Fundamentals Of Mechanics Of Flight: Stability And Control Of Aircraft

Explain the concept of whole aircraft lift/drag and analyze their impact on the aerodynamic performance of the aircraft.

Critically review the aircraft related operational factors affecting its performance.

Analyse the stability of an aircraft’s airframe and contrast quantifiable factors affecting the control of the aircraft.

Evaluate and compare the operation and purpose of different types of wind-tunnels.

Describe the motion of aircraft using standard terminology, static and dynamic stability, and control of an aircraft.

? Neatly sketch an airplane and mark on it the three body axes. Name the aerodynamic forces and moments about these axes; show their convention positive direction. Then, neatly sketch different views (two views) of an airplane to show positive angle of attack, positive angle of sideslip and aerodynamic forces with respect to velocity vector.

? What is the static and dynamic stability of an airplane? Discuss in details the longitudinal, lateral and directional stabilities

? Describe the static margin and briefly explain the effect of the static margin on aircraft stability and control.

? Describe the spiral mode and dutch-roll mode.

Body Axes of an Airplane

This report presents findings from a research conducted about fundamentals of mechanics of flight. As a member of a team working in a reputable aviation company, it is very important to understand the basics of flight mechanics, which mainly comprises of the stability and control of an aircraft. This report focuses on different aspects of the stability and control of an aircraft, including effects of different components or devices on the stability and control of the aircraft. Other areas discussed in the report include different types of aircraft stability, static margin and its effect on stability and control of an aircraft, spiral and Dutch-roll modes, stability and controllability of an aircraft, and control surfaces of an aircraft. Information in this report is very essential in understanding how different devices of an aircraft are used to produce forces that help in controlling the aircraft and making it stable.

Figure 1 below is a sketch showing an airplane’s three body axes.

Figure 1: Body axes of an airplane

Figure 2 below is a sketch showing aerodynamic forces and positive angle of attack (AOA) of an airplane

Figure 2: Aerodynamic forces and positive AOA

Figure 3 below is a sketch showing aerodynamic forces and positive sideslip angle

Figure 3: Aerodynamic forces and positive sideslip angle

An airplane has three main axes: longitudinal axis, lateral axis and vertical axis. There are also four main aerodynamic forces that act on the body of an airplane. These are: thrust (the aerodynamic force pushing the airplane forward), drag (aerodynamic force pulling the airplane backwards), lift (aerodynamic force pushing the airplane upwards) and weight (aerodynamic force pulling the airplane downwards). Thrust acts in the opposite direction of drag while lift acts in the opposite direction of weight.

An airplane is said to be in static stability if the sum of all forces that are acting on it and all moments created equals zero i.e. positive forces equal negative forces and positive moments equal negative moments. In this state, the airplane experiences zero accelerations and it continues flying in a steady condition. A deflection caused by control surfaces or a gust of wind creates an unbalance of force or moment thus disturbing the airplane and causing it to accelerate. When this kind of disturbance occurs, the airplane will tend to return to its steady condition of flight. This tendency of returning to the steady condition of flight after experiencing a disturbance is what is referred to as static stability. The static stability can be positive, neutral or negative. An airplane is said to have static stability if it tends to return to equilibrium or steady condition of flight after disturbance. An airplane in negative static stability is the one that continues flying in the direction or condition of disturbance. An airplane in neutral static stability is the one that fly in the direction of disturbance but remains in equilibrium.

Aerodynamic Forces

Dynamic stability deals with an airplane continuing to fly in the resulting motion or conditions with time after experiencing disturbance. This means that dynamic stability is defined by the time history of the airplane’s resulting motion. An airplane with positive dynamic stability is the one whose motion’s amplitude decreases with time while the one with negative dynamic stability is the one whose motion’s amplitude increases with time. Negative dynamic stability is also referred to as dynamic instability. Neutral dynamic stability is the tendency of an airplane to go back to the original flight condition (level and direction) after experiencing a disturbance that put it in a new position.

An airplane is said to have longitudinal stability if it has the capability to keep a constant angle of attack (AOA) in relation to relative wind (that is, there is no tendency of the airplane’s nose going down and causing it to dive nor going up and causing it to stall). In other words, an airplane attains longitudinal stability when it is in pitch motion. The longitudinal stability is mainly controlled by the horizontal stabilizer whose action depends on the aircraft’s AOA and speed.

