Atmosphere Layers PDF

MODULE 1 Layers of Atmosphere Troposphere - The troposphere starts at the Earth's surface and extends 8 to 14.5 kilometers high (5 to 9 miles). This part of the atmosphere is the densest. Almost all weather is in this region. Stratosphere - The stratosphere starts just above the troposphere and ex...

MODULE 1 Layers of Atmosphere Troposphere - The troposphere starts at the Earth's surface and extends 8 to 14.5 kilometers high (5 to 9 miles). This part of the atmosphere is the densest. Almost all weather is in this region. Stratosphere - The stratosphere starts just above the troposphere and extends to 50 kilometers (31 miles) high. The ozone layer, which absorbs and scatters the solar ultraviolet radiation, is in this layer. Mesosphere - The mesosphere starts just above the stratosphere and extends to 85 kilometers (53 miles) high. Meteors burn up in this layer Thermosphere - The thermosphere starts just above the mesosphere and extends to 600 kilometers (372 miles) high. Aurora and satellites occur in this layer. Ionosphere - The ionosphere is an abundant layer of electrons and ionized atoms and molecules that stretches from about 48 kilometers (30 miles) above the surface to the edge of space at about 965 km (600 mi), overlapping into the mesosphere and thermosphere. This dynamic region grows and shrinks based on solar conditions and divides further into the sub-regions: D, E and F; based on what wavelength of solar radiation is absorbed. The ionosphere is a critical link in the chain of Sun-Earth interactions. This region is what makes radio communications possible. Exosphere - this is the upper limit of our atmosphere. It extends from the top of the thermosphere up to 10,000 km (6,200 mi) The Atmosphere -Air is a mixture of several gases. For practical purposes it is sufficient to say that air is a mixture of one-fifth oxygen and fourth-fifths nitrogen. Pure, dry air contains about 78 percent (by volume) nitrogen, 21 percent oxygen and almost 1 percent argon. In addition, it contains about 0.03 percent carbon dioxide and traces of several other gases such as hydrogen, helium, neon, etc. The atmosphere is the whole mass of air extending upward for hundreds of miles. It may be compared with a pile of blankets. Air in the higher altitudes, like the top blanket of the pile, is under much less pressure than the air at the lower altitudes. The air at the earth’s surface may be compared with the bottom blanket in a pile because it supports the weight of all the layers above it. Air has weight and can be weighed. If an unsealed glass jar is weighed, it is heavier than it will be after the air has been exhausted with a vacuum pump. This experiment is illustrated in Fig. 1. The difference in weight is the weight of the air which was in the unsealed jar. Perfectly dry air weights 0.07651 lb. per cu. ft. At sea level when the temperature is 59˚F (15˚C), at 40˚ latitude, with a barometric pressure of 14.69 psi (29.92 inches of mercury). Figure 2 represents the cross section of an open container. The arrows show that the air everywhere exerts a uniform pressure in every direction. If the inside pressure against the bottom were greater than the outside pressure, the container would tend to bulge out at the bottom. Likewise, if the inside pressure against the bottom were less than the outside pressure, the bottom would tend to bend upward. Pressure - The atmosphere may be compared to a pile of blankets. Air in the higher altitudes, like the top blanket of pile, is under much less pressure than the air at the lower altitudes. The air at the earth’s surface may be compared to the bottom blanket, because it supports the weight of all the layers above it. Pressure may be defined as force acting upon a unit area. - For example, if a force of 5 pounds is acting against an area of 1 square inch, we say that there is a pressure of 5 psi (pounds per square inch). Atmosphere pressure can be measured by means of a mercury BAROMETER. This is a glass tube, closed at one end and filled with mercury. It is than inverted such that the open end is under the surface of some mercury in a dish. Atmosphere pressure, acting on the surface of the mercury in the dish, will support a column of mercury within the tube. The height of the mercury column above the level in the dish is measured and is an indication of the atmospheric pressure. If atmospheric pressure decreases (due to increasing altitude for example) the height of the mercury column in the glass tube will be lower; and vice versa when the atmosphere pressure were to increase. Atmospheric pressure at sea level under standard conditions is 29.92 inches of mercury. As altitude increases, the atmospheric pressure acting on the body will decrease. This is because less weight of air is acting on the body. However this atmospheric pressure DOES NOT decrease at a steady rate with increase altitude but tends to decrease faster at low altitudes. In practice half the weight of air in the atmosphere is below an altitude of about 18,000 feet. There climbing to this altitude will cause the atmospheric pressure to reduce to half its sea-level value. Climbing a further 18,000 feet to 36,000 feet, will only cause the pressure to reduce to a quarter of its sea-level value. Temperature Temperature is considered a measure of hotness or coldness of a body. (Vibration of molecules which produces heat).There are two scales normally used for measuring temperature. The