Jeppsen PPL C3 PDF - Four Forces of Flight

SECTION A FOUR FORCES OF FLIGHT The science of aerodynamics deals with the motion of air and the forces acting on bodies moving relative to the air. When you study aerodynamics, you are learning about why and how an airplane flies. Although aerodynamics is a complex subject, exploring the fundamenta...

SECTION A FOUR FORCES OF FLIGHT The science of aerodynamics deals with the motion of air and the forces acting on bodies moving relative to the air. When you study aerodynamics, you are learning about why and how an airplane flies. Although aerodynamics is a complex subject, exploring the fundamental principles which govern flight can be an exciting and rewarding experience. The challenge to understand what makes an airplane fly begins with learning the four forces of 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 wing. The airplane is supported in flight by lift. Weight, which opposes lift, is caused by the downward pull of gravity. Thrus Thrust is the forward force which propels the airplane through the air. It varies with the amount of engine power being used. Opposing thrust is drag, which is a backward, or retarding, force which limits the speed of the airplane. In unaccelerated flight, the four forces are in equilibrium. Unaccelerated flight means that the airplane is maintaining a constant airspeed and is neither accelerating nor decelerating. [Figure 3-1] Figure 3-1. In straightand-level, unaccelerated flight, lift is equal to and directly opposite weight and thrust is equal to and directly opposite drag. Notice that the arrows which represent the opposing forces are equal in length, but all four arrows are not the same length. This indicates that all four forces are not equal but that the opposing forces are equal to each other. The arrows which show the forces acting on an airplane are often called vectors The magnitude of a vector is indivectors. cated 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 time, they combine to create a resultant. [Figure 3-2] The four forces acting on an airplane in flight are lift, weight, thrust, and drag. These forces are in equilibrium during unaccelerated flight. FOUR FORCES OF FLIGHT SECTION A Figure 3-2. When vertical and horizontal forces are applied, as shown on the left, the resultant acts in a diagonal direction. As shown on the right, the resultant of two opposing forces, which are equal in magnitude, is zero. Resultant is Zero LIFT Lift is the key aerodynamic force. It is the force which opposes weight. In straight-and-level, unaccelerated flight, when weight and lift are equal, an airplane is in a state of equilibrium. If the other aerodynamic factors remain constant, the airplane neither gains nor loses altitude. When an airplane is stationary on the ramp, it is also in equilibrium, but the aerodynamic forces are not a factor. In calm wind conditions, the atmosphere exerts equal pressure on the upper and lower surfaces of the wing. Movement of air about the airplane, particularly the wing, is necesIn straight-and-level, unaccelerated flight, sary before the aerodynamic force of lift lift equals weight and thrust equals drag. becomes effective. Knowledge of some of the basic principles of motion will help you to understand the force of lift. NEWTON'S LAWS OF FORCE AND MOTION In the 17th century, Sir Isaac Newton, a physicist and mathematician presented principles of motion which, today, help to explain the creation of lift by an airfoil. Newton' Newton'ss three law thre laws oof motio motion are as follows: Newton's first law: A body at rest tends to remain at rest, and a body in motion tends to remain moving at the same speed and in the same direction. For example, an airplane at rest on the ramp will remain at rest unless a force is applied which is strong enough to overcome the airplane's inertia. Newton's second law: When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force. This law may be expressed by the formula: Force = mass x acceleration (F = ma). Newton's third law: For every action there is an equal and opposite reaction. This principle applies whenever two things act upon each other, such as the air and the propeller, or the air and the wing of an airplane. 3-3 CHAPTER AERODYNAMIC 3 PRINCIPLES BERNOULLI'S PRINCIPLE Daniel Bernoulli, a Swiss mathematician, expanded on Newton's ideas and further explored the motion of fluids in his 1738 publication Hydrodynamica. It was in this text that Bernoulli's equation, which describes the basic principle of airflow pressure differential, first appeared. Bernoulli's Bernoulli' principle principle, simply stated, says, "as the velocity of a fluid (air) increases, its internal pressure decreases." Bernoulli's principle is derived from Newton's second law of motion which states the requirement of an unbalanced force (in this case, pressure) to produce an acceleration (velocity change). One way you can visualize Bernoulli's principle is to imagine air flowing through a tube which is narrower in the middle than at the ends. This type of device is usually called a venturi venturi. [Figure 3-3] Figure 3-3. As the air enters the tube, it is traveling 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. Throughout this process, the total energy of the airstream is conserved. An increase in velocity (kinetic energy) is accompanied by a decrease in static pressure (potential energy). AIRFOILS airfoi is any surface, such as a wing, which provides aerodynamic force when it An airfoil interacts with a moving stream of air. Some of the terms used to describe the wing, and the interaction of the airflow about it, are defined in figures 3-4 and 3-5. Circulation of the airstream about the airfoil is an important factor in the generation of lift. Circulatory flow affects the pressure distribution on an airfoil. [Figure 3-6] The physical principles just discussed help explain the circulation of air around a wing and the pressure distribution on the wing's surface. A combination of the forces described by these principles create the total lift generated by an airfoil. The airplane wing's shape is designed to take advantage of both Newton's laws and Bernoulli's principle. The greater curvature on the upper portion of an airfoil causes air to accelerate as it passes over the wing. Upwash The angle formed by the wing chord line and relative wind is called the angle of the deflection of the oncoming airstream upward and over the wing attack. Trailing Edge Leading Edge the part of the airfoil which meets the airflow first Downwash Figure 3-4. As an airfoil moves through the air, it alters the airflow around it. 3-4 - the portion of the airfoil where the airflow over the upper surface rejoins the lower surface airflow the downward deflection of the airstream as it passes over the wing and past the trailing edge FOUR FORCES OF FLIGHT SECTION Camber the characteristic curve of the airfoil's upper and lower surfaces Chord Line Relative Wind the airflow which is parallel to and opposite the flight path of the airplane Angle of Attack A an imaginary straight line drawn through the airfoil from the leading edge to the trailing edge the angle between the chord line of the airfoil and the direction of the relative wind You shouldn't confuse the actual flight path with the flight attitude of the airplane. For example, the airplane's fuselage may be parallel to the horizon while the aircraft is descending. This airplane is in a level flight attitude, while the actual flight path is forward and down. Notice that the relative wind is upward and back, parallel to and opposite the flight path. As the angle of attack increases, lift also increases. Notice that lift acts perpendicular to the relative wind, regardless of angle of attack. Figure 3-5. Chord line and camber are terms which help define the wing's shape, while flight path and relative wind describe the movement of the wing with respect to the surrounding air. The angle of attack is one factor which determines the amount of lift generated by the wing. According to Bernoulli's theorem, the increase in speed of air on the top of an airfoil produces a drop in pressure and this lowered pressure is a component of total lift. In addition to the lowered pressure, a downward-backward flow of air also is generated from the top surface of the wing. The reaction to this downwash results in an upward force on the wing which demonstrates Newton's third law of motion. This action/reaction Angle of Attack: -8° Angle of Attack: +4° Figure 3-6. As air circulates around the wing's surface, there are regions along the surface where the pressure is less than atmospheric, and regions where the pressure is greater than atmospheric. The specific pressure distribution varies with angle of attack. Angle of Attack: +10° Low Pressure High Pressure 3-5 CHAPTER 3 AERODYNAMIC PRINCIPLES Figure 3-7. As angle of attack increases, CL also increases. This continues to a point where CL peaks. This point of maximum lift is called CLmax. In this example, CLmax occurs at about 17°. If the maximum lift angle is exceeded, lift decreases rapidly and the wing stalls. principle also is apparent as the airstream strikes the lower surface of the wing when inclined at a small angle (the angle of attack) to its direction of motion. The air is forced downward and therefore causes an upward reaction resulting in positive lift. An airplane always stalls when the critical angle of attack is exceeded regardless of airspeed, flight attitude, or weight. coefficient oof lif lift (CL) is a way to measure lift as it relates to angle of attack. CL is The coefficien determined by wind tunnel tests and is based on airfoil design and angle of attack. Every airplane has an angle of attack where maximum lift occurs. [Figure 3-7] STALLS A stal stall is caused by the separation of airflow from the wing's upper surface. This results in a rapid decrease in lift. For a given airplane, a stall always occurs at the same angle, regardless of airspeed, flight attitude, or weight. This angle is the stalling or critica criticall angle oof attack. angl attack [Figure 3-8] Stall characteristics vary with different airplanes. However, in training airplanes during most normal maneuvers, the onset of a stall is gradual. The first indications may be provided by a mushy feeling in the flight controls, a stall warning device, or a slight buffeting of the airplane. To recover from a stall, you must restore the smooth airflow by decreasing the angle of attack to a point below the critical angle of attack. 3-6 Figure 3-8. Increasing the angle of attack beyond CLmax causes progressive disruption of airflow from the upper surface of the wing. At first the airflow begins to separate at the trailing edge. As the angle of attack is further increased, the airflow separation progresses forward until the wing is fully stalled. FOUR FORCES OF FLIGHT SECTION A WING DESIGN FACTORS Wing design is based on the anticipated use of the airplane, cost, and other factors. The main design considerations are wing planform, camber, aspect ratio, and total wing area. Camber, as noted earlier, affects the difference in the velocity of the airflow between the upper and lower surfaces of the wing. If the upper camber increases and the lower camber remains the same, the velocity differential increases. The Boundary Layer Examining the boundary layer can lead to a better understanding of the cause of airflow separation from the wing. The boundary layer is a thin layer of air next to the surface of an airfoil which shows a reduction in speed due to the air's viscosity or stickiness. The boundary layer can be described as either laminar or turbulent based on the type of airflow. Laminar flow begins near the leading edge and consists of smooth laminations of air sliding over one another. At some point along the airfoil, this laminar layer transitions to a thicker turbulent flow with higher velocities. Figure A depicts the development of the boundary layer on a flat plate. The velocity profiles can help you visualize the local velocity of the airstream in the boundary layer and provide a comparison between the laminar and turbulent airflow. Proceeding back from the leading edge of the airfoil, pressure decreases with distance. This favorable pressure gradient (high to low) assists the flow of the boundary layer. At the point where the local velocity of the air at the surface is zero, the pressure gradient reverses and an adverse pressure gradient exists (low to high). As the angle of attack increases, the unfavorable pressure gradient grows longer, and the airflow begins to separate from the wing. [Figure B] When the airflow does not adhere to the surface near the leading edge, a stall occurs. The high velocity airflow of the turbulent boundary layer helps to prevent the airflow separation which can cause a stall. 