2: Aerodynamics and the Principles of Flight

The Axes of an Aircraft

Airfoils and Aerodynamic Shapes

The Aerodynamic Forces

The Balance of Forces

Maneuverability, Controllability, and Stability

Aerodynamic Efficiency

Load Factors and Maneuvering Speed

Stalls and Spins

Wing Shapes

High-Lift Devices

Ground Effect

Principles of Rotorcraft Flight

Gyroplane Aerodynamics

Glider Aerodynamics

Weight-Shift Control Aerodynamics

The Axes of an Aircraft

A fixed-wing aircraft has three axes which are perpendicular to each other. These axes intersect at the center of gravity (CG) of the aircraft.

Airfoils and Aerodynamic Shapes

An airfoil is a specially shaped surface, designed to produce aerodynamic lift as air flows over it. For an airfoil to produce lift, wind must strike it at an angle, called the angle of attack. See Figure 2-1.

Figure 2-1. Angle of attack

When air, which is a viscous fluid, strikes an airfoil, it adheres to the surface and splits, some passing over the top and the rest passing below. The top of the airfoil drops away from the air flowing over it, and the air speeds up, just as water speeds up as it flows down a hill. The lower surface of the airfoil rises into the path of the air flowing below it, and the air slows down just as water slows down when it is forced over a rise.

Bernoulli’s principle explains the relationship between pressure and velocity in air flowing over the wing. If no energy is added to or taken from the air as it flows over an airfoil, the pressure will decrease as the velocity increases and will increase as the velocity decreases. When the air flowing over the top of the wing speeds up, its pressure drops and air above it is pulled down to fill this low pressure. Since for every action there is an equal and opposite reaction, the force with which the wing pulls the air down is exactly balanced by a force which pulls the wing up. The air below the wing is slowed down, and its pressure increases, forcing air away from it. The same force that pushes the air away pushes the wing up. The result of the air being pulled down to the top and pushed away from the bottom causes the air flowing over the wing to be forced down at an angle called the downwash angle. The weight of the air forced down is exactly the same as the lift causing the airplane to be forced up. It is determined by five factors:

  1. The cross-sectional shape of the airfoil.
  2. The surface area of airfoil.
  3. The angle the air strikes the airfoil (angle of attack).
  4. The speed of the air moving over the airfoil.
  5. The density of the air.

Some terms that are associated with airfoils:

Chord line—A line that passes through an airfoil from the leading edge to the trailing edge.

Leading edge—The front of an airfoil, the portion that passes through the air first.

Trailing edge—The rear of an airfoil, the portion that passes through the air last.

Center of pressure—The point on the chord line of an airfoil at which all the aerodynamic forces are thought to be concentrated.

Relative wind—The direction from which the wind is moving when it reaches the airfoil.

Downwash angle—The angle the air is flowing when it leaves the airfoil.

Angle of attack—The angle between the chordline of an airfoil and the relative wind. The angle of attack varies in flight.

Angle of incidence—The angle between the chordline of an airfoil and the longitudinal axis of an aircraft. The angle of incidence is fixed.

Upper camber—The contour of the top surface of an airfoil.

Lower camber—The contour of the bottom surface of an airfoil.

Mean camber—A line drawn from the leading edge to the trailing edge of an airfoil, equidistant at all points from the upper and lower cambers. The shape of the mean camber line determines the aerodynamic characteristics of a wing.

See Figure 2-2.

Figure 2-2. Airfoil nomenclature

The angle of attack directly controls the distribution of the pressures above and below the wing. As the angle of attack is increased, the pressure differential becomes greater until the stalling angle of attack is reached. At this time, the smooth airflow over the top of the wing breaks away and the air becomes turbulent. This lowers the pressure differential.

An asymmetrical airfoil is one in which the upper camber and lower cambers are different, and a symmetrical airfoil is one whose upper and lower cambers are the same on either side of the chord line. Because of the shape of an asymmetrical airfoil, air flowing over the wing directly in line with the chord line (at zero angle of attack) will still produce a pressure on the upper surface that is lower than that of the surrounding air.