Celsius scale, formerly called Centigrade scale has 100 divisions between the freezing and boiling points of pure water. Water freezes at 0ͦ C and boils at 100ͦ C. The Fahrenheit scale is also based on the freezing and boiling points of water, but its freezing point is 32ͦ F. There are 180 divisions on this scale, so water boils at 212ͦ F. The standard sea level temperature is 15ͦ C or 59ͦ F. The temperature decreases as altitude increases. Up to about 36,000 ft above the earth’s surface, the decrease is quite regular, about 2ͦ C or 3ͦ F per 1000 ft. This is also known as the temperature lapse rate. Above this altitude, the temperature remains at approximately -57ͦ C. This sudden check in the fall of temperature has resulted in the lower part of the atmosphere being divided into two layers. The one nearer the earth, in which the temperature is falling being called the TROPOSPHERE, the higher one in which the temperature is constant, the STRATOSPHERE. The surface dividing the two is called the TROPOPAUSE. Density -The density of the air is the MASS of air per unit volume. -We have seen that both the temperature and pressure of the atmosphere decrease with altitude. Density also decreases with increasing altitude but at slower rate as compared to the fall of pressure as altitude increases. This is due to the restoring effect of the decreasing temperature on density as altitude increases. BERNOULLI’S PRINCIPLE - The basic principle of pressure differential of subsonic airflow was discovered by Daniel Bernoulli, a Swiss physicist. Bernoulli’s Principle, simply stated, says that “an increase in the speed of fluid simultaneously with decrease in pressure.” As the air enters the tube, it is travelling at a known velocity and pressure. When the airflow enters the narrow portion, the velocity increases and the pressure decreases. Then, as the airflow continues through the tube to the wider portion, both the velocity and pressure return to their original values. Newton’s law of motion Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. As the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased. The remaining lift is provided by the wing’s lower surface as air striking the underside is deflected downward. According to Newton’s Third law of Motion, “for every action there is an equal and opposite reaction.” The air that is deflected downward also produces an upward (lifting) reaction. (Law of Interaction) There are many other factors that determine the lifting capacity of a wing. Before we look into these, let’s first discuss some basic terminology. MODULE 2 Four forces acting on an airplane in flight During flight, the four forces acting on the airplane are lift, weight, thrust and drag. -Lift is the upward force created by the effect of airflow as it passes over and under the wings. It supports the airplane in flight. -Weight opposes lift. It is caused by the downward pull of gravity. -Thrust is the forward force which propels the airplane through the air. It varies with the amount of engine power is being used. -Opposing thrust is drag, which is a backward or retarding, force that limits the speed of the airplane. The arrows which show the forces acting on an airplane are often called vectors. The magnitude of vector is indicated by the arrow’s length, while the direction is shown by the arrow’s orientation. When two or more forces act on an object at the same, they combine to create a resultant. LIFT - The upward force created by the wings moving through the air, which sustains the airplane in flight. WEIGHT - The weight of the airplane is not constant. It varies with the equipment installed, passengers, cargo, and fuel load. During the course of a flight, the total weight of the airplane decreases as fuel is consumed. Additional weight reduction may also occur during some specialized flight activities, such as crop dusting, firefighting or sky diving flights. -In contrast, the direction in which the force of weight acts is constant. It always acts straight down toward the center of the earth. THRUST - Thrust is the forward acting force which opposes drag and propels the airplane. - In most airplanes, the force is provided when the engine turns the propeller. - Each propeller blade is cambered like the airfoil shape of a wing. - This shape, plus the angle of attack of the blades, produces reduced pressure in front of the propeller and increased pressure behind it. - As is the case with the wing, this produces a reaction force in the direction of the lesser pressure. - This is how the propeller produces thrust, the force which moves the airplane forward. - During straight and level, unaccelerated flight, the force of thrust and drag are equal. - You increase thrust by using the throttle to increase power. - When you increase power, thrust exceeds drag, causing the airplane to accelerate. - This acceleration, however, is accompanied by a corresponding increase in drag. The airplane continues to accelerate only while the force of thrust exceeds the force of drag. - When drag again equals thrust, the airplane ceases to accelerate and maintains a constant airspeed. - However, the new airspeed is higher than the previous one. -When you reduce thrust, the force of drag causes the airplane to decelerate. - But as the airplane slows, drag diminishes. - When drag has decreased enough to equal thrust, the airplane no longer decelerates. - Once again, it maintains a constant airspeed. - Now, however, it is slower