Helicopter controllability refers to its ability to be rotated about its principal axes under the action of the control moments, which are created by deflection of the control command levers. The controllability is characterized by control effectiveness, control sensitivity, control lag, and the forces on the command levers.
Control effectiveness. By control effectiveness we mean the magnitude of the control moment per degree of deflection of the tilt control and per degree of change of the tail rotor pitch. Control effectiveness, and controllability as well, is divided into longitudinal, lateral, and directional. Longitudinal
control effectiveness is found from the ratio — of the longitudinal control
moment to the tilt control longitudinal deflection angle л. The lateral
control effectiveness is defined by the ratio (where x is the tilt control
transverse deflection angle). The directional control effectiveness is
found from the ratio _JSL (where Дф is the tail rotor pitch change) .
Longitudinal and lateral control effectiveness depends on tail rotor rpm, vertical location of the helicopter center of gravity, and horizontal hinge offset. The higher the main rotor rpm and the larger the blade thrust and centrifugal forces, the larger the longitudinal and lateral control moments and the greater the control effectiveness. For these reasons the main rotor rpm cannot be reduced markedly, since the control effectiveness decreases.
The lower the helicopter center of gravity, the larger the main rotor thrust arm relative to the center of gravity and the larger the control moment and the higher the effectiveness. This means that cargo should be located as low as possible in the helicopter.
In speaking of the deflection of the tilt control relative to the universal axis, we must bear in mind that the deflection angles are severely limited and do not exceed л = 4 – 6° in the longitudinal direction and X < 4° in the lateral direction.
The longitudinal and lateral control effectiveness of the Mi-4 helicopter is
——~—— «450 kgfm/deg. n X
Control sensitivity. Control sensitivity is equal to the ratio of the angular rate of rotation of the helicopter around any axis to the tilt control deflection angle. Lateral control sensitivity is usually greater than the
longitudinal and directional control sensitivities. The control sensitivity is usually greater for light than for heavy helicopters. The control sensitivity depends on the control effectiveness and the damping moment. The greater the control effectiveness, the higher the sensitivity; the larger the damping moment, the lower the control sensitivity.
Control lag. The control moment which leads to rotation of the helicopter about any axis (longitudinal or transverse) is created by main rotor thrust force vector deviation. A characteristic feature of these moments is the large magnitude of the thrust force and the small magnitude of the arm of this force relative to the axis of rotation. Consequently, in order to create a control moment we must impart to the large mass of air discharged by the main rotor additional momentum in a new direction in order to obtain a new direction of the thrust force. A comparatively long time is spent on this.
This time equals approximately the time for a single rotation of the main rotor and amounts to 0.2 – 0.3 seconds.
Consequently, this time is required for the helicopter to begin rotation
about the longitudinal or lateral axes after the control stick is displaced.
This is then the control lag. This lag will be longer, the larger the helicopter moment of inertia relative to the axis of rotation and the lower the main rotor rpm. The lag in the longitudinal control is greater than in
the lateral. For comparison we take the control lag for an airplane. The
airplane control moment is created by comparatively small forces with large arms. Therefore, the creation of the control moments for an airplane requires about one tenth of the time of that for a helicopter. This characteristic must be considered in helicopter piloting techniques.
Control stick force. An attempt is made to have the main rotor blades momentless. This means that with variation of the pitch the blade center of pressure shifts very little and the blade moment about its longitudinal axis scarcely changes at all. But small moment changes still arise. These variations are transmitted through the pitch control horns to the blades, from the
blade to the tilt control, and from there to the cyclic pitch control lever. High-frequency force pulsations develop on the stick and it begins to vibrate.
To eliminate these vibrations, inertial or hydraulic dampers which absorb small blade oscillations are connected into the control linkage system.
In the inertial dampers the energy of the oscillatory motions is expended on rotating the pendulum, and in the hydraulic dampers on overcoming the friction forces of the piston and the fluid forced through the piston. Dampers in the control system are used on the light helicopters. Hydraulic boosters are used on the intermediate and heavy helicopters, which create mechanical /177 forces by fluid pressure on the hydraulic booster piston. These forces are used to deflect the tilt control or change the tail rotor pitch. Each control loop has its own hydraulic booster.
Irreversible boosters are most widely used at the present time; they can deflect the control organs without forces from the pilot. When moving the command lever, the pilot displaces only the slide valve piston, which regulates the fluid flowrate into the booster. Consequently, there are no forces at all on the command levers, i. e., the stick and pedals move without any resistance. This means that the pilot does not feel the helicopter control and cannot define exactly the magnitude of the command lever deflection.
In addition to the boosters, artifical feel mechanisms are provided in the helicopter control system to create definite forces on the stick to give a "control feel." These artificial feel units are provided in each control loop and consist of springs. When the command lever is deflected, one of these springs is compressed and pilot effort is expended in this compression.
The magnitude of the force increases as the stick is deflected. If these forces are applied for brief periods they do not create any serious inconvenience (bearing in mind that the pilot very rarely deflects the stick fully to restore equilibrium). However, if it is necessary to alter the flight regime, for example to transition from hover to flight at maximal speed, then the stick must be moved nearly full forward and held in this position. In this
case the pilot must apply a large force to the stick for a long period. This soon causes fatigue. A trimming mechanism or a load-relieving mechanism is used to remove or regulate the force on the stick.