Secondary Flight Controls System:

The basic secondary controls are flaps,slats,spoilers and trim tabs. They are used to changed the lift and drag characteristics of the aircraft or in some aircraft to assist the primary flight controls.  

Wing Flaps and Slats (High Lift Devices): the wing are hinged or sliding airfoil which increases the wing area,chamber and down wash. Flaps are located near the trailing edge on the both sides of the wing. 

On the large transport category airplane that need large amount of lift multiple slotted flaps are used in conjunction with leading edge slats and flaps. the slats are added on the both sides of the leading edge. The slats further increase the chamber and also helps to operate the wing at much higher angle of attack by further improving the airflow condition.

The slats when extended from a slot,air flows through the slot and over the main wing,smoothing out the airflow over the wing and delaying the onset of the stall.

This increases in the lift produced by the wing will also decrease the stall speed. This will reduce the take-off,landing speeds and improve maneuvering capability at low speed during take-off and landing. How much flaps should be used during takeoff and landing depends upon aircraft weight,runway length,temperatures etc.

Types of trailing edge flaps:

Spoilers: The Spoilers are small rectangular metal deflectors hinged on the upper surface of the wing. they are normally flushed along the smooth upper chamber of the wing. On the large transport category aircraft,they are also raised to augment the roll during turn or as the ground spoilers immediately after landing. If required they are raised in the air as flight spoilers(speed break) to reduce the speed quickly and or increase the rate of descend. 

Trim Tab: The trim tab is small movable secondary control surface hinged to primary control surfaces. The trim tab is moved in the direction opposite that of the primary control surfaces. The trim tabs can be positioned by mechanical or electrical means by the pilot to correct any out of balance effect. 

The trim tab relieves the pilot for continuous application of the force on the primary controls. In the big jet airplane,elevators trim tab is eliminated by the movable stabilizer. Types of tabs are

1. Trim Tab
2. Balance Tab
3. Sporing Tab
4. Servo Tab

Maneuvers of Aircraft.

Following are the maneuvers of aircraft:

Take-Off: The take off is a portion of an aircraft movement,during which the aircraft accelerates  on ground from state of rest to initial climb. 

In general ,during take off aircraft should obtain sufficient lift to support the weight of the aircraft. The aircraft lifts up (airborne) after reaching calculated of speed and then increases the AOA by up movement of the elevators. In some big jet aircraft,the high lift devices are extended during takeoff to lower the stall speed,which decreases the take-off speed and required runway length. 

The take off speed is always higher than stall speed by about 120% of stall speed. 

Climb: The climb is a portion of an aircraft flight,between lift-off and cruising altitude. During climb,thrust needed is greater than the drag,aircraft climbs when the lift is greater than weight. Rate of climb depends upon the engine power than AOA. 

In case of engine failure in the flight,the climb performance will decrease due to the loss of thrust and additional drag of the propeller. 

Types of Climb: Best angle of climb of speed is at the best altitude versus distance ratio or for clearing obstructions. 

Best rate of climb speed ia at the best versus time ratio. 

Optimum climb speed is best overall economy (flight time,speed and fuel burn)

Cruise: The cruise is a level flight condition at optimum speed in clean configuration from top of climb to top of descent. The cruise altitude varies with types of aircraft,sector distance,take-off weight,ATC,upper-wind and many other factors. 

If the airplane cruises lower than optimum cruising altitude,the engines,the engine burn more fuel,which may not be cost effective.

When the aircraft is heavy for high altitude cruise,the aircraft does the step climb-cruise. Initially aircraft flies at lower cruising level. The aircraft after burning the fuel in the low level cruise becomes lighter for higher optimum cruising level then step-climbs to higher cruising level .

Types of Cruise(speed): The different types of the cruise are optimum speed cruise,fixed Mach number cruise,long range,maximum endurance and high speed cruise,engines out cruise etc.

Straight and level flight: In straight and level and un-accelerated flight(constant speed,altitude and heading). 

1. Lift acting vertically upwards through the center of pressure equals weight
2. The weight of the airplane acting vertically down wards through the center of gravity.
3. The thrust,pulling horizontally forwards equal drag. 
4. Drag force acting parallel to flight path in the opposite directions. 

Glide: Glide is a portion of aircraft flight,in which an aircraft descends under the influence of gravity. Among the four forces,aircraft is now deprived of the thrust. Lift is now not vertical, but at right angles to the path of the glide. 

Glide Angle: To obtain maximum distance,the aircraft must fly at AOA which gives best L/D ratio. The flatter the glide angle,the aircraft will glide larger distance. 

