2. So obviously the flybar who does the control of tilting the main rotor blades to any lateral direction, 360 degrees.

Fig. 13 The Flybar, Flybarpaddle Assembly and its' functions

Now we will discuss the functions of the flybar, flybar paddle, main rotor blades in motion. As rotor turns, those two paddle are shaped like an airfoil. These are controlled by a universal link and rod to the swash plate. When the swash plate tilts in the front, the link rod pulls down and rotates the flybar assembly. The paddle then pushes the flybar downward because of the aerodynamic forces ( see Fig. 14 ). But, because of the law of gyroscopic precession, the flybar will not drop down or push the flybar down in THAT position, instead the application of force will react 90 degrees after the clockwise motion. Remember it is also a rotating mass so the law still applies. So the flybar will drop in front or where the swash plate is tilted.

Then as we go back to Fig. 13 as reference, the main rotor blade also change the angle of attack ( i.e. pitch, angle of incidence ). When blade #2 reaches paddle on side 1 position as the whole rotor system rotates clockwise, then it will change the angle of incidence too like the flybar shown on Fig. 14 and will push the main rotor blade on that position. The effect will be 90 degrees after the application of force same as the flybar paddle. I hope this didn't make your head spin. Just keep in mind the law of gyroscopic precession.

Fig. 14 The Flybar Assembly and Main Rotor in Motion

It's not easy to understand how the main rotor blades tilt by the application of force with the gyroscopic procession at work. It is also hard to explain and make illustrations about it but for the sake of better understanding, I made a 3D model in autocad and will illustrate the functions:

1) The first drawing represents a rotating model helicopter main rotor mass. This is a fixed pitch type, meaning the blade is constant pitch, not variable. The flybar rotates by control input and hence changes the pitch of the paddle. The change in pitch created a downward force that tends to pull down the flybar. But due to the gyroscopic effect the flybay will not tilt down to that position because of the resistance of the rotating flybar.

2) Hence, the flybar will tilt 90 degrees after the application of force as you can see below. By tilting the flybar, the whole rotor is tilted also so the main rotor blade pitch is changed as you can see. The change in blade pitch creates a downward force in the that area but will not tilt because of gyroscopic effect.

3) Then the reaction will occur 90 degrees after the application of force. You can see the angle of tilt below to show the area where reaction occurs. Then the cycle continues with the paddle and flybar rotates and changes the pitch. I hope the illustrations are helpful. Honestly I was not very familiar with this function when I was just learning to fly rc helicopters. This is very important to keep in mind while learning to hover.

Now the tail rotor. Simply just to counter act the torque generated by the main rotor, shaft and engine. Without it the whole fuselage will spin opposite the direction of the main rotor blades' rotation. So in Newton's Third Law States that " For every action, there is an equal an opposite reaction". It will not be possible to control the helicopter for sure. The pitch of the tail rotor blade is variable in order to control the degree of pitch. Aside for counteracting the main rotor torque, it is also used as directional control of the helicopter in the Y-axis (vertical axis). And for stability, controlling it alone manually to stabilize the fuselage is almost impossible because the variable conditions that contributes. To mention a few, the throttle, specially during hovering you will tend to adjust the throtle to compensate for the wind conditions. Tilting the main rotors to balance the heli will affect the required power, hence constant adjustment to the throttle is necessary.

Fig. 15 How Main Rotor Torque is Counteracted by Tail Rotor

So the appearance of gyros ( not gyroscopic effect ), a small rotating disk housed in a casing becames necessary to effectively control the tail rotor. This gizmo is connected to the tail rotor servo to creates a dampening effect and stabilized the fuselages rotation. When an external force is applied to the fuselage, either a gust of wind, main rotor downwash, the tail rotors' will counteract by applying an opposite force. Its like an autopilot. To illustrate further using an example of driving a car. The steering wheel is our means of control to steer tha car left or right. We balance the cars' direction by constantly steering the wheel. If the car is moving to the right for example, maybe a strong wind is causing it to go to the right, we will counteract by applying an opposite force. So this is how a gyro works.

There are some more to consider in the tail rotor's stabilizing effect. We call it translating tendency. It is the natural reaction of the helicopter to drift either left or right depending on the main rotors' rotation and tail rotors' thrust. When the helicopter is in a hovering mode, the tail rotor counteracts the main rotors' torque to keep the fuselage steady. But the tail rotor is a creating a wind on the perpendicular side of the fuselage so its' trying to drift the whole aircraft away.

Fig. 16 Helicopter in Hover Mode Tend to Side-Slip

To counteract the drifting effect, we will again apply a counteracting force using the main rotor cyclic control. We should tilt the main rotor on the opposite direction where the aircraft is drifting to balance everything ( see Fig. 17 ).

Fig. 17 Main Rotor is Tilted to Counteract Translating Tendency ( or Side-Slip )

When the helicopter is very close to the ground, the controls are very sensitive due to thephenomenon called "ground effect" ( See Fig. 18 ). The rotor downwash hits the ground which creates a dampening effect.

Fig. 18 Ground Effect Due to Main Rotor Down Wash