Lateral stability refers to the tendency of an airplane to go back to its original attitude from motion about longitudinal axis of the airplane (also known as rolling motion). In the process of an airplane gaining lateral stability, the wing on one side rolls up while the wing on the opposite side rolls down. This means that the wings are not perpendicular (at right angle) to the direction of gravitational acceleration hence lift and gravity are also not matching. The rolling creates a vertical lift component and a horizontal side load that cause the airplane to slip on one side. The airplane is then said to be laterally stable if it is able to return to its original flight condition by the help of the sideslip load that is generated. Lateral stability can be attained using sweptback wings or wings that are included upwards.

Directional stability is the airplane’s stability about the vertical axis. Airplanes are designed such that once they are in a steady condition flight-path, they can continue in this direction and level even without the control inputs of the pilot (this is referred to as auto-pilot mode). But even in this mode, the airplane can still experience a skid due to external or natural factors that create aerodynamic forces. When an airplane experiences this kind of imbalance when in auto-pilot mode and it automatically recovers back to steady condition, it is said to have good directional balance performance. The directional stability is primarily controlled by the vertical stabilizer. Some of the design parameters sued to improve an airplane’s directional balance include extended fuselage, sweptback wings and big dorsal fin.

Stability of an Aircraft

Static margin is the distance measured between aerodynamic centre or neutral point of an aircraft and its centre of mass. This distance is expressed as a percentage (%) of mean aerodynamic chord. This distance affects the aircraft’s static longitudinal stability. An aircraft always has a fixed neutral point that is determined by the design of the aircraft and it can only attain static longitudinal stability if its centre of gravity is in front of the neutral point. The static margin of an aircraft usually varies from 5 to 40%. There different techniques that can be used to improve the static longitudinal stability of an aircraft including moving the wingback or moving the centre of gravity in front. Static margin affects stability of the aircraft. An increase in static margin causes a corresponding increase in stability of an aircraft meaning that static margin is directly proportional to an aircraft’s stability.

An aircraft with positive static margin is the one whose centre of gravity is in front of the neutral point making it to be longitudinally stable. On the other hand, an aircraft with negative static margin is the one whose centre of gravity is behind its neutral point making it longitudinally unstable. Static longitudinal stability of the aircraft is attained using control inputs of suitable control surfaces that will move the centre of gravity in front of the neutral point. In general, the stability and controllability of an aircraft are significantly affected by the position of centre of gravity of the aircraft in relation to its neutral point. Large static margin means that the aircraft is unstable hence its degree of response to the pilot control inputs is slow (i.e. it has low controllability – less controllable). On the other hand, small static margin means that the aircraft is more stable hence it has a high degree of response (quick response) to the pilot control inputs (i.e. it has high controllability – more controllable). The pilot can attain the desired static margin using control surfaces to move the aircraft’s centre of gravity in front of the neutral point.

Spiral mode is a type of mode that is non-oscillatory in nature. The mode typically consists of yaw movement with marginal roll. The half-life of spiral mode is in the minute order. In most cases, the spiral mode in excited by sideslip disturbance that usually follows a roll disturbance causing the wing to drop. For instance, assuming that an airplane is in a straight-and-level flight-path and it experiences a disturbance that makes it develop a small positive roll angle. If the airplane is not checked, a small positive sideslip velocity will develop producing lift that generates yawing moment, which can make the aircraft to turn and start moving in the direction of sideslip. In other words, spiral mode should be checked because it can become fatal. The pilot should always control or stop spiral mode immediately it happens. However, spiral mode is usually invisible when it starts developing. With time, the airplane continues to roll off its true vertical to a point where the aircraft can no longer be supported by the lift produced. This causes a significant increase in speed of the airplane and dropping of the nose. If the situation is not controlled, spiral dive starts. Unchecked spiral mode causes the airframe’s structural failure. The condition can be corrected automatically using true horizon. An airplane in spiral mode usually has more kinetic energy that should be released using control surfaces. The characteristics of spiral mode mainly depends on the airplane’s directional static stability and lateral static stability. There are two types of spiral modes: stable spiral mode and unstable spiral mode. The airplane should always be in stable spiral mode to avoid complications that may turn fatal if unchecked on time.

