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
angle is reached, the air ceases to flow smoothly over the top surface of the airfoil and begins to burble, or eddy. This means that air breaks away from the upper camber line of the wing. What was formerly the area of decreased pressure is now filled by this burbling air. When this occurs, the amount of lift drops and drag becomes excessive, The force of gravity exerts itself, and the nose of the aircraft drops. Thus we see that the burble point is the stalling angle.
As we have seen, the distribution of the pressure forces over the airfoil varies with the angle of attack. The application of the resultant force, that is, the center of pressure, varies correspondingly.
As this angle increases, the center of pressure moves forward; and as the angle decreases, the center of pressure moves back. The unstable travel of the center of pressure is characteristic of practically all airfoils.
Angle of Incidence The acute angle which the wing chord makes with the longitudinal axis of the aircraft is called the angle of incidence (figure 2-6), or the angle of wing setting. The angle of incidence in most cases is a fixed, built-in angle. When the leading edge of the wing is higher than the trailing edge, the angle of incidence is said to be positive. The angle of incidence is negative when the leading edge is lower than the trailing edge of the wing.
Longitudinal axis Angle of Chora line incidence of wing FIGURE 2-6. Angle of incidence.
Wing Area Wing area is measured in square feet and includes the part blanked out by the fuselage. Wing area is adequately described as the area of the shadow cast by the wing at high noon. Tests show that lift and drag forces acting on a wing are roughly proportional to the wing area. This means that if the wing area is doubled, all other variables remaining the same, the lift and drag created by the wing is doubled. If the area is tripled, lift and drag are tripled.
Shape of the Airfoil The shape of the airfoil determines the amount of turbulence or skin friction that it will produce. The shape of a wing consequently affects the efficiency of the wing.
Airfoil section properties differ from wing or aircraft properties because of the effect of the wing planform. A wing may have various airfoil sections from root to tip, with taper, twist, and sweepback.
The resulting aerodynamic properties of the wing are determined by the action of each section along the span.
Turbulence and skin friction are controlled mainly by the fineness ratio, which is defined as the ratio of the chord of the airfoil to the maximum thickness. If the wing has a high fineness ratio, it is a very thin wing. A thick wing has a low fineness ratio. A wing with a high fineness ratio produces a large amount of skin friction. A wing with a low fineness ratio produces a large amount of turbulence. The best wing is a compromise between these two extremes to hold both turbulence and skin friction to a minimum.
Efficiency of a wing is measured in terms of the lift over drag (L/D) ratio. This ratio varies with the angle of attack but reaches a definite maximum value for a particular angle of attack. At this angle, the wing has reached its maximum efficiency. The shape of the airfoil is the factor which determines the angle of attack at which the wing is most efficient; it also determines the degree of efficiency Research has shown that the most efficient airfoils for general use have the maximum thickness occurring about one-third of the way back from the leading edge of the wing.
High-lift wings and high-lift devices for wings have been developed by shaping the airfoils to produce the desired effect. The amount of lift produced by an airfoil will increase with an increase in wing camber. Camber refers to the curvature of an airfoil above and below the chord line surface. Upper camber refers to the upper surface, lower camber to the lower surface, and mean camber to the mean line of the section. Camber is positive when departure from the chord line is outward, and negative when it is inward. Thus, high-lift wings have a large positive camber on the upper surface and a slight negative camber on the lower surface. Wing flaps cause an ordinary wing to approximate this 32
same condition by increasing the upper camber and by creating a negative lower camber.
It is also known that the larger the wingspan as compared to the chord, the greater the lift obtained.
This comparison is called aspect ratio. The higher the aspect ratio, the greater the lift. In spite of the benefits from an increase in aspect ratio, it was found that definite limitations were of structural and drag considerations.
On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the aircraft off the ground. Thus, modern aircraft have airfoils which strike a medium between extremes, with the shape varying according to the aircraft for which it is designed.