Air flowing over a symmetrical airfoil at zero angle of attack will not produce a pressure differential across the airfoil. The location of the center of pressure in a symmetrical airfoil remains relatively constant as the angle of attack changes, but in an asymmetrical airfoil the center of pressure moves forward as the angle of attack increases and rearward as the angle of attack decreases. The changes in the center of pressure affect the aerodynamic balance and controllability of the aircraft. See Figure 2-3.

Figure 2-3. Airfoil shapes

The Aerodynamic Forces

A fixed-wing aircraft in flight is acted on by four basic forces:

Lift—an aerodynamic force produced by an airfoil which acts perpendicular to the relative wind. Lift is concentrated at the center of pressure.

Weight—acts vertically downward toward the center of the earth, and is concentrated at the CG.

Thrust—produced by the engine or engine-propeller combination. It acts forward parallel to the axis of the engine.

Drag—a combination of forces that acts rearward, parallel to the relative wind. Total drag is made up of induced drag, caused by the production of lift, and parasite drag, caused by the friction of the air passing over the aircraft surface.

See Figure 2-4.

Figure 2-4. The four forces in flight

The two lift variables the pilot controls are airspeed and angle of attack. To maintain level flight at a lower airspeed, the pilot must increase the angle of attack. To prevent the aircraft climbing when the speed is increased, the angle of attack must be reduced.

The amount of lift increases as the angle of attack is increased until the angle of attack becomes so great that the air flowing over the surface breaks away and burbles. This is the critical angle of attack.

The actual weight of an aircraft is caused by gravity, and it always acts directly toward the center of the earth. The tail load of a normal aircraft acts downward, and must be overcome by lift in the same way as weight. The apparent weight of an aircraft is the increase in weight caused by acceleration. In a coordinated steep turn with a bank angle of 60°, there is an acceleration force of 2 Gs. A 3,600-pound airplane in a 2 G turn has an apparent weight of 7,200 pounds. It is the apparent weight that causes an airplane to stall at a higher than normal airspeed when in a high-G turn.

There are two basic types of drag: induced and parasite.

Induced drag is caused by the production of lift and it is affected by the same factors which affect lift.

Induced drag is greatly affected by the airspeed. It varies inversely as the square of the airspeed. If the airspeed is reduced to one-half its original value, the induced drag increases four times. In order for an aircraft to fly slowly, its angle of attack must be high, and a high angle of attack produces a large amount of induced drag.

Parasite drag is caused by the friction of the air flowing over the aircraft, and it is not related to the production of lift. Parasite drag increases proportional to the square of the airspeed.

Parasite drag can be further classified into:

Form drag—caused by the frontal area of the aircraft.

Profile drag—caused by the viscous nature of the air passing over the aircraft’s surfaces.

Interference drag—caused by interference of the airflow between adjacent parts of the aircraft.

Because induced drag decreases as airspeed increases and parasite drag increases as airspeed increases, there is an airspeed and an angle of attack at which the induced and parasite drag are the same. This is the airspeed at which the total drag is the least and the lift-to-drag ratio is maximum (L/DMAX). At airspeeds above that for L/DMAX, the total drag increases due to the increase in parasite drag, and at airspeeds below that for L/DMAX, the total drag increases because of the increase in induced drag. See Figure 2-5.

Figure 2-5. Drag vs. airspeed

Thrust is a force which imparts a change in the velocity of a mass. Thrust, produced by a turbojet engine or an engine-propeller combination, provides the force used to move an aircraft through the air. Thrust acts forward, in line with the propeller shaft. If the engine is mounted well below or well above the longitudinal axis of the aircraft, a change in power will produce a rotational force about the aircraft’s lateral axis. If the thrust line is below the longitudinal axis, an increase in power will produce a nose-up rotational force.

The Balance of Forces

When an airplane, weight-shift control, or powered parachute is flying at a steady speed, is not rising or descending, and is not speeding up or slowing down, all of the forces are in balance. The upward force of lift is exactly the same as the downward forces of weight and the aerodynamic load on the tail. Thrust is exactly the same as drag.

