Flight Theory, Airfoils, and Stability

5-36 saturate it at 75 °F. If the temperature drops and the absolute humidity remain constant, the relative humidity will increase. This is because less water vapor is required to saturate the air at the lower temperature. Dew Point The dew point is the temperature to which humid air must be cooled at constant pressure to become saturated. If the temperature drops below the dew point, condensation occurs. People who wear eyeglasses have experience going from cold outside air into a warm room and having moisture collect quickly on their glasses. This happens because the glasses were below the dew point temperature of the air in the room.

The air immediately in contact with the glasses was cooled below its dew point temperature, and some of the water vapor was condensed out. This principle is applied in determining the dew point. A vessel is cooled until water vapor begins to condense on its surface. The temperature at which this occurs is the dew point. Vapor Pressure Vapor pressure is the portion of atmospheric pressure that is exerted by the moisture in the air, which is expressed in tenths of an inch of mercury. The dew point for a given condition depends on the amount of water pressure present; thus, a direct relationship exists between the vapor pressure and the dew point.

Standard Atmosphere If the performance of an aircraft is computed, either through flight tests or wind tunnel tests, some standard reference condition must be determined first in order to compare results with those of similar tests. The conditions in the atmosphere vary continuously, and it is generally not possible to obtain exactly the same set of conditions on two different days or even on two successive flights. For this reason, a set group of standards must be used as a point of reference. The set of standard conditions presently used in the United States is known as the U.S. Standard Atmosphere.

The standard atmosphere approximates the average conditions existing at 40° latitude, and is determined on the basis of the following assumptions. The standard sea level conditions are: Pressure at 0 altitude (P0) = 29.92 "Hg Temperature at 0 altitude (T0) = 15 °C or 59 °F Gravity at 0 altitude (G0) = 32.174 fps/s The U.S. Standard Atmosphere is in agreement with the International Civil Aviation Organization (ICAO) Standard Atmosphere over their common altitude range. The ICAO Standard Atmosphere has been adopted as standard by most of the principal nations of the world.

Aircraft Theory of Flight

Before a technician can consider performing maintenance on an aircraft, it is necessary to understand the pieces that make up the aircraft. Names like fuselage, empennage, wing, and so many others, come into play when describing what an airplane is and how it operates. For helicopters, names like main rotor, anti-torque rotor, and autorotation come to mind as a small portion of what needs to be understood about rotorcraft. The study of physics, which includes basic aerodynamics, is a necessary part of understanding why aircraft operate the way they do.

Four Forces of Flight

During flight, there are four forces acting on an airplane. These forces are lift, weight, thrust, and drag. [Figure 5-53] Lift is the upward force created by the wing, weight is the pull of gravity on the mass, thrust is the force created by the airplane’s propeller or turbine engine, and drag is the friction caused by the air flowing around the airplane. All four of these forces are measured in pounds. Any time the forces are not in balance, something about the airplane’s condition is changing. The possibilities are as follows: 1. When an airplane is accelerating, it has more thrust than drag.

2. When an airplane is decelerating, it has less thrust than drag. 3. When an airplane is at a constant velocity, thrust and drag are equal. 5-37 Lift Weight Drag Thrust 4. When an airplane is climbing, it has more lift than weight. 5. When an airplane is descending, it has more weight than lift. 6. When an airplane is at a constant altitude, lift and weight are equal. Bernoulli’s Principle and Subsonic Flow The basic concept of subsonic airflow and the resulting pressure differentials was discovered by Daniel Bernoulli, a Swiss physicist. Bernoulli’s principle, as we refer to it today, states that “as the velocity of a fluid increases, the static pressure of that fluid will decrease, provided there is no energy added or energy taken away.” A direct application of Bernoulli’s principle is the study of air as it flows through either a converging or a diverging passage, and to relate the findings to some aviation concepts.

A converging shape is one whose cross-sectional area gets progressively smaller from entry to exit. A diverging shape is just the opposite, with the cross-sectional area getting larger from entry to exit. Figure 5-54 shows a converging shaped duct, with the air entering on the left at subsonic velocity and exiting on the right. Notice that the air exits at an increased velocity and a decreased static pressure when looking at the pressure and velocity gauges, and the indicated velocity and pressure. The unit leaving must increase its velocity as it flows into a smaller space, because a unit of air must exit the duct when another unit enters.