3-7 CHAPTER 3 AERODYNAMIC PRINCIPLES Airfoil Design and Wind Tunnels Orville and Wilbur Wright constructed a wind tunnel in 1901 and tested several hundred airfoil shapes to determine optimum performance before their aircraft was built. Today, aircraft designers use wind tunnels to test specific designs and organizations such as the National Aeronautics and Space Administration (NASA) use wind tunnels to perform research on the development of airfoil and aircraft shapes. Figure A shows a NASA researcher preparing a test of an MD-11 aircraft in a 12-foot pressure wind tunnel. One of the largest wind tunnels in the world measures 80 feet by 120 feet and is located at NASA's Ames Research Center in Silicon Valley, California. [Figure B] There is, of course, a limit to the amount of camber which can be used. After a certain point, air will no longer flow smoothly over the airfoil. Once this happens, the lifting capacity diminishes. The ideal camber varies with the airplane's performance specifications, especially the speed range and the Figure 3-9. Aspect ratio is the load-carrying requirements. span of the wing, wingtip to wingtip, divided by its average chord. In general, the higher the aspect ratio, the higher the lifting efficiency of the wing. For example, gliders may have an aspect ratio of 20 to 30, while typical training aircraft have an aspect ratio of about 7 to 9. Aspec Aspect ratio rati is the relationship between the length and width of a wing. It is one of the primary factors in determining lift/drag characteristics. At a given angle of attack, a higher aspect ratio produces less drag for the same amount of lift. [Figure 3-9] Win are Wing area is the total surface area of the wings. Most wings don't produce a great amount of lift per square foot, so wing area must be sufficient to support the weight of the airplane. For example, in a training aircraft at normal operating speed, the wings produce 5ft only about 10.5 pounds of Average lift for each square foot of Chord wing area. This means a wing area of 200 square feet is required to support an airplane weight of 2,100 pounds during straight-and-level flight. Wing Area = 180 sq ft Aspect Ratio = 7.2 36 ft Span Wing Area = 100sqft Aspect Ratio = 25 2ft Average Chord 50 ft Span 3-8 FOUR FORCES OF SECTION A FLIGHT Planform refers to the shape of the airplane's wing when viewed from above or below. Planfor Each planform design has advantages and disadvantages. [Figure 3-10] The elliptical wing is ideal for flight at slow speeds since it provides a minimum of drag for a given aspect ratio. This type of planform is difficult to construct, though, and its stall characteristics are not as favorable as those of the rectangular wing. The rectangular wing is not as efficient as the elliptical wing, but it has a tendency to stall first at the wing root which provides adequate stall warning and aileron effectiveness. Tapering provides a decrease in drag and increase in lift which is most effective at high speeds. A highly tapered wing has a tendency [to stall first slightly inboard of the wingtip. A good compromise on planform for low-speed aircraft is a combination of both rectangular and tapered configurations. The rectangular inboard section exhibits good stall characteristics and is cost effective. The tapered outboard portion allows for a reduction in weight and an increase in aspect ratio. Sweptback wings, including delta wings, are efficient at high speeds but low-speed performance is degraded by this design. [ Figure 3-10. Each planform design has its own specific aerodynamic characteristics. 3-9 CHAPTER 3 AERODYNAMIC PRINCIPLES The Swept Wing Sweepback is an important design feature of high-speed aircraft. This characteristic allows the airplane to fly at higher speeds without reaching the critical Mach number. This is the speed at which the wing experiences supersonic airflow. Although high-speed performance is facilitated, sweepback degrades performance at low speeds. A significant part of the air velocity is flowing spanwise, not contributing to lift. This raises the stall speed and will also cause the wingtips to stall first. By employing variable-sweep wings, the Grumman F-14 Tomcat changes its flying configuration to meet different aerodynamic and performance requirements during such operations as takeoff and landing or a highspeed air-to-air engagement. [Figure A] A wing with forward sweep has the same effect on the airflow as a sweptback wing. Forward sweep also will reduce the critical Mach number over the wing. A forward-swept wing has the advantage of a spanwise flow directed inboard and does not have the problem of the wingtips stalling first as with aft-swept wings. Therefore, the forward-swept wing is more efficient at slow speeds. One drawback, though, is the tendency of the wing to twist more than a sweptback wing when high flight loads are applied, which can cause structural failure. The Grumman Corporation's forward-swept wing research airplane, the X-29, uses high-tech composite construction which makes the wing lightweight and rigid to prevent twisting in flight. [Figure B]. Studies have shown that the X-29 is approximately 30% to 40% more efficient at producing lift than most conventional fighters with aftswept wings. Once the design of the wing is determined, the wing must be mounted on the airplane. Usually it is attached to the fuselage with the chord line inclined upward at a slight angle, which is called the angl angle oof incidence incidence. [Figure 3-11] When wing twist, or washout, is incorporated into the wing design, the wingtip has a lower angle of incidence than the wing root. Wing twist is used by airplane designers to prevent undesirable stall characteristics in some wing designs which have the tendency to stall first at the wingtips and then stall inward toward the root. This is an undesirable characteristic, since the disrupted airflow near the wingtip can reduce aileron effectiveness to such an extent that it may be impossible to control the airplane about its longitudinal axis. [Figure 3-12] Longitudinal Axis Figure 3-11. Angle of incidence refers to the angle between the wing chord line and a line parallel to the longitudinal axis of the airplane 3-10 Angle of Incidence FOUR FORCES OF SECTION A FLIGHT Another method sometimes used to ensure positive control during the stall is installation of stal stall strips strips, which consist of two metal strips attached to the leading edge of each wing near the fuselage. These strips disrupt the airflow at high angles of attack, causing the wing area directly behind them to stall before the wingtips stall. Higher Angle of Attack at Root Figure 3-12. Incorporating wing twist into wing design creates a lower angle of incidence at the wingtip than the wing root (usually about two or three degrees). This results in the wingtip having a lower angle of attack than the root during the approach to a stall. Thus, the wingtip and ailerons will still be flying and effectively provide positive control when the wing root has stalled. Wing Root Inboard End Stalls First Lower Angle of Attack at Tip Wingtip PILOT CONTROL OF LIFT The amount of lift generated by an airplane is controlled by the pilot as well as determined by aircraft design factors. For example, you can change the angle of attack and the airspeed or you can change the shape of the wing by lowering the flaps. Anytime you do something to increase lift, drag also increases. Drag is always a by-product of lift. CHANGING ANGLE OF ATTACK You have direct control over angle of attack. During flight at normal operating speeds, if you increase the angle of attack, you increase lift. Anytime you change the pitch of the airplane during flight, you change the angle of attack of the wings. At the same time, you are changing the coefficient of lift. CHANGING AIRSPEED The faster the wing moves through the air, the greater the lift. Actually, lift is proportional to the square of the airplane's speed. For example, at 200 knots, an airplane has four times the lift of the same airplane traveling at 100 knots, if the angle of attack and other factors are constant. On the other hand, if the speed is reduced by one-half, lift is decreased to one-quarter of the previous value. Although airspeed is an important factor in the production of lift, it is only one of several factors. The airspeed required to sustain an aircraft in flight depends on the flap position, the angle of attack, and the weight. ANGLE OF ATTACK AND AIRSPEED The relationship between angle of attack and airspeed in the production of lift is not as complex as it may seem. Angle of attack establishes the coefficient of lift for the airfoil. 3-11 CHAPTER AERODYNAMIC 3 NORMAL TAKEOFF CHECKLIST - 1. FLAPS 0o-10o. PRINCIPLES.. According to National Transportation Safety Board (NTSB) records, during a four and a half year period from 1988 through 1993, there were eighty-seven accidents in which failure to properly use an aircraft procedures checklist was specifically attributed as a cause or factor. Forty-three accidents occurred during the approach and landing phase of flight, while thirty-five occurred during takeoff. Landing gear was indicated in thirty-six accidents, fuel systems in twenty-eight, and flaps were involved in eleven of the reports. The most serious accidents involving misuse of checklists occurred when flaps were improperly configured, usually during takeoff. This type of accident frequently involves some form of distraction or disruption of normal flight routine. When a distraction occurs while using a written checklist, it is easy to resume the checklist by starting with the last item completed. Properly using checklists can help you manage workload and maintain situational awareness during flight operations. At the same time, lift is proportional to the square of the airplane's speed. Since you can control both angle of attack and airspeed, you can control lift. Total lift depends on the combined effects of airspeed and angle of attack. When speed decreases, you must increase the angle of attack to maintain the same amount of lift. Conversely, if you want to maintain the same amount of lift at a higher speed, you must decrease the angle of attack. HIGH-LIFT DEVICES High-lift devices are designed to increase the efficiency of the airfoil at low speeds. The most common high-lift device is the trailing-edge flap. When properly used, flap flapss increase the lifting efficiency of the wing and decrease stall speed. This allows you to fly at a reduced speed while maintaining sufficient control and lift for sustained flight. Remember, though, that when you retract the flaps, the stall speed increases. The ability to fly at slow speeds is particularly important during the approach and landing phases. For example, an approach with full flaps permits you to fly at a fairly steep descent angle without gaining airspeed which allows the airplane to touch down at a slower speed. In addition, you can land near the approach end of the runway, even when Flaps allow you to steepen the angle of there are obstacles along the approach path. descent on an approach without increasing airspeed. In training airplanes, configuratio configuration normally refers to the position of the landing gear and flaps. When the gear and flaps are up, an airplane is in a clean configuration. If the gear is fixed rather than retractable, the airplane is considered to be in a clean configuration when the flaps are in the up position. During flight, you can change configuration by raising or lowering the gear, or by moving the flaps. Lowering