Types of Aircraft Stability

Dutch-roll is a lateral-directional motion of an airplane comprising of an out-of-phase combination of roll and yaw oscillations. The Dutch roll can happen accidently or naturally (as a result of directional stability). Dutch roll usually happens at high altitude especially if the yaw dampener has been turned off. Therefore the first step to control Dutch roll is to ensure that the yaw dampener is turned on. In other words, one artificial way of increasing Dutch roll stability is by installing a yaw damper. The next step is for the pilot to try and reduce yawing oscillations. This can be achieved by ensuring that the rudder pedals are held in neutral position. The next step is to apply spoiler (aileron) control in the opposite direction to the roll. The last step is to accelerate the airplane to high speed or descend to where the air is denser so as to improve directional stability. There are three main causes of Dutch roll mode: weaker yaw stability, strong roll stability, and dihedral effect and sweepback. The main characteristic of a Dutch roll mode is the airplane experiencing a series of out-of-phase turns. In this case, the airplane usually rolls in one direction while at the same time yawing in the other direction, making it unstable and less controllable.

The experimental data for coefficient of pitching moment, yawing moment and rolling moment shows that the three aircraft models have longitudinal stability, lateral stability and directional stability. An aircraft is said to have longitudinal stability if it has a positive AOA and coefficient of pitching moment. An aircraft is said to have lateral stability if it has a positive sideslip angle and coefficient of yawning moment. An aircraft is also said to have directional stability of it has a positive yaw angle and coefficient of rolling moment. By looking at the experimental data obtained for the three aircraft models and drawing the graphs of C_m against alpha, C_n against beta and C_t against beta, the graphs provide nearly linear curves for each parameters. This means that coefficient of pitching moment is directly proportional to alpha, coefficient of yawning moment is directly proportional to beta, and coefficient of rolling moment is directly proportional to beta. Therefore the three aircraft models have longitudinal, lateral and directional stability.

Stability is the ability of an airplane to return to the desired, steady or neutral position after experiencing a disturbance or turbulence without the input of the pilot. The airplane returns to the neutral position by the airframe itself. In other words, stability enables the airplane to recover from different upsetting forces and maintain steady flight-path without the input of the pilot. Different airplanes have varied levels of stability. For example, a fighter airplane has a different level of stability with that of a training airplane. A training airplane is designed to have high stability because the learners do not have adequate skills to control the airplane and return it to stable conditions in case it gets disturbed. In general, stability refers to the ability of an airplane to generate appropriate forces that enable it to recover back to the original steady condition flight-path after experiencing some disturbance that pushed it out of equilibrium.

Longitudinal Stability

Controllability is the response quality of an airplane to the pilot control inputs or commands. The airplane must respond accordingly to the pilot’s control inputs for it to record best performance. The pilot controls the airplane by moving the flight controls that creates aerodynamic forces to make the airplane follow a desired flight-path. An airplane with high controllability is the one that responds promptly and easily to the movement of controls. It is also important to note that stability is always a compromise for controllability. This means that an airplane with high stability has low controllability, and vice versa.

Longitudinal control refers to the control of pitching motion of an airplane. Pitching motion is basically where an airplane moves from side to side. The control surface of longitudinal control is the elevator (also referred to as stabilator). The pilot can control pitching motion by deflecting the elevator. The elevator is usually fixed on horizontal stabilizer on the tail of the airplane. When the elevator is deflected up or down, it causes the lift generated on the tail of the airplane to either increase or decrease. As a result, the nose of the airplane tilts either upwards or downwards. This is what is referred to as horizontal control. During horizontal control, the pilot will continue deflecting the elevator until when the airplane has returned to the original steady condition flight-path or stabilizes in the resultant motion.

Roll control refers to the mechanisms used to stabilize the airplane when it experiences rolling motion. It is a type of control that is performed to attain lateral stability of an airplane. The main control surfaces of roll control are spoilers or ailerons. These surfaces are found on the wing of the airplane. During roll control, the airplane can be turned left or right by moving the spoilers or ailerons. Ailerons and spoilers are fixed on both the left and right wing, and they move in opposite direction. This means that when the left-wing aileron or spoiler is moving upwards, the right-wing aileron or spoiler will be moving downwards, and vice versa. This opposite movement of the aileron and spoiler causes a decrease in lift on one wing and an increase of lift on the opposite wing simultaneously. This makes it possible for the airplane to roll either left or right because of the unbalanced aerodynamic forces and moments. Just like longitudinal control, the pilot accomplishes roll control by continuing to tilt the ailerons or spoilers until when original steady flight conditions or desirable resultant motion conditions are reached.