Some form of energy must be expended to cause an aircraft to rise. Chemical energy is converted into mechanical energy in the engine; this in turn is converted into thrust to pull the aircraft through the air so aerodynamic lift can be produced. When the engine power is increased in an aircraft trimmed for straight-and-level unaccelerated flight, these things happen:

  1. The additional power produces more thrust.
  2. The additional thrust causes the aircraft to begin to accelerate. For an airplane, this increases the downward tail load which increases the angle of attack. The airplane begins to increase its altitude.
  3. The increased angle of attack increases the drag until the new drag force balances the increased thrust, and the acceleration stops with the airplane flying at the same airspeed it had before the power was increased.
  4. The upward force of lift is equal to the combined downward force of weight and tail load, and the airplane climbs at a steady speed.

During a steady climb, the rate of climb for an airplane, weight-shift control, and powered parachute is determined by the difference between the power available and the power required. The more excess power available, the greater the rate of climb. The angle of climb is determined by the difference between the available thrust and the drag. The greater the thrust, the greater the angle of climb.

Since an aircraft is held at altitude by an expenditure of energy, it will descend if the energy being expended is decreased. If the power is slightly decreased in an airplane cruising at altitude in straight-and-level unaccelerated flight, these things will happen:

  1. The reduction in power decreases the thrust.
  2. The decreased thrust causes a momentary deceleration. For an airplane, the upward lift and downward tail load both decrease and the nose drops, and the airplane begins to descend and accelerate.
  3. As the airplane descends, the forward vector of the descent angle acts as thrust to make up for the propeller thrust reduction.
  4. The airspeed builds up enough to increase the tail load, and the nose rises enough to hold the air-speed the same as it was before the power was reduced. The drag is now the same as the engine thrust plus the thrust vector from the descent angle, and all acceleration stops.
  5. The upward force of lift is now equal to the combined downward force of weight and tail load, and the airplane descends at a constant power and constant airspeed.

The climbing and descending of weight-shift control and powered parachute is similar, but the trim generally maintains the aircraft at a constant airspeed with smooth and gradual throttle advances and reductions in power.

An aircraft turns when the lift vector is tilted and the horizontal component of the lift pulls the nose around in a curved path. In straight-and-level flight, lift acts upward and balances the weight of the aircraft. When the airplane is banked, the lift vector tilts and the vertical component is no longer equal to the weight, and the aircraft begins to descend. To complicate this, the centrifugal force caused by the turn increases the apparent weight of the aircraft. To prevent the aircraft descending in this turn, the tilted lift vector must be increased enough to equal the resultant of the weight and the centrifugal force. This is done by increasing the angle of attack. See Figure 2-6.

Figure 2-6. Forces in a turn

All aircraft turn by banking, but an airplane rudder is still an important flight control for the airplane. When turning an airplane to the right, the left aileron is lowered and the right aileron is raised. The lift and induced drag on the left wing both increase at the same time the lift and induced drag on the right wing decrease. The increased induced drag causes the nose to momentarily yaw to the left before the wing rises enough to bank the aircraft and produce a horizontal component of the lift to pull the nose around to the right. This momentary yawing to the left is called adverse yaw. Adverse yaw is minimized by differential aileron movement. The aileron moving upward travels farther than the one moving downward. This causes the upward-moving aileron to produce enough parasite drag to counter the induced drag produced by the downward-moving aileron. When rolling out of a steep-banked turn, the slower airspeed, higher angle of attack, and higher wing loading require a greater aileron deflection than when rolling into the turn. This produces adverse yaw, but this time the yaw is in the direction of the turn, and it requires more rudder pressure to counteract it.

The weight-shift control has a slight amount of adverse yaw when a bank is started, but the nose angle quickly stabilizes the wing to track directly into the relative wind for a coordinated turn.

A powered parachute is designed to track directly into the wind with no noticeable adverse yaw while banking.