the flaps affects the chord line and increases the angle of attack for the section of the wing where the flaps are attached. [Figure 3-13] Figure 3-13. Flaps increase lift (and drag) by increasing the wing's effective camber and changing the chord line, which increases the angle of attack. In some cases, flaps also increase the area of the wing. Most flaps, when fully extended, form an angle of 35° to 40° relative to the wing. 3-12 Chord Line Angle of Attack FLAPS U Chord Line FLAPS D Angle of Attack FOUR FORCES OF FLIGHT SECTION A There are several common types of flaps. The plai plain fla flap is attached to the wing by a hinge. When deflected downward, it increases the effective camber and changes the wing's chord line. Both of these factors increase the lifting capacity Figure 3-14. Plain flap of the wing. [Figure 3-14] The spli split fla flap is hinged only to the lower portion of the wing. This type of flap also increases lift, but it produces greater drag than the plain flap because of the turbulent wake it causes. [Figure 3-15] Figure 3-15. Split flap Figure 3-16. Slotted flap Figure 3-17. Fowler flap The slotte slotted flap fla is similar to the plain flap. In addition to changing the wing's camber and chord line, it also allows a portion of the higher pressure air beneath the wing to travel through a slot. This increases the velocity of the airflow over the flap and provides additional lift. The high energy air from the slot accelerates the upper surface airflow and delays airflow separation to a higher angle of attack. [Figure 3-16] Another type of flap is the Fowle Fowler flap flap. It is attached to the wing by a track and roller system. When extended, the Fowler flap moves rearward as well as down. This rearward motion increases the total wing area, as well as the camber and chord line. [Figure 3-17] Although the amount of lift and drag created by a specific flap system varies, a few general observations can be made. As the flaps are extended, at first they will produce a relatively large amount of lift for a small increase in drag. However, once the flap extension reaches approximately the midpoint, this relationship reverses. Now, a significant increase in drag will occur for a relatively small increase in lift. Because of the large increase in drag beyond the half-flap position, most manufacturers limit the takeoff setting to half flaps or less. Vortex Generators Vortex generators are small airfoil-like surfaces on the wing which project vertically into the airstream. [Figure A] Vortices are formed at the tip of these generators just as they are on ordinary wingtips. These vortices add energy to the boundary layer (the layer of air next to the surface of the wing) to prevent airflow separation. This reduces stall speeds and can increase takeoff and landing performance. Although most commonly seen on high-speed aircraft, vortex generators also are used on some light general aviation aircraft. Many animals, including bats, owls, beetles, flies, moths and even dolphins, have mechanisms for controlling lift and drag through control of the boundary layer. Blood vessels in the wings of a worker bee stabilize the membranes and increase the energy of the turbulent boundary layer flow. [Figure B] 3-13 CHAPTER AERODYNAMIC 3 PRINCIPLES Many high-speed, high-performance airplanes employ high-lift devices to the leading edge of the wing to increase lift at slow speeds. Leading-edge flaps are used to increase the wing camber which provides additional lift. Fixed slots and movable slats conduct the flow of high energy air beneath the wing into the airflow on the upper surface of the wing which delays airflow separation to a higher angle of attack. WEIGHT Weight is the force of gravity which acts vertically through the center of the airplane toward the center of the earth. The weight of the airplane is not a 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, fire fighting, or sky diving flights. THRUST Thrust is the forward-acting force which opposes drag and propels the airplane. In most ion airplanes, this force is provided when the engine turns the propeller. The same physical principles involved in the generation of lift also apply when describing the force of thrust. As explained previously in this chapter, Newton's second law states that an unbalanced force, F, acting on a mass, m, will accelerate, a, the mass in the direction of the force (F = ma). In the case of airplane thrust, the force is provided by the expansion of the burning gases in the engine which turns the propeller. A mass of air moves through the propeller, a rotating airfoil, and is accelerated opposite to the direction of the flight path. The equal and opposite reaction illustrated by Newton's third law is thrust, a force on the airplane in the direction of flight. Figure 3-18. It is easy to visualize the creation of form drag by examining the airflow around a flat plate. Streamlining decreases form drag by reducing the airflow sep aration. During straight-and-level, unaccelerated flight, the forces 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, the airspeed is slower than the one previously flown. DRAG 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 par asite or induced. PARASITE DRAG 3-14 Parasit dra Parasite drag is caused by any aircraft surface which deflects or interferes with the smooth airflow around the airplane. Parasite drag normally is divided into three types: form drag, interference drag, and skin friction drag. FOUR FORCES OF FLIGHT SECTION A Figure 3-19. Design features such as wheel fairings and retractable landing gear can reduce both form and interference drag. For drag Form dra results from the turbulent wake caused by the separation of airflow from the surface of a structure. The amount of drag is related to both the size and shape of the structure which protrudes into the relative wind. [Figure 3-18] Interferenc Interference drag dra occurs when varied currents of air over an airplane meet and interact. Placing two objects adjacent to one another may produce turbulence 50% to 200% greater than the parts tested separately. An example of interference drag is the mixing of the air over structures such as wing and tail surface brace struts and landing gear struts. [Figure 3-19] Ski Skin friction frictio dra drag is caused by the roughness of the airplane's surfaces. Even though these surfaces may appear smooth, under a microscope, they may be quite rough. A thin layer of air clings to these rough surfaces and creates small eddies which contribute to drag. [Figure 3-20] 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. [Figure 3-21] Figure 3-20. Skin friction drag can be minimized by employing a glossy, tat finish to surfaces, and by eliminating protruding rivet heads, roughness, and other irregularities. Figure 3-21. If airspeed is doubled, parasite drag increases fourfold. This is the same formula that applies to lift. Because of its rapid increase with increasing airspeed, parasite drag is predominant at high speeds. At low speeds, near a stall, parasite drag is at its low point. 3-15 CHAPTER 3 AERODYNAMIC PRINCIPLES INDUCED DRAG Induce dra Induced drag is generated by the airflow circulation around the wing as it creates lift. The high pressure air beneath the wing joins the low pressure air above the wing at the trailing edge and wingtips. This causes a spiral or vortex which trails behind each wingtip whenever lift is being produced. These wingti wingtip vortices vortice have the effect of deflecting the airstream downward in the vicinity of the wing, creating an increase in downwash. Therefore, the wing operates in an average relative wind which is inclined downward and rearward near the wing. Because the lift produced by the wing is perpendicular to the relative wind, the lift is inclined aft by the same amount. The component of lift acting in a rearward direction is induced drag. [Figure 3-22] High pressure air joins low pressure air at the trailing edge of the wing and wingtips. Wingtip vortices develop. The downwash increases behind the wing. Induced Drag The average relative wind is inclined downward and rearward and lift is inclined aft. The rearward component of lift is induced drag. Figure 3-22. The formation of induced drag is associated (with the downward deflection of the airstream near the wing. 3-16 FOUR FORCES OF FLIGHT SECTION A Figure 3-23. Induced drag is inversely proportional to the square of the speed. If speed is decreased by half, induced drag increases fourfold. It is the major cause of drag at reduced speeds near the stall; but, as speed increases, induced drag decreases. As the air pressure differential increases with an increase in angle of attack, stronger vortices form and induced drag is increased. Since the wing usually is at a low angle of attack at high speed, and a high angle of attack at low speed, the relationship of induced drag to speed also can be plotted. [Figure 3-23] TOTAL DRAG 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. [Figure 3-24] Figure 3-24. The low point on the total drag curve shows the airspeed at which drag is minimized. This point, where the lift-to-drag ratio is greatest, is referred to as L/Dmax. At this speed, the total lift capacity of the airplane, when compared to the total drag of the airplane, is most favorable. This is important in airplane performance. 3-17 CHAPTER AERODYNAMIC 3 PRINCIPLES GROUND EFFECT The phenomenon of groun ground effect effec is associated with the reduction of induced drag. During takeoffs or landings, when you are flying very close to the ground, the earth's surface actually alters the three-dimensional airflow pattern around the airplane. This causes a reduction in wingtip vortices and a decrease in upwash and downwash. Since ground effect restricts the downward deflection Ground effect is the result of the earth's surof the airstream, induced drag decreases. When face altering the airflow patterns about the the wing is at a height equal to its span, the airplane. In ground effect, an airplane may decline in induced drag is only about 1.4%; become airborne before it reaches its recommended when the wing is at a height equal to one-fourth takeoff speed. its span, the loss of induced drag is about 24%. [Figure 3-25] With the reduction of induced drag in ground effect, the amount of thrust required to produce lift is reduced. What this means is that your airplane is capable of lifting off at lower-than-normal speed. Although you might initially think that this is desirable, consider what happens as you climb out of ground effect. The power (thrust) required to sustain flight increases significantly as the normal airflow around the wing returns In ground effect, induced drag decreases and induced drag is suddenly increased. and excess speed in the flare may cause If you attempt to climb out of ground floating when the aircraft is within one effect before reaching the speed for norwingspan above the surface. mal climb, the airplane might sink back to the surface. Ground effect is noticeable in the landing phase of flight, too, just before touchdown. Within one wingspan above the ground, the decrease in induced drag makes your airplane seem to float on the cushion of air beneath it. Because of this, power reduction usually is required during the flare to help the airplane land. Although all airplanes may experience ground effect, it is more noticeable in low-wing airplanes, simply because the wings are closer to the ground. Figure 3-25. When you are flying in ground effect, the effects of upwash, downwash, and wingtip vortices decrease. This results in a reduction of induced drag. Ground effect is most noticeable near the surface, and it decreases rapidly until it becomes negligible at a height approximately equal to the wingspan of the aircraft. 