Airplanes and Aerodynamics

1.Flight Controls

            It should be noted that flight control systems and characteristics can vary greatly depending on the type of aircraft flown. The most basic flight control system designs are mechanical and date back to early aircraft. They operate with a collection of mechanical parts, such as rods, cables, pulleys, and sometimes chains to transmit the forces of the flight deck controls to the control surfaces. Mechanical flight control systems are still used today in small general and sport category aircraft where the aerodynamic forces are not excessive.Aircraft flight control systems consist of primary and secondary systems. The ailerons, elevator ,or stabilator,, and rudder constitute the primary control system and are required to control an aircraft safely during flight. Wing flaps, leading edge devices, spoilers, and trim systems constitute the secondary control system and improve the performance characteristics of the airplane or relieve the pilot of excessive control forces.Movement of any of the three primary flight control surfaces ,ailerons, elevator or stabilator, or rudder,, changes the airflow and pressure distribution over and around the airfoil. These changes affect the lift and drag produced by the airfoil/ control surface combination, and allow a pilot to control the aircraft about its three axes of rotation. A properly designed aircraft is stable and easily controlled during normal maneuvering. Control surface inputs cause movement about the three axes of rotation. The types of stability an aircraft exhibits also relate to the three axes of rotation.

The rudder, elevators, and ailerons are the main parts of a fixed-wing aircraft that control its flight. The ailerons are connected to the end of both wings, and when they are moved, they turn the plane around its longitudinal axis. The elevator is connected to the edge of the horizontal stabilizer that is at the back. When it is moved, it changes the pitch of the plane, which is its angle with respect to the horizontal axis. The rudder is attached to the vertical stabilizer's trailing edge with a hinge. When the rudder is moved, the plane turns around its vertical axis ,yaw,.

The primary flight control surfaces that move the aircraft about its longitudinal axis are the ailerons. In other words, when the ailerons move in flight, the aircraft rolls. Ailerons are typically located on each wing's outboard trailing edge. They are incorporated into the wing and are calculated as part of the surface area of the wing. When the flight deck control wheel or control stick is moved to the right, the aileron mounted on the right wing deflects upward while the aileron mounted on the left wing deflects downward. The right aileron's upward deflection reduces the camber of the wing, resulting in less lift on the right wing. In contrast, a downward deflection of the left aileron causes an increase in camber and, as a result, an increase in lift on the left wing. The difference in lift between the wings causes the aircraft to roll to the right. Roll spoilers mounted on the upper surface of the wing supplement ailerons on some aircraft.

The rudder is a primary flight control surface that controls rotation about an aircraft's vertical axis. This movement is known as "yaw" The rudder is a movable surface attached to the trailing edge of a vertical stabilizer or fin. The rudder, unlike a boat, is used to overcome adverse yaw caused by turning or, in the case of a multi-engine aircraft, by engine failure, and it also allows the aircraft to be intentionally slipped when necessary. The rudder in most aircraft is controlled by the flight deck rudder pedals, which are mechanically linked to the rudder. A rudder pedal deflection causes a corresponding rudder deflection in the same direction; for example, pushing the left rudder pedal results in a rudder deflection to the left. As a result, the aircraft nose moves to the left due to rotation about the vertical axis. Hydraulic actuators are frequently used in large or high-speed aircraft to help overcome mechanical and aerodynamic loads on the rudder surface. The effectiveness of the rudder increases as the aircraft speed increases. As a result, at slow speeds, a large amount of rudder input may be required to achieve the desired results. At higher speeds, smaller rudder movements are required, and rudder travel is automatically limited in many more sophisticated aircraft when the aircraft is flown above Maneuvering Speed to prevent deflection angles that could potentially result in structural damage to the aircraft.

An elevator is a primary flight control surface that controls movement about an aircraft's lateral axis. This movement is known as "pitch" Most planes have two elevators, one on the trailing edge of each half of the horizontal stabilizer. The elevators move up or down in response to manual or autopilot control inputs. Elevators in most installations move symmetrically, but in some fly-by-wire controlled aircraft, they move differentially to meet control input demands. Some aircraft models have provisions to "disconnect" the right and left elevators from one another in the event of a control surface jam, whereas others use separate hydraulic systems to power the left and right elevators, ensuring that at least one surface remains operational in the event of a hydraulic system failure. The elevators are activated by moving the control column or control stick forward or backward. The elevator surface is deflected downwards when the pilot moves the controls forward. This raises the camber of the horizontal stabilizer, resulting in more lift. The increased lift on the tail surface causes rotation around the aircraft's lateral axis, resulting in a nose down change in aircraft attitude. An aft movement of the flight deck controls has the opposite effect.