The rate of turn at any given airspeed depends on the amount of the sideward force causing the turn, that is, the horizontal lift component. The horizontal lift component varies in proportion to the amount of bank. The rate of turn at a given airspeed increases as the angle of bank is increased. On the other hand, when a turn is made at a higher airspeed at a given bank angle, the inertia is greater, and the centrifugal force created by the turn becomes greater, causing the turning rate to become slower. Therefore, at a given angle of bank, a higher airspeed will make the radius of the turn larger because the airplane will be turning at a slower rate.

Maneuverability, Controllability, and Stability

Maneuverability is the quality of an aircraft that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers. Controllability is the capability of an aircraft to respond to the pilot’s control, especially with regard to flight path and attitude. Stability is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium, and to return or continue on the original flight path. An aircraft can have two basic types of stability: static and dynamic, and three conditions of each type of stability: positive, neutral, and negative.

Positive static stability is the condition of stability in which restorative forces are set up that will tend to return the aircraft to its original condition anytime it is disturbed from a condition of straight-and-level flight. If an aircraft has negative static stability, anytime it is disturbed from a condition of straight-and-level flight, forces are set up that will tend to cause it to depart further from its original condition. Negative static stability is a highly undesirable characteristic as it can cause loss of control. An aircraft with neutral static stability produces neither forces that tend to return it to its original condition, nor cause it to depart further from this condition. See Figure 2-7.

Figure 2-7. Static stability

Positive dynamic stability is a condition in which the forces of static stability decrease with time. Positive dynamic stability is desirable. Negative dynamic stability causes the forces of static stability to increase with time. Negative dynamic stability is undesirable. Neutral dynamic stability causes an aircraft to hunt back and forth around a condition of straight-and-level flight, with the corrections getting neither larger or smaller. Neutral dynamic stability is also undesirable. See Figure 2-8.

Figure 2-8. Static and dynamic stability

An aircraft is longitudinally stable if it returns to a condition of level flight after the control wheel is momentarily moved forward and then released. The location of the center of pressure relative to the center of gravity determines the longitudinal stability, and thus the controllability of an aircraft. An aircraft is given positive longitudinal stability by locating the center of gravity ahead of the center of pressure, and balancing this nose-down moment with a nose-up moment caused by the downward aerodynamic tail load. Negative longitudinal static stability would result if the center of pressure were forward of the center of gravity, the aircraft would have a tendency to nose up and enter a stalled condition. The amount of downward tail load is determined by the airspeed. When the nose momentarily drops, the airspeed increases and the tail load increases enough to return the nose to level flight. When the nose momentarily rises, the airspeed decreases and the tail load decreases enough for the nose to drop back to level flight attitude.

Phugoid oscillation is a long-period oscillation in which the pitch attitude, airspeed, and altitude vary, but the angle of attack remains relatively constant. It is a gradual interchange of potential and kinetic energy about some equilibrium airspeed and altitude. An aircraft experiencing longitudinal phugoid oscillation is demonstrating positive static stability, and it is easily controlled by the pilot.

An aircraft will return to level flight after a wing drops if it has positive lateral stability. The wings of most aircraft have a positive dihedral angle. This is the angle produced by the wing tips being higher than the wing roots. If the left wing drops in flight, the aircraft will momentarily begin to slip to the left, and the angle of attack of the left wing will increase and the angle of attack of the right wing will decrease. The increased angle of attack will cause the left wing to rise back to level-flight attitude.

Directional stability, which is the tendency of the nose of an aircraft to turn into the relative wind, is achieved by the vertical area of the fuselage and vertical tail surfaces behind the center of gravity. If a wing drops and the aircraft begins to slip to the side, the directional stability will cause the nose to yaw into the relative wind.

Dihedral effect rights an aircraft when a wing drops, and directional stability causes the nose to yaw into the direction of the low wing. These two forces oppose each other, and an aircraft with a strong static directional stability and weak dihedral effect will have spiral instability. When the wing drops, the nose will yaw toward the low wing and the airplane will begin to turn. The increased speed of the wing on the outside of the turn will increase the angle of bank, and the strong directional stability will force the nose to a low pitch angle. This will cause the aircraft to enter a descending spiral.