3-18 FOUR FORCES OF FLIGHT SECTION A SUMMARY CHECKLIST During flight, the four forces acting on the airplane are lift, weight, thrust, and drag. The four forces are in equilibrium during unaccelerated flight. Lift is the upward force created by the effect of airflow as it passes over and under the wing. The airplane wing's shape is designed to take advantage of both Newton's laws and Bernoulli's principle. According to Bernoulli's principle, the increase in speed of air on the top of an airfoil produces a drop in pressure and this lowered pressure is a component of total lift. The reaction to downwash from the top surface of the wing and the airstream striking the wing's lower surface causes an upward reaction in positive lift according to Newton's third law of motion. Planform, camber, aspect ratio, and wing area are some of the design factors which affect a wing's lifting capability. A stall is caused by the separation of airflow from the wing's upper surface. For a given airplane, a stall always occurs at the critical angle of attack, regardless of airspeed, flight attitude, or weight. Total lift depends on the combined effects of airspeed and angle of attack. When speed decreases, you must increase the angle of attack to maintain the same amount of lift. Flaps increase lift (and drag) by increasing the wing's effective camber and changing the chord line which increases the angle of attack. Flap types include plain, split, slotted, and Fowler. Weight is the force of gravity which acts vertically through the center of the airplane toward the center of the earth. Thrust is the forward-acting force which opposes drag and propels the airplane. Drag acts in opposition to the direction of flight, opposes the forward-acting force of thrust, and limits the forward speed of the airplane. Parasite drag is caused by any aircraft surface which deflects or interferes with the smooth airflow around the airplane. Parasite drag normally is divided into three types: form drag, interference drag, and skin friction drag. If airspeed is doubled, parasite drag increases fourfold. Induced drag is generated by the airflow circulation around the wing as it creates lift. Induced drag increases with flight at slow airspeeds as the angle of attack increases. The phenomenon of ground effect occurs close to the ground where the earth's surface restricts the downward deflection of the airstream from the wing, decreasing induced drag. 3-19 CHAPTER 3 AERODYNAMIC PRINCIPLES KEY TERMS Critical Angle of Attack Lift Weight Aspect Ratio Thrust Wing Area Drag Planform Vectors Angle of Incidence Newton's Three Laws of Motion Stall Strips Bernoulli's Principle Flaps Venturi Configuration Airfoil Plain Flap Leading Edge Split Flap Trailing Edge Slotted Flap Upwash Fowler Flap Downwash Parasite Drag Relative Wind Form Drag Camber Interference Drag Chord Line Skin Friction Drag Angle of Attack Induced Drag Coefficient of Lift Wingtip Vortices Stall Ground Effect QUESTIONS 1. Select the true statement(s) regarding the four forces of flight. a. b. c. 3-20 During accelerated flight, thrust and drag are equal. The four forces are in equilibrium during unaccelerated flight. In straight-and-level unaccelerated flight, all four forces are equal in magnitude. FOUR FORCES OF SECTION A FLIGHT 2. Refer to the following illustration and identify the aerodynamic terms associated with the airfoil. 3. Describe how Newton's laws of motion and Bernoulli's principle explain the generation of lift by an airfoil. 4. True/False. As airspeed increases, the angle of attack at which an airfoil stalls also increases. 5. Determine the aspect ratio of the following planforms. Wing Span = 196 ft Average Chord = 28 ft Wing Span = 35 ft Average Chord = 5 ft Wing Span = 37 ft Average Chord = 11 ft 6. Identify three methods you can use to control lift during flight. 7. Will the wing's angle of attack increase or decrease when trailing edge flaps are lowered? 8. Is it more desirable for the wing root or wingtips to stall first and why? 9. List the three forms of parasite drag and provide examples of aircraft features which reduce parasite drag. 10. Explain why induced drag increases as airspeed decreases. 11. The reduction in induced drag due to ground effect is most noticeable when the airplane is within what distance from the earth's surface? 3-21 SECTION B STABILITY Deviation Amplitude Although no airplane is completely stable, all airplanes must have desirable stability and An airplane said to be inherently stable handling characteristics. An inherently stable will require less effort to control. airplane is easy to fly and reduces pilot fatigue. This quality is essential throughout a wide range of flight conditions — during climbs, descents, turns, and at both high and low airspeeds. An aircraft's inherent stability also affects its ability to recover from stalls and spins. In fact, stability, maneuverability, and controllability are all interrelated design characteristics. Seconds Figure 3-26. An airplane that exhibits positive dynamic stability experiences a series of progressively smaller oscillations after a disturbance, such as an updraft.The amount of time that it takes for the oscillations to cease is a measure of the degree of stability. After a significant disturbance, oscillations for typical light airplanes normally damp to half of the original deviation in 20 to 30 seconds. SECTION B STABILITY Stability Stabilit is the characteristic of an airplane in flight that causes it to return to a condition of equilibrium, or steady flight, after it is disturbed. For example, if you are flying a stable airplane that is disrupted while in straight-and-level flight, it has a tendency to return to the same attitude. The initial tendency to return to the position from which it was displaced is termed positive positiv stati static stability. stability However, since the aircraft doesn't immediately return to the original position, but instead does so over a period of time through a series of successively smaller oscillations, the aircraft also displays positiv positivee dynamic stability dynami stability. [Figure 3-26] Since an inherently stable platform is highly desirable in training aircraft, they are normally designed to possess both positive static and positive dynamic stability. Maneuverabilit is the characteristic Maneuverability of an airplane that permits you to maneuver it easily and allows it to withstand the stress resulting from the maneuvers. An airplane's size, weight, flight control system, structural strength, and thrust determine its maneuverability. Controllabilit Controllabilityy is the capability of an airplane to respond to your control inputs, especially with regard to attitude and flight path. Stability, maneuverability, and controllability all refer to movement of the aircraft about one or more of three axes of rotation. Longitudinal Axis Lateral Axis THREE AXES OF FLIGHT Since an aircraft operates in a three dimensional environment, aircraft movement takes place around one or more of three axes of rotation. They are called the longitudinal longitudinal,, lateral, and vertical lateral vertica axes axe of flight. The common reference point for the three axes is the airplane's cente center of gravit (CG), which is the theoretical gravity point where the entire weight of the airplane is considered to be concentrated. Since all three axes pass through this point, you can say that the airplane always moves about its CG, regardless of which axis is involved. The ailerons, elevator, and rudder create aerodynamic forces which cause the airplane to rotate about the three axes. [Figure 3-27] Vertical Axis Figure 3-27. Ailerons control roll movement about the longitudinal axis; the elevator controls pitch movement about the lateral axis; and the rudder controls yaw movement about the vertical axis. 3-23 CHAPTER 3 AERODYNAMIC PRINCIPLES LONGITUDINAL AXIS When you deflect the ailerons to begin a turn, they create an immediate rolling movement about the longitudinal axis. Since the ailerons always move in opposite directions, the aerodynamic shape of each wing and the associated production of lift is affected differently. [Figure 3-28] The rolling movement about the longitudinal axis will continue as long as the ailerons are deflected. To stop the roll, you must relax control pressure and return the ailerons to their original, or neutral, position. This is called neutralizing the controls. Figure 3-28. Deflected ailerons alter the chord line and change the effective camber of the outboard section of each wing. In this example, the angle of attack increases for the right wing, causing a corresponding increase in lift. At the same time, you can see that the left wing will lose some of its lift because of a decrease in its angle of attack. The airplane will roll to the left, because the right wing is producing more lift than the left wing. Longitudinal Axis LATERAL AXIS Since the horizontal stabilizer is an airfoil, the action of the elevator (or stabilator) is quite similar to that of an aileron. Essentially, the chord line and effective camber of the stabilizer are changed by deflection of the elevator. Movement of the control wheel fore or aft causes motion about the lateral axis. Typically, this is referred to as an adjustment to pitch, or a change in pitch attitude. For example, when you move the control wheel forward, it causes movement about the lateral axis that decreases the airplane's pitch attitude. [Figure 3-29] A decrease in pitch attitude decreases the angle of attack. Conversely, an increase in pitch attitude increases the angle of attack. 3-24 SECTION B STABILITY Lateral Axis Figure 3-29. When you push forward on the control wheel, the elevator is lowered and the angle of attack of the stabilizer increases which causes it to produce more lift. The lifting force created by the stabilizer causes the airplane to pivot forward about its lateral axis. Pitch Down Increased Angle of Attack Increased Lift Chord Line Relative Wind VERTICAL A X I S When you apply pressure on the rudder pedals, the rudder deflects into the airstream. This produces an aerodynamic force that rotates the airplane about its vertical axis. This is referred to as yawing the airplane. The rudder may be displaced either to the left or right of center, depending on which rudder pedal you depress. [Figure 3-30] Figure 3-30. Since the vertical stabilizer also is an airfoil, deflection of the rudder alters the stabilizer's effective camber and chord line. In this case, left rudder pressure causes the rudder to move to the left. With a change in the chord line, the angle of attack is altered, generating an aerodynamic force toward the right side of the vertical fin. This causes the tail section to move to the right, and the nose of the airplane to yaw to the left. Elevator HORIZONTAL STABILIZER Yaw movement about the vertical axis is produced by the rudder. Vertical Axis TOP VIEW OF VERTICAL STABILIZER Increased Angle of Attack Relative Wind -Chord line Increased Lift Yaw Left Rudder LONGITUDINAL STABILITY The longitudinal longitudina stabilit stability of an airplane involves the pitching motion or tendency of the aircraft to move about its lateral axis. An airplane which is longitudinally stable will tend to return to its trimmed angle of attack after displacement. This is desirable because an airplane with this characteristic tends to resist either excessively nose-high or noselow pitch attitudes. If an airplane is longitudinally unstable, it has the tendency to climb or dive until a stall or a steep dive develops. As a result, a longitudinally unstable airplane is very dangerous to fly. 3-25 CHAPTER AERODYNAMIC 3 PRINCIPLES BALANCE An important consideration when designing a longitudinally stable airplane is the balance between the center of gravity and the center of pressure of the wing. The center cente of of pressure is a point along the wing chord line where lift is considered to be concentrated. pressur For this reason, the center of pressure is sometimes referred to as the center cente of o lift. lift. On a typical cambered wing, this point The longitudinal stability of an airplane is along the chord line changes position with determined primarily by the location of the different flight attitudes. It moves forward center of gravity in relation to the center of as angle of attack increases and aft as angle pressure (lift). of attack decreases. As a result, pitching tendencies created by the position of the center of pressure in relation to the CG vary. For example, with a high angle of attack and the center of pressure in a forward position (closer to the CG) the nose-down pitching tendency is decreased. The reverse is true as the angle of attack is decreased and the center of pressure moves further aft of the CG. [Figure 3-31] Center of Pressure Center of Gravity Figure 3-31. To maintain balance, and aid longitudinal stability, most aircraft are designed so that, during normal operations, the center of pressure remains aft of the center of gravity. CENTER OF GRAVITY POSITION The position of the center of gravity (CG), which is determined by the distribution of weight either by design or by the pilot, can also affect the longitudinal stability of an airplane. If the CG is too far forward, the airplane is very nose heavy; if the CG is too far aft, the airplane may become tail heavy. To achieve longitudinal stability, most airplanes are designed so they're slightly nose heavy. This is accomplished during the engineering and development phase by placing the center of gravity slightly forward of the center of pressure. 