Flight control systems are classified as either primary or secondary flight controls. Primary flight controls, which include ailerons, elevators ,or, in some installations, stabilators,, and rudder, are required to safely control an aircraft during flight. Secondary flight controls, which include high lift devices such as slats and flaps, as well as flight spoilers and trim systems, are intended to improve the aircraft's performance characteristics or to relieve excessive control loading.

Flaps are a type of high lift device that consists of a hinged panel or panels mounted on the wing's trailing edge. They increase the camber and, in most cases, the chord and surface area of the wing when extended, resulting in an increase in both lift and drag and a decrease in stall speed. These factors contribute to improved takeoff and landing performance. Many different flap designs and configurations are in use. Large aircraft may use multiple types, with different flap designs on the inboard and outboard sections of the wing. Some of the more common flap designs are described below: Plain Flap - The rear portion of the wing's aerofoil rotates downward on a simple hinge arrangement mounted at the flap's front. Split Flap - From the leading edge of the flap, the rear portion of the lower surface of the wing aerofoil hinges downwards, while the upper surface remains immobile. Slotted Flap - Similar to a Plain Flap, but with a gap between the flap and the wing to force high pressure air from beneath the wing over the flap's upper surface. This reduces boundary layer separation and keeps the airflow over the flap laminar. A Fowler Flap is a split flap that slides backwards level for a short distance before hinged downwards. It increases chord ,and thus wing surface area, first, then camber. This results in a flap that can optimize both takeoff ,partial extension for optimal lift, and landing performance ,full extension for optimal lift and drag,. Most large aircraft have this type of flap or a variation of it. Double Slotted Fowler Flap - This design improves the performance of the Fowler flap by incorporating the slotted flap's boundary layer energizing features.

Trim Systems are categorized as "secondary" flight control systems. Trimming an aircraft is the process of adjusting the aerodynamic forces on the control surfaces so that the aircraft maintains the set attitude without any control input. While aerodynamic forces affect all axes of rotation, not all aircraft types can be trimmed in all three axes. Almost all aircraft designs include some form of pitch axis trim, and the majority include some form of yaw axis trim. Roll axis trim is found on many aircraft, but it is the least common installation of the three. Trim systems come in a variety of configurations, and more than one type may be found on a given aircraft. The trim tab, which can be installed in both fixed and flight adjustable configurations, is the most commonly used trim system. Other types of trim systems include adjustable springs, anti-servo tabs on stabilator-equipped aircraft, and a trimable horizontal stabilizer.

2. Aerodynamic Forces

Understanding basic aerodynamic concepts is essential for understanding the operation of an aircraft's major components and subcomponents. In unaccelerated straight-and-level flight, four forces act on an aircraft. Thrust, lift, weight, and drag are examples of these forces. Thrust is the forward force generated by the engine/propeller. It opposes or overcomes the drag force. It is said to act parallel to the longitudinal axis in general. As will be explained later, this is not always the case. Drag is a backward, retarding force caused by the wing, fuselage, and other protruding objects disrupting airflow. Drag acts in the opposite direction of thrust, parallel to the relative wind. The combined weight of the aircraft, the crew, the fuel, and the cargo or baggage is referred to as weight. Because of the force of gravity, weight pulls the aircraft downward. It acts vertically downward through the aircraft's center of gravity to oppose lift ,CG,. Lift is produced by the dynamic effect of the air acting on the wing and acts perpendicular to the flight path through the wing's center of lift ,CL,. An aircraft moves in three dimensions and is controlled by rotating it around one or more axes. The longitudinal, or roll, axis runs from nose to tail of the aircraft, passing through the CG. The lateral or pitch axis runs across the aircraft in a straight line through the wing tips, passing through the CG once more. The vertical, or yaw, axis runs vertically through the aircraft, intersecting the CG. All control movements cause the aircraft to move around one or more of these axes, allowing for flight control. CG is one of the most important aspects of aircraft design. It is the precise point at which the mass or weight of an aircraft is said to center; that is, a point around which the aircraft would remain relatively level if suspended or balanced. The position of an aircraft's CG determines the aircraft's stability in flight. The aircraft becomes more dynamically unstable as the CG moves rearward ,towards the tail,. It is critical in aircraft with fuel tanks in front of the CG that the CG is set with the fuel tank empty. Otherwise, as the fuel runs out, the plane becomes unstable. The CG is calculated during initial design and construction and is further influenced by onboard equipment installation, aircraft loading, and other factors.

