Rudder Rudder control tab Outboard aileron Stabilizer actuated elevator tab Elevator control tab
- Balance tab Elevator
• Outboard spoilers
- Outboard flap Inboard aileron Stabilizer 20
- Fillet flap
- Inboard flap
- Inboard spoilers Control tab Leading edge flaps Vortex generators FIGURE 1-34. Control surfaces on a large turbojet aircraft.
board flaps at the trailing edge of the wing. The aileron hinge supports extend aft and are attached to aileron hinge bearings to support the aileron.
The outboard ailerons are made up of a nose spar and ribs covered with aluminum honeycomb panels. A continuous hinge attached to the forward edge of the nose is grooved to mate with the hem of a fabric seal.
The outboard ailerons are located in the trailing edge of each outboard wing section. Hinge supports extend aft from the wing and are attached to the aileron hinge bearing to support the aileron. The nose of the aileron extends into a balance chamber in the wing and is attached to balance panels.
Aileron balance panels (figure 1-35) reduce the force necessary to position and hold the ailerons.
The balance panels may be made of aluminum honeycomb skin bonded to an aluminum frame, or of Hinge Balance panel aluminum skin-covered assemblies with hat-section stiffeners. Clearance between the aileron nose and wing structure provides a controlled airflow area necessary for balance panel action. Seals attached to the panels control air leakage.
Air loads on the balance panels (figure 1-35) depend on aileron position. When the ailerons are moved during flight to either side of the streamline position, differential pressure is created across the balance panels. This differential pressure acts on the balance panels in a direction that assists aileron movement. Full balance panel force is not required for small angles of aileron displacement because the manual force necessary to rotate the control tab through small angles is slight. A controlled air bleed is progressively decreased as the aileron displacement angle is increased. This action increases the differential air pressure on the balance panels as Control tab Vent gap
AILERON WING -Aileron hinge point FIGURE 1-35. Aileron balance panel.
the ailerons rotate from the streamline position. The increasing load on the balance panel counteracts the increasing load on the ailerons.
Auxiliary Wing Flight Surfaces The ailerons are the primary wing flight surfaces.
Auxiliary wing flight surfaces include trailing edge flaps, leading edge flaps, speed brakes, spoilers, and leading edge slats. The number and type of auxiliary wing flap surfaces on an aircraft vary widely, depending on the type and size of aircraft.
Wing flaps are used to give the aircraft extra lift.
They reduce the landing speed, thereby shortening the length of the landing rollout to facilitate landing in small or obstructed areas by permitting the gliding angle to be increased without greatly increasing the approach speed. In addition, the use of flaps during takeoff reduces the length of the takeoff run.
Most flaps are hinged to the lower trailing edges of the wings, inboard of the ailerons. Leading edge flaps are also used, principally on large high-speed aircraft. When they are in the "up" (or retracted) position, they fair in with the wings and serve as part of the wing trailing edge. When in the "down" (or extended) position, the flaps pivot on the hinge points and drop to about a 45° or 50° angle with the wing chord line. This increases the wing camber and changes the airflow, providing greater lift.
Some common types of flaps are shown in figure
1-36. The plain flap (figure 1-36A) forms the trailing edge of the wing when the flap is in the up (or 21
- Retracted Fore flap A. Plain flap (down) Mid flap Aft flap FIcURE 1-37. Triple-slotted trailing edge flaps.
B. Split flap (down) flap area. The resulting slots between flaps prevents separation of the airflow over the flap area.
The leading edge flap (figure 1-38) is similar in operation to the plain flap; that is, it is hinged on the bottom side, and, when actuated, the leading edge of the wing extends in a downward direction C. Fowler flap (down) to increase the camber of the wing. Leading edge flaps are used in conjunction with other types of FICURE 1-36. Wing flaps. flaps.
retracted) position. It contains both the upper and lower surface of the wing trailing edge.
The plain split flap (figure 1-36B) is normally housed flush with the undersurface of the wing. It is similar to a plain flap except that the upper surface of the wing extends to the flap trailing edge and does not droop with the flap. This flap is also called the split-edge flap. It is usually just a braced, flat metal plate hinged at several points along its leading edge.