In a diverging duct, just the opposite would happen. From the entry point to the exit point, the duct is spreading out and the area is getting larger. [Figure 5-55] With the increase in cross-sectional area, the velocity of the air decreases and the static pressure increases. The total energy in the air has not changed. What has been lost in velocity, which is kinetic energy, is gained in static pressure, which is potential energy. In the discussion of Bernoulli’s principle earlier in this chapter, a venturi was shown in Figure 5-46. In Figure 5-56, a venturi is shown again, only this time a wing is shown tucked up into the recess where the venturi’s converging shape is.

There are two arrows showing airflow. The large arrow shows airflow within the venturi, and the small arrow shows airflow on the outside heading toward the leading edge of the wing. In the converging part of the venturi, velocity would increase and static pressure would decrease. The same thing would happen to the air flowing around the wing, with the velocity over the top increasing and static pressure decreasing. In Figure 5-56, the air reaching the leading edge of the wing separates into two separate flows. Some of the air goes over the top of the wing and some travels along the bottom. The air going over the top, because of the curvature, has farther to travel. With a greater distance to travel, the air going over the top must move at a greater velocity. The higher velocity on the top causes the static pressure on the top to be less than it is on the bottom, and this difference in static pressures is what creates lift.

For the wing shown in Figure 5-56, imagine it is 5 ft. wide and 15 ft. long, for a surface area of 75 ft 2 (10,800 in2). If the difference in static pressure between the top and bottom is 0.1 psi, there will be 1⁄10 lb of lift for each square inch of surface area. Since there are 10,800 in2 of surface area, there would be 1,080 lb of lift (0.1 × 10,800). Lift and Newton’s Third Law Newton’s third law identifies that for every force there is an equal and opposite reacting force. In addition to Bernoulli’s principle, Newton’s third law can also be used to explain the lift being created by a wing. As the air travels around a wing and leaves the trailing edge, the air is forced to move in a downward direction. Since a force is required to make something change direction, there must be an equal and opposite reacting force.

In this case, the reacting force is what we call lift. In order to calculate lift based on Newton’s third law, Newton’s second law and the formula “Force = Mass × Acceleration” would be used. The mass would be the weight of air flowing over the wing every second, and the acceleration would be the change in velocity the wing imparts to the air. The lift on the wing as described by Bernoulli’s principle, and lift on the wing as described by Newton’s third law, is 5-38 Airflow Subsonic 14 psi 10 psi 300 mph 400 mph

Pressure

Velocity Low High Airflow Subsonic 10 psi 14 psi 400 mph 300 mph

Pressure

Velocity Low High not separate or independent of each other. They are just two different ways to describe the same thing, namely the lift on a wing. Airfoils An airfoil is any device that creates a force, based on Bernoulli’s principles or Newton’s laws, when air is caused to flow over the surface of the device. An airfoil can be the wing of an airplane, the blade of a propeller, the rotor blade of a helicopter, or the fan blade of a turbofan engine. The wing of an airplane moves through the air because the airplane is in motion, and generates lift by the process previously described. By comparison, a propeller blade, helicopter rotor blade, or turbofan engine fan blade rotates through the air.

These rotating blades could be referred to as rotating wings, as is common with helicopters when they are called rotary wing aircraft. The rotating wing can be viewed as a device that creates lift, or just as correctly, it can be viewed as a device that creates thrust. In Figure 5-57 an airfoil, or wing, is shown, with some of the terminology that is used to describe a wing. The terms and their meaning are as follows: Camber The camber of a wing is the curvature which is present on top and bottom surfaces. The camber on the top is much more pronounced, unless the wing is a symmetrical airfoil, which has the same camber top and bottom. The bottom of the wing, more often than not, is relatively flat. The increased camber on top is what causes the velocity of the air to increase and the static pressure to decrease. The bottom of the wing has less velocity and more static pressure, which is why the wing generates lift.

Chord Line The chord line is an imaginary straight line running from the wing’s leading edge to its trailing edge. The angle between the chord line and the longitudinal axis of the airplane is known as the angle of incidence. Relative W ind The relative wind is a relationship between the direction of airflow and the aircraft wing. In normal flight circumstances, the relative wind is the opposite direction of the aircraft flightpath. • If the flightpath is forward then the relative wind is backward. • If the flightpath is forward and upward, then the relative wind is backward and downward. • If the flightpath is forward and downward, then the relative wind is backward and upward.