Aerodynamic Efficiency

The efficiency of a wing is demonstrated by the relationship between the lift and the drag produced at any given angle of attack. The angle of attack chart (see FAA Figure 19) shows the important relationships between the coefficient of lift CL, the coefficient of drag CD, and the lift over drag ratio L/D:

  1. The CL increases smoothly with the angle of attack until the stalling angle of attack is reached at 20°. Beyond this angle of attack the lift drops off totally.
  2. The CD increases steadily with the angle of attack.
  3. The L/D curve increases with the angle of attack until the L/DMAX is reached; then it decreases. The L/DMAX is the most efficient angle of attack to fly. At L/DMAX the airfoil produces the greatest lift for the least drag, and this is the angle of attack at which the aircraft will travel the maximum horizontal distance for each foot of altitude lost. In FAA Figure 19, L/DMAX is reached at an angle of attack of 6°.
  4. At angles of attack below that which provides L/DMAX, the majority of the drag is parasite drag, and at angles of attack above that which produces L/DMAX, the majority of the drag is induced drag. Flight at any airspeed other than that which produces L/DMAX will cause the total drag for the lift to increase.

The drag chart (see FAA Figure 20) shows the relationship between parasite drag, induced drag, and total drag for any airspeed, and thus at any angle of attack. Velocity increases from left to right, but angle of attack increases from right to left. Induced drag increases as the angle of attack increases, and parasite drag increases as the angle of attack decreases. The least total drag is produced at the airspeed at which the induced and parasite drag are equal, and this is the airspeed at which the airplane has the highest lift over drag ratio, L/DMAX, and it is the airspeed at which the aircraft will have its maximum glide range in still air. See Figure 2-9.

Figure 2-9. Drag vs. airspeed

Load Factors and Maneuvering Speed

Load factor is the ratio of the amount of load imposed on an aircraft structure to the weight of the structure itself. Load factors imposed on an aircraft in flight are measured by accelerometers and are expressed in units of G. A 1 G load factor is one in which the load on the structure is equal to the weight of the structure. An aircraft resting on the ground imposes a 1 G load on its landing gear.

When an aircraft is maneuvered in flight, the inertia forces increase its apparent weight, and the structure must support a weight greater than that of the airplane at rest. If an accelerometer in an airplane weighing 2,675 pounds indicates 2.5 in a steep turn, the wings are supporting a load of 2.5 × 2,675, or 6,687.5 pounds. An aircraft stalls when its wing reaches a critical angle of attack, regardless of its airspeed.

Load factor increases in a coordinated banked turn because of the centrifugal force acting on the aircraft as it is pulled around in the turn. The increased load factor requires a higher angle of attack to provide the needed lift, and as a result, there is a correlation between the angle of bank, the load factor, and the increase in stall speed.

When an airplane is flying in a coordinated 60°-banked level turn, the load factor is 2, and the stall speed has increased by approximately 40%. If the airplane normally stalls at 60 knots, it will stall at 84 knots in a coordinated 60°-banked turn. This is the reason it is so important to teach students to recognize the onset of an accelerated stall.

By using FAA Figure 18, it is found that an airplane with a normal stalling speed of 62 knots can be forced into an accelerated stall at 124 knots (an increase of 100%). This will occur at a bank angle of 75°, and at this bank angle the load factor is 4 Gs.

The design maneuvering speed (VA) of an aircraft is the maximum airspeed at which an aircraft may be safely stalled. This airspeed is not marked on the airspeed indicator, but it is found in the Pilot’s Operating Handbook (POH). For aircraft that do not have a POH, this speed is approximately 1.7 times the normal stalling speed. When flying into severe turbulence, the pilot should reduce the airspeed to the maneuvering speed.

Airplanes certificated in the normal category can tolerate a load factor of +3.8 and -1.52. Airplanes certificated in the utility category can tolerate load factors of +4.4 and -1.76.