Aircraft requiring extra wing area to aid lift often use Fowler faps (figure 1-36C). This system houses the flaps flush under the wings much as does the plain split flap system. But, instead of the flaps hinging straight down from a stationary hinge line, worm-gear drives move the flaps leading edge rearward as the flaps droop. This action provides normal flap effect, and, at the same time, wing area is increased when the flaps are extended.
An example of a triple-slotted segmented flap used on some large turbine aircraft is shown in figure 1-37. This type of trailing edge flap system provides high lift for both takeoff and landing.
Each flap consists of a foreflap, a mid-flap, and an aft-flap. The chord length of each flap expands as the flap is extended, providing greatly increased FIGURE 1-38. Cross section of a leading edge flap.
Figure 1-34 shows the location of the leading edge flaps on a large multi-engine turbine aircraft.
Three Kruger-type flaps are installed on each wing.
The flaps are machined magnesium castings with integral ribs and stiffeners. The magnesium casting of each flap is the principal structural component and consists of a straight section with a hollow core called the torque tube extending from the straight section at the forward end.
Each leading edge flap has three gooseneck hinges attached to fittings in the fixed wing leading edge, and a hinged fairing is installed on the trailing edge of each flap. Figure 1-39 shows a typical leading edge flap in retracted position, with an outline of the extended position.
Speed brakes, sometimes called dive flaps or dive brakes, serve to slow an aircraft in flight. These brakes are used when descending at a steep angle or when approaching the runway for a landing. The brakes themselves are manufactured in many 22
Hinge point Actuator
• Flap extended Flap retracted
- Retractable nose FICURE 1-39. Leading edge flap.
shapes, and their location depends on the design of the aircraft and the purpose of the brakes.
The brake panels may be located on certain parts of the fuselage or on the wing surfaces. Brakes on the fuselage are small panels that can be extended into the smooth airflow to create turbulence and drag. Wing-type brakes may be multiple-finger channels extending above and below the wing surfaces to break up smooth airflow. Usually speed brakes are controlled by electrical switches and actuated by hydraulic pressure.
Another type of air brake is a combination of spoilers and speed brakes. A typical combination consists of spoiler flaps located in the upper wing surfaces ahead of the ailerons. When the operator wishes to use both air brakes and spoilers, he can slow the flight speed and maintain lateral control as wel.
Spoilers are auxiliary wing flight control surfaces, mounted on the upper surface of each wing, which operate in conjunction with the ailerons to provide lateral control.
Most spoiler systems can also be extended symmetrically to serve a secondary function as speed brakes. Other systems are equipped with separate ground and fight spoilers. Most spoiler panels are bonded honeycomb structures with aluminum skin.
They are attached to the wing structure by machined hinge fittings which are bonded into the spoiler panel.
Tabs One of the simplest yet most important devices to aid the pilot of an aircraft is the tab attached to a control surface. Although a tab does not take the place of a control surface, it is mounted on or attached to a movable control surface and causes easier movement or better balance of the control surface.
All aircraft, except a few of the very lightest types, are equipped with tabs that can be controlled from the cockpit. Tabs on some of these aircraft are usually adjustable only when the aircraft is on the ground. Figure 1-40 shows the location of a typical rudder tab.
Hinge line
• Rudder trailing Vertical edge stabilizer
• Tab Fuselage Aircraft movement FIGURE 1-40. Typical location of rudder control tab.
LANDING GEAR The landing gear is the assembly that supports the aircraft during landing or while it is resting or moving about on the ground. The landing gear has shock struts to absorb the shock of landing and taxiing. By means of a gear-retraction mechanism, the landing gear attached to the aircraft structure and enables the gear to extend and retract. The landing gear arrangement either has a tailwheel or a nosewheel. Landing gear arrangements having a nosewheel are usually equipped for nosewheel steering. Nosewheel aircraft are protected at the fuselage tail section with a tail skid or bumper. By means of wheels and tires (or skia), the landing gear forms a stabilizing contact with the ground during landing and taxiing. Brakes installed in the wheels enable the aircraft to be slowed or stopped during movement on the ground.