Therefore, the relative wind is parallel to the flightpath, and travels in the opposite direction. Angle of Attack The angle between the chord line and the relative wind is the angle of attack. As the angle of attack increases, the lift on the wing increases. If the angle of attack becomes too great, the airflow can separate from the wing and the lift 5-39 Airflow Velocity increase Airplane path Relative wind Lower camber Trailing edge Upper camber Chord lineLeading edge Angle of attack will be destroyed. When this occurs, a condition known as a stall takes place. There are a number of different shapes, known as planforms that a wing can have. A wing in the shape of a rectangle is very common on small general aviation airplanes. An elliptical shape or tapered wing can also be used, but these do not have as desirable a stall characteristic. For airplanes that operate at high subsonic speeds, sweptback wings are common, and for supersonic flight, a delta shape might be used.

The aspect ratio of a wing is the relationship between its span, or a wingtip to wingtip measurement, and the chord of the wing. If a wing has a long span and a very narrow chord, it is said to have a high aspect ratio. A higher aspect ratio produces less drag for a given flight speed, and is typically found on glider type aircraft. The angle of incidence of a wing is the angle formed by the intersection of the wing chord line and the horizontal plane passing through the longitudinal axis of the aircraft. Many airplanes are designed with a greater angle of incidence at the root of the wing than at the tip, and this is referred to as washout. This feature causes the inboard part of the wing to stall before the outboard part, which helps maintain aileron control during the initial stages of a wing stall.

Boundary Layer Airflow The boundary layer is a very thin layer of air lying over the surface of the wing and, for that matter, all other surfaces of the airplane. Because air has viscosity, this layer of air tends to adhere to the wing. As the wing moves forward through the air the boundary layer at first flows smoothly over the streamlined shape of the airfoil. Here the flow is called the laminar layer. As the boundary layer approaches the center of the wing, it begins to lose speed due to skin friction and it becomes thicker and turbulent. Here it is called the turbulent layer. The point at which the boundary layer changes from laminar to turbulent is called the transition point. Where the boundary layer becomes turbulent, drag due to skin friction is relatively high. As speed increases, the transition point tends to move forward. As the angle of attack increases, the transition point also tends to move forward. With higher angles of attack and further thickening of the boundary layer, the turbulence becomes so great the air breaks away from the surface of the wing. At this point, the lift of the wing is destroyed and a condition known as a stall has occurred.

In Figure 5-58, view A shows a normal angle of attack and the airflow staying in contact with the wing. View B shows an extreme angle of attack and the airflow separating and becoming turbulent on the top of the wing. In view B, the wing is in a stall. Boundary Layer Control One way of keeping the boundary layer air under control, or lessening its negative effect, is to make the wing’s surface as smooth as possible and to keep it free of dirt and debris. As the friction between the air and the surface of the wing increases, the boundary layer thickens and becomes more turbulent and eventually a wing stall occurs. With a smooth and clean wing surface, the onset of a stall is delayed and the wing can operate at a higher angle of attack. One of the reasons ice forming on a wing can be such a serious problem is because of its effect on boundary layer air. On a high-speed airplane, even a few bugs splattered on the wing’s leading edge can negatively affect boundary layer air.

Other methods of controlling boundary layer air include wing leading edge slots, air suction through small holes on the wing’s upper surface, and the use of devices called vortex generators. A wing leading edge slot is a duct that allows air to flow from the bottom of the wing, through the duct, to the top of the wing. As the air flows to the top of the wing, it is directed along the wing’s surface at a high velocity and helps keep the boundary layer from becoming turbulent and separating from the wing’s surface. Another way of controlling boundary layer air is to create suction on the top of the wing through a large number of 5-40 View A View B small holes. The suction on the top of the wing draws away the slow-moving turbulent air, and helps keep the remainder of the airflow in contact with the wing.

V ortex generators are used on airplanes that fly at high subsonic speed, where the velocity of the air on the top of the wing can reach Mach 1. As the air reaches Mach 1 velocity, a shock wave forms on the top of the wing, and the subsequent shock wave causes the air to separate from the wing’s upper surface. V ortex generators are short airfoils, arranged in pairs, located on the wing’s upper surface. They are positioned such that they pull high-energy air down into the boundary layer region and prevent airflow separation. Wingtip Vortices Wingtip vortices are caused by the air beneath the wing, which is at the higher pressure, flowing over the wingtip and up toward the top of the wing. The end result is a spiral or vortex that trails behind the wingtip anytime lift is being produced. This vortex is also referred to as wake turbulence, and is a significant factor in determining how closely one airplane can follow behind another on approach to land. The wake turbulence of a large airplane can cause a smaller airplane, if it is following too closely, to be thrown out of control. V ortices from the wing and from the horizontal stabilizer are quite visible on the MD-11 shown in Figure 5-59.