Lateral Stability

Yaw control is the mechanism of stabilizing an airplane when its nose is experiencing a side to side movement. The yaw motion is caused by the rudder’s deflection hence yaw control is also performed using rudder. Yaw is basically the rotation of the airplane around the vertical axis. The yaw control is a directional stability control that is performed by controlling the rudder to apply pressure either on the left or right side of the aircraft. Upon application of pressure, the rudder tends to spin sideways pushing the tail of the airplane to move either to the left or right direction. This left-right movement of the tail also causes the nose and entire body of the airplane to move left-right thus controlling yaw. When the airplane in in yaw motion, the pilot will continue applying pressure on the rudder until when the airplane reaches the desired new motion condition or regains its original steady conditions.

Control surfaces are the devices or components used by the pilot for adjusting and controlling the direction and level of an airplane (i.e. flight-path). These control surfaces are used to move the airplane left, right, up or down. In other words, the control surfaces are used in controlling the longitudinal, lateral and directional stability of the airplane. Figure 4 below shows various devices of an airplane, including control surfaces.

Figure 4: Control surfaces of an airplane.

Some of the control surfaces associated with the tail of an airplane include the following:

Elevator: this is a primary control surface fixed on the tail of an airplane. It is used for adjusting the airplane’s pitch angle and AOA. These two parameters are very essential determinants of the stability and controllability of the airplane. They contribute to the airplane’s pitch stability/control and orientation. The elevators move up when the stick is pulled backward by the pilot and the elevators move down when the stick is pushed forward. When the elevator is pushed up, the tail gets pushed down causing the nose of the airplane to pitch up thus increasing the AOA of the wings. This generates more drag and lift. When the stick is centered, the elevators go back to neutral thus stopping change in pitch. When the elevator is pushed down, the tail gets pushed up causing the nose of the airplane to pitch down thus reducing the AOA of the wings. This generates less drag and lift.

Directional Stability

Rudder: this is a primary control surface fixed on the tail of the airplane (usually on the airplane’s vertical stabilizer). The rudder is used for yaw control along the airplane’s vertical axis. When pressure is applied on rudder, the nose of the airplane moves either to the left or right until the desired flight-path is achieved. When the pilot pushes the right pedal, the rudder gets deflected to the right thus pushing the tail of the airplane to the left and causing the nose to move to the right. When the pilot pushes the left pedal, the rudder gets deflected to the left thus pushing the tail of the airplane to the right and causing the nose to move to the left. When the pedal is centered, the rudder goes back to neutral thus stopping yaw.

Stabilizer: there are two types of stabilizers – horizontal stabilizer and vertical stabilizer. The two control surfaces are used for providing control and stability of the airplane. The stabilizer are used in adjusting for changes in center of gravity or center of pressure that is caused by changes in attitude and speed of the airplane, dropping payload or cargo or fuel consumption. The stabilizers can also create a negative or positive lift.

Trim tab: this is a secondary control surface that is fixed on the airplane’s trailing edge and is used for counterbalancing the aerodynamic forces and stabilizing the airplane until it attains the desired attitude with minimal use of control force.

Some of the control surfaces associated with the wing of an airplane include the following:

Aileron: this is a primary control surface that is fixed on the airplane wing’s trailing edge. The movement of the ailerons is in opposite directions (when the aileron on the left wing moves down, the one on the right wing moves up, and vice versa). A lowered aileron increases lift while a raised one reduces lift generated on the wing.

Spoiler: this is a secondary control surface that are used for disrupting flow of air over the airplane wing thus reducing lift. The spoiler is very useful in reducing altitude without having to gain undue airspeed.

Flap: this is a secondary control surface that is fixed on the trailing edge of each wing of the airplane. Deflecting the flap downwards causes an increase in the wing’s effective curvature. Flabs are used to increase the maximum lift coefficient thus reducing the stalling speed of the airplane. The flaps are usually used when the airplane is flying at low speed, when the airplane has a high AOA and during landing.

Static Margin

Slat: this is a secondary control surface that is used for increasing lift of the airplane. The slats alter the flow of air over the wing thus decreasing the stalling speed. Slats can either be fixed or retractable.

Conclusion

This report has discussed different aspects of stability and control of an aircraft. The fundamental basic of mechanics of flight is that an aircraft has three main axes: longitudinal axis, lateral axis and vertical axis. There are also three main forces acting on the aircraft: lift, weight, thrust and drag. A pilot has to use different control surfaces to balance these forces by changing the position of center of gravity relative to the neutral point. There are also different types of control including pitching control, yawing control and rolling control. Pitch is controlled using elevator, roll is controlled using ailerons or spoiler, and yaw is controlled using rudder. There are also primary and secondary control surfaces that are used in controlling and stabilizing the aircraft. The aircraft can also be controlled automatically or manual through control inputs of the pilot.

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