SKIN AND FAIRING The smooth outer cover of the aircraft is referred to as skin. The skin covers the fuselage, wings, empennage, nacelles, and pods. The material used for the skin covering is usually sheet aluminum alloy, treated so that it will not corrode. Magnesium and stainless steel may also be used to a limited extent. The thickness of the skin materials covering a structural unit may differ, depending on the load and streases imposed within and throughout the structure. To smooth out the airflow over the angles formed by the wings and other structural units with the fuselage, shaped and rounded panels or metal skin are attached. This paneling or skin is called fairing. Fairing is sometimes referred to as a fillet.
Some fairing is removable to provide access to aircraft components, whereas other fairing is riveted to the aircraft structure.
ACCESS AND INSPECTION DOORS Access doors permit normal or emergency entrance into or exit from the aircraft. Also, they provide access to servicing points and manually operated drains. Inspection doors provide access to a particular part of the aircraft being inspected or maintained. Access or inspection doors are either hinged or removable. They are fastened in the closed position with catch and locking mechanisms, screws, quick-release devices, or cowling type fasteners. Access and inspection doors that are removable often have a stenciled identification number that is identical to a number stenciled near the opening that they cover. Other access and inspection doors have a stenciled nomenclature to identify the opening that they cover.
HELICOPTER STRUCTURES Like the fuselages in fixed-wing aircraft, helicopter fuselages may be welded truss or some form of monocoque construction. Although their fuselage configurations may vary a great deal, most helicopter fuselages employ structural members similar to those used in fixed-wing aircraft. For example, most helicopters have such vertical/widthwise braces as bulkheads, formers, rings, and frames. They are also provided with such lengthwise braces as stringers and longerons. In addition, the gussets, joiners, and skin hold the other structural members together.
The basic body and tail boom sections of a typical helicopter are of conventional, all-metal, riveted structures incorporating formed aluminum alloy bulkheads, beams, channels, and stiffeners. Stressed skin panels may be either smooth or beaded. The firewall and engine deck are usually stainless steel.
The tail boom is normally of semimonocoque construction, made up of formed aluminum bulkheads, extruded longerons, and skin panels or of welded tubular steel.
The major structural components of one type of helicopter are shown in figure 1-41. The members of a helicopter's tail group vary widely, depending on the individual type and design. In this case, a stabilizer is mounted on a pylon to make up the group. In other cases, the stabilizer may be mounted on the helicopter tail cone or fuselage. In either case, both the pylon and stabilizer usually contain aluminum alloy structural members covered with magnesium alloy skin. The types of structural members used, however, usually vary. A pylon usually has bulkheads, formers, frames, stringers, and beams, making it somewhat of a blend of aircraft wing and fuselage structural members. The stabilizer is usually built more like an aircraft wing, with ribs and spars.
In a typical helicopter, the tail, body, and tail boom are constructed of all-metal stressed skin and metal reinforcing members. The helicopter cabin is normally a plexiglass enclosure which is supported by aluminum tubing in some models.
A large single-rotor helicopter is shown in figure
1-42. It is all-metal and is basically composed of two major sections, the cabin and the tail cone. The cabin section is further divided into passenger or cargo compartments, which provide space for the crew, passengers, cargo, fuel and oil tanks, controls, and powerplant. In multi-engine helicopters, the powerplants are usually mounted in separate engine nacelles.
As shown in figure 1-42, the aft section of a typical single-rotor helicopter consists of the tail cone, the fin, the tail-cone housing, the tail-rotor pylon, and the tail-end fairing. The tail cone is bolted to the rear of the forward section and supports the tail rotor, tail-rotor drive shafts, stabilizers, tail-cone housing, and tail-rotor pylon. The tail cone is of magnesium alloy and aluminum alloy construction. The tail-cone housing is bolted to the aft end of the tail cone. Trim stabilizers extend out 24
Pylon Main rotor • Tail rotor Stabilizer Main gear Forward section Tail gear —Tail cone FIGURE 1-41. Typical helicopter structural components.
