In the first article of this series, we looked at the basics of flight controls. In it, we discussed the various flight controls in an aircraft and how their movement affects the flight.

We also looked at the hinge moments which oppose the pilot input on a flight control surface. In this piece, we will focus on how the flight controls are moved, how the hinge moments can be reduced, and how trimming of flight controls works.

Aerodynamic balancing – reducing hinge moments

The hinge moment is an undesired moment generated by the natural aerodynamic force which occurs on the control surface. The hinge moment tries to oppose the pilot control input, which needs to be counteracted by the pilot if he or she wants to control the aircraft.

From this point on, to make things a little easier, we will call the aerodynamic force the control force.

In smaller aircraft with smaller control surfaces and less forward speed, the control forces are small enough for a person with average strength to overcome it. However, in a larger aircraft with greater speed, the control forces can be so large that the flight controls become extremely stiff, making the aircraft difficult to control.

To make the movement of flight controls easier, designers have come up with various design features. Let us look at some of them.

Inset hinge

In this design, the hinge point is moved a little back on the control surface. This reduces the distance between the control force component and the hinge point. This reduction in distance decreases the moment generated by the force, reducing the stick forces.

When designing an inset hinge, the control force component should never go forward of the hinge point. This could lead to a control overbalance which could lead to an overcontrol situation.

Horn balance

The horn balance is a very common type of design found in almost all aircraft with manual controls. It can even be found in large aircraft such as ATRs and Dash 8s and even in the rudder of the Boeing 737.

A horn balance design involves making a part of the control surface extend forward of the hinge line. As the control surface is deflected, the extended area also deflects, but the force generated by it generates a moment that opposes the control force component. For example, if the pilot moves the control surface upwards, the control force tries to pull the control surface downwards by creating an anti-clockwise moment. At this point, the horn balance, which is forward of the hinge line, also generates a downward force generating a clockwise moment that opposes the control force. This reduces the effort the pilot must put in to control the aircraft.

As with the inset hinge design, the horn balance must be carefully designed so as not to cause an overbalance situation.

In larger aircraft, inset hinges and horn balance might not be enough to control the aircraft. Moreover, designs like horn balance have extra area exposed to the airflow, which adds to the drag. This is not acceptable for high-performance aircraft. So, other methods of controlling the aircraft came about. This involves the use of control tabs.

Balance tab

The balance tab is a small movable control tab that is attached to the main movable control surface. The mechanics of the system is such that when the pilot commands an input, the control surface moves. When this surface moves, the tab moves in the opposite direction.

The control force generated by the main movable control surface is thus opposed to the force generated by the tab. Even though the force generated by the tab is small when compared to the control force, the distance between the tab force and the hinge line is far greater. Hence, the moment generated by the tab force is enough to overcome the effects of the control force.

Servo tab

In heavier aircraft, the movable control surface might be too heavy for the pilot to make a control input. To fix this problem, a servo tab can be used. This system is very similar to the balance tab. The only difference is that the pilot controls no longer move the control surface but the small tab. The main control surface is simply free to pivot on the hinge point.

For example, if the pilot wants the control surface to move up, he puts in the necessary cockpit control inputs. When done so, the tab moves downwards (opposite direction of the command). The deflection of the tab generates a force, in this case, an up force which generates a moment forcing the main control surface, which is allowed to freely pivot about the hinge point to move upwards.

This greatly reduces the stick forces as the pilot is no longer directly controlling the bulky main control surface. The disadvantage of this type of control system is the reduced effectiveness at low speeds. At low speeds, the force on the tab may be too small for it to make a considerable effect on the main control surface.

The Boeing 707 uses servo tabs in its elevator controls.

Spring tab

The spring tab is an upgraded servo tab. In a spring tab control mechanism, a spring of known tension is placed between the pilot input mechanism and the control tab. At low speeds, the movement of the cockpit control cannot overcome the spring tension, and thus the tab does not move, making the tab and the control surface move together.

At high speeds, the control inputs are higher. This compresses the spring, and the balance tab moves like and acts like a servo tab. In a spring tab system, the stick forces are higher at lower speeds, and the stick forces are smaller at higher speeds.

The actuation of flight controls

The most basic form of aircraft control actuation involves the use of control cables. These cables run from pilot controls to the control surface and move them.

Most general aviation aircraft have cable-run controls. Even some turboprops have such control systems. However, cable systems are highly impractical when it comes to flight control actuation of large aircraft. Hence, these aircraft are built with hydraulically actuated powered flight controls.

