Table of Contents
The basic functions of an aircraft’s structure are to transmit and resist five types of applied loads that act on aircraft in flight, namely: tension, compression, shear, bending, and torsion (Megson, 2007). In order to withstand these loads, the structure of the aircraft’s components should provide an aerodynamic shape and protect passengers, payload and systems from environment conditions, encountered in flight (Megson, 2007). The fuselage covering is supported by longitudinal stiffening members and transverse frames to enable it resist bending, compressive and torsion loads without buckling (Megson, 2007). The main function of the wings’ ribs is to help the covering to resist the distributed aerodynamic pressure loads, concentrated undercarriage and additional wing store loads into the whole aircraft’s structure and redistribute the stress around undercarriage wells, inspection panels and fuel tanks (Megson, 2007). The cross-sectional shape’s maintenance helps to hold the combination of abovementioned loads (FAA, 2008). The skin and spar web develop the shear stresses, which can resist shear and torsion loads (FAA, 2008). The combined action of the skin and stringers reacts to axial and landing loads (Megson, 2007). In order to restrain large inertia reactions, manufacturers provide the most circular sectional shape for the pressurized fuselage that supports concentrated loads such as wing reactions, tailplane reactions and undercarriage reactions (FAA, 2008). The airplane needs support when parked, taxiing, taking off or landing to withstand compression stress (Megson, 2007). The landing gear provides this support and is equipped with wheels and skis (FAA, 2008).
In order to efficiently control flight performance, a pilot should manage the balance between the forces of thrust, drag, lift and weight (FAA, 2008). This depends on his ability to plan and coordinate flight controls (FAA, 2008). These can be divided into primary and secondary flight controls, which are normally a small airfoil section that are hinged and mounted on the trailing edge of the main airfoil (Swatton, 2011). The effective angle of attack is changed by the movement of flight control. Primary flight controls enable the pilot to rotate the airplane around its three axes. The ailerons cause the airplane to roll about its longitudinal axis, providing lateral stability; the elevators cause the airplane to pitch around its lateral axis, providing longitudinal stability; the rudder allows the airplane to yaw around its vertical axis, providing directional stability (Swatton, 2011; Private Pilot Ground School, 2006). The secondary flight controls include the following equipment: flaps, which increase lift and induce drag for any given angle of attack, spoilers, which reduce lift and increase drag, and trim systems (Private Pilot Ground School, 2006).These systems include the following elements: trim tabs, which are used to correct airplane’s tendency to abnormal attitude without application of pressure to the control column or the rudder pedals, balance tab, which moves to an opposite direction from primary controls to counterbalance some of their air pressure, and servo tabs, which relieve control pressure (Private Pilot Ground School, 2006).
The pilot has no direct control over the location of forces, which act on the airplane in flight and cannot independently change it without changing the effect of others (Swatton, 2006).
Aerodynamic Principles of the Management of Flight Controls
Airplane’s tendency to rotate about its center of gravity in aerodynamic terms is called a “moment” (FAA, 2008). Therefore, a moment means the equality of the force applied to the distance, of which it was applied (Private Pilot Ground School, 2006). Thereby, this distance from a reference point of line to the applied force is defined as “a moment arm” (FAA, 2008)
Flight controls can be deployed by the pilot to control aerodynamic moments, which are produced by pitch, roll and yaw (Swatton, 2011). The nose-down pitching moment, which is caused by the wings’ generated lift, is counterbalanced by tailplane (Swatton, 2011). When the stick is pushed forward and tailplane’s lift is increased, the elevators increase its camber and an upward force on the underside of the control forces as well (Megson, 2007). The whole tailplane’s angle of incidence, or angle of attack, can be alerted by stabilator to provide the required pitch and trim control by moving the control column (Swatton, 2011). Stabilator combines functions of stabilizer and elevator and is fitted with an anti-balance tab to provide some resistance to the pilot’s input force by deflecting, in order to prevent him from over controlling the airplane (Megson, 2007). To yaw an airplane, the rudder should be deflected in the particular side, against which the opposing airflow will be produced. Its equivalent airspeed square is inversely proportional to the angular deflection of the rudder (Swatton, 2011). The turning moment of the rudder is affected by the location of center of gravity and consequently by the length of the moment arm (Swatton, 2011). Decreasing of the moment arm is caused by increasing of center of gravity and increasing of aerodynamic force for a given rudder angular deflection (FAA, 2008).
In the cases of engine failure, the rudder requires a large angular deflection to generate sufficient strength to produce required moment, which will counteract the emerged yawing moment (Swatton, 2011). This yawing effect is most difficult to control during take-off and take-off climb path, because during these phases, the engines are producing maximum take-off thrust (FAA, 2008). Imbalance between the torque, produced by the operative engine, and the lack of torque, produced by the inoperative engine, causes rolling moment in addition to yawing moments (Swatton, 2011). In these cases, the ailerons are used to control an airplane about the roll axis and are pushed down by the pilot from the one side, which causes the opposite wing’s aileron to go up (Megson, 2007). Rolling moment causes the angle of attack of the up-going wing to decrease and the down-going wing to increase as an opposition to initial disturbance, which has an effect of “dumping” roll (Swatton, 2011).
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In addition, the rate of roll for any given aileron deflection depends on their surface area, the airspeed of the airplane and the distance of the ailerons from the longitudinal axis (Swatton, 2011). Ailerons’ size is limited by the torsion stiffness of the wings and induced drag that they cause (Megson, 2007). A rate of roll depends on the level of the indicated airspeed and in order to achieve its highest rate at low speeds by the pilot, the ailerons are mounted at the end of the trailing edge of the wing to help him succeed the required level (Swatton, 2011).