Gravity is the pulling force that draws all bodies to the center of the earth. The CG can be thought of as the point at which all of the aircraft's weight is concentrated. The aircraft would balance in any attitude if it were supported at its exact CG. It should be noted that CG is extremely important in an aircraft because its position has a significant impact on stability. The allowable CG location is determined by the overall design of the aircraft. The distance traveled by the center of pressure ,CP, is determined by the designers. It is critical to understand that the weight of an aircraft is concentrated at the CG, while the aerodynamic forces of lift occur at the CP. When the CG is forward of the CP, the aircraft has a natural tendency to pitch nose down. A nose up pitching moment is created if the CP is forward of the CG. To maintain flight equilibrium, designers set the aft limit of the CG forward of the CP for the corresponding flight speed. Weight and lift have a direct relationship. This is a simple relationship, but it is critical for understanding flying aerodynamics. The upward force on the wing acting perpendicular to the relative wind and perpendicular to the aircraft's lateral axis is referred to as lift. Lift is required to counteract the weight of the aircraft. When the lift force equals the weight force in stabilized level flight, the aircraft is in equilibrium and neither accelerates upward nor downward. Vertical speed will decrease if lift becomes less than weight. When the lift exceeds the weight, the vertical speed increases.

Air flowing around the surface of a solid object applies a force to it. It doesn't matter if the object is moving through a stationary mass of air, if the object is stationary and the air is moving, or if both are moving. The component of this force that is perpendicular to the direction of the oncoming flow is called lift. Drag force, which is the surface force component parallel to the flow direction, is always present with lift. The dynamic effect of air passing over an aircraft's wing ,aerofoil, produces a force ,lift, perpendicular to the flightpath through the wing's center of lift. Lift is the force that opposes weight's downward force.

To begin moving, thrust must be applied and must be greater than drag. The plane keeps moving and gaining speed until thrust and drag are equal. To keep a constant airspeed, thrust and drag must be equal, just as lift and weight must be equal to keep a constant altitude. When the aircraft is in level flight, the engine power is reduced, the thrust is reduced, and it slows down. The aircraft continues to decelerate as long as the thrust is greater than the drag. To some extent, as the aircraft slows, the drag force decreases. The aircraft will continue to slow until thrust equals drag again, at which point it will stabilize. Similarly, as engine power is increased, thrust exceeds drag and airspeed increases. The aircraft will continue to accelerate as long as the thrust remains greater than the drag. The aircraft maintains a constant airspeed when drag equals thrust. Straight-and-level flight can be maintained at a variety of speeds. If the aircraft is to remain level in all speed regimes, the pilot must coordinate AOA and thrust. An important aspect of the lift principle ,for a given airfoil shape, is that lift varies with AOA and airspeed. As a result, a large AOA at low airspeeds produces the same amount of lift as a low AOA at high airspeeds. Flight speed regimes are classified into three types: low-speed flight, cruising flight, and high-speed flight. When airspeed is low, the AOA must be relatively high to maintain the balance of lift and weight; otherwise, as thrust and airspeed decrease, lift becomes less than weight and the aircraft begins to descend. To maintain level flight, the pilot can increase the AOA to generate a lift force equal to the aircraft's weight. Even though the aircraft will be flying slower, it will remain level. The AOA is adjusted to maintain lift weight equality. The airspeed will naturally adjust until drag equals thrust, at which point it will stabilize ,assuming the pilot is not trying to hold an exact speed. Straight-and-level flight in the slow-speed regime provides some intriguing conditions in terms of force equilibrium. There is a vertical component of thrust that helps support the aircraft when it is in a nose-high attitude. For one thing, wing loading is typically lower than expected. When thrust is increased in level flight, the aircraft accelerates and lift increases. Unless the AOA is reduced just enough to maintain the relationship between lift and weight, the aircraft will begin to climb. This decrease in AOA must be coordinated with an increase in thrust and airspeed. Otherwise, if the AOA is reduced too quickly, the aircraft will descend; if it is reduced too slowly, the aircraft will climb. To maintain level flight, the AOA must vary as the airspeed varies due to thrust. It is even possible to have a slightly negative AOA at very high speeds and level flight. In order to maintain altitude as thrust and airspeed decrease, the AOA must increase. If speed is reduced sufficiently, the required AOA will increase to the critical AOA. Any further increase in AOA will cause the wing to stall. As a result, extra caution is required at low speeds and low thrust settings to avoid exceeding the critical angle of attack. If the plane has an AOA indicator, it should be used to keep track of how close the plane is to the critical AOA. Some aircraft have the ability to change the thrust direction rather than the AOA. This is done by either pivoting the engines or vectoring the exhaust gases.