Upwash and downwash refer to the effect an airfoil has on the free airstream. Upwash is the deflection of the oncoming airstream, causing it to flow up and over the wing. Downwash is the downward deflection of the airstream after it has passed over the wing and is leaving the trailing edge. This downward deflection is what creates the action and reaction described under lift and Newton’s third law. Axes of an Aircraft An airplane in flight is controlled around one or more of three axes of rotation. These axes of rotation are the longitudinal, lateral, and vertical. On the airplane, all three axes intersect at the center of gravity. As the airplane pivots on one of these axes, it is in essence pivoting around the center of gravity (CG).

The center of gravity is also referred to as the center of rotation. On the brightly colored airplane shown in Figure 5-60, the three axes are shown in the colors red (vertical axis), blue (longitudinal axis), and orange (lateral axis). The flight control that makes the airplane move around the axis is shown in a matching color. The rudder, in red, causes the airplane to move around the vertical axis and this movement is described as being a yaw. The elevator, in orange, causes the airplane to move around the lateral axis and this movement is described as being a pitch. The ailerons, in blue, cause the airplane to move around the longitudinal axis and this movement is described as being a roll.

Aircraft Stability

When an airplane is in straight-and-level flight at a constant velocity, all the forces acting on the airplane are in equilibrium. If that straight-and-level flight is disrupted by a disturbance in the air, such as wake turbulence, the airplane might pitch up or down, yaw left or right, or go into a roll. If the airplane has what is characterized as stability, once the disturbance goes away, the airplane will return to a state of equilibrium. Static Stability The initial response that an airplane displays after its equilibrium is disrupted is referred to as its static stability. If the static stability is positive, the airplane will tend to return to its original position after the disruptive force is removed.

If the static stability is negative, the airplane will continue to move away from its original position after the disruptive force is removed. If an airplane with negative static stability has the nose pitch up because of wake turbulence, the tendency will be for the nose to continue to pitch up even after the turbulence goes away. If an airplane tends to remain in a displaced position after the force is removed, but does not continue to move toward even greater displacement, its static stability is described as being neutral. Dynamic Stability The dynamic stability of an airplane involves the amount of time it takes for it to react to its static stability after it has been displaced from a condition of equilibrium. Dynamic stability involves the oscillations that typically occur as the airplane tries to return to its original position or attitude. Even though an airplane may have positive static stability, it may have dynamic stability which is positive, neutral, or negative.

Imagine that an airplane in straight-and-level flight is disturbed and pitches noseup. If the airplane has positive static stability, the nose will pitch back down after the disturbance is removed. If it immediately returns to straight- 5-41 Vertical axis Longitudinal axis Lateral axis CG axis is a matching color. and-level flight, it is also said to have positive dynamic stability. The airplane, however, may pass through level flight and remain pitched down, and then continue the recovery process by pitching back up. This pitching up and then down is known as an oscillation. If the oscillations lessen over time, the airplane is still classified as having positive dynamic stability. If the oscillations increase over time, the airplane is classified as having negative dynamic stability.

If the oscillations remain the same over time, the airplane is classified as having neutral dynamic stability. A, the displacement from equilibrium goes through three oscillations and then returns to equilibrium. In view B, the displacement from equilibrium is increasing after two oscillations, and will not return to equilibrium. In view C, the displacement from equilibrium is staying the same with each oscillation. Longitudinal Stability Longitudinal stability for an airplane involves the tendency for the nose to pitch up or pitch down, rotating around the lateral axis, which is measured from wingtip to wingtip. If an airplane is longitudinally stable, it will return to a properly trimmed angle of attack after the force that upset its flightpath is removed.

