Generally, AFC (Air Flight Control) consist of primary and secondary. The elevators, ailerons, rudder, and stabilator included in the primary control and are compulsory to governor in an aircraft with safety during the operation of the flight. Other devices like trim, spoilers, leading edge devices, and wing flaps are included in the secondary control and which act for the improvement in the performance also helps for pilot to minimize the operation of control with other devices.
Mechanical flight control scheme is the straightforward controlling method designed till date, it was designed with basic mechanical devices. Devices like Chains, cables, rods, pulley and conveyor useful for transmit the forces on the force surface. Its old traditional control scheme yet still in use for small and sport type of aircrafts, specially in the aircrafts where aerodynamic forces not much extreme. As the pilot presses the control gear in the forward/backward direction, the cable is attainment tensed with connection from the linkages and it leads the Controller of plane to change.
Table 1 Mechanical flight control component and functioning
|
S.No
|
Item
|
Purpose
|
|
1
|
cable
|
Act for transmitting the power
|
|
2
|
Control stick
|
To make commands for the rest of the circuit
|
|
3
|
Turnbuckle
|
Help to regulate the Cable span
|
|
4
|
Pulley
|
Help in leader the in radial way
|
|
5
|
Fairlead
|
Help in leader the Cable
|
|
6
|
Cable connector
|
Connection to the cable
|
The connection from the cockpit control surface using linkage may be mechanical specially in the case where the aircraft dimensions and flight enclose permit. In such circumstance the hinge moment produced by the external refraction is squat sufficient to be simply weighed by the muscular exertion of the pilot. There are two kind of mechanical schemes are used: cable-pulley and the push-pull rods.
Generally, the sequence of connection using rod links which attach the control surface to the cockpit (cabin) input. The use of bell-crank levers are very common which are helpful to provide the direction of rod positioning. The use of bell crank lever is very important for the directing the direction of transition and also provides the coupling among the stick movement and elevator refraction. The major linkage are must be rigid to the evade any unsolicited deflection throughout the flight and owing to fuselage bounciness. Second, axial unpredictability throughout compression must be excepted; the instability load P of a rod is given by:
The orientation length is connected to the true length of the pole, implying that to expand the shakiness load the length must be diminished, or the poles must be as often as possible obliged by slide guides, or the directing must be hindered with bell-cranks. Examination of the model of system plan is occasionally essential, for the reason that vibrations of the rods may cause oscillation with deflections of the surface. Such problems are normally face in helicopters, for the reason that vibrations produced by the foremost rotor which can make a melodramatic resonance of the flight control rods.
Exactly same function described above may be happened with cable pulley system. In cable pulley system few cables are used instead of rods. So, in mechanical flight control system where pulleys are used to change the track of the positions, furnished with idlers to decrease somewhat slack owing to construction elasticity, cable strands reduction or thermal growth. Many a time the cable-pulley construction is favored, since it is additional stretchy and permits reaching more remote areas of the airplane. Here in figure 2 illustrate the cabin connected with the help of rod.
Discuss the advantages and disadvantages of the chosen mechanical system compare this to other types of system.
Advantage:
- Simple control compared to other complicated control schemes
- Easy to understand and implement
- Low cost compared to hydraulic and electronics control
- Fewer component required in whole setup
- Less maintenance due to fewer components
Disadvantage:
- higher in weight compared to the other control schemes
- Responds typically not much sluggish though not accurate like electronics control
- No automatic response
- High wear and tear of parts
- Cable may break during control may cause fatal accident
- Very expensive to manufacture and maintain the cable systems, because of weigh
In bigger aircraft the control loads because of aerodynamic forces interim on the control plane are very high in simple mechanical controls. That’s why two mechanical solution are proposed. One in which attempt is made to extract the high mechanical strength through pulleys and levers but maximum reduction in force is restricted by mechanical components
Mostly the maintenance of the mechanical flight control system is arising in the cockpit control to the control surfaces which may comprise but is not restricted to the ailerons, flaps, leading edge devices, elevators, fixed point and adjustable trimming devices cables, push-pull rods, Pulleys, fairleads, and bell cranks. Generally, manufacturer publish the maintenance requirement of mechanical flight control system because FAA made mandatory annual inspection and maintenance requirement which is important for the health of aircraft generally ranges from quarterly, or yearly. Very important part of maintenance in mechanical flight control system is the proper lubrication of parts according to schedule and after inspection. Improper lubrication may cause high wear and tear on mechanical devices. Rigging of mechanical flight control system may include proper positioning of control surface and the tension adjustment in the cable connection, springs and the linkages. Many a times rigging is the neglected during maintenance task because its very difficult process in many aircrafts. Calibrated tension meter is required for the adjustment of the tension in cable. High tension in cable may cause break down and lose may lead to delay in response. Control lever must be check against the free movement, linkages must be properly lubricated, rigging must be done and pulley shaft must be checked during the maintenance process.
References:
Moir, Ian and Allan Seabridge, Aircraft systems: mechanical, electrical, and avionics subsystems integration(John Wiley & Sons, 2011) vol 52
Thomas B and William R Ferrell, 'Remote manipulative control with transmission delay' (1963)(1) IEEE Transactions on Human Factors in Electronics25
Flight Control System by Pritam Ashutosh Available online [https://pritamashutosh.wordpress.com/2012/11/17/flight-control-system/]