3.Angle of Attack

Another important concept to understand is angle of attack ,AOA,. Since the early days of flight, AOA is fundamental to understanding many aspects of airplane performance, stability, and control. The AOA is defined as the acute angle between the chord line of the airfoil and the direction of the relative wind.

Straight-and-level flight may be sustained at a wide range of speeds. The pilot coordinates AOA and thrust in all speed regimes if the aircraft is to be held in level flight. An important fact related to the principal of lift ,for a given airfoil shape, is that lift varies with the AOA and airspeed. Therefore, a large AOA at low airspeeds produces an equal amount of lift at high airspeeds with a low AOA. The speed regimes of flight can be grouped in three categories: low- speed flight, cruising flight, and high-speed flight.

When the airspeed is low, the AOA must be relatively high if the balance between lift and weight is to be maintained. If thrust decreases and airspeed decreases, lift will become less than weight and the aircraft will start to descend. To maintain level flight, the pilot can increase the AOA an amount that generates a lift force again equal to the weight of the aircraft. While the aircraft will be flying more slowly, it will still maintain level flight. The AOA is adjusted to maintain lift equal weight. The airspeed will naturally adjust until drag equals thrust and then maintain that airspeed ,assuming the pilot is not trying to hold an exact speed,.In level flight, when thrust is increased, the aircraft speeds up and the lift increases. The aircraft will start to climb unless the AOA is decreased just enough to maintain the relationship between lift and weight. The timing of this decrease in AOA needs to be coordinated with the increase in thrust and airspeed. Otherwise, if the AOA is decreased too fast, the aircraft will descend, and if the AOA is decreased too slowly, the aircraft will climb.As the airspeed varies due to thrust, the AOA must also vary to maintain level flight. At very high speeds and level flight, it is even possible to have a slightly negative AOA. As thrust is reduced and airspeed decreases, the AOA must increase in order to maintain altitude. If speed decreases enough, the required AOA will increase to the critical AOA. Any further increase in the AOA will result in the wing stalling. Therefore, extra vigilance is required at reduced thrust settings and low speeds so as not to exceed the critical angle of attack. If the airplane is equipped with an AOA indicator, it should be referenced to help monitor the proximity to the critical AOA.

All other factors being constant, for every AOA there is a corresponding airspeed required to maintain altitude in steady, unaccelerated flight ,true only if maintaining level flight,. Since an airfoil always stalls at the same AOA, if increasing weight, lift must also be increased.

4.Stalls

An aircraft stall results from a rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical AOA. A stall can occur at any pitch attitude or airspeed. Stalls are one of the most misunderstood areas of aerodynamics because pilots often believe an airfoil stops producing lift when it stalls. In a stall, the wing does not totally stop producing lift. Rather, it cannot generate adequate lift to sustain level flight. In most straight-wing aircraft, the wing is designed to stall the wing root first. The wing root reaches its critical AOA first making the stall progress outward toward the wingtip. By having the wing root stall first, aileron effectiveness is maintained at the wingtips, maintaining controllability of the aircraft. Various design methods are used to achieve the stalling of the wing root first. In one design, the wing is “twisted” to a higher AOA at the wing root. Installing stall strips on the first 20–25 percent of the wing’s leading edge is another method to introduce a stall prematurely.

The wing never completely stops producing lift in a stalled condition. If it did, the aircraft would fall to the Earth. Most training aircraft are designed for the nose of the aircraft to drop during a stall, reducing the AOA and “unstalling” the wing. The nose-down tendency is due to the CL being aft of the CG. The CG range is very important when it comes to stall recovery characteristics. If an aircraft is allowed to be operated outside of the CG range, the pilot may have difficulty recovering from a stall. The most critical CG violation would occur when operating with a CG that exceeds the rear limit. In this situation, a pilot may not be able to generate sufficient force with the elevator to counteract the excess weight aft of the CG. Without the ability to decrease the AOA, the aircraft continues in a stalled condition until it contacts the ground.

The stalling speed of a particular aircraft is not a fixed value for all flight situations, but a given aircraft always stalls at the same AOA regardless of airspeed, weight, load factor, or density altitude. Each aircraft has a particular AOA where the airflow separates from the upper surface of the wing and the stall occurs. This critical AOA varies from approximately 16° to 20° depending on the aircraft’s design. But each aircraft has only one specific AOA where the stall occurs.

There are three flight situations in which the critical AOA is most frequently exceeded: low speed, high speed, and turning.

​​Low speed is not necessary to produce a stall. The wing can be brought into an excessive AOA at any speed. For example, an aircraft is in a dive with an airspeed of 100 knots when the pilot pulls back sharply on the elevator control.