3-26 Your control over CG location is largely determined by what you put into the airplane, and where you put it. This includes the weight of items such as fuel, passengers, and baggage. For example, if you load heavy baggage into an aft baggage compartment, it might cause the CG to shift to an unfavorable position which can result in severe control problems. As you might expect, for an airplane to be controllable during flight, the CG must be located within a reasonable distance forward or aft of an optimum position. All airplanes have forward and aft limits for the position of the CG. The distance between these limits is the CG range. [Figure 3-32] STABILITY Center of Gravity (CG) FORWARD CG LIMIT SECTION B Approved CG Range AFT CG LIMIT When the CG is within the approved CG range, the airplane not only is controllable, but its longitudinal stability also is satisfactory. If the CG is located near the forward or aft limit of the approved CG range, a slight loss of longitudinal stability may be noticeable, but stabilator (or elevator) effectiveness is still adequate to control the airplane during all approved maneuvers. However, loading an aircraft in such a way as to move the CG too far forward or aft could result in a situation in which the capability of the stabilator (or elevator) to control the aircraft is exceeded. Figure 3-32. An airplane must be loaded so the effect of weight distribution does not adversely affect longitudinal balance. Loading limitations must be observed to ensure that the position of the CG remains within the approved range as published in the aircraft's pilot's operating handbook (POH). CG TOO FAR FORWARD If you load your airplane so the CG is forward of the forward CG limit, it will be too nose heavy. Although this tends to make the airplane seem stable, adverse side effects include longer takeoff distance and higher stalling speeds. The condition gets progressively worse as the CG moves to an extreme forward position. Eventually, stabilator (or elevator) effectiveness will be insufficient to lift the nose. [Figure 3-33] Figure 3-33. If the CG is well forward of the approved CG range, stabilator (or elevator) effectiveness will be insufficient to exert the required tail-down force needed for a nose-high landing attitude. During landing, this may cause the nosewheel to strike the runway before the main gear. Approved CG Range Extreme Forward CG 3-27 CHAPTER CG AERODYNAMIC 3 TOO FAR PRINCIPLES AFT A CG located aft of the approved CG range is even more dangerous than a CG that is too far forward. With an aft CG, the airplane becomes tail heavy and very unstable in pitch, regardless of speed. CG limits are established during initial testing and airworthiness certification. An airplane loaded to its aft CG limit will be One of the criteria for determining the CG less stable at all speeds. range in light airplanes is spin recovery capability. If the CG is within limits, a normal category airplane must demonstrate that it can be recovered from a one-turn spin; and a utility category airplane that is approved for spins must be recoverable from a fully developed spin. The aft CG limit is the most critical factor. As the CG moves aft, stabilator (or elevator) effectiveness decreases. When the CG is at the aft limit, stabilator effectiveness is adequate; but, when the CG is beyond the aft limit, the stabilator may be ineffective for stall or spin recovery. [Figure 3-34] Approved CG Range Extreme Aft CG Figure 3-34. If the CG is too far aft, you will not have enough stabilator (or elevator) effectiveness to raise the tail and lower the nose of the airplane. As a result, you may be unable to recover from a stall or spin. As a pilot, there are certain actions you can take to prevent an aft CG position. You can make sure the heaviest passengers and baggage, or cargo, are loaded as far forward as practical. Lighter passengers and baggage normally should be loaded in aft seats or compartments. The main thing you must An airplane becomes progressively more do is follow the airplane manufacturer's difficult to control as the CG moves aft. If loading recommendations in the POH. If the CG is beyond the aft limit, it will be diffiyou do this, your airplane will be loaded so cult to lower the nose to recover from a stall or spin. the CG is within the approved range where longitudinal stability is adequate and, at the same time, where you can control the airplane during all approved maneuvers. Two important points to remember are that a CG beyond acceptable limits adversely affects longitudinal stability, and the most hazardous condition is an extreme aft CG position. You will learn more about the effects of adverse loading in the section on weight and balance in Chapter 8. 3-28 STABILITY SECTION HORIZONTAL STABILIZER When the airplane is properly loaded, the CG remains forward of the center of pressure and the airplane is slightly nose heavy. The nose-heavy tendency is offset by the position of the horizontal stabilizer, which is designed with a negative angle of attack. This produces a downward force, or negative lift on the tail, to counteract the nose heaviness. The downward force is called the tail-dow tail-down force, force and is the balancing force in most flight conditions. [Figure 3-35] Figure 3-35. Longitudinal stability is also aided by the horizontal stabilizer which, due to a negative angle of attack, produces a downward force to counteract nosedown tendencies. Center of Pressure -Tail Down Force Center of Gravity Negative , Angle of Longitudinal Axis Additional forces are exerted on horizontal tail Chord surfaces of most aircraft by Line downwash from the proTail Down peller and the wings. [Figure Force 3-36] T-tail designs are not subject to the same downwash HORIZONTAL STABILIZER effect, simply because the horizontal tail surface is above most, or all, of the downwash. With the exception of T-tail airplanes, the strength of the downward force on the horizontal stabilizer is related to angle of attack, speed of the airplane, and power setting in single engine propeller-driven airplanes. Any variance in the strength of the downwash, such as a power change, affects the horizontal tail's contribution to longitudinal stability. Wing Downwash Figure 3-36. The downwash from the propeller and the wings passing over the horizontal stabilizer influences the longitudinal stability of the airplane. Prop Downwash 3-29 CHAPTER 3 AERODYNAMIC PRINCIPLES The Canard Design Although the tail-down force created by the horizontal stabilizer is excellent for longitudinal stability and balance, it is aerodynamically inefficient. The wings must support the negative lift created by the tail, and the negative angle of attack on the stabilizer increases drag. If an airplane design permitted two lifting surfaces, aerodynamic efficiency would be much greater. A canard is a stabilizer that is located in front of the main wings. Canards are something like miniature forward wings. They were used in the pioneering days of aviation, most notably on the Wright Flyer, and are now reappearing on several original designs. The Beechcraft Starship (see photo) employs a variable sweep canard design. The canard provides longitudinal stability about the lateral axis by lifting the nose of the airplane. Since both the main wings and the canard produce positive lift, the design is aerodynamically efficient. A properly designed canard is also stall/spin resistant. The canard stalls at a lower angle of attack than the main wings. In doing so, the canard's angle of attack immediately decreases after it stalls. This breaks the stall and effectively returns the canard to a normal lift-producing angle of attack before the main wings have a chance to stall. Ailerons remain effective throughout the stall because they are attached to the main wings. In spite of its advantages, the canard design has limitations in total lift capability. Critical design conditions also must be met to maintain adequate longitudinal stability throughout the flight envelope. POWER EFFECTS If you reduce power during flight, a definite nose-down pitching tendency occurs due to the reduction of downwash from the wings and the propeller which reduces elevator effectiveness. Although this is a destabilizing factor, it is a desirable characteristic because it tends to result in a nose-down attitude during power reductions. The nosedown attitude helps you maintain, or regain, airspeed. Increasing power has the opposite effect. It causes increased downwash on the horizontal stabilizer which decreases its contribution to longitudinal stability and causes the nose of the airplane to rise. The influence of power on longitudinal stability also depends on the overall design of the airA power reduction in airplanes, other than Tplane. Since power provides thrust, the aligntails, will decrease the downwash on the ment of thrust in relation to the longitudinal horizontal stabilizer from the wings and proaxis, the CG, the wings, and the stabilizer are all peller slipstream. This is what causes the nose to pitch factors. The thrustline thrustlin is determined by where down after a power reduction. the propeller is mounted and by the general direction in which thrust acts. In most light general aviation airplanes, the thrustline is parallel to the longitudinal axis and above the CG. This creates a slight pitching moment around the CG. If thrust is decreased, the pitching moment is reduced and the nose heaviness tends to decrease. An increase in thrust increases the pitching moment and increases nose heaviness. [Figure 3-37] Notice that these pitching tendencies are exactly the reverse of the pitching tendencies resulting from an increase or decrease in downwash. This thrustline design arrangement minimizes the destabilizing effects of power changes and improves longitudinal stability. 