than the one previously flown. DRAG - As you have seen, drag is associated with lift. It is caused by any aircraft surface that defects or interferes with the smooth airflow around the airplane. A highly cambered, large surface area wing creates more drag (and lift) than a small, moderately cambered wing. If you increase airspeed, or angle of attack, you increase drag (and lift). Drag acts in opposition to the direction of flight, opposes the forward-acting force of thrust, and limits the forward speed of the airplane. Drag is broadly classified as either parasite or induced. Types of Drag Parasite drag - includes all drag created by the airplane, except that drag directly associated with the production of lift. It is created by the disruption of the flow of air around the airplane’s surfaces. Parasite drag normally divided into three types: form drag, skin friction drag and interference drag. Form drag is created by any structure which protrudes into the relative wind. The amount of drag created is related to both the size and shape of the structure. For example, a square strut will create substantially more than a smooth or rounded strut. Stream lining reduces form drag. Skin friction drag is caused by a roughness of the airplane surfaces. Even though the surface may appear smooth under a microscope they may be quite rough. A thin layer of air clings to this rough surface and these rough surfaces and creates small eddies which contributes to drag. Interference drag occurs when varied currents of air over an airplane meet and interact. This interaction creates additional drag. One example of this type of drag is the mixing of the air where the wing and airframe join. Each type of parasite drag varies with the speed of the airplane. The combined effect of all parasite drag varies proportionately to the square of the airspeed. For example, a particular airplane at a constant altitude has four times as much parasite drag at 160 knots as it does at 80 knots. Induced drag is the main by-product of the production of lift. It is directly related to the angle of attack of the wing. The greater the angle, the greater the induced drag. Since the wing usually is at a low angle of attack at high speed, and high angle of attack at low speed, the relationship of induced drag to speed also can be plotted. Total drag - for an airplane is the sum of parasite and induced drag. The total drag curve represents these combined forces and is plotted against airspeed. Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. - Wingtip vortices are sometimes named trailing or lift-induced vortices because they also occur at points other than at the wing tips. - Indeed, vortices is trailed at any point on the wing where the lift varies spanwise (a fact described and quantified by the lifting-line theory); it eventually rolls up into large vortices near the wingtip, at the edge of flap devices, or at other abrupt changes in wing planform. -Wingtip vortices are associated with induced drag, the imparting of downwash, and are a fundamental consequence of three-dimensional lift generation. - Careful selection of wing geometry (in particular, aspect ratio), as well as of cruise conditions, are design and operational methods to minimize induced drag. -Wingtip vortices form the primary component of wake turbulence. - Depending on ambient atmospheric humidity as well as the geometry and wing loading of aircraft, water may condense or freeze in the core of the vortices, making the vortices visible. -An aircraft in flight is free to rotate in three dimensions: pitch, nose up or down about an axis running from wing to wing; yaw, nose left or right about an axis running up and down; and roll, rotation about an axis running from nose to tail. - The axes are alternatively designated as lateral, vertical, and longitudinal. These axes move with the vehicle and rotate relative to the Earth along with the craft. - These definitions were analogously applied to spacecraft when the first manned spacecraft were designed in the late 1950s. -These rotations are produced by torques (or moments) about the principal axes. - On an aircraft, these are intentionally produced by means of moving control surfaces, which vary the distribution of the net aerodynamic force about the vehicle's center of mass. - Elevators (moving flaps on the horizontal tail) produce pitch, a rudder on the vertical tail produces yaw, and ailerons (flaps on the wings that move in opposing directions) produce roll. - On a spacecraft, the moments are usually produced by a reaction control system consisting of small rocket thrusters used to apply asymmetrical thrust on the vehicle. MODULE 3 Lateral - pitch - elevator Longitudinal - roll - ailerons Vertical - yaw - rudder MODULE 4 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 anybody when its equilibrium is disturbed is referred to as stability. The two types of stability are static and dynamic. Static Stability - 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—the initial tendency of the aircraft to return to the original state of equilibrium after being disturbed. Neutral static stability—the initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed. Negative static stability—the initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed. Dynamic Stability - 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—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—once displaced, the displaced object neither