Gravity and centrifugal force prevent an immediate alteration of the flight path, but the aircraft’s AOA changes abruptly from quite low to very high. Since the flight path of the aircraft in relation to the oncoming air determines the direction of the relative wind, the AOA is suddenly increased, and the aircraft would reach the stalling angle at a speed much greater than the normal stall speed.

5.Spins

A spin is not different from a stall in any element other than rotation. Since spin recoveries are usually affected with the nose much lower than is common in stall recoveries, higher airspeeds and consequently higher load factors are to be expected. The load factor in a proper spin recovery usually is found to be about 2.5 Gs. A spin is simply a stall with a yawing motion. In a fully developed spin the wing is stalled fully. One wing is stalled more than the other and consequently generating an imbalance in the forces acting on the airplane which causes the rotation about the vertical axis.

A spin occurs when one wing stalls, or loses lift, more than the other wing, causing the aircraft to enter a rotating descent. In this situation, the aircraft is rotating around its vertical axis and descending at the same time. A spin is a very dangerous situation for pilots as it can result in a loss of altitude, reduced airspeed, and ultimately, a crash. To understand the aerodynamics of a spin, we need to examine the forces acting on an aircraft and how they change during a spin.

In normal flight, an aircraft is subject to four main forces: lift, weight, thrust, and drag. Lift is generated by the wings and opposes the weight of the aircraft. Thrust is generated by the engines and opposes drag, which is the resistance of the air. These forces must be balanced for the aircraft to remain in level flight. When an aircraft enters a spin, this balance is upset, and the forces acting on the aircraft change.

The key to understanding a spin is to understand the stall. A stall occurs when the angle of attack of the wing exceeds a certain critical angle, causing the airflow over the wing to separate and reducing lift. In a spin, one wing stalls while the other does not, causing the aircraft to enter a rolling motion. The stalled wing generates less lift, while the other wing continues to generate lift, causing the aircraft to roll in the direction of the stalled wing.

As the aircraft begins to roll, the rudder becomes more effective, and the aircraft starts to yaw in the opposite direction. The yawing motion causes the aircraft to enter a spiral descent, and the angle of attack of the stalled wing increases even further. This increases the rolling motion, and the spin becomes more pronounced.

During a spin, the airflow over the wings is disrupted, and the lift generated by the wings is greatly reduced. The reduced lift causes the aircraft to lose altitude rapidly, and the airspeed decreases, making it difficult to recover. The reduced airflow over the rudder and elevator also reduces their effectiveness, making it difficult to control the aircraft.

To recover from a spin, the pilot must take specific actions. The first step is to reduce the angle of attack by lowering the nose of the aircraft. This allows the airflow over the wings to reattach and generate lift. The pilot must also use the rudder to counteract the yawing motion and stop the spin. Once the spin has stopped, the pilot can then use the elevator to pull the aircraft out of the dive.

Aerodynamically, a spin is a complex and dangerous situation. The key to understanding a spin is to understand the stall and how it affects the airflow over the wings. The reduced lift and disrupted airflow make it difficult to control the aircraft, and recovering from a spin requires specific actions by the pilot.