The weight and balance of an airplane, which is based on both the design characteristics of the airplane and the way it is loaded, is a major factor in determining longitudinal stability. There is a point on the wing of an airplane, called the center of pressure or center of lift, where all the lifting forces concentrate. In flight, the airplane acts like it is being lifted from or supported by this point. This center of lift runs from wingtip to wingtip. There is also a point on the airplane, called the center of gravity, where the mass or weight of the airplane is concentrated. For an airplane to have good longitudinal stability, the center of gravity is typically located forward of the center of lift. This gives the airplane a nosedown pitching tendency, which is balanced out by the force generated at the horizontal stabilizer and elevator. The center of gravity has limits within which it must fall. If it is too far forward, the forces at the tail might not be able to compensate and it may not be possible to keep the nose of the airplane from pitching down.

In Figure 5-62, the center of lift, center of gravity, and center of gravity limits are shown. It can be seen that the center of gravity is not only forward of the center of lift, it is also forward of the center of gravity limit. At the back of the airplane, the elevator trailing edge is deflected upward to create a downward force on the tail, to try and keep the nose 5-42 Time Positive static and positive dynamic stability Positive static and negative dynamic stability Positive static and neutral dynamic stability Time Time A B C Aft CG limitForward CG limit Center of lift Center of gravity of the airplane up. This airplane would be highly unstable longitudinally, especially at low speed when trying to land.

It is especially dangerous if the center of gravity is behind the aft limit. The airplane will now have a tendency to pitch noseup, which can lead to the wing stalling and possible loss of control of the airplane. Lateral Stability Lateral stability of an airplane takes place around the longitudinal axis, which is from the airplane’s nose to its tail. If one wing is lower than the other, good lateral stability will tend to bring the wings back to a level flight attitude. One design characteristic that tends to give an airplane good lateral stability is called dihedral. Dihedral is an upward wing angle, with respect to the horizontal, and it is usually just a few degrees.

Imagine a low wing airplane with a few degrees of dihedral experiencing a disruption of its flightpath such that the left wing drops. When the left wing drops, this will cause the airplane to experience a sideslip toward the low wing. The sideslip causes the low wing to experience a higher angle of attack, which increases its lift and raises it back to a level flight attitude. The dihedral on a wing is shown in Figure 5-63. Directional Stability Movement of the airplane around its vertical axis, and the airplane’s ability to not be adversely affected by a force creating a yaw type of motion, is called directional stability. The vertical fin gives the airplane this stability, causing the airplane to align with the relative wind. In flight, the airplane acts like the weather vane we use around our home to show the direction the wind is blowing. The distance from the pivot point on a weather vane to its tail is greater than the distance from its pivot point to the nose. So, when the wind blows, it creates a greater torque force on the tail and forces it to align with the wind. On an airplane, the same is true. With the CG being the pivot point, it is a greater distance from the CG to the vertical stabilizer than it is from the CG to the nose. [Figure 5-64] Dutch Roll The dihedral of the wing tries to roll the airplane in the opposite direction of how it is slipping, and the vertical fin will try to yaw the airplane in the direction of the slip. These two events combine in a way that affects lateral and directional stability. If the wing dihedral has the greatest effect, the airplane will have a tendency to experience a Dutch roll. A Dutch roll is a small amount of oscillation around both the longitudinal and vertical axes. Although this condition is not considered dangerous, it can produce an uncomfortable feeling for passengers.

Commercial airliners typically have yaw dampers that sense a Dutch roll condition and cancel it out.

Flight Control Surfaces

The purpose of flight controls is to allow the pilot to maneuver the airplane, and to control it from the time it starts the takeoff roll until it lands and safely comes to a halt. Flight controls are typically associated with the wing and the vertical and horizontal stabilizers, because these are the parts of the airplane that flight controls most often attach to. In flight, and to some extent on the ground, flight controls provide the airplane with the ability to move around one or more of the three axes. Flight controls function by changing the shape or aerodynamic characteristics of the surface they are attached to.

Flight Controls & the Lateral Axis The lateral axis of an airplane is a line that runs below the wing, from wingtip to wingtip, passing through the airplane’s center of gravity. Movement around this axis is called pitch, and control around this axis is called longitudinal control. The flight control that handles this job is the elevator attached to the horizontal stabilizer, a fully moving horizontal stabilizer, or on a v-tail configured airplane, it is called ruddervators. An elevator on a Cessna 182 can be seen in Figure 5-65 . In Figure 5-66 , a fully moving horizontal stabilizer, known as a stabilator, can be seen on a Piper Cherokee Cruiser PA-28-140, and Depending on the airplane being discussed, movement around the lateral axis happens as a result of the pilot moving the