B F D H
E TAIL CONE SECTION H. Fin J. Tail cone CABIN SECTION K. Tail-cone housing A. Engine compartment E. Fuel tank compartment L. Pylon B. Cockpit F. Baggage compartment M. Tail-end fairing C. Transmission compartment G. Electrical and radio D. Cabin compartment FIGURE 1-42. Location of major helicopter components.
on both sides of the tail cone forward of the housing.
Helicopter structural members are designed to carry a load or, stated differently, to resist stress. A single member of the helicopter structure may be subjected to a combination of stresses. In most cases it is desirable for structural members to carry end loads rather than side loads; that is, to be subjected to tension or compression rather than bending. Structural members are usually combined into a truss to carry end loads. In a typical Pratt truss, the longitudinal and vertical members are tubes or rods capable of carrying compression loads.
Nonstructural members that are not removable from the helicopter are usually attached by riveting or spot welding. Riveting is the most common method of attaching aluminum alloy sheets together.
Parts that can be removed from the helicopter structure are usually bolted together.
Transparent materials are used for windshields and windows and sometimes to cover parts requiring frequent visual inspection. Transparent plastic sheet and laminated glass are the materials most commonly used.
Some helicopter manufacturers use impregnated glass cloth laminate (hiber glass) as a lightweight substitute for certain metal parts, since fiber glass is simple to manufacture, has a high strength-weight ratio, and resists mildew, corrosion, and rot.
CHAPTER 2
ASSEMBLY AND RIGGING
GENERAL This chapter includes both assembly and rigging since the subjects are directly related. Assembly involves putting together the component sections of the aircraft, such as wing sections, empennage units, nacelles, and landing gear. Rigging is the final adjustment and alignment of the various component sections to provide the proper aerodynamic reaction.
Two important considerations in all assembly and rigging operations are: (1) Proper operation of the component in regard to its aerodynamic and mechanical function, and (2) maintaining the aircraft's structural integrity by the correct use of materials, hardware, and safetying devices. Improper assembly and rigging may result in certain members being subjected to loads greater than those for which they were designed.
Assembly and rigging must be done in accordance with the requirements prescribed by the aircraft manufacturer. These procedures are usually detailed in the applicable maintenance or service manuals. The Aircraft Specification or Type Certificate Data Sheets also provide valuable information regarding control surface travel.
The rigging of control systems varies with each type of aircraft, therefore, it would be impracticable to define a precise procedure. However, certain principles apply in all situations and these will be discussed in this chapter. It is essential that the aircraft manufacturer's instructions be followed when rigging an aircraft.
THEORY OF FLIGHT Numerous comprehensive texts have been written about the aerodynamics involved in the flight of an aircraft. It is unnecessary that a mechanic be totally versed on the subject. However, he must understand the relationships between the atmosphere, the aircraft, and the forces acting on it in flight, in order to make intelligent decisions affecting the flight safety of both airplanes and helicopters.
Understanding why the aircraft is designed with a particular type of primary and secondary control system, and why the surfaces must be aerodynamically smooth, becomes essential when maintaining today's complex aircraft.
AERODYNAMICS Theory of flight deals with aerodynamics. The term aerodynamics is derived from the combination of two Greek words- "aer" meaning air, and "dyne" meaning force of power. Thus, when aero is joined with dynamics, we have aerodynamics, meaning the study of objects in motion through the air and the forces that produce or change such motion.
Aerodynamics is the science of the action of air on an object. It is further defined as that branch of dynamics which deals with the motion of air and other gases, with the forces acting upon an object in motion through the air, or with an object which is stationary in a current of air. In effect, aerodynamics is concerned with three distinct parts. These parts may be defned as the aircraft, the relative wind, and the atmosphere.
THE ATMOSPHERE Before discussing the fundamentals of the theory of flight, there are several basic ideas that must be considered. An aircraft operates in the air; therefore, the properties of air that affect aircraft control and performance must be understood.
Air is a mixture of gases composed principally of nitrogen and oxygen. Since air is a combination of gases, it follows the laws of gases. Air is considered a fluid because it answers the definition of a fluid, namely, a substance which may be made to flow or change its shape by the application of moderate pressure. Air has weight, since something lighter than air, such as a balloon filled with helium, will rise in the air.