6.Ground Effect

The term "Ground Effect" refers to the positive influence on the lifting characteristics of an aircraft wing's horizontal surfaces when it is close to the ground. This effect is caused by the distortion of the airflow beneath such surfaces caused by the ground's proximity. It is applicable to both fixed-wing and rotary-wing aircraft. Ground Effect lift is primarily caused by a reduction in the amount of induced drag generated, which improves the lift/drag ratio. The form of the wing tip vortex, which is always generated when an aerofoil moves through the air because pressure beneath a wing is always higher than pressure above it, is modified when generated close to the ground. As the airflow is pushed outwards, vortices near the ground become elliptical rather than circular. This raises the effective aspect ratio of the wing above the geometric aspect ratio, reducing induced drag. When in hover, the ground effect is similar to the fixed wing case for a rotary wing. Because proximity to the ground changes the velocity of the downwash, a lower angle of attack is required to maintain a hover. Furthermore, ground proximity affects rotor blade tip vortices, causing them to be forced outwards. The overall effect of an improved lift/drag ratio when a rotary wing is in ground effect, as with the fixed wing case, is that a given amount of lift is produced at a lower angle of attack than would be required in free air. Because the 'lift bonus' attributable to ground effect is primarily a result of reduced induced drag, how this changes with height above ground is effectively a proxy for changes in the lift coefficient. Induced drag increases nonlinearly with distance from the ground, reaching its free air value at a height above ground equal to the wing span of a fixed-wing aircraft or the rotor diameter of a helicopter. The extent to which a wing is swept back will affect the detail, but not the principle, of this change in ground effect. Taking the wingspan of a fixed-wing aircraft, which is usually expressed in meters, and converting it to feet, which is the standard unit of measurement for height above ground. The wingspan of most modern twin aisle aircraft is around 200 feet. In the rotary wing case, larger helicopter rotor diameters typically range from 55 to 60 feet. Ground effect is greatest when the wind is calm and the surface is smooth and level. The effect on grass, an uneven surface, and occasionally on water is likely to be much lower. Unsurprisingly, fixed-wing aircraft with a low wing fuselage attachment experience the greatest ground effect. In normal flight operations, ground effect awareness is important during the landing flare because it will exacerbate any tendency of an aircraft to float if either airspeed is above the threshold or pitch control is not optimal. It is also considered in the design of Fly-By-Wire systems in terms of the normal law transition from flight to ground status and vice versa. In calm conditions, landing an aircraft significantly smaller than the preceding one should be approached with caution if landing beyond the previous aircraft's touchdown position. A landing or take off on an intersecting runway soon after a much larger aircraft has landed or rotated for take off may necessitate similar considerations. Ground effect may cause an initial airborne state that cannot be sustained as distance from runway surface increases and the lift premium from ground effect decreases if either rotation for take off or an attempt to conduct a go around after touchdown is initiated at too low a speed for the aircraft configuration or weight.

7.Airplane Turn

An airplane turns as a result of the horizontal component of lift. An airfoil generates lift perpendicular to its wingspan.. As the angle of bank increases, lift shifts into the horizontal plane and is instead used to turn the aircraft. The relationship between horizontal, the lift that turns an airplane, and vertical component of lift, the lift that keeps you in the air, change with bank angle. The total lift generated is the same as the vertical component in straight level and unaccelerated flight. The total lift component generated by the wing is also described as load factor ,total aerodynamic load on the wing, and it is the product of vertical and horizontal lift vectors.

8.Airplane Stability

Stability is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium and to return to or to continue on the original flight path. It is primarily an aircraft design characteristic. The flight paths and attitudes an aircraft flies are limited by the aerodynamic characteristics of the aircraft, its propulsion system, and its structural strength.These limitations indicate the maximum performance and maneuverability of the aircraft. If the aircraft is to provide maximum utility, it must be safely controllable to the full extent of these limits without exceeding the pilot’s strength or requiring exceptional flying ability. If an aircraft is to fly straight and steady along any arbitrary flight path, the forces acting on it must be in static equilibrium. The reaction of any body when its equilibrium is disturbed is referred to as stability. The two types of stability are static and dynamic.

Static stability refers to the initial tendency, or direction of movement, back to equilibrium. In aviation, it refers to the aircraft’s initial response when disturbed from a given pitch, yaw, or bank.

Positive static stability is when the initial tendency of the aircraft to return to the original state of equilibrium after being disturbed.

Neutral static stability is when the initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed.

Negative static stability exists where the initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed.

Static stability has been defined as the initial tendency to return to equilibrium that the aircraft displays after being disturbed from its trimmed condition. Occasionally, the initial tendency is different or opposite from the overall tendency, so a distinction must be made between the two. Dynamic stability refers to the aircraft response over time when disturbed from a given pitch, yaw, or bank. This type of stability also has three subtypes.

Positive dynamic stability is when over time, the motion of the displaced object decreases in amplitude and, because it is positive, the object displaced returns toward the equilibrium state.

Neutral dynamic stability is when once displaced, the displaced object neither decreases nor increases in amplitude. A worn automobile shock absorber exhibits this tendency.

Negative dynamic stability is when over time, the motion of the displaced object increases and becomes more divergent.

Stability in an aircraft affects two areas significantly:

Maneuverability which is the quality of an aircraft that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers. It is governed by the aircraft’s weight, inertia, size and location of flight controls, structural strength, and powerplant. It too is an aircraft design characteristic.

Controllability is the capability of an aircraft to respond to the pilot’s control, especially with regard to flight path and attitude. It is the quality of the aircraft’s response to the pilot’s control application when maneuvering the aircraft, regardless of its stability characteristics.

9.Torque and P-Factor

Torque reaction involves Newton’s Third Law of Physics— for every action, there is an equal and opposite reaction. As applied to the aircraft, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the aircraft in the opposite direction. Most United States built aircraft engines rotate the propeller clockwise, as viewed from the pilot’s seat. The discussion here is with reference to those engines.