Power operated controls

The power-operated controls of an aircraft consist of a hydraulic servo actuator and a slide valve.

The hydraulic actuator consists of a slide valve, an actuating rod, and a piston assembly. In a fully powered control system, one side of the piston assembly is bolted to the airframe, with the other side connected to the control surface.

When the pilot makes a control input, the actuating rod moves. This action causes hydraulic fluid under pressure to enter the piston housing. One side of the piston receives the fluid under pressure, while the opposite side is connected to the return line. As one end of the hydraulic actuator is held firm by the airframe structure, the piston remains stationary while the whole actuator assembly moves, making the control surface connected to it deflect.

The direction of the control deflection will depend on the side of the piston to which the hydraulic pressure is applied.

The powered flight controls give no feel to the pilot as it is fully hydraulically actuated. Without the feel, a pilot may overcontrol the aircraft and cause undue stress on the airframe. To avoid this from happening, in powered flight control systems, an artificial feel unit is provided, which gives the pilot an “artificial feel” of the controls.

Artificial feel systems

The artificial feel systems are used in aircraft with fully powered controls. The mission of the system is to give the pilot a feel of the aircraft. In an aircraft, the faster you fly, the more effective the controls become, and the less input the pilot has to put to maneuver the aircraft, while the slower you fly, the less effective the controls become and the higher the input requirement becomes.

So, one could say that the feel of the aircraft is connected to the speed of the aircraft. And this forms the basis of the artificial feel system.

In an aircraft, the speed is measured by the pitot tube, which measures the total pressure (static + dynamic), and the static port, which measures the static pressure. Thus, when static pressure is omitted from the total pressure, we can find the dynamic pressure or the pressure rise due to speed.

The artificial feel system consists of a Q pot, where the “Q” stands for dynamic pressure. In the Q pot, dynamic pressure is fed to an upper section of a diaphragm, and static pressure is fed to the lower section of the same diaphragm. This diaphragm is connected and controls a selector valve which has a hydraulic fluid pressure port and a fluid pressure return port.

If the aircraft speed were to increase, the dynamic pressure would increase, which forces the diaphragm down. This action moves the selector valve such that the hydraulic return port closes while, at the same time, the pressure port opens. This causes hydraulic fluid under pressure to gush through the cylinder.

This hydraulic fluid is then routed to a piston which is connected to the pilot controls. At high speeds, the hydraulic fluid places the piston such that it opposes the pilot's inputs making it harder for him or her to move the controls. At lowers speeds, the hydraulic fluid moves out of the system through the return line, and there is less resistance to pilot controls.

Aircraft trimming

Trimming is used to maintain the aircraft's attitude and speed at a fixed value with little to no intervention of controls by the pilot. There are two main types of trim systems. They are:

• Trim tabs
• All movable horizontal stabilizers or a variable incidence tailplane

Trim tabs

A trim tab is a tab attached to the end of the control surface. It works like a balance tab. To operate the trim tab, the pilot must first move the control surface. Once moved, the trim tab can be moved by the pilot by either using a trim wheel or an electric trim switch on the control column.

The tab must be moved opposite side of the control surface to generate an opposing moment to the control forces generated by the control surface. Pilots trim the aircraft by feel. Trim control should be used until the pilot is no longer required to apply a considerable amount of force to keep the aircraft's attitude and speed.

Pilots should not try to fly the aircraft with the trim. The trim is not a primary control surface. It is there to assist the pilot to fly. So, before trimming the aircraft, the pilot must get his speed and attitude locked using the primary controls and then trim to zero out the stick forces.

Variable incidence tailplane

This type of trim system is found in large high-speed aircraft to trim the aircraft in the pitch axis.

In such a trim system, the entire horizontal stabilizer moves to trim the aircraft. When the pilot moves the controls, the elevator moves like a normal aircraft. However, when he or she moves the trim switches or turns the trim wheel, the horizontal stabilizer moves in proportion with the help of a jack screw. As the tailplane moves, the elevator is neutralized such that when the trimming is complete, the elevator and the tailplane moves to a new angle and remain in line with each other, greatly reducing drag.

This type of trimming is found in almost all heavy jet transport aircraft.

The advantages of a variable incidence tailplane are as follows:

• Reduced drag due to the fact when trimmed there is no tab protruding out.
• The aircraft can be trimmed at a greater CG and speed range.
• The trimming does not reduce the effective range of pitch control as the elevator is always neutral and free to move to whichever angle is desired by the pilot.