During the power-off gliding,the power comes from the potential energy. To give up this potential energy the aircraft must loose height. The potential energy is the energy the body possesses due to the position in relation to suitable level(weight). The space shuttle during re-entry to earth atmosphere is an example of glide. 

Landing: Landing is bringing the aircraft to touch down point at the lowest possible vertical velocity (sink rate) and horizontal velocity. To lower the approach and the landing speed,lift,must be as large as possible,thus devices like flaps and slats are extended. 

Landing speed is always higher than the stall speed,by about 130% of stall speed. 

Taxing: Movement of aircraft  in the ground to reach the runway or parking. 

Primary Flight Controls

The primary flight controls are aileron,elevator and rudder. The pilot use these primary controls to intentionally maneuver the airplane on one or more axes. conventional flight controls consist of a control wheel or side stick and rudder pedals in the cockpit. 

These controls wheel side sticks and rudder pedals are duplicated on the left and right. seats in the cockpit. The elevator and aileron are both moved by controls wheel or side stick.The rudder is connected to foot pedals. The Conventional flight controls are moved through a system of cables or rods. 

In Delta winged aircraft "Elevon" a combination of ailerons and elevators is used. In some aircraft "Flaperon" a combination flaps and aileron is used for the roll. In V-tail aircraft "ruddervator" is used as rudders and elevators.

Elevator: The longitudinal control about its lateral axis is controlled by the elevators. This movement is called "Pitch". The elevators are movable surfaces hinged behind the tail plane(horizontal Stabilizer) may be at or near the top of the vertical tail plane or at the rear end of the fuselage. The elevators control the angle of attack of the wings.

When back pressure is applied (pull ) on the control wheel,the elevators are raised and nose rises,increasing the AOA. The reverse action takes place,when forward pressure is applied(push),the elevators are lower and nose lowers,decreasing the AOA. The elevators and stabilizer are located on either side of the fuselage near the tail of the airplane.

Aileron: The lateral control about its longitudinal axis is controlled by the two ailerons. This movement is called "Roll"

The ailerons are movable surfaces. They are hinged at outer trailing edge of each wing so that whwn one is moving upward on one wing the other aileron is downward on opposite wing. To turn left the pilot applies left pressure to the control column or side stick. Then the left aileron deflects upward and the right aileron deflects downward.

The force of the airflow is altered by these control changes ,causing the left wing to lower and the right wing to rise. This differential in lift causes the aircraft to roll to the left.

Rudder: Directional control about its vertical axis is controlled by the rudder. This motion is called "Yaw". The rudder is hinged vertically to the fixed surface vertical stabilizer or fin.
Pushing the right rudder pedal forward by the right foot will yaw the airplane to the right. Conversely if the left rudder pedal is pressed forward by the left foot the airplane will yaw to the left. The directional control is not used for turning the airplane in the normal condition of the flight. Pressure on the rudder is used to counter the adverse yaw. Rudder plays a big role to control the yaw during an engine failure on multi-engine aircraft.

Hydraulic (powered ) control: Because of the huge sizes of the primary control surfaces in the big aircraft,it will be impossible to move the control surfaces by the human force. Thus controls are boosted by hydraulically or electrically actuated systems as commanded by the pilot in the cockpit.

Fly-By-Wire: A fly by wire (FBW) systems replaces manual control of the aircraft with an electronic interface. The movements of flight controls are converted to electronic signals and flight control computers determine how to move the actuators at a each control surface to provide the expected response. The actuators are usually hydraulic,but electric actuators are also used.

The FBW system can save weight,improve reliability,These are carefully developed in order to produce maximum operational effect without compromising safety.

Artificial feel devices: With purely mechanical flight control systems,the aerodynamic forces on the control surfaces are transmitted through the mechanisms and are felt directly by the pilot. This gives feedback of airspeed and aids flight safety. With hydro-mechanical flight control systems, the load on the surfaces cannot be felt and there is a risk of over stressing the aircraft through excessive control surface movement. To overcome this problem artificial feel systems are used by a spring device to give increased resistance at higher speeds. 

Axes of the Airplane in Flight

The airplane axes are imaginary lines around which the airplane is free to rotate or move around. The three axes intersects at the center of gravity and each one is perpendicular to the other two. 

 

Longitudinal Axis: The imaginary line that extends lengthwise through the fuselage,from nose to tail is the longitudinal axis. Motion about the longitudinal axis is roll(lateral stability). 