A load factor chart, or V-g diagram, (see FAA Figure 17) shows the relationship between airspeed and load factor for all types of operations:

  1. Point A is the normal wings-level stall speed of this airplane. This is the airspeed at which the 1 G horizontal line crosses the dashed maximum-lift capability curve.
  2. Point C is the design maneuvering airspeed, VA. This is the highest airspeed the airplane can be stalled without exceeding the 3.8 G load factor.
  3. Point D is the maximum structural cruising airspeed, VNO. This is marked on the airspeed indicator as the upper limit of the green arc or the bottom of the yellow arc.
  4. Point E is the never-exceed airspeed, VNE. This is marked on an airspeed indicator with a red radial line.
  5. The horizontal dashed line between points A and C show the positive limit load factor, which for this airplane is 3.8 Gs.
  6. The red-shaded area all around the outside of the V-g diagram shows the airspeeds and load factors that will cause structural damage.
  7. The darker shaded area is the caution speed range, and is the limiting speed for flight in which abrupt maneuvers may be executed, or in which turbulent air may be encountered.
  8. The straight lines marked -30 fps, -15 fps, +15 fps, and +30 fps are the lines showing the load factors produced at the various indicated airspeeds by sharp-edged wind gusts of 30 and 15 feet per second positive and 15 and 30 feet per second negative.

Stalls and Spins

A stall is a flight condition that occurs when the angle of attack becomes so high that the air flowing over the top of the wing can no longer flow smoothly and it breaks away. This disturbs the area of low pressure above the airfoil and causes the wing to lose lift. A wing does not stall because of low airspeed or high gross weight, but when a specific angle of attack is reached, regardless of the speed or weight. This is the reason it is important to teach accelerated stalls and approach to landing stalls.

By using the load factor/stall speed chart in FAA Figure 18, we can determine the percent increase in stall speed and the load factor produced by any degree of bank in a coordinated turn. To use this chart, follow the vertical line upward from 60° bank angle until it intersects the load factor curve, and then project a line horizontally to the left until it intersects the load factor index at 2 G. Follow the vertical line upward from the 60° bank angle until it intersects the stall speed increase curve, then project a line horizontally to the left until it intersects the percent increase in stall speed index at 40 percent. A 60° banked coordinated turn will produce a 2 G load factor, and it will cause a 40 percent increase in the aircraft’s stall speed.

A spin is a flight condition in which one wing is more stalled than the other. A spin is entered by increasing the angle of attack until the aircraft stalls. At this instant, the rudder is deflected and the aircraft yaws about its vertical axis. Yawing causes the outside wing to speed up and come out of the stall a little, while the inside wing is driven deeper into the stall. The lift produced by the outside wing has a forward component which causes the aircraft to rotate as it descends in a nose-down attitude at a slow airspeed.

Wing Shapes

It is desirable for a wing to stall in the root area before it stalls near the tips. This provides a warning of an impending stall and allows the ailerons to be effective during the stall. A rectangular wing stalls in the root area first. The stall begins near the tip and progresses inboard on a highly tapered wing or a sweptback wing. See Figure 2-10.

Figure 2-10. Stall progression patterns based on different wing shapes.

An elliptical wing produces the best lift coefficient and a minimum of induced drag for a given aspect ratio, but an elliptical wing does not have aileron effectiveness during the stall. The main disadvantage of an elliptical wing for general aviation aircraft is its high production cost.

The wings of most modern high-performance airplanes are sweptback wings. Sweepback increases the critical Mach number, the airspeed at which there is the first indication of air flowing over the surface at the speed of sound. By increasing the critical Mach number, sweepback delays the onset of compressibility effects. One major disadvantage of a sweptback wing is that it increases the dutch roll tendencies of the airplane. Dutch roll is a coupled lateral-directional oscillation which is usually dynamically stable, but it is objectionable in an airplane because of its oscillatory nature. Dutch roll is normally prevented by the use of yaw dampers in the flight control system.

Aspect ratio is the ratio of the span of an aircraft wing to its mean, or average, chord. Generally speaking, the higher the aspect ratio, the more efficient the wing. But for practical purposes, structural considerations normally limit the aspect ratio for all except high-performance sailplanes. A high aspect ratio wing has a low stall speed, and at a constant air velocity and high angle of attack, it has less drag than a low aspect ratio wing.

Considering the wings described in FAA Figure 21, we see the following relationships:

Aircraft 1 has an aspect ratio of 40/6 or 6.7.

