In aerospace engineering, control linkages are mechanisms that connect the controls (yokes, sticks, levers) to the control surfaces (rudder, ailerons, flaps).
These linkages must transfer commands from the controls to the surfaces via rigid members and flexible cables. The flexibility of the cables is important as it ensures smooth transfer of command.
Control linkages can be made of many different materials depending on the design. Some common ones are steel, aluminum, polymer matrix composites (PMCs), and fiber reinforced polymer (FRP) sheets.
The design of a linkage depends on several factors such as load type and magnitude, surface velocities required for regulation, and whether or not there will be continuous or discontinuous motion.
This article will discuss some details about some common types of control linkages in aerospace engineering.
Description of cable
A cable connects the control lever to the part that moves, such as a rudder or an aileron. The term “cable” refers to the fact that this connection consists of only a flexible wire, with no rigid member such as a rod or shaft incorporated.
The entire length of the cable is not necessarily connected to the control lever. Part of it may be disconnected and attached to the moving part. This is done in order to reduce vibration and stress on the system.
The strength of a cable connection depends on its diameter, material, and whether or not it is reinforced with another material. These factors determine how much force it can resist before tearing or breaking.
Combination of rigid and flexible members
In some cases, the control linkage consists of a rigid member and a flexible cable. In other cases, the control linkage consists of a rigid member and a flexible member that is part of the control surface.
In still other cases, the control linkage consists of a rigid member, a flexible cable, and a flexible member that is part of the control surface. All of these combinations provide adequate performance for flight characteristics.
When only one of these combinations does not produce adequate performance, it is most likely the rigid member or the rigid members that need to be improved. This is important to recognize because replacing an internal flexible member with an external one can have adverse effects on the airplane.
Monitoring if internal flexible members are deteriorating and replacing them before a flight is important to ensure safe flight operations.
Design considerations for the linkage
In addition to considerations about the control system itself, there are a number of other design considerations for the linkage. These include:
Number of degrees of freedom – The number of possible motions or positions the linkage can take depends on the design. For example, a quad-copter has four rotational degrees of freedom and a vertical and horizontal axis, giving it six total possible positions. A helicopter has two rotational degrees of freedom and a vertical axis, giving it three total possible positions.
Number of links – The stability and smoothness of the system depends on the number of links in the linkage. A system with more links will be more stable and smooth than one with fewer links.
Lengths of links – The lengths of the links in the linkage play an important role in determining how smooth the motion is. Shortening the length of a link will make the motion more instantaneous, whereas making them longer will make the motion smoother.
Rigid member failure mode
In this failure mode, the rigid member breaks or cracks, but the cable remains intact. In this situation, the control linkage is no longer functional.
Since the cable remains intact, some movement of the control surface may still be possible. However, without the rigid member to connect the cable to the control surface, no movement will be transmitted.
Because of this, partial loss of control linkage functionality may result in increased difficulty in controlling an airplane during flight. This may lead to serious issues such as loss of altitude or uncontrolled spinning.
Because of the high stress placed on them during flight, cables are regularly inspected and replaced if necessary. Rigid members such as rods and tubes are inspected via radiography to look for cracks. Materials used to make them are also tested for strength and durability.
Cable failure mode
In some cases, a cable can break due to external forces. This is called a failure mode. If a cable breaks, the control linkage may not function properly, which can be dangerous.
Cables can break due to overstress conditions or cut conditions. In the first case, the internal components of the cable are stronger than the surrounding materials, so it breaks after being stretched and strained enough.
In the second case, a cutting device such as a knife or saw is able to penetrate and sever the cable. This is particularly dangerous if it happens to be located inside of the airplane!
Airplane cables are tested for their strength as well as their susceptibility to breaking in response to external forces.
Rigid member design considerations
When designing the rigid member component, there are a few things to consider. First, you need to make sure the linkage can move the control rod without interfering with other controls or causing undue stress on the linkage.
Second, you need to make sure the linkage can move the control rod without breaking or wearing out. The material used for the linkage needs to be strong enough to withstand pulling and pushing forces.
Third, you need to make sure there is enough room for movement of the control rod within the linkage. If there is not enough space for movement, then it may break the linkage or cause other problems with operation of other controls.
The design of the rigid member component varies depending on what works best for operation of the aircraft. Some require thicker links or rods while others require thinner ones.
Cable design considerations
When designing an aircraft control linkage, engineers must take into account the type of movement required, the forces that will be applied and received, and the space available.
For example, a control linkage for a wing flap must allow the wing to be raised and lowered. This requires a moving part that can move freely, as well as enough length to move the wing.
Additionally, cables must be strong enough to handle all applied forces such as pressure and tension. A weak cable would result in poor performance and possibly damage to the aircraft.
Finally, due to limited space availability on an aircraft, other components such as wires or rods may need to be placed between the control links. It is important to consider these when designing new components so they do not disrupt function or damage other parts.
Applications besides aircraft controls
Besides being used in aircraft controls, linkages similar to control linkages can be designed and manufactured for a variety of applications.
Some examples include robotics, machine tools, medical devices, and transportation vehicles. In all of these applications, the design and function of control linkages remains the same.
Robotics use control linkage systems to coordinate movements between robotic arms and other coordinating devices. Machine tools use them to coordinate movement between moving parts that produce manufactured products. Medical devices use them to regulate flow of fluids and gases within the device. Transportation vehicles like cars and boats use them to coordinate movements such as opening and closing windows or doors or shifting gears.