When the aircraft is airborne, this force is acting around the longitudinal axis, tending to make the aircraft roll. To compensate for roll tendency, some of the older aircraft are rigged in a manner to create more lift on the wing that is being forced downward. The more modern aircraft are designed with the engine offset to counteract this effect of torque. This effect is occurring at all times but is more prevalent the faster the engine and propeller spin faster.

When an aircraft is flying with a high AOA, the “bite” of the downward moving blade is greater than the “bite” of the upward moving blade. This moves the center of thrust to the right of the prop disc area, causing a yawing moment toward the left around the vertical axis. Proving this explanation is complex because it would be necessary to work wind vector problems on each blade while considering both the AOA of the aircraft and the AOA of each blade.This asymmetric loading is caused by the resultant velocity, which is generated by the combination of the velocity of the propeller blade in its plane of rotation and the velocity of the air passing horizontally through the propeller disc. With the aircraft being flown at positive AOAs, the right ,viewed from the rear, or downswinging blade, is passing through an area of resultant velocity, which is greater than that affecting the left or upswinging blade. Since the propeller blade is an airfoil, increased velocity means increased lift. The downswinging blade has more lift and tends to pull ,yaw, the aircraft’s nose to the left.

When the aircraft is flying at a high AOA, the downward moving blade has a higher resultant velocity, creating more lift than the upward moving blade.This might be easier to visualize if the propeller shaft was mounted perpendicular to the ground ,like a helicopter. If there were no air movement at all, except that generated by the propeller itself, identical sections of each blade would have the same airspeed. With air moving horizontally across this vertically mounted propeller, the blade proceeding forward into the flow of air has a higher airspeed than the blade retreating with the airflow. Thus, the blade proceeding into the horizontal airflow is creating more lift, or thrust, moving the center of thrust toward that blade. Visualize rotating the vertically mounted propeller shaft to shallower angles relative to the moving air ,as on an aircraft,. This unbalanced thrust then becomes proportionately smaller and continues getting smaller until it reaches the value of zero when the propeller shaft is exactly horizontal in relation to the moving air.

10.Load Factor

In aerodynamics, the maximum load factor, at a given bank angle, is a proportion between lift and weight and has a trigonometric relationship. The load factor is measured in Gs ,acceleration of gravity, a unit of force equal to the force exerted by gravity on a body at rest and indicates the force to which a body is subjected when it is accelerated. Any force applied to an aircraft to deflect its flight from a straight line produces a stress on its structure. The amount of this force is the load factor. While a course in aerodynamics is not a prerequisite for obtaining a pilot’s license, the competent pilot should have a solid understanding of the forces that act on the aircraft, the advantageous use of these forces, and the operating limitations of the aircraft being flown.

For example, a load factor of 3 means the total load on an aircraft’s structure is three times its weight. Since load factors are expressed in terms of Gs, a load factor of 3 may be spoken of as 3 Gs, or a load factor of 4 as 4 Gs.

If an aircraft is pulled up from a dive, subjecting the pilot to 3 Gs, he or she would be pressed down into the seat with a force equal to three times his or her weight. Since modern aircraft operate at significantly higher speeds than older aircraft, increasing the potential for large load factors, this effect has become a primary consideration in the design of the structure of all aircraft.

The problem of load factors in aircraft design becomes how to determine the highest load factors that can be expected in normal operation under various operational situations. These load factors are called “limit load factors.” For reasons of safety, it is required that the aircraft be designed to withstand these load factors without any structural damage. Although the Code of Federal Regulations (CFR) requires the aircraft structure be capable of supporting one and one-half times these limit load factors without failure, it is accepted that parts of the aircraft may bend or twist under these loads and that some structural damage may occur.

The above considerations apply to all loading conditions, whether they be due to gusts, maneuvers, or landings. The gust load factor requirements now in effect are substantially the same as those that have been in existence for years. Hundreds of thousands of operational hours have proven them adequate for safety. Since the pilot has little control over gust load factors (except to reduce the aircraft’s speed when rough air is encountered), the gust loading requirements are substantially the same for most general aviation type aircraft regardless of their operational use. Generally, the gust load factors control the design of aircraft which are intended for strictly non acrobatic usage.

Any aircraft, within the limits of its structure, may be stalled at any airspeed. When a sufficiently high AOA is imposed, the smooth flow of air over an airfoil breaks up and separates, producing an abrupt change of flight characteristics and a sudden loss of lift, which results in a stall.