PRESSURE The deeper a diver goes beneath the surface of the ocean, the greater the pressure becomes on his body due to the weight of the water overhead. Since air also has weight, the greater the depth from the 27
outer surface of the atmosphere, the greater the pressure. If a 1-in. square column of air extending from sea level to the "top" of the atmosphere could be weighed, it would be found to weigh about 14.7 Ibs. Thus, atmospheric pressure at sea level is 14.7
p.s.i. (pounds per square inch). However, pounds per square inch is rather a crude unit for the mẹasurement of a light substance such as air. Therefore, atmospheric pressure is usually measured in terms of inches of mercury.
The apparatus for measuring atmospheric pressure is shown in figure 2-1. A glass tube, 36 in.
long, open at one end, and closed at the other, is filled with mercury. The open end is sealed temporarily and then submerged into a small container partly filled with mercury, after which the end is unsealed. This allows the mercury in the tube to descend, leaving a vacuum at the top of the tube.
Some of the mercury flows into the container while a portion of it remains in the tube. The weight of the atmosphere pressing on the mercury in the open container exactly balances the weight of the mercury in the tube, which has no atmospheric pressure pushing down on it due to the vacuum in the top of the tube. As the pressure of the surrounding air decreases or increases, the mercury column lowers or rises correspondingly. At sea level the height of the mercury in the tube measures approximately
29.92 in., although it varies slightly with atmospheric conditions.
An important consideration is that atmospheric pressure varies with altitude. The higher an object rises above sea level, the lower the pressure. Various atmospheric conditions have a definite relation to flying. The effect of temperature, altitude, and density of the air on aircraft performance is discussed in the following paragraphs.
DENSITY Density is a term that means weight per unit volume. Since air is a mixture of gases, it can be compressed. If the air in one container is under one-half as much pressure as the air in another identical container, the air under the greater pres.
sure weighs twice as much as that in the container under lower pressure. The air under greater pressure is twice as dense as that in the other container.
For equal weights of air, that which is under the greater pressure occupies only half the volume of that under half the pressure.
The density of gases is governed by the following rules:
Vacuum - Зв-
29.92 inches 24 Atmospheric pressure acting on surface of mercury
0.491 lbs.
mercury FIGURE 2-1. Measurement of atmospheric pressure.
(1) Density varies in direct proportion with the pressure.
(2) Density varies inversely with the temperature.
Thus, air at high altitudes is less dense than air at low altitudes, and a mass of hot air is less dense than a mass of cool sir.
Changes in density affect the aerodynamic performance of aircraft. With the same horsepower, an aircraft can fly faster at a high altitude where the density is low than at a low altitude where the density is great. This is because air offers less resistance to the aircraft when it contains a smaller number of air particles per unit volume.
HUMIDITY Humidity is the amount of water vapor in the air.
The maximum amount of water vapor that air can hold varies with the temperature. The higher the temperature of the air, the more water vapor it can absorb. By itself, water vapor weighs approximately 28
five-eighths as much as an equal amount of perfectly dry air. Therefore, when air contains water vapor it is not as heavy as air containing no moisture.
Assuming that the temperature and pressure remain the same, the density of the air varies inversely with the humidity. On damp days the air density is less than on dry days. For this reason, an aircraft requires a longer runway for takeoff on damp days than it does on dry days.
BERNOULLI'S PRINCIPLE AND SUBSONIC FLOW Bernoulli's principle states that when a fluid (air) flowing through a tube reaches a constriction, or narrowing of the tube, the speed of the fluid flowing through that constriction is increased and its pressure is decreased. The cambered (curved) surface of an airfoil (wing) affects the airlow exactly as a constriction in a tube affects airflow. This resemblance is illustrated in figure 2-2.
Normal Decreased Normal pressure pressure pressure A Normal flow Increased flow Normal flow FICURE 2-2. Bernoulli's principle.
Diagram A of figure 2-2 illustrates the effect of air passing through a constriction in a tube. In B, the air is flowing past a cambered surface, such as an airfoil, and the effect is similar to that of air passing through a restriction.