decreases nor increases in amplitude. A worn automobile shock absorber exhibits this tendency. Negative dynamic stability—over time, the motion of the displaced object increases and becomes more divergent. Stability in an aircraft affects two areas significantly: Maneuverability—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—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. Longitudinal Stability (Pitching) - In designing an aircraft, a great deal of effort is spent in developing the desired degree of stability around all three axes. - But longitudinal stability about the lateral axis is considered to be the most affected by certain variables in various flight conditions. - Longitudinal stability is the quality that makes an aircraft stable about its lateral axis. It involves the pitching motion as the aircraft’s nose moves up and down in flight. - A longitudinally unstable aircraft has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall. - Thus, an aircraft with longitudinal instability becomes difficult and sometimes dangerous to fly. Static longitudinal stability, or instability in an aircraft, is dependent upon three factors: 1. Location of the wing with respect to the CG 2. Location of the horizontal tail surfaces with respect to the CG 3. Area or size of the tail surfaces In analyzing stability, it should be recalled that a body free to rotate always turns about its CG. - To obtain static longitudinal stability, the relation of the wing and tail moments must be such that, if the moments are initially balanced and the aircraft is suddenly nose up, the wing moments and tail moments change so that the sum of their forces provides an unbalanced but restoring moment which, in turn, brings the nose down again. - Similarly, if the aircraft is nose down, the resulting change in moments brings the nose back up. - The Center of Lift (CL) in most asymmetrical airfoils has a tendency to change its fore and aft positions with a change in the AOA. - The CL tends to move forward with an increase in AOA and to move aft with a decrease in AOA. - This means that when the AOA of an airfoil is increased, the CL, by moving forward, tends to lift the leading edge of the wing still more. - This tendency gives the wing an inherent quality of instability. (NOTE: CL is also known as the center of pressure (CP).) - Most aircraft are designed so that the wing’s CL is to the rear of the CG. - This makes the aircraft “nose heavy” and requires that there be a slight downward force on the horizontal stabilizer in order to balance the aircraft and keep the nose from continually pitching downward. - Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight negative AOA. - The downward force thus produced holds the tail down, counterbalancing the “heavy” nose. - It is as if the line CG-CL-T were a lever with an upward force at CL and two downward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer). To better visualize this physics principle: If an iron bar were suspended at point CL, with a heavy weight hanging on it at the CG, it would take downward pressure at point T to keep the “lever” in balance. - Even though the horizontal stabilizer may be level when the aircraft is in level flight, there is a downwash of air from the wings. - This downwash strikes the top of the stabilizer and produces a downward pressure, which at a certain speed is just enough to balance the “lever.” - The faster the aircraft is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer (except T-tails). [Figure 5-24] - In aircraft with fixed-position horizontal stabilizers, the aircraft manufacturer sets the stabilizer at an angle that provides the best stability (or balance) during flight at the design cruising speed and power setting. - If the aircraft’s speed decreases, the speed of the airflow over the wing is decreased. - As a result of this decreased flow of air over the wing, the downwash is reduced, causing a lesser downward force on the horizontal stabilizer. - In turn, the characteristic nose heaviness is accentuated, causing the aircraft’s nose to pitch down more. [Figure 5-25] This places the aircraft in a nose-low attitude, lessening the wing’s AOA and drag and allowing the airspeed to increase. - As the aircraft continues in the nose-low attitude and its speed increases, the downward force on the horizontal stabilizer is once again increased. - Consequently, the tail is again pushed downward and the nose rises into a climbing attitude. - As this climb continues, the airspeed again decreases, causing the downward force on the tail to decrease until the nose lowers once more. - Because the aircraft is dynamically stable, the nose does not lower as far this time as it did before. - The aircraft acquires enough speed in this more gradual dive to start it into another climb, but the climb is not as steep as the preceding one. - After several of these diminishing oscillations, in which the nose alternately rises and lowers, the aircraft finally settles down to a speed at which the downward force on the tail exactly counteracts the tendency of the aircraft to dive. - When this condition is attained, the aircraft is once again in balanced flight and continues in stabilized flight as long as this attitude and airspeed are not changed. - A similar effect is noted upon closing the throttle. - The downwash of the wings is reduced and the force at T in Figure 5-23 is not enough to hold the horizontal stabilizer down. - It seems as