CENTER OF GRAVITY Gravity is the pulling force that tends to draw all bodies in the earth's sphere to the center of the earth. The center of gravity may be considered as a point at which all the weight of the aircraft is concentrated. If the aircraft were supported at its LIFT THRUST exact center of gravity, it would balance in any position. Center of gravity is of major importance in an aircraft, for its position has a great bearing upon stability.
The center of gravity is determined by the general design of the aircraft. The designer estimates how far the center of pressure will travel. He then fixes the center of gravity in front of the center of pressure for the corresponding flight speed in order to provide an adequate restoring moment for flight equilibrium.
THRUST AND DRAG An aircraft in flight is the center of a continuous battle of forces. Actually, this conflict is not as violent as it sounds, but it is the key to all maneuvers performed in the air. There is nothing mysterious about these forces; they are definite and known. The directions in which they act can be calculated; and the aircraft itself is designed to take advantage of each of them. In all types of flying, flight calculations are based on the magnitude and direction of four forces: weight, lift, drag, and thrust. (See fig. 2-7.)
DRAG
GRAVITY FIGURE 2-7. Forces in action in flight.
Weight is the force of gravity acting downward upon everything that goes into the aircraft, such as the aircraft itself, the crew, the fuel, and the cargo.
Lift acts vertically and by so doing counteracts the effects of weight.
Drag is a backward deterrent force and is caused by the disruption of the airflow by the wings, fuselage, and protruding objects.
Thrust produced by the powerplant is the forward force that overcomes the force of drag.
Notice that these four forces are only in perfect balance when the aircraft is in straight and level unaccelerated flight.
The force of lift and drag are the direct result of the relationship between the relative wind and the aircraft. The force of lift always acts perpendicular to the relative wind, and the force of drag always acts parallel to the relative wind and in the same direction. These forces are actually the components that produced a resultant lift force on the wing as shown in figure 2-8.
Lift Resultant Drag FICURE 2-8. Resultant of lift and drag.
Weight has a definite relationship with lift, and thrust with drag. This relationship is quite simple, but very important in understanding the aerodynamics of flying. As stated previously, lift is the upward force on the wing acting perpendicular to the relative wind. Lift is required to counteract the aircraft's weight, caused by the force of gravity acting on the mass of the aircraft. This weight force acts downward through a point called the center of gravity which is the point at which all the weight of the aircraft is considered to be concentrated. When the lift force is in equilibrium with the weight force, the aircraft neither gains nor loses altitude.
If lift becomes less than weight, the aircraft loses altitude. When the lift is greater than weight, the aircraft gains altitude.
Drag must be overcome in order for the aircraft to move, and movement is essential to obtain lift.
To overcome the drag and move the aircraft forward, another force is essential. This force is thrust.
Thrust is derived from jet propulsion or from a propeller and engine combination. Jet propulsion theory is based on Newton's third law of motion which states that for every action there is an equal and opposite reaction. For example, in firing a gun the action is the bullet going forward while the reaction is the gun recoiling backwards. The turbine engine causes a mass of air to be moved backward at high velocity causing a reaction forward that moves the aircraft.
In a propeller/engine combination, the propeller is actually two or more revolving airfoils mounted on a horizontal shaft. The motion of the blades through the air produces lift similar to the lift on the wing, but acts in a horizontal direction, pulling the aircraft forward.
Before the aircraft begins to move, thrust must be exerted. It continues to move and gain speed until thrust and drag are equal. In order to maintain a steady speed, thrust and drag must remain equal, just as lift and weight must be equal for steady, horizontal flight. We have seen that increasing the lift means that the aircraft moves upward, whereas decreasing the lift so that it is less than the weight causes the aircraft to lose altitude. A similar rule applies to the two forces of thrust and drag. If the
r.p.m. of the engine is reduced, the thrust is lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft travels more and more slowly until its speed is insufficient to support it in the air.
Likewise, if the r.p.m. of the engine is increased, thrust becomes greater than drag, and the speed of the aircraft increases. As long as the thrust continues to be greater than the drag, the aircraft continues to accelerate. When drag equals thrust, the aircraft flies at a steady speed.