As the air flows over the upper surface of an airfoil, its speed or velocity increases and its pressure decreases. An area of low pressure is thus formed.
There is an area of greater pressure on the lower surface of the airfoil, and this greater pressure tends to move the wing upward. This difference in pressure between the upper and lower surfaces of the wing is called lift. Three-fourths of the total lift of an airfoil is the result of the decrease in pressure over the upper surface. The impact of air on the under surface of an airfoil produces the other onefourth of the total lift.
An aircraft in flight is acted upon by four forces:
(1) Gravity, or weight, the force that pulls the aircraft toward the earth.
(2) Lift, the force that pushes the aircraft upward.
(3) Thrust, the force that moves the aircraft forward.
(4) Drag, the force that exerts a braking action.
MOTION Motion is the act or process of changing place or position. An object may be in motion with respect to one object and motionless with respect to another. For example, a person sitting quietly in an aircraft flying at 200 knots is at rest or motionless with respect to the aircraft; however, the person is in motion with respect to the air or the earth, the same as is the aircraft.
Air has no force or power, except preasure, unless it is in motion. When it is moving, however, its force becomes apparent. A moving object in motionless air has a force exerted on it as a result of its own motion. It makes no difference in the effect then, whether an object is moving with respect to the air or the air is moving with respect to the object.
The flow of air around an object caused by the movement of either the air or the object, or both, is called the relative wind.
Velocity and Acceleration The terms "speed" and "velocity" are often used interchangeably, but they do not mean the same.
Speed is the rate of motion, and velocity is the rate of motion in a particular direction in relation to time.
An aircraft starts from New York City and flies 10 hrs. at an average speed of 260 m.p.h. At the end of this time the aircraft may be over the Atlantic Ocean, the Pacific Ocean, the Gulf of Mexico, or, if its flight were in a circular path, it may even be back over New York. If this same aircraft flew at a velocity of 260 m.p.h. in a southwestward direction, it would arrive in Los Angeles in about 10 hrs. Only the rate of motion is indicated in the first example and denotes the speed of the aircraft.
In the last example, the particular direction is in- 29
cluded with the rate of motion, thus, denoting the velocity of the aircraft.
Acceleration is defined as the rate of change of velocity. An aircraft increasing in velocity is an example of positive acceleration, while another aircraft reducing its velocity is an example of negative acceleration. (Positive acceleration is often referred to as acceleration and negative acceleration as deceleration.)
Newton's Laws of Motion The fundamental laws governing the action of air about a wing are Newton's laws of motion.
Newton's first law is normally referred to as the law of inertia. It simply means that a body at rest will not move unless force is applied to it. If it is moving at uniform speed in a straight line, force must be applied to increase or decrease that speed.
Since air has mass, it is a "body" in the meaning of the law. When an aircraft is on the ground with its engines stopped, inertia keeps the aircraft at rest. An aircraft is moved from its state of rest by the thrust force created by the propeller, by the expanding exhaust gases, or both. When it is flying at uniform speed in a straight line, inertia tends to keep the aircraft moving. Some external force is required to change the aircraft from its path of flight.
Newton's second law, that of force, also applies to objects. This law states that if a body moving with uniform speed is acted upon by an external force, the change of motion will be proportional to the amount of the force, and motion will take place in the direction in which the force acts. This law may be stated mathematically as follows:
Force = mass X acceleration (F = ma).
If an aircraft is flying against a headwind, it is slowed down. If the wind is coming from either side of the aircraft's heading, the aircraft is pushed off course unless the pilot takes corrective action against the wind direction.
Newton's third law is the law of action and reaction. This law states that for every action (force) there is an equal and opposite reaction (force).
This law is well illustrated by the action of a swimmer's hands. He pushes the water aft and thereby propels himself forward, since the water resists the action of his hands. When the force of lift on an aircraft's wing equals the force of gravity, the aircraft maintains level fight.
The three laws of motion which have been discussed are closely related and apply to the theory of flight. In many cases, all three laws may be operating on an aircraft at the same time.
AIRFOILS An airfoil is a surface designed to obtain a desirable reaction from the air through which it moves.