if the force at T on the lever were allowing the force of gravity to pull the nose down. - This is a desirable characteristic because the aircraft is inherently trying to regain airspeed and reestablish the proper balance. - Power or thrust can also have a destabilizing effect in that an increase of power may tend to make the nose rise. - The aircraft designer can offset this by establishing a “high thrust line” wherein the line of thrust passes above the CG. [Figures 5-26 and 5-27] - In this case, as power or thrust is increased a moment is produced to counteract the down load on the tail. - On the other hand, a very “low thrust line” would tend to add to the nose-up effect of the horizontal tail surface. Conclusion: with CG forward of the CL and with an aerodynamic tail-down force, the aircraft usually tries to return to a safe flying attitude. - The following is a simple demonstration of longitudinal stability. Trim the aircraft for “hands off” control in level flight. - Then, momentarily give the controls a slight push to nose the aircraft down. - If, within a brief period, the nose rises towards the original position, the aircraft is statically stable. - Ordinarily, the nose passes the original position (that of level flight) and a series of slow pitching oscillations follows. - If the oscillations gradually cease, the aircraft has positive stability; if they continue unevenly, the aircraft has neutral stability; if they increase, the aircraft is unstable. MODULE 5 Lateral Stability (Rolling) - Stability about the aircraft’s longitudinal axis, which extends from the nose of the aircraft to its tail, is called lateral stability. - Positive lateral stability helps to stabilize the lateral or “rolling effect” when one wing gets lower than the wing on the opposite side of the aircraft. - There are four main design factors that make an aircraft laterally stable: dihedral, sweepback, keel effect, and weight distribution. Dihedral - Some aircraft are designed so that the outer tips of the wings are higher than the wing roots. - The upward angle thus formed by the wings is called dihedral. [Figure 5-28] When a gust causes a roll, a sideslip will result. - This sideslip causes the relative wind affecting the entire airplane to be from the direction of the slip. - When the relative wind comes from the side, the wing slipping into the wind is subject to an increase in AOA and develops an increase in lift. - The wing away from the wind is subject to a decrease in angle of attack, and develops a decrease in lift. - The changes in lift effect a rolling moment tending to raise the windward wing, hence dihedral contributes to a stable roll due to sideslip. Sweepback and Wing Location - Many aspects of an aircraft’s configuration can affect its effective dihedral, but two major components are wing sweepback and the wing location with respect to the fuselage (such as a low wing or high wing). - As a rough estimation, 10° of sweepback on a wing provides about 1° of effective dihedral, while a high wing configuration can provide about 5° of effective dihedral over a low wing configuration. - A sweptback wing is one in which the leading edge slopes backward. [Figure 5-30] - When a disturbance causes an aircraft with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is more perpendicular to the relative airflow. - As a result, the low wing acquires more lift, rises, and the aircraft is restored to its original flight attitude. Keel Effect and Weight Distribution - A high wing aircraft always has the tendency to turn the longitudinal axis of the aircraft into the relative wind, which is often referred to as the keel effect. - These aircraft are laterally stable simply because the wings are attached in a high position on the fuselage, making the fuselage behave like a keel exerting a steadying influence on the aircraft laterally about the longitudinal axis. - When a high-winged aircraft is disturbed and one wing dips, the fuselage weight acts like a pendulum returning the aircraft to the horizontal level. - Laterally stable aircraft are constructed so that the greater portion of the keel area is above the CG. [Figure 5-31] - Thus, when the aircraft slips to one side, the combination of the aircraft’s weight and the pressure of the airflow against the upper portion of the keel area (both acting about the CG) tends to roll the aircraft back to wings-level flight. Directional Stability (Yawing) - Stability about the aircraft’s vertical axis (the sideways moment) is called yawing or directional stability. - Yawing or directional stability is the most easily achieved stability in aircraft design. - The area of the vertical fin and the sides of the fuselage aft of the CG are the prime contributors that make the aircraft act like the well known weather vane or arrow, pointing its nose into the relative wind. - In examining a weather vane, it can be seen that if exactly the same amount of surface were exposed to the wind in front of the pivot point as behind it, the forces fore and aft would be in balance and little or no directional movement would result. - Consequently, it is necessary to have a greater surface aft of the pivot point than forward of it. - Similarly, the aircraft designer must ensure positive directional stability by making the side surface greater aft than ahead of the CG. [Figure 5-32] - To provide additional positive stability to that provided by the fuselage, a vertical fin is added. - The fin acts