CONTROL BALANCING
The forces that act at the control surface’s center of gravity and aerodynamic center must be balanced around the hingeline in order to regulate control pressure, prevent control flutter, and provide control-free stability. Control-free is the situation where the controls are not being manipulated by the pilot (hands off). Aerodynamic balance concerns balancing the forces that act at the aerodynamic center. Mass balance concerns balancing the forces that act at the center of gravity.
Aerodynamic balance is used to keep control pressures (associated with higher velocities) within reasonable limits. As the trailing edge of the control surface is deflected in one direction, the leading edge deflects into the airstream forward of the hingeline . The force on the leading edge creates a moment that reduces the force required to deflect the control surface, so the pilot may control the airplane more easily.
The relationship of the control surface CG to the
hingeline will determine the control-free stability of the
airplane. Stability is more desirable in transport and
bomber type airplanes and therefore the control
surface CG is usually located forward of the hingeline. This keeps the control surface aligned with the fixed surface ahead of it when struck by gusts from turbulence. For high performance airplanes, the CG is located on or aft of the hingeline. With the CG aft of the hingeline, the control tends to float into the relative wind and cause a greater displacement which allows a faster response to control action and makes the airplane more maneuverable. To gain a balance between control response and stability, the control CGs are located on the hingeline. To locate the CG on the hingeline, weights are placed inside the control surface in the area forward of the hingeline (shielded horn and overhang). This technique is called mass balancing.
CONTROL FEEL
There are several basic types of control systems used to move the control surfaces: conventional, power-boosted, and full power (fly-by-wire).
In conventional controls the forces applied to the stick and rudder pedals are transferred directly to the control surfaces via push-pull tubes, pulleys, cables and levers. If an external force moves the control surfaces, the stick or rudder pedal will move in the cockpit. This action is called reversibility and gives the pilot feedback. Feedback is the force that the pilot feels in his hands or feet for a given deflection of the stick or rudder pedals. Without feedback the pilot would tend to over control and possibly overstress the airplane.
Power-boosted controls have mechanical linkages with hydraulic, pneumatic, or electrical boosters to assist the pilot in moving the controls in the same way power steering assists a car driver. The degree to which the controls are boosted varies depending upon the mission and design of the airplane. These systems have some reversibility, and the pilot receives some control feel through the cockpit controls. If the boost system fails, the pilot can still control the airplane, although the control forces will be greatly increased.
With a full-power or fly-by-wire control system, the pilot has no direct connection with the control surfaces. The controls of a full power system are connected to hydraulic valves or electrical switches which control the movement of the control surfaces. The fly-by-wire system uses computer commands to displace the controls. These systems are not reversible. Movement of the control stick causes the control surfaces to move, but movement of the control surfaces will not cause the control stick to move. Since these systems are not reversible, they require an artificial means of producing control feel.
Artificial feel is the use any device used to create or enhance control feedback under various flight condi- tions such as airspeed and acceleration changes.
servo trim tab are used to provide artificial feel. It moves in the opposite direction as the aileron, thus helping the pilot deflect the aileron, making the air- plane easier to maneuver.
Artificial feel is provided to the rudder by an anti-servo trim tab. When the rudder is displaced, the anti-servo tab moves in the same direction at a faster rate. The more that a rudder pedal is pressed, the greater the resistance that the pilot will feel.
Because trim tabs do not provide the desired type of artificial feel, the in some cases elevator may use a neutral trim tab that maintains a constant angle to the elevator when the control surface is deflected. The elevator uses both a bobweight and a downspring to provide the pilot with some artificial feel. The downspring increases the force required to pull the stick aft at low airspeeds when required control pressures are extremely light. The bobweight increases the force required to pull the stick aft during maneuvering flight.
