Flight Control Systems ( part-2 )

3 AIRCRAFT CONTROL SYSTEMS

A flight control system is required for all flight operations, and these systems have changed over the years. On the first flights, there were articulated surfaces for simple control, with pilot cables and pulleys to control the system. This technique has been used for about 10 years and is now being used in small airplanes.
Due to the transition to large aircraft and the increase of flight organs, in these systems based on the pilot muscle power, this method is not sufficient for aerodynamic moments, resulting in deviations in the surfaces. The solution to this problem is aerodynamic balancing, but aircraft dimensions and flight organs are now growing even further and a control power system is needed for an aerodynamic surface. Today, two categories of control systems are used; 'Full mechanical control' (for gliders and general aviation) and '' Power or servo assisted control system '' (for large aircraft).
Contribution to the control systems of servomechanisms has had a nice additional effect; actively using control technology, providing control over flight control operators directly.

• Do not balance high-performance aircraft in unstable conditions.
• Automatic speed cut and acceleration.
• Acceleration at high angles.
• Meeting basic body deficiencies.

Along with the aircraft categories, aerodynamic surface control also changes. The plane has different control surfaces. Primary flight controls (rotation and deflection control), secondary controls; lifting and lowering.
Primary flight control capability is necessary for safety and is applied in this respect to considerably contemporary unstable military aircraft [2].

2.1 Direct mechanical control

If the aircraft size and the flight envelope allow, it may be entirely mechanical to control a surface cabin joint assembly, in which case the joint formed by surface bending will be low enough and contrast with the pilot muscle strength.
There are two types of mechanical systems: push-pull bars and cable-pulley. In the first case the bars are connected to the cabin entrance to the control surface. The crank arms are used to change the direction of the bars.

Figure 1 : shows the push pull control rods.
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The main requirements of a push-pull bar system are: All connections must be rigid first, so deviation and body elasticity during flight are not required, axial imbalance must be eliminated during compression [2].
The reference length is related to the actual length of the bar, which means that the length must be reduced to increase the variable load or the bars must be tightly constrained by the slide guides or the orientation must be cut with bell-crank.
Finally, a formal analysis of the system layout is sometimes necessary, because the vibrations in the bars reveal vibrational deviations on the surfaces. This problem is especially important in helicopters because vibrations generated by the main rotor can cause unwanted resonance in the flight control rods. Cable strap loosening or thermal stress is often preferred, cable-pulley response because it is more flexible and allows the aircraft to reach more remote areas. In Figure 2, the cables are connected where the cabin column is connected by a quarter circle with rods [2].

Figure 2 :. Cable and pulley control for elavator
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2.2 Hydraulic Control

If the effect of the pilots is not direct enough to control, the main option is the pilot assisted power system. A few control surfaces in the aircraft are operated by electric motors: already considered in the previous section, reliability, safety, power per unit of weight and flexibility in terms of operation with a hydraulic system was seen as a more appropriate solution then spread on the most modern aircraft. The pilot sends or requests signals over the parts of the car, the valve opens the connection points of the high pressure hydraulic fluid and more drive wheels work at one side. The valve is positioned next to these operators, the signal is sent in 2 different ways; mechanical or electrical; the mechanical signal is used in push-pull rods, or more commonly the cable pulley is used, the electrical signal is a solution in more modern and sophisticated vehicles.
The basic principle of hydraulic control is simple. Power control in two ways is different.

• The system should control the surface in a proportional manner, the surface must be a function of the rotation pilot.
• The pilot must activate the control valve with a small effort and there must be irritation in the maneuver intensity.
  The first problem is solved by using (hydraulic) servo mechanisms, the components must be proportional to an actuator motion connected to the actuator in various ways; In Figure 3, two of them are drawn, the second requires the actuator, including the hydraulic circuit.

Figure 3 : Classical hydraulic servomechanism.
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In both cases the control valve housing is a solid cylinder and a spool valve for driving connected to the column mechanics of the cabin.

In the first case, the cylinder is hinged to the aircraft, so that the pulley change and the opening of the ports are moved in one direction or the other, and the piston rod is also connected to a lever in the pulley valve. When the piston movement brings the roller to the neutral position, the actuator stops, then the deviation is proportional to the demand.

In the latter case the piston is restricted to the aircraft; cabin control is via the spool bar valve. In the hydraulic circuit an emergency valve is on a transfer piece to the control valve; if the transfer pressure drops, for example a pump and engine failure, the emergency valve switches to the other position and the control valve opens the tank inlet; this process makes the hydraulic system lock by manual operation of the pilot cylinder.

Now it is clear that the pilot is working very hard under normal hydraulic operating conditions, the contrast required for the mechanical friction of the connection and control valve movement, the pilot never forces the aircraft in load conditions. For this reason, artificial power systems are introduced. The cab can be activated directly with the control rod or pedals. A simple solution is to deflect a spring system with force proportional to the pilot's request. This solution is not sensitive to actual flight conditions. A more advanced bow feeling is called 'Q feeling'. This system reads the data for the pitot-static probe, reads the dynamic pressure, or reads the difference between the total pressure or the static pressure, which is proportional to the air velocity V and air density ρ (proportional) [2].
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RESOURCES ;
[2] Politecnico Di Milano - Dipartimento di Ingegneria Aerospaziale Aircraft systems –
lecture notes, versıon (2004) Chapter 6 – Flight Control System.

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