similar to the feather on an arrow in maintaining straight flight. - Like the weather vane and the arrow, the farther aft this fin is placed and the larger its size, the greater the aircraft’s directional stability. - If an aircraft is flying in a straight line, and a sideward gust of air gives the aircraft a slight rotation about its vertical axis (i.e., the right), the motion is retarded and stopped by the fin because while the aircraft is rotating to the right, the air is striking the left side of the fin at an angle. - This causes pressure on the left side of the fin, which resists the turning motion and slows down the aircraft’s yaw. - In doing so, it acts somewhat like the weather vane by turning the aircraft into the relative wind. - The initial change in direction of the aircraft’s flight path is generally slightly behind its change of heading. - Therefore, after a slight yawing of the aircraft to the right, there is a brief moment when the aircraft is still moving along its original path, but its longitudinal axis is pointed slightly to the right. - The aircraft is then momentarily skidding sideways and, during that moment (since it is assumed that although the yawing motion has stopped, the excess pressure on the left side of the fin still persists), there is necessarily a tendency for the aircraft to be turned partially back to the left. That is, there is a momentary restoring tendency caused by the fin. - A minor improvement of directional stability may be obtained through sweepback. - Sweepback is incorporated in the design of the wing primarily to delay the onset of compressibility during high-speed flight. - In lighter and slower aircraft, sweepback aids in locating the center of pressure in the correct relationship with the CG. - A longitudinally stable aircraft is built with the center of pressure aft of the CG. - Because of structural reasons, aircraft designers sometimes cannot attach the wings to the fuselage at the exact desired point. - If they had to mount the wings too far forward, and at right angles to the fuselage, the center of pressure would not be far enough to the rear to result in the desired amount of longitudinal stability. - By building sweepback into the wings, however, the designers can move the center of pressure toward the rear. - The amount of sweepback and the position of the wings then place the center of pressure in the correct location. - When turbulence or rudder application causes the aircraft to yaw to one side, the opposite wing presents a longer leading edge perpendicular to the relative airflow. The airspeed of the forward wing increases and it acquires more drag than the back wing. The additional drag on the forward wing pulls the wing back, turning the aircraft back to its original path. - The contribution of the wing to static directional stability is usually small. The swept wing provides a stable contribution depending on the amount of sweepback, but the contribution is relatively small when compared with other components. Free Directional Oscillations (Dutch Roll) - Dutch roll is a coupled lateral/directional oscillation that is usually dynamically stable but is unsafe in an aircraft because of the oscillatory nature. - The damping of the oscillatory mode may be weak or strong depending on the properties of the particular aircraft. - If the aircraft has a right wing pushed down, the positive sideslip angle corrects the wing laterally before the nose is realigned with the relative wind. - As the wing corrects the position, a lateral directional oscillation can occur resulting in the nose of the aircraft making a figure eight on the horizon as a result of two oscillations (roll and yaw), which, although of about the same magnitude, are out of phase with each other. - In most modern aircraft, except high-speed swept wing designs, these free directional oscillations usually die out automatically in very few cycles unless the air continues to be gusty or turbulent. - Those aircraft with continuing Dutch roll tendencies are usually equipped with gyro-stabilized yaw dampers. - Manufacturers try to reach a midpoint between too much and too little directional stability. - Because it is more desirable for the aircraft to have “spiral instability” than Dutch roll tendencies, most aircraft are designed with that characteristic. Spiral Instability - exists when the static directional stability of the aircraft is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium. - When the lateral equilibrium of the aircraft is disturbed by a gust of air and a sideslip is introduced, the strong directional stability tends to yaw the nose into the resultant relative wind while the comparatively weak dihedral lags in restoring the lateral balance. - Due to this yaw, the wing on the outside of the turning moment travels forward faster than the inside wing and, as a consequence, its lift becomes greater. - This produces an overbanking tendency which, if not corrected by the pilot, results in the bank angle becoming steeper and steeper. - At the same time, the strong directional stability that yaws the aircraft into the relative wind is actually forcing the nose to a lower pitch attitude. - A slow downward spiral begins which, if not counteracted by the pilot, gradually increases into a steep spiral dive. - Usually the rate of divergence in the spiral motion is so gradual the pilot can control the tendency without any difficulty. - Many aircraft are affected to some degree