3-30 SECTION B STABILITY Thrustline Nose-Down Pitching Tendency With Increased Power Figure 3-37. Airplanes with the thrustline parallel to the longitudinal axis and above the CG produce a pitching moment about the CG which partially counteracts downwash effects on the stabilizer. An increase in power, or thrust, increases downwash on the stabilizer and produces a nose-up pitching tendency. At the same time, the increased thrust also creates a nose-down pitching tendency because the thrustline is above the CG. High power settings combined with low airspeed produce a situation in which increased downwash and decreased airspeed reduce the overall stabilizing effect of the horizontal stabilizer. Additionally, the extension of high-lift devices, such as flaps, can increase downwash and its debilitating effects on longitudinal stability. Therefore, it is particularly important to maintain precise aircraft control during power-on approaches or goarounds since longitudinal stability may be reduced. LATERAL STABILITY Stability about an airplane's longitudinal axis, which extends nose to tail, is called lat latera stability eral stability. If one wing is lower than the opposite wing, lateral stability helps return the wings to a level attitude. This tendency to resist lateral, or roll, movement is aided by specific design characteristics. Four of the most common design features that influence lateral stability are weight distribution, dihedral, sweepback, and keel effect. Two of these, sweepback and keel effect, also help provide directional stability about the vertical axis. You have no control over the design features that help maintain lateral stability, but you can control the distribution of weight and improve lateral stability. For example, most training airplanes have two fuel tanks, one inside each wing. Before you takeoff on a long flight, you normally fill both tanks. If you use fuel from only one tank, you will soon notice that the airplane wants to roll toward the wing with the full tank. The distribution of weight is uneven and lateral stability is affected. You can prevent the imbalance by switching tanks before a significant difference in weight can occur. DIHEDRAL The most common design for lateral stability is known as wing dihedral. Dihedra Dihedral is the upward angle of the airplane's wings with respect to the horizontal. When you look at an airplane, dihedral makes the wings appear to form a spread-out V. Dihedral usually is just a few degrees. If an airplane with dihedral enters an uncoordinated roll during gusty wind conditions, one wing will be elevated and the opposite wing will drop. This causes an immediate sideslip downward toward the low wing. Since the relative wind is now coming from the side, the low wing experiences an increased angle of attack while the high wing's angle 3-31 CHAPTER 3 AERODYNAMIC PRINCIPLES Aircraft Enters Sideslip Angle of Attack and Lift Increases on Low Wing (Wing into the Wind) Decreased Lift Increased Lift Figure 3-38. Wing dihedral is a major contributor to lateral stability. Aircraft Rolls Level of attack is reduced. The increased angle of attack on the low wing produces more lift for that wing and tends to roll the aircraft back toward a level flight attitude. [Figure 3-38] The fuselage also is a contributor to lateral stability, although in varying degrees depending primarily on the placement of the wings. During a sideslip, airflow creates both an upwash and downwash as it passes around the fuselage just ahead of the wing. The upwash effect tends to roll a high-wing airplane toward the upright position, and therefore is a stabilizing factor. In a low-wing airplane, however, the downwash is destabilizing since it tends to contribute to a roll in the direction of the sideslip. [Figure 3-39] This effect produces a movement equivalent to 3° to 4° of negative dihedral in a typical low wing aircraft, while a high-wing aircraft may experience an effect approximating 2° to 3° of positive dihedral. This accounts for the greater dihedral normally found on most lowwing general aviation aircraft. [Figure 3-40] From an operational standpoint, it's important to note that, in certain situations, the propeller slipstream can reduce the lateral stability of the airplane by reducing the effect of wing dihedral. At high power settings and low airspeeds, propwash increases 3-32 SECTION B STABILITY Figure 3-39. Upwash created by a sideslipping condition tends to contribute to lateral stability in the high-wing aircraft (left). The low-wing aircraft (right) experiences a downwash during a sideslip which is laterally destabilizing. Sideslip Sideslip the effectiveness of the inboard sections of the wings which decreases the effect of dihedral, thereby reducing lateral stability. This warrants particular attention since highpower, low-airspeed conditions also contribute to a degradation of longitudinal stability. SWEEPBACK In many airplanes, the leading edges of the wings do not form right angles with the longitudinal axis. Instead, the wings are angled backward from the wing root to the wingtips. This design characteristic is referred to as wing sweep or sweepback sweepback. In high performance airplanes with pronounced sweepback, the design is used primarily to maintain the center of lift aft of the CG and reduce wave drag when operating at speeds Figure 3-40. Since they are inherently more stable laterally, high-wing aircraft such as the Cessna 172 on the left are designed with less dihedral than the typical low-wing aircraft, as exhibited by the Beechcraft Bonanza on the right. 3-33 CHAPTER 3 AERODYNAMIC at or above the speed of sound, or Mach one. [Figure 3-41] In light training airplanes, the main purpose of sweepback design is to improve lateral stability. Sweepback also may aid slightly in directional stability. If an airplane rotates about its vertical axis or yaws to the left, the right wing has less sweep and a slight increase in drag. The left wing has more sweep and less drag. This tends to force the airplane back into alignment with the relative wind. KEEL EFFECT Lateral stability also is provided by the vertical fin and side area of the fuselage reacting to the airflow very much like the keel of a ship. Kee effec Keel effect is the steadying influence exerted by the side area of the fuselage and vertical stabilizer. [Figure 3-42] Figure 3-41. The swept wing Upper Wing arrangement moves the low wing more perpendicular to the airflow during a sideslip, thereby increasing lift and drag on that wing. At the same time, the upper wing experiences an effective increased sweep, decreasing lift and drag. This out-of-balance situation causes the aircraft to roll out of the sideslip. Side Force Keel Area Figure 3-42. In this example, as the aircraft rolls to the right, a side force is applied to the right side of the fuselage. Since the majority of the surface area lies above the CG, the keel effect tends to roll the aircraft back toward an upright position. 3-34 CG PRINCIPLES SECTION STABILITY B DIRECTIONAL STABILITY Stability about the vertical axis is called directional directiona stability. stability The primary contributor to directional stability is the vertical tail which causes an airplane in flight to act much like a weather vane. You can compare the pivot point on the weather vane to the center of gravity of the airplane. The nose of the airplane corresponds to the weather vane's arrowhead, and the vertical fin on the airplane acts like the tail of the weather vane. [Figure 3-43] Figure 3-43. An airplane must have more surface area behind the CG than it has in front of it. When an airplane enters a sideslip, the greater surface area behind the CG helps keep the airplane aligned with the relative wind. How Does a Flying Wing Maintain Directional Stability? In most aircraft designs, the primary source of directional stability is the vertical tail. Aircraft designed without tail assemblies, such as flying wings, must somehow still maintain directional stability and control, but how is this possible? One of the most notable flying wing designs is employed by the U.S. Air Force B-2 Stealth bomber manufactured by Northrop Corporation. [Figure A] The B-2 uses a fourtimes redundant fly-by-wire flight control system that is controlled by approximately 200 computer processors. The actual flight controls are primarily located on the trailing edge of wing (fuselage). Yaw control is accomplished using "drag rudders" located near each wingtip. The drag rudders extend or retract independently to control the direction of nose movement. (Note the extended drag rudder on the left wing of the B-2 in figure A.) If both drag rudders are extended simultaneously, they act as speed brakes. Pitch and roll control is accomplished through the use of "elevons" located inboard of the drag rudders. Although not employed on the B-2, some stability can be derived from bending the trailing edge of the wing upward. In addition, the tips of a swept wing can be twisted to a negative angle of attack to act as a horizontal tail. Although the B-2 didn't fly until 1989, the flying wing concept is not new. In fact, the Northrop Corporation developed flying wing prototypes as early as the 1940's. These included the prop-powered B-35, first flown in 1946 [Figure B], and the jet-powered YB-49 which flew in 1947 [Figure C]. 3-35 CHAPTER 3 AERODYNAMIC PRINCIPLES INTERACTION OF LATERAL AND DIRECTIONAL STABILITY For ease-of-understanding, lateral and directional stability have been discussed separately up to this point. However, it is impossible to yaw an aircraft without also creating a rolling motion. This interaction between the lateral and directional stabilizing design elements can sometimes uncover some potentially undesirable side effects. Two of the most common are Dutch roll and spiral instability. Dutc Dutch roll rol is a combination of rolling/yawing oscillations caused either by your control input or by wind gusts. Dutch roll will normally occur when the dihedral effects of an aircraft are more powerful than the directional stability. After a disturbance resulting in a yawing motion and sideslip, the dihedral effect will tend to roll the aircraft away from the direction of the initial yaw. However, due to weak directional stability, the rolling movement may overshoot the level position and reverse the sideslip. This motion continues to repeat, creating an oscillation that can be felt by the pilot as side-to-side wagging of the aircraft's tail. If Dutch roll tendency is not effectively dampened, it is considered objectionable. The alternative to an airplane that exhibits Dutch roll tendencies is a design that has better directional stability than lateral stability. If directional stability is increased and lateral stability is decreased, the Dutch roll motion is adequately suppressed. However, this design arrangement tends to cause spiral instability. Spira instabilit Spiral instability is associated with airplanes that have strong directional stability in comparison with lateral stability. When an airplane susceptible to spiral instability is disturbed from a condition of equilibrium and a sideslip is introduced, the strong directional stability tends to yaw the airplane back into alignment with the relative wind. Due to the yaw back into the relative wind, the outside wing travels faster than the inside wing and, as a result, more lift is generated by the outside wing. The rolling moment increases the angle of bank, which increases the sideslip. The comparatively weak dihedral effect lags in restoring lateral stability and the yaw forces the nose of the airplane down while the angle of bank continues to increase, tightening the spiral. Spiral instability is normally easily overcome by the pilot. However, if left uncorrected, the motion could increase into a tight spiral dive, sometimes referred to as a graveyard spiral. As you can see, even a well-designed airplane may have some undesirable characteristics. Generally, increased dihedral reduces spiral instability while an increased vertical tail surface increases spiral instability. Since Dutch roll is considered less tolerable than spiral instability, designers attempt to minimize the Dutch roll tendency. The compromise results in a small degree of spiral instability which generally is considered acceptable. STALLS The inherent stability of an airplane is particularly important as it relates to the aircraft's ability to recovery from stalls and spins (which can result from aggravated stalls). Familiarization with the causes and effects of stalls is especially important during flight at slow airspeeds, such as during takeoff and landing, where the margin above the stall speed is small. It's important to understand the variables that affect stall development. As indicated earlier in this chapter, a stall will always occur when the maximum lift, or critical angle of attack (CLmax) is exceeded. If an airplane's speed is too slow, the required angle of attack to maintain lift may be exceeded, causing a stall. It's important to note, however, that the airspeed at which an aircraft may be stalled is not fixed. For example, although the extension of flaps increases drag, it also increases the wing's ability to produce lift, thereby reducing the stall speed. Stall speed also can be affected by a number of other factors such as weight and environmental conditions. As aircraft weight increases, a higher angle of attack is required to maintain the same airspeed since some of the lift must be used to support the increased weight. This causes an increase in the aircraft's stall speed. The