Aircraft 2 has an aspect ratio of 35/5 or 7.

Aircraft 3 has an aspect ratio of 48/6 or 8.

Aircraft 3 has the highest aspect ratio of those in FAA Figure 21. If we consider only the aspect ratio, this wing will generate the greatest lift of all those given here.

Aircraft 4 has an aspect ratio of 30/6 or 5.

Aircraft 4 has the lowest aspect ratio of those given here. If we consider only the aspect ratio, this wing will have the greatest drag of all those given in the FAA figure.

High-Lift Devices

An aircraft wing stalls at a specific angle of attack, but the stalling angle of attack can be increased by the use of high-lift devices such as flaps, slots, and slats. High-lift devices allow a wing to fly at a higher angle of attack before it stalls; therefore, these devices allow lift to be produced at slower airspeeds than would be possible without them.

Slots are fixed ducts between the bottom and the top of the leading edge of a wing ahead of the aileron. At high angles of attack, high-pressure air from below the wing is ducted back over the top of the wing. Here, its high velocity creates the needed low pressure that holds the air against the surface and prevents it from burbling until a much higher angle of attack is reached. Allowing a higher angle of attack before the stall occurs allows the aircraft to land at a slower airspeed.

Flaps, which are the most widely used high-lift devices on general aviation aircraft, modify the airfoil shape by increasing the camber of the wing. This increases both the lift and the drag produced by the wing. It is common practice in airplanes, on takeoff, to lower the flaps less than half way. This increases the lift more than it increases the drag, and the airplane is able to take off in a shorter distance than it could without the use of flaps. For landing, the flaps can be fully extended. This increases the drag more than the lift, and the aircraft can descend at a steep approach angle without building up excessive airspeed. The most commonly used flaps are seen in FAA Figure 23:

  1. The plain, or simple, flap is a hinged portion of the wing trailing edge inboard of the ailerons. This is the least effective type of flap, but its economy of construction makes it popular on light airplanes.
  2. The split trailing edge flap creates a large amount of drag and produces the least change in pitching moment.
  3. The slotted flap produces a gap between the wing and the flap leading edge. High-energy air from below the wing flows through this gap and accelerates the air on the upper surface of the flap. This delays flow separation over the flap.
  4. The Fowler flap extends out from the rear of the wing on tracks, and increases the area of the wing as well as changing its camber. The Fowler flap produces the largest increase in lift coefficient with the smallest increase in drag, and it causes the greatest change in pitching moment of all of the flaps listed here.

Ground Effect

An aircraft can be flown just clear of the ground at a slightly slower airspeed than is required to sustain level flight at higher altitude because of a phenomenon known as ground effect. Ground effect is caused by the ground interfering with the pattern of the air flowing over the aircraft wing.

When the aircraft is flown at an altitude of less than one half of its wing span above the surface, the aerodynamic characteristics of the wing change. As an aircraft is flown into ground effect, the air spilling over the wing tips is reduced. This changes the spanwise flow of air, which lowers the induced angle of attack and decreases the induced drag without increasing the parasite drag. With decreased drag, less thrust is required in ground effect than out of ground effect.

The lift coefficient for a given angle of attack is higher in ground effect, and as a result, the wing produces more lift at the same angle of attack in ground effect than out of ground effect. If a constant angle of attack is held as an aircraft enters ground effect, the lift will increase and the induced drag will decrease. There will be no increase in the total drag. When an aircraft leaves ground effect on takeoff, the following occurs:

  1. A greater angle of attack is required to maintain the same lift coefficient.
  2. The aircraft experiences an increase in induced drag and thrust required, in a powered aircraft.
  3. The aircraft experiences a decrease in stability and has a nose-up pitching moment.
  4. There is a reduction in static pressure in the pitot-static system which increases the indicated airspeed.