by this characteristic, although they may be inherently stable in all other normal parameters. -This tendency explains why an aircraft cannot be flown “hands off” indefinitely. - Much research has gone into the development of control devices (wing leveler) to correct or eliminate this instability. The pilot must be careful in application of recovery controls during advanced stages of this spiral condition or excessive loads may be imposed on the structure. - Improper recovery from spiral instability leading to inflight structural failures has probably contributed to more fatalities in general aviation aircraft than any other factor. - Since the airspeed in the spiral condition builds up rapidly, the application of back elevator force to reduce this speed and to pull the nose up only “tightens the turn,” increasing the load factor. - The results of the prolonged uncontrolled spiral are inflight structural failure, crashing into the ground, or both. - Common recorded causes for pilots who get into this situation are loss of horizon reference, inability to control the aircraft by reference to instruments, or a combination of both. Aerodynamic Forces in Flight Maneuvers Forces in Climbs - For all practical purposes, the wing’s lift in a steady state normal climb is the same as it is in a steady level flight at the same airspeed. - Although the aircraft’s flight path changed when the climb was established, the AOA of the wing with respect to the inclined flight path reverts to practically the same values, as does the lift. - There is an initial momentary change as shown in Figure 5-36. - During the transition from straight-and-level flight to a climb, a change in lift occurs when back elevator pressure is first applied. - Raising the aircraft’s nose increases the AOA and momentarily increases the lift. - Lift at this moment is now greater than weight and starts the aircraft climbing. - After the flight path is stabilized on the upward incline, the AOA and lift again revert to about the level flight values. - If the climb is entered with no change in power setting, the airspeed gradually diminishes because the thrust required to maintain a given airspeed in level flight is insufficient to maintain the same airspeed in a climb. - When the flight path is inclined upward, a component of the aircraft’s weight acts in the same direction as, and parallel to, the total drag of the aircraft, thereby increasing the total effective drag. - Consequently, the total effective drag is greater than the power, and the airspeed decreases. - The reduction in airspeed gradually results in a corresponding decrease in drag until the total drag (including the component of weight acting in the same direction) equals the thrust. [Figure 5-37] - Due to momentum, the change in airspeed is gradual, varying considerably with differences in aircraft size, weight, total drag, and other factors. - Consequently, the total effective drag is greater than the thrust, and the airspeed decreases. - Generally, the forces of thrust and drag, and lift and weight, again become balanced when the airspeed stabilizes but at a value lower than in straight-and-level flight at the same power setting. - Since the aircraft’s weight is acting not only downward but rearward with drag while in a climb, additional power is required to maintain the same airspeed as in level flight. - The amount of power depends on the angle of climb. - When the climb is established steep enough that there is insufficient power available, a slower speed results. - The thrust required for a stabilized climb equals drag plus a percentage of weight dependent on the angle of climb. - For example, a 10° climb would require thrust to equal drag plus 17 percent of weight. - To climb straight up would require thrust to equal all of weight and drag. Therefore, the angle of climb for climb performance is dependent on the amount of excess thrust available to overcome a portion of weight. - Note that aircraft are able to sustain a climb due to excess thrust. - When the excess thrust is gone, the aircraft is no longer able to climb. At this point, the aircraft has reached its “absolute ceiling.” Forces in Descents - As in climbs, the forces that act on the aircraft go through definite changes when a descent is entered from straight-and- level flight. For the following example, the aircraft is descending at the same power as used in straight-and-level flight. - As forward pressure is applied to the control yoke to initiate the descent, the AOA is decreased momentarily. - Initially, the momentum of the aircraft causes the aircraft to briefly continue along the same flight path. - For this instant, the AOA decreases causing the total lift to decrease. - With weight now being greater than lift, the aircraft begins to descend. - At the same time, the flight path goes from level to a descending flight path. - Do not confuse a reduction in lift with the inability to generate sufficient lift to maintain level flight. - The flight path is being manipulated with available thrust in reserve and with the elevator. - To descend at the same airspeed as used in straight-and-level flight, the power must be reduced as the descent is entered. - Entering the descent, the component of weight acting forward along the flight path increases as the angle of descent increases and, conversely, when leveling off, the component of weight acting along the flight path decreases as the angle of descent decreases.