distribution of weight also affects the stall speed of an aircraft. For example, a 3-36 SECTION B STABILITY forward CG creates a situation which requires the tail to produce more downforce to balance the aircraft. This, in turn, causes the wings to produce more lift than if the CG was located more rearward. So, you can see that a more forward CG also increases stall speed. Any modification to the wing surface also can affect the stall speed of the aircraft. Although manmade high-lift devices can decrease stall speed, the opposite can occur due to natural factors. Snow, ice or frost accumulation on the wing's surface not only changes the shape of the wing, disrupting the airflow, but also increases weight and drag, all of which will increase stall speed. Another environmental factor that can affect stall speed is turbulence. The unpredictable nature of turbulence encounters can significantly and suddenly cause an aircraft to stall at a higher airspeed than the same aircraft in stable conditions. This occurs when a vertical gust changes the direction of the relative wind and abruptly increases the angle of attack. During takeoff and landing operations in gusty conditions, an increase in airspeed usually is necessary in order to maintain a wide margin above stall. TYPES OF STALLS There are three basic types of stalls that will normally be practiced during training to familiarize you with stall recognition and recovery in particular flight regimes. Power-of Power-off stalls stall are practiced to simulate the conditions and aircraft configuration you will most likely encounter during a normal landing approach. Power-on Power-o stalls stall are normally encountered during takeoff, climb-out, and go-arounds when the pilot fails to maintain proper control due to premature flap retraction or excessive nosehigh trim. To help you understand how stalls may occur at higher than normal stall speed, your accelerated stall stalls and show you associated recovery techniques. instructor may demonstrate accelerate Most stalls are practiced while maintaining coordinated flight. However, uncoordinated, or crossedcontrol, inputs can be very dangerous when operating near a stall. One type of stall, sometimes referred to as the crossed-control crossed-contro stal stall is most likely to occur when a pilot tries to compensate for overshooting a runway during a turn from base to final while on landing approach. [Figure 3-44] As the airplane begins to overshoot the runway, the pilot displaces the control wheel to the right in an attempt to return to the extended runway centerline. The pilot adds excessive right rudder to increase the turn rate. The upper wing begins to travel faster and produce more lift than the low wing. The angle of bank increases. The pilot moves the control wheel toward the left to counter the increasing angle of bank. The airplane is now crosscontrolled. The nose begins to drop and the pilot applies back pressure to arrest the rate of descent. The slow moving right wing drops further, increasing angle of attack until it stalls. The airplane rolls to the right due to the continued generation of lift by the left wing. Figure 3-44. If a stall occurs during a skidding turn close to the ground, such as during the turn to final, there may not be sufficient altitude for recovery. 3-37 CHAPTER 3 AERODYNAMIC PRINCIPLES STALL RECOGNITION There are a number of ways to recognize that a stall is imminent. Ideally, you should be able to detect the first signs of an impending stall and make appropriate corrections before it actually occurs. If you have a good understanding of the types of stalls, recognition is much easier. Recovery at the first indication of a stall is quite simple; but, if you allow the stalled condition to progress, recovery becomes more difficult. A typical indication of a stall is a mushy feeling in the flight controls and less control effect as the aircraft's speed decreases. The reduction in control effectiveness is primarily due to reduced airflow over the flight control surfaces. In fixed-pitch propeller airplanes, a loss of revolutions per minute (r.p.m.) may be noticeable as you approach a stall in power-on conditions. Also, a reduction in the sound of air flowing along the fuselage is usually evident. Just before the stall occurs, buffeting, uncontrollable pitching, or vibrations may begin. Finally, your kinesthetic sense (ability to recognize changes in direction or speed) may also provide a warning of decreased speed or the beginning of a sinking feeling. STALL RECOVERY Your primary consideration after a stall occurs should be to regain positive control of the aircraft. If you do not recover promptly by reducing the angle of attack, a secondary stall and/or spin may result. A secondar secondary stal stall is normally caused by poor stall recovery technique, such as attempting flight prior to attaining sufficient flying speed. If you encounter a secondary stall, you should apply normal stall recovery procedures. The following basic guidelines should be used to effect a proper stall recovery. 1. Decrease the angle of attack. Depending on of the type of aircraft, you may find that a different amount of forward pressure on the control wheel is required. Too little forward movement may not be enough to regain lift; too much may impose a negative load on the wing, hindering recovery. 2. Smoothly apply maximum allowable power. If you are not already at maximum allowable power, increase the throttle to minimize altitude loss and increase airspeed. 3. Adjust the power as required. As the airplane recovers, you should maintain coordinated flight while adjusting the power to a normal level. You can usually prevent an accidental stall by knowing when you are most susceptible to a stall and recognizing the indicators of an impending stall. Unless you are practicing stalls and stall recoveries, don't wait for the stall to fully develop — apply stall recovery techniques at the first indication of an impending stall. SPINS The spin is one of the most complex of all flight maneuvers. A spi spin may be defined as an aggravated stall which results in the airplane descending in a helical, or corkscrew, path. Single-engine, normal category airplanes are prohibited from intentional spins. This is indicated by a placard with words such as "No acrobatic maneuvers, including spins, approved." However, during aircraft certification tests, normal category airplanes must demonstrate recovery from a one-turn spin or a three-second spin, whichever takes longer. The recovery must take place within one additional turn with normal control inputs. Since airplanes in the normal category have not been tested for more than oneturn/three second spins, their performance characteristics beyond these limits are unknown. 3-38 SECTION B STABILITY Acrobatic category airplanes must fully recover from fully developed spins within one and one-half additional turns. Certification in this category also requires six turns or three seconds, whichever takes longer, before the recovery control inputs are applied. Utility category airplanes may be tested under the one-turn (normal) criteria or they may satisfy the six-turn (acrobatic) spin requirements. However, spins in utility category airplanes may be approved only with specific loading, such as a reduced weight and with a forward CG position. It is extremely important for you to understand all of the operating limitations for your airplane. Applicable limitations are placarded in the aircraft and/or included in the POH. PRIMARY CAUSES A stalled aircraft is a prerequisite for a spin. However, a properly executed stall is essentially a coordinated maneuver where both wings are equally or nearly equally stalled. In contrast, a spin is an uncoordinated maneuver with the wings unequally stalled. [Figure 3-45] In this case, the wing that is more completely stalled will often drop before the other, and the nose of the aircraft will yaw in the direction of the low wing. Increasing CL and CD Figure 3-45. The relative coefficients of lift and drag for each wing during a spin are depicted in this graph. Note that the upgoing wing experiences more lift (or a lesser stalled condition) and less drag. The opposite wing is forced down and back due to less lift (more stall) and increased drag. Increasing Angle of Attack Typically, the cause of an inadvertent spin is exceeding the critical angle of attack while performing an uncoordinated maneuver. The lack of coordination is normally caused by either too much or not enough rudder control for the amount of aileron being used. The result is a crossed-control condition. If you do not initiate the stall recovery promptly, the airplane is more likely to enter a full stall that To enter a spin, an airplane must first be may develop into a spin. The spin that occurs stalled. Although both wings are in a stalled from crossed-controlling usually results in rotacondition during a spin, one wing is stalled tion in the direction of the rudder being applied, more than the other. regardless of which wing is raised. In a skidding turn, where both aileron and rudder are applied in the same direction, rotation will be in that direction. However, in a slipping turn, where opposite aileron is held against the rudder, the resultant spin will usually occur in the direction opposite the aileron that is being applied. 3-39 CHAPTER 3 AERODYNAMIC PRINCIPLES Coordinated use of the flight controls is important, especially during flight at slow airspeeds. Although most pilots are able to maintain coordination of the flight controls during routine maneuvers, this ability often deteriorates when distractions occur and their attention is divided between important tasks. Distractions that have caused problems include preoccupation with situations inside or outside the cockpit, maneuvering to avoid other aircraft, and maneuvering to clear obstacles during takeoffs, climbs, approaches, or landings. Because of this, you will be required to learn how to recognize and cope with these distractions by practicing "flight at slow airspeeds with realistic distractions" during your flight training. In addition, although you are not required to demonstrate flight proficiency in spins during private pilot training, you will need to exhibit knowledge of the situations where unintentional spins may occur, as well as the general spin recognition and recovery procedures for the airplane you use for your practical test. TYPES OF SPINS In general, there are three main types of spins. The most common is the upright, or erec erectt spin which is characterized by a slightly nose down rolling and yawing motion in the spi same direction. In an inverte inverted spin, spin the aircraft is spinning upside down with yaw and Figure 3-46. A spin may be characterized as erect, inverted, or flat, depending on the roll and yaw motion of the aircraft. ERECT SPIN Roll and Yaw in Same Direction 3-40 INVERTED SPIN FLAT SPIN Roll and Yaw in Opposite Directions Yaw Only SECTION B STABILITY STALL /SPIN ACCIDENT PREVENTION During a recent four-year period, 171 stall/spin accidents occurred, 73 percent of which were fatal. By far the majority of the accidents were a result of unintentional stalls and spins that occurred close to the ground. Are these accidents preventable? Consider that human error has been cited as a contributing factor in 90 to 95 percent of all stall/spin accidents, and the answer is a resounding yes. A review of related accident reports suggest that adhering to the following guidelines can help you avoid an accidental stall/spin. Pay attention to aircraft loading. An aircraft with an aft CG is more prone to stall/spin entry. Do not takeoff with snow, ice, or frost on the wings. If an emergency that requires a forced landing occurs immediately after takeoff, don't attempt to return to the runway. Select a suitable landing site straight ahead or slightly off to the side. Maintain coordinated flight as much as possible. Particularly avoid skidding turns near the ground. Use a somewhat higher than normal airspeed during takeoffs and landings in gusty winds. Always concentrate on flying the aircraft and avoid prolonged distractions. roll occurring in opposite directions. Inverted spins are most likely to occur during aerobatic maneuvers. The third type of spin can be the most deadly. In a fla flat spin spin, the aircraft simply yaws about its vertical axis with a pitch attitude approximately level with the horizon. Although it sounds fairly benign, recovery is usually very difficult or impossible except in specialized aerobatic aircraft. Most general aviation aircraft are designed to prevent entry into flat spins provided the loading and CG location are within approved limits. [Figure 3-46] WEIGHT AND BALANCE CONSIDERATIONS Even minor weight and balance changes can affect an aircraft's spin characteristics. Heavier weights generally result in slow spin rates initially; but, as the spin progresses, heavier weights tend to cause an increasing spin rate and longer recovery time. Distribution of weight is even more significant. Forward center of gravity positions usually inhibit the high angles of attack necessary for a stall. Thus, an airplane with a forward CG tends to be more stall and spin resistant than an aircraft with an aft CG. In addition, spins with aft CG positions are more likely to become flat. In a training airplane, the addition of a back seat passenger or a single suitcase to an aft baggage compartment can affect the CG enough to change the characteristics of a spin. In addition, any concentration of weight, or unbalanced weight distribution, that is particularly far from the CG is undesirable. This type of loading may occur with tip tanks or outboard wing tanks. If the fuel in these tanks becomes unbalanced, an asymmetrical condition exists. The worst asymmetric condition is full fuel in the wing on the outside of the spin and no fuel in the tanks on the inside of the turn. Once the spin is developed, the momentum (inertial force) makes recovery unlikely. 