The relative motion of the air over an object that produces lift also produces drag. Drag is the resistance of the air to objects moving through it. If an aircraft is flying on a level course, the lift force acts vertically to support it while the drag force acts horizontally to hold it back. The total amount of drag on an aircraft is made up of many drag forces, 34
but for our purposes, we will only consider threeparasite drag, profile drag, and induced drag.
Parasite drag is made up of a combination of many different drag forces. Any exposed object on an aircraft offers some resistance to the air, and the more objects in the airstream, the more parasite drag. While parasite drag can be reduced by reducing the number of exposed parts to as few as practical and streamlining their shape, skin friction is the type of parasite drag most difficult to reduce. No surface is perfectly smooth. Even machined surfaces when inspected under magnification have a ragged uneven appearance. These ragged surfaces deflect the air near the surface causing resistance to smooth airflow. Skin friction can be reduced by using glossy flat finishes and eliminating protruding rivet heads, roughness, and other irregularities.
Profile drag may be considered the parasite drag of the airfoil. The various components of parasite drag are all of the same nature as profile drag.
The action of the airfoil that gives us lift also causes induced drag. Remember that the pressure above the wing is less than atmospheric, and the pressure below the wing is equal to or greater than atmospheric pressure. Since fluids always move from high pressure toward low pressure, there is a spanwise movement of air from the bottom of the wing outward from the fuselage and upward around the wing tip. This flow of air results in spillage" over the wing tip, thereby setting up a whirlpool of air called a vortex (figure 2-9). The air on the upper surface has a tendency to move in toward the fuselage and off the trailing edge. This air current forms a similar vortex at the inner portion of the trailing edge of the wing. These vortices increase drag, because of the turbulence produced, and constitute induced drag.
FIGURE 2-9, Wing tip vortices.
Just as lift increases with an increase in angle of attack, induced drag also increases as the angle of attack becomes greater. This occurs because as the angle of attack is increased, there is a greater pressure difference between the top and bottom of the wing. This causes more violent vortices to be set up, resulting in more turbulence and more induced drag.
AXES OF AN AIRCRAFT Whenever an aircraft changes its attitude in flight, it must turn about one or more of three axes.
Figure 2-10 shows the three axes, which are imaginary lines passing through the center of the aircraft. The axes of an aircraft can be considered as imaginary axles around which the aircraft turns like a wheel. At the center, where all three axes intersect, each is perpendicular to the other two.
The axis which extends lengthwise through the fuselage from the nose to the tail is called the longitudinal axis. The axis which extends crosswise, from wing tip to wing tip, is the lateral axis. The axis which passes through the center, from top to bottom, is called the vertical axis.
Motion about the longitudinal axis resembles the roll of a ship from side to side. In fact, the names used in describing the motion about an aircraft's three axes were originally nautical terms. They have been adapted to aeronautical terminology because of the similarity of motion between an aircraft and a ship.
Thus, the motion about the longitudinal axis is called roll; motion along the lateral (crosswing) axis is called pitch. Finally, an aircraft moves about its vertical axis in a motion which is termed yaw.
This is a horizontal movement of the nose of the aircraft.
Roll, pitch, and yaw-the motions an aircraft makes about its longitudinal, lateral, and vertical axes are controlled by three control surfaces. Roll is produced by the ailerons, which are located at the trailing edges of the wings. Pitch is affected by the elevators, the rear portion of the horizontal tail assembly. Yaw is controlled by the rudder, the rear portion of the vertical tail assembly.
STABILITY AND CONTROL An aircraft must have sufficient stability to maintain a uniform flight path and recover from the various upsetting forces. Also, to achieve the best performance, the aircraft must have the proper response to the movement of the controls.
• Rudder Longitudinal axis Elevator
- Aileron Vertical axis Lateral axis Longitudinal axis Normal attitude Banking (roll) control Vie " A affected by aileron movement Lateral axis Normal attitude Normal attitude View B Climb and dive (pitch) control affected by elevator movement Vertical axis View C Directional (YAW) control affected by rudder movement FIGURE 2-10. Motion of an aircraft about its axes.