Lateral Axis: The imaginary line that extends crosswise,wing tip to wing tip,is the lateral axis. Motion about the lateral axis is pitch (longitudinal stability)

Vertical Axis: The imaginary line,which passes vertically through the center of gravity,is the vertical axis. Motion about the vertical axis is Yaw (vertical or directional stability) 

An aircraft is in state of equilibrium condition if all the force acting opposite direction cancels each other exactly. Equilibrium simple means that the existing state of affairs remains unchanged ,that there is no pitching ,yawing,rolling or any change of velocity.

Relative Wind and Angle of Attack

Relative Wind: The relative wind is the velocity and direction of the airflow with reference to the airplane. The relative wind flows opposite the direction of the air in motion. If the airplane is straight and level,the relative wind is horizontal,on descend it is from slightly upward and on climb it is from slightly downward. 

Angle of Attack: α (AOA): Angle of attack is the angle formed between the chordline and direction of the relative wind striking the airfoil. The Greek letter α (alpha) is used to denote this angle. The positive lift can be obtained if AOA increased. Each AOA produces a particular lift coefficient,which increases up to a maximum value. The lift begins to decrease at a critical angle of attack,which varies with the airplanes type and its configurations. 

The airflow about a symmetrical airfoil at 0 degree AOA attack will have a streamline pattern over the upper and lower surfaces of the airfoil. The airflow about a chambered airfoil has the ability to produce non symmetrical pressure distribution even at 0 degree AOA. Because of the positive chamber,the distance from the leading edge stagnation point to trailing edge stagnation point is greater over the upper surface than over the lower surface. 

Angle of Incidence: The angle of incidence is the angle formed by the intersection of the wing chord line and the longitudinal axis of the aircraft. It is fixed angle for a particular airplane and mostly designed with positive angle of incidence. 

Pitch Angle(Attitude): Angle of Attack (AOA) is sometimes confused with pitch angle(attitude) or flight path angle. Pitch angle is the angle between the airplane nose and the horizon. 

Four Basics Forces of Flight.

An aircraft in straight and level accelerated  flight has four forces acting on it. These forces are :

1. Lift, an upward acting force

2.Drag,retarding force of the resistance to lift

3.Weight, the downward effect that gravity has on the aircraft

4. Thrust, the forward acting force provided by the propulsion system

LIFT(L): As the wing moves through the air,the airflow will be over and under the wing. As air passes over the upper surface of wing,because of the wing chambered shape,the air has to travel at higher speed. This results in decrease in pressure on the upper surface of the wing. This will create a component of force acting upward perpendicular to the relative wind(Bernoulli's principle)

Down wash Airflow: If the wing is tilted upward,the airflow under wing is deflected downward below and behind the wing of an airplane. This is called down wash. Due to this aerodynamic action,the wing receives an additional upward counter aerodynamics force. The lift of a wing is proportional to the amount of air diverted down time the downward velocity of that air. According to Newtons third law of motion the counter aerodynamics force is produced due to airflow deflected downwards. 

In fixed wing aircraft the speed of the airflow over the wings is determined by the air speed of the airplane. This is not so with (helicopters) rotor wing aircraft. The speed of the airflow over the rotors is determined not only by the airspeed of the helicopters,but also by the speed of the rotation of the rotor blades. To move the helicopters in any horizontal in any horizontal direction,entire rotor disk must be tilted in the desired directions. 

Drag (D):The force is required to move or accelerate any airfoil(airplane) in the air. The application of the force to an airfoil produce equal and opposite force (Newtons Third Law of Motion). 

This opposite aerodynamic force or the resistance an aircraft experiences in moving forward through the air is called Drag. This is the component of force acting in the opposite direction to the line of flight. the force is paralleled to the relative wind an opposes the motion of the airplane through air.

At subsonic speed there are two kinds of drag. 

1. Induced Drag
2. Parasite Drag. 

Induced Drag: The induced drag is unavoidable part of the lift generation. This is created as aresult of the generation of lift. The pressure difference between the upper and lower surfaces of the wing results is downward component"down wash". This results in wing vortex being formed at each wing tip,causing an induced drag. As the angle of attack increases,so does drag,at a critical point the angle of attack can become so great that the airflow is broken over the upper surface of the wing and lift is lost while drag increases. 

Parasite Drag: Parasitic Drag is caused by form resistance(due to shape), skin friction,interference and all other elements that are not contributing to lift (engines,fuselage,landing gear,tail section,antennas and the shape that cause local airflow separations). The higher the airspeed,the greater the parasite drag. The Parasite drag may be further classified into:-

1. Form Drag is created by the form or shape of structure as it resists motion of the aircraft through the air. The drag arises to airflow separations from surface due to shape of the body. The streamline is necessary to decrease parasite drag. 
2. Skin friction is due to viscosity of air. The airflow has tendency to cling its surface of an airfoil. The best way to reduce skin friction is by cleaning and polishing the surface. 