3-41 CHAPTER 3 AERODYNAMIC PRINCIPLES INCIPIENT SPIN Lasts about 4 to 6 seconds in light aircraft. Approximately 2 turns Less Stalled Chord Line Relative Wind Less Angle of Attack FULLY DEVELOPED SPIN Airspeed, vertical speed, and rate of rotation are stabilized. Small, training aircraft lose approximately 500 feet per 3 second turn. More Drag More Stalled Relative Wind Chord Line Greater Angle of Attack Figure 3-47. The incipient spin usually occurs rapidly in light airplanes (about 4 to 6 seconds) and consists of approximately the first two turns. At about the half-turn point, the airplane is pointed almost straight down but the angle of attack is usually above that of the stall because of the inclined flight path. As the one-turn point approaches, the nose may come back up and the angle of attack continues to increase. As the airplane continues to rotate into the second turn, the flight path becomes more nearly vertical, and the pitching, rolling, and yawing motions become more stabilized. This is the beginning of the fully developed spin. The last phase, recovery, occurs when anti-spin forces overcome pro-spin forces. During recovery, the angle of attack on both wings decreases below the critical angle and the rotation rate slows. This phase can range from one-quarter of a turn to several turns. RECOVERY Wings regain lift. Training aircraft usually recover in about 1/4 to 1/2 of a turn after anti-spin inputs are applied. SPIN PHASES Many different terms may be used to describe a spin. In light, training airplanes, a complete spin maneuver consists of three phases — incipient, fully developed, and recovery. The incipien incipient spi spin is that portion of a spin from the time the airplane stalls and rotation starts until the spin is fully developed. The full fully developed develope spi spin occurs after the incipient stage when the angular rotation rates, airspeed, and vertical speed are stabilized from turn to turn and the flight path is close to vertical. For this reason, the fully developed stage is often referred to as the steady-state portion of the spin. Spi Spin recover recovery is the final stage of the spin and is the phase when the application of anti-spin forces result in a slowing and/or eventual cessation of rotation coupled with a decrease in angle of attack below C Lmax. [Figure 3-47] 3-42 SECTION STABILITY B SPIN RECOVERY While some characteristics of a spin are predictable, every airplane spins differently. In addition, the same airplane's spin behavior changes with variations in configuration, loading, and several other factors. Therefore, it's easy to understand why spin recovery techniques vary for different aircraft and why you must follow the recovery procedures outlined in the POH for your airplane. The following is a general recovery procedure for erect spins, but it should not be applied arbitrarily without regard for the manufacturer's recommendations. 1. Move the throttle to idle. This will eliminate thrust and minimize the loss of altitude. 2. Neutralize the ailerons. 3. Determine the direction of rotation. This is most easily and accurately accomplished by referencing the turn coordinator. 4. Apply full opposite rudder. Ensure that you apply the rudder opposite the direction of rotation. 5. Briskly apply elevator (or stabilator) forward to approximately the neutral position. Some aircraft require merely a relaxation of back pressure; others require full forward elevator (or stabilator) pressure. 6. As rotation stops (indicating the stall has been broken), neutralize the rudder. If you don't neutralize the rudder when rotation stops, you could enter a spin in the opposite direction. 7. Gradually apply aft elevator (or stabilator) to return to level flight. Applying the elevator too quickly may result in a secondary stall, and possibly another spin. Also, make sure you adhere to aircraft airspeed and load limits during the recovery from the dive. Normally, the recovery from an incipient spin requires less time (and altitude) than the recovery from a fully developed spin. As a rule of thumb, small aircraft authorized for spins will lose approximately 500 feet of altitude per each 3-second turn and recover in 1/4 to 1/2 of a turn. More altitude will be lost and recoveries may be prolonged at higher altitudes due to the less dense air. It should be clear that you, as an applicant for a private pilot certificate, are not required to demonstrate flight proficiency in spin entries or spin recovery techniques. Even though your flight instructor may demonstrate a spin at some point during your training, you should never intentionally enter a spin, even after you become certificated as a private pilot, unless you obtain additional training from an experienced instructor. The emphasis in stall/spin training for private pilots is awareness of conditions that could lead to an unintentional stall or spin and to provide you with some general recovery procedures. SUMMARY CHECKLIST Most training aircraft are designed to display both positive static and positive dynamic stability. All aircraft movement takes place around the longitudinal, lateral, and vertical axes, all of which pass through the center of gravity. 3-43 CHAPTER 3 AERODYNAMIC PRINCIPLES Longitudinal stability relates to movement about the airplane's lateral axis. Longitudinal stability is influenced by the relationship between the center of pressure and the center of gravity as well as the effects of power changes and the design of the horizontal stabilizer. Stability around the aircraft's longitudinal axis is referred to as lateral stability. Wing dihedral, sweepback, keel effect, and weight distribution are design features that affect an airplane's lateral stability. Directional stability, or stability about the vertical axis, of most aircraft is maintained by the vertical tail. Dutch roll is most likely to occur on aircraft with weak directional stability and strong lateral stability. Aircraft with strong directional stability and weak lateral stability are susceptible to spiral instability. A stall will always occur when the critical angle of attack, or CLmax, is exceeded. This can occur at any airspeed and in any configuration or attitude. A spin will not develop unless both wings are stalled. A normal, erect spin results in the airplane entering a nose-low autorotative descent with one wing stalled more than the other. KEY TERMS 3-44 Stability Sweepback Positive Static Stability Keel Effect Positive Dynamic Stability Directional Stability Maneuverability Dutch Roll Controllability Spiral Instability Longitudinal Axis Power-Off Stalls Lateral Axis Power-On Stalls Vertical Axis Accelerated Stalls Center of Gravity (CG) Crossed-Control Stall Longitudinal Stability Secondary Stall Center of Pressure Spin Center of Lift Erect Spin CG Range Inverted Spin Tail-Down Force Flat Spin Thrustline Incipient Spin Lateral Stability Fully Developed Spin Dihedral Spin Recovery SECTION B STABILITY QUESTIONS 1. Referring to the airplane diagram below, identify the three axes of flight and the type of movement associated with each axis. Match the following control surface with the associated aircraft movement. 2. Roll movement A. Elevator (or stabilator) 3. Pitch movement B. Ailerons 4. Yaw movement C. Rudder 5. In relation to the center of gravity, in which direction would the center of pressure normally move as angle of attack is increased on a cambered wing? 6. What factors can affect the longitudinal stability of an airplane at high power settings and low airspeed? 7. Why are high wing aircraft normally designed with less dihedral than low wing aircraft? 8. Does the propwash resulting from high power settings increase or decrease the contribution of wing dihedral to the lateral stability of an airplane? 9. An aircraft with strong directional stability and weak lateral stability is prone to what type of undesirable side effect? 10. True/False. When landing in gusty winds, airspeed should be increased above normal to help guard against a stall. 11. List the basic guidelines for stall recovery. 12. Recall the general procedures for recovery from an erect spin. 3-45 SECTION C AERODYNAMICS OF MANEUVERING FLIGHT The extent to which an airplane can perform a variety of maneuvers is primarily a matter of design and a measure of its overall performance. Although aircraft design and performance may differ, the aerodynamic forces acting on any maneuvering aircraft are essentially the same. Understanding the aerodynamics of maneuvering flight can help you perform precise maneuvers while maintaining your airplane within its design limitations. CLIMBING FLIGHT The aerodynamic forces acting on an airplane established in a stabilized climb are in equilibrium; however, since the flight path is inclined, the relationship between these forces is altered. For example, the total force of weight no longer acts perpendicular to the flight path, but is comprised of two components. Although one component still acts 90° to the flight path, a rearward component of weight acts in the same direction as drag, opposing thrust. [Figure 3-48] A transition from level flight into a climb normally combines a change in pitch attitude with an increase in power. If you attempt to climb just by pulling back on the control wheel to raise the nose of the airplane, momentum will cause a brief increase in altitude, but airspeed will soon decrease. The amount of thrust generated by the propeller for cruising flight at a given airspeed is not enough to maintain the same airspeed in a climb. Excess thrust, not excess lift, is necessary for a sustained climb. In fact, as the angle of climb steepens, thrust will not only oppose drag, but also will increasingly Rearward Component of Weight Figure 3-48. In a climb, the rearward component of weight is opposed by thrust, while the component of weight acting perpendicular to the flight path is supported by lift. Component of Weight Acting Perpendicular to Flight Path AERODYNAMICS OF MANEUVERING FLIGHT SECTION C When Is No Lift Required To Fly? During a normal sustained climb, a component of weight is opposed by lift. However, in a true sustained vertical climb, such as the one performed by the General Dynamics F-16C pictured in figure A, the wings supply no vertical lift, and thrust is the only force opposing weight. Depending on an aircraft's thrust-to-weight ratio, a sustained vertical climb may be m