Principles of Rotorcraft Flight

Dissymmetry of lift is the difference in lift that exists between the two halves of a helicopter rotor disc in forward flight. The advancing blade (the blade whose tip is moving in the same direction as the heli-copter) has an airspeed equal to the tip speed plus the forward speed of the helicopter. The airspeed of the retreating blade (the blade whose tip is moving in the direction opposite to that of the helicopter) is equal to its tip speed minus the forward speed of the helicopter. Since lift is a function of the airspeed, the lift on the side of the advancing blade is greater than the lift on the side of the retreating blade.

Dissymmetry of lift is compensated for by allowing the blades to flap. As the lift of the advancing blade increases, the blade flaps upward, decreasing its angle of attack. At the same time, the retreating blade flaps downward, increasing its angle of attack. The changes in the lift caused by the differences in angle of attack counteract the changes in lift caused by the differences in airspeed.

During forward cruising flight, the individual blades operate at unequal airspeeds and unequal angles of attack, but with equal lift moments.

The forward speed of a rotorcraft is restricted primarily by dissymmetry of lift. As the forward speed increases, the difference in the speeds of the advancing and retreating blades becomes greater. At a high forward speed, the slow airspeed of the retreating blade causes it to have such a high angle of attack that it stalls. The stall begins at the tip and spreads inboard as forward airspeed increases.

A helicopter can maintain a constant altitude as long as the vertical component of lift equals its weight. When a helicopter makes a banked turn, the lift must be increased. The total lift vector is always a bisector of the coning angle, and when the disc is tilted, the total lift vector tilts so that it has both vertical and horizontal components. The total lift vector must be increased enough so it is equal to the helicopter weight plus the additional load caused by centrifugal force in the turn.

On the ground with the engine not running, rotor blades are acted on only by gravity. When they are spinning, but producing no lift, centrifugal force holds them out flat. When they are producing lift, they try to rise, but the balance of centrifugal force and lift cause them to assume an upward angle, called the coning angle.

Thrust produced by the tail rotor of a single-rotor helicopter will cause the helicopter to drift. To compensate for this drift, some helicopters have their mast rigged slightly away from the vertical position. This offset gives the lift produced by the main rotor a slight horizontal component.

As the blades of a helicopter rotor flap upward, the center of mass of the blades shifts in closer to their center of rotation, their mass-arm shortens. This shift, called the Coriolis effect, causes the blades to try to increase their rotational velocity.

Gyroplane Aerodynamics

Rotation of the blades of a gyroplane rotor is produced by the horizontal component of the rotor lift. In flight, air flows upward through the rotor disc and causes the blades to produce lift that acts at right angles to the direction of the air through the rotor disc.

Drag acts parallel to the wind through the rotor, and the resultant of the lift and drag forces lies at a slight angle ahead of the axis of rotation of the rotor. This is called the autorotation angle.

An autorotative force, perpendicular to the axis of rotation, causes the blades of a gyroplane to rotate with no power applied to them. The driving force for the rotors of a gyroplane is caused by air flowing upward through its rotors, and is not affected by the movement of the gyroplane over the ground.

A gyroplane can safely descend vertically or move backward with respect to ground references as long as air is moving upward through its rotors.

Glider Aerodynamics

Almost all gliders use an asymmetrical airfoil in which the upper camber is greater than the lower camber. This characteristic of an airfoil section produces good lift at slow airspeeds.

The minimum drag and the highest L/D ratio (L/DMAX) of a glider occur at the angle of attack and airspeed where the parasite drag and the induced drag are the same.

A glider will travel the maximum distance through the air when it is operating at an airspeed that produces the L/DMAX.

The L/D ratio of a glider is an index of its performance, and it is an aerodynamic function. For a given airfoil section, the L/DMAX is determined only by the angle of attack and is not affected by the weight of the glider. The L/DMAX always occurs at the same angle of attack.

A glider carrying water ballast can increase its lift by flying faster, rather than by increasing its angle of attack.

A higher airspeed is required for a heavy glider to obtain the same glide ratio it would have when it is lightly loaded.

Weight-Shift Control Aerodynamics

The explanations for the answers given describe the concepts that should be understood before taking the test. Many of these questions are based on older weight-shift and PPC designs and unique characteristics of specific designs. The answers given are the best of the choices provided.

[10-2024]