A study of this effect has revealed that an aircraft’s stalling speed increases in proportion to the square root of the load factor. This means that an aircraft with a normal unaccelerated stalling speed of 50 knots can be stalled at 100 knots by inducing a load factor of 4 Gs. If it were possible for this aircraft to withstand a load factor of nine, it could be stalled at a speed of 150 knots.

Since the load factor is squared as the stalling speed doubles, tremendous loads may be imposed on structures by stalling an aircraft at relatively high airspeeds.Increased load factors are a characteristic of all banked turns. As noted in the section on load factors in steep turns, load factors become significant to both flight performance and load on wing structure as the bank increases beyond approximately 45°.

11.Velocity Vs. G-Loads

Velocity and G-loads are related in that as velocity increases, the G-loads also increase.

G-load, or gravitational force, is a measure of the force exerted on an object due to gravity. When an object moves at a constant velocity, it experiences a constant G-load of 1 (equivalent to the force of gravity on Earth). However, when an object accelerates or decelerates, the G-load increases or decreases accordingly.

For example, when a car accelerates from 0 to 60 miles per hour (mph) in a few seconds, the G-load on the driver and passengers increases as they are pushed back into their seats. Similarly, when a plane takes off or performs a high-G maneuver, the G-load on the pilot and passengers increases as they are pushed down into their seats.

As velocity increases, the G-load also increases because the acceleration required to maintain that velocity increases. This relationship is particularly important in aviation and spaceflight, where high velocities and G-loads can have significant effects on the human body and equipment. Pilots and astronauts must be trained to withstand and operate under these conditions to ensure safety and mission success.

The flight operating strength of an aircraft is presented on a graph whose vertical scale is based on load factor. The diagram is called a Vg diagram—velocity versus G loads or load factor. Each aircraft has its own Vg diagram that is valid at a certain weight and altitude.

Exceeding the positive G-load limits can have several negative effects on both the pilot and the aircraft.

When a pilot experiences positive G-forces beyond the limits of their body's tolerance, blood is forced away from their brain, resulting in a condition known as "G-LOC" (G-induced loss of consciousness). The pilot  may lose consciousness for a few seconds, which can be very dangerous during high-speed or low-altitude flight. In addition, excessive G-forces can cause fatigue, tunnel vision, blackouts, and even permanent injury or death.

Excessive positive G-loads can also damage the aircraft. The high forces can cause structural damage, such as bending or breaking of parts, and can also damage or impair the operation of vital systems such as hydraulics and electronics. In some cases, exceeding the positive G-load limits can cause the aircraft to break apart in mid-air, resulting in a catastrophic failure.

To prevent exceeding the positive G-load limits, aircraft are designed and tested to withstand specific G-loads, and pilots are trained to maintain safe G-load limits during flight. Flight crews use special equipment such as G-suits to help mitigate the effects of G-forces, and many aircraft are equipped with systems that automatically limit the maximum G-loads that can be experienced. It is critical for pilots to understand and adhere to safe G-load limits to ensure the safety of themselves and their aircraft.

The lines of maximum lift capability (curved lines) are the first items of importance on the Vg diagram. The aircraft in Figure 5-53 is capable of developing no more than +1 G at 64 mph, the wing level stall speed of the aircraft. Since the maximum load factor varies with the square of the airspeed, the maximum positive lift capability of this aircraft is 2 G at 92mph,3Gat112mph,4.4Gat137mph,and so forth.Any load factor above this line is unavailable aerodynamically (i.e., the aircraft cannot fly above the line of maximum lift capability because it stalls). The same situation exists for negative lift flight with the exception that the speed necessary to produce a given negative load factor is higher than that to produce the same positive load factor.

If the aircraft is flown at a positive load factor greater than the positive limit load factor of 4.4, structural damage is possible. When the aircraft is operated in this region, objectionable permanent deformation of the primary structure may take place and a high rate of fatigue damage is incurred. Operation above the limit load factor must be avoided in normal operation.

There are two other points of importance on the Vg diagram. One point is the intersection of the positive limit load factor and the line of maximum positive lift capability. The airspeed at this point is the minimum airspeed at which the limit load can be developed aerodynamically. Any airspeed greater than this provides a positive lift capability sufficient to damage the aircraft. Conversely, any airspeed less than this does not provide positive lift capability sufficient to cause damage from excessive flight loads. The usual term given to this speed is “maneuvering speed,” since consideration of subsonic aerodynamics would predict minimum usable turn radius or maneuverability to occur at this condition. The maneuver speed is a valuable reference point, since an aircraft operating below this point cannot produce a damaging positive flight load. Any combination of maneuver and gust cannot create damage due to excess airload when the aircraft is below the maneuver speed.