Thus, we can say that any part of the aircraft which converts air resistance into a force useful for flight is an airfoil. The blades of a propeler are so designed that when they rotate, their shape and position cause a higher pressure to be built up behind them than in front of them so that they will pull the aircraft forward. The profile of a conventional wing, shown in figure 2-3, is an excellent example of an airfoil. Notice that the top surface of the wing profile has greater curvature than the lower surface.
100 MPH
14.7 1b/in* 115 MPH " 14.54 1b/in° 105 MPH
14.67 1b/in° FIGURE 2-3. Airflow over a wing section.
The difference in curvature of the upper and lower surfaces of the wing builds up the lift force.
Air flowing over the top surface of the wing must reach the trailing edge of the wing in the same amount of time as the air flowing under the wing.
To do this, the air passing over the top surface moves at a greater velocity than the air passing below the wing because of the greater distance it must travel along the top surface. This increased velocity, according to Bernoulli's principle, means a corresponding decrease in pressure on the surtace.
Thus, a pressure differential is created between the upper and lower surfaces of the wing, forcing the wing upward in the direction of the lower pressure.
The theoretical amount of lift of the airfoil at a velocity of 100 m.p.h. can be determined by sampling the pressure above and below the airfoil at the point of greatest air velocity. As shown in figure 2-3, this pressure is 14.54 p.s.1, above the airfoil. Subtracting this pressure from the pressure below the airfoil, 14.67, gives a difference in pressure of 0.13 p.s.i. Multiplying 0.13 by 144 (number of square inches in a square foot) shows that each 30
square foot of this wing will lift 18.72 pounds. angle of attack decreases. The effect of increasing Thus, it can be seen that a small pressure differen- angle of attack on the center of pressure is shown tial across an airfoil section can produce a large in figure 2-5.
lifting force. Within limits, lift can be increased by increasing the angle of attack, the wing area, the freestream velocity, or the density of the air, or by changing the shape of the airfoil. Angle of attack = 0° Angle of Attack Relative Resultant • Negative pressure Before beginning the discussion on angle of airstream pattern attack and its effect on airfoils, we shall first consider the terms "chord" and "center of pressure." The chord of an airfoil or wing section is an • Positive pressure pressure Center of Angle of attack = 6° imaginary straight line which passes through the Negative pressure pattern section from the leading edge to the trailing edge, moves forward as shown in figure 2-4. The chord line provides one side of an angle which ultimately forms the angle of attack. The other side of the angle is formed by a line indicating the direction of the relative air- Angle of attack = 12° stream. Thus, angle of attack is defined as the angle between the chord line of the wing and the direc- Center of pressure tion of the relative wind. This is not to be confused moves forward with the angle of incidence, which is the angle between the chord line of the wing and the longitudinal axis of the aircraft.
Resultant lift Angle of attack = 18° Wing completely | L,ift stalled Chord Angle of attack Drag Relative airstream Center of pressure FIGURE 2-4. Positive angle of attack. FIGURE 2-5. Effect of increasing angle of attack.
On each minute part of an airfoil or wing surface, a small force is present. This force is different in magnitude and direction from any forces acting on other areas forward or rearward from this point.
It is possible to add all of these small forces mathematically, and the sum is called the resultant force (lift). This resultant force has magnitude, direction, and location, and can be represented as a vector, as shown in figure 2-4. The point of intersection of the resultant force line with the chord line of the airfoil is called the center of pressure. The center of pressure moves along the airfoil chord as the angle of attack changes. Throughout most of the flight range, the center of pressure moves forward with increasing angle of attack and rearward as the The angle of attack changes as the aircraft's attitude changes, Since the angle of attack has a great deal to do with determining lift, it is given primary consideration when designing airfoils. In a properly designed airfoil, the lift increases as the angle of attack is increased.
When the angle of attack is increased gradually toward a positive angle of attack, the lift component increases rapidly up to a certain point and then suddenly begins to drop off. During this action the drag component increases slowly at first and then rapidly as lift begins to drop off.
When the angle of attack increases to the angle of maximum lift, the burble point is reached. This is known as the critical angle. When the critical 31