Compressibility Drag: This is the other form of drag due to shock wave build up (on the wing) at higher Mach Number speed. This has considerable effect on jet aircraft performance due increase in drag,loss of lift and change in position of Center of Pressure. 

Lift and Drag Ratio: This aerodynamic parameter is very important in the aircraft performance. 

Speed of Sound: 

Drag Equation is 

 is the drag force, which is by definition the force component in the direction of the flow velocity,

${\displaystyle \rho }$ is the mass density of the fluid,

${\displaystyle u}$ is the flow velocity relative to the object,

${\displaystyle A}$ is the reference area, and

${\displaystyle C_{D}}$ is the drag coefficient – a dimensionless coefficient related to the object's geometry and taking into account both skin friction and form drag, in general ${\displaystyle C_{D}}$ depends on the Reynolds number.

Aerofoil and its Components

If the wing of an airplane is sliced through the leading edge to trailing edge ,the side view would be an airfoil. an airfoil is a device which gets a useful reactions from air moving over its surface. When an airfoil is moved through the air,it is capable of producing lift. Airplane wings,control surfaces,horizontal tail surfaces,vertical tail surfaces,propellers and helicopters rotor blades are all examples of an airfoil. 

The front edge of the airfoil is called the leading edge and tapered rear edge is called the trailing edge. When the mass of air moves through the streamline shape of airfoil,the total air reaction is the source of aerodynamic force. 

 Chord Line: It is the straight line between the maximum curvature of leading edge and trailing edge of the airfoil or the width of the airfoil (wing).

Stagnation Point: When an airplane moves through the mass of air, the particles of air are deflected above and below the aerofoil IE. the wing. The stagnation point is the location on an aerofoil at the leading edge where the airflow divides with some air passing over the top of the aerofoil and rest passing below it. At the stagnation point the velocity of air is almost zero.

As the angle of attack increase the stagnation points moves further down of an aerofoil. This causes greater distance on the top surface from stagnation point to trailing edge. This will result in creation of additional lift. 

Wingspan: The wingspan is the distance from left wing tip to right tip or the length of the wing.

Wing Area: The wing area is a measurement of total surface of the wing. (Wing area=wing span*chord)

Aspect Ratio: The aspect ratio is a ratio of the length of wingspan to its chord (AR=span/chord). The AR has a big role to minimize the induced drag(wing tip vortices).The wing with high aspect ratio generates more lift and less induces drag. Thus,the gliders have wing with high aspect ratio.

Wingtip Vortices: A wingtip vortex is spinning air produced at the tip of the wing ant time the aircraft is producing lift. It is pressure difference on the top and bottom of the wing. Wing tip vortices create the wake turbulence during takeoff and landing of a big jet aircraft. The condensation of moisture as white visible trail on clear sky of a high flying jet is due to wing tip vortices.

Chamber: The wing chamber is defined as the amount of the curvature of an airfoil surface from leading edge to trailing edge. The subsonic airplanes have normally high upper chamber and almost flat lower chamber. The shape of the wing chamber line has very important in determine the aerodynamic characteristics of an airfoil.
When chamber airfoil is moved through the air it provide lift even at 0 degree angle of attack. When the upper and lower chambers of an airfoil are identical,the airfoil is said to be symmetrical. For the symmetrical airfoil and angle of attack has to be positive to produce lift.

Winglet: The winglet is vertical extension on the tips,outboard end of the wing. The winglet increases the efficiency of the wing. It reduces the induced drag by relocating wing tip vortex. The winglet also increases the effective aspect ratio.

Arrow showing Winglet.

Skin Friction: The skin friction is the resistance caused due to the tendency of the air particles to cling on the surface of an airfoil. There are two reasons for this tendency.
1. roughness of the surface of an airfoil
2. due to viscosity of the air.




Boundary Layer: The boundary layer is very thin layer of air adjacent to the wing surface tends to adhere to the wing because of its viscosity.

The viscosity is the resistance to flow,and causes the aerodynamic drag. The air velocity in the boundary layer varies from zero on the surface of the airfoil to the velocity of free stream at the outer edge of the boundary layer.

Various method have been developed to control the boundary. One of then is vortex generator. Vortex generator are small plates standing into the air stream in a row along the airfoil. They tend to delay the breakaway of the boundary layer by re energizing it.