Three terms that appear in any discussion of stability and control are: (1) Stability, (2) maneuverability, and (3) controllability. Stability is the characteristic of an aircraft which tends to cause it to fly (hands off) in a straight and level flight path.
Maneuverability is the ability of an aircraft to be directed along a desired flight path and to withstand the stresses imposed. Controllability is the quality of the response of an aircraft to the pilot's commands while maneuvering the aircraft.
Static Stability An aircraft is in a state of equilibrium when the sum of all the forces acting on the aircraft and all the moments is equal to zero. An aircraft in equilibrium experiences no accelerations, and the aircraft continues in a steady condition of flight. A gust of wind or a deflection of the controls disturbs the equilibrium, and the aircraft experiences acceleration due to the unbalance of moment or force.
The three types of static stability are defined by the character of movement following some disturbance from equilibrium. Positive static stability exists when the disturbed object tends to return to equilibrium. Negative static stability or static instability exists when the disturbed object tends to continue in the direction of disturbance. Neutral static stability exists when the disturbed object has neither the tendency to return nor continue in the displacement direction, but remains in equilibrium in the direction of disturbance. These three types of stability are illustrated in figure 2-11.
Dynamic Stability While static stability deals with the tendency of a displaced body to return to equilibrium, dynamic stability deals with the resulting motion with time.
If an object is disturbed from equilibrium, the time history of the resulting motion defines the dynamic stability of the object. In general, an object demonstrates positive dynamic stability if the amplitude of motion decreases with time. If the amplitude of motion increases with time, the object is said to possess dynamic instability.
Any aircraft must demonstrate the required degrees of static and dynamic stability. If an aircraft were designed with static instability and a rapid rate of dynamic instability, the aircraft would be very difficult, if not impossible, to fly. Usually, positive dynamic stability is required in an aircraft design to prevent objectionable continued oscillations of the aircraft.
Tendency to return to equilibrium
- Equilibrium A. Positive Static Stability Equilibrium Tendency to continue in displacement direction B. Negative static stabiliy or static instability Equilibrium encountered at any point of displacement C. Neutral static stability FIGURE 2-11. Static stability.
Longitudinal Stability When an aircraft has a tendency to keep a constant angle of attack with reference to the relative wind-that is, when it does not tend to put its nose down and dive or lift its nose and stall--it is said to have longitudinal stability. Longitudinal stability refers to motion in pitch. The horizontal stabilizer is the primary surface which controls longitudinal stability. The action of the stabilizer depends upon the speed and angle of attack of the aircraft.
Figure 2-12 illustrates the contribution of tail hift to stability. If the aircraft changes its angle of attack, a change in lift takes place at the aerody.
namic center (center of pressure) of the horizontal stabilizer.
Increase in lift Aerodynamic center of horizontal tail FIGURE 2-12. Producing tail lift.
Under certain conditions of speed, load, and angle of attack, the flow of air over the horizontal stabilizer creates a force which pushes the tail up or down. When conditions are such that the airflow creates equal forces up and down, the forces are said to be in equilibrium. This condition is usually found in level flight in calm air.
Directional Stability Stability about the vertical axis is referred to as directional stability. The aircraft should be de.
signed so that when it is in straight and level flight it remains on its course heading even though the pilot takes his hands and feet off the controls. If an aircraft recovers automatically from a skid, it has been well designed and possesses good directional balance. The vertical stabilizer is the primary surface which controls directional stability.
As shown in figure 2-13, when an aircraft is in a sideslip or yawing, the vertical tail experiences a change in angle of attack with a resulting change in lift [not to be confused with the lift created by the wing]. The change in lift, or side force, on the vertical tail creates a yawing moment about the center of gravity which tends to return the aircraft to its original fight path.
Relative wind Tail moment / arm Center of gravity Change in tail lift FIGURE 2-13. Contribution of vertical tail to directional stability.
Sweptback wings aid in directional stability. If the aircraft yaws from its direction of flight, the wing which is farther ahead offers more drag than the wing which is aft. The effect of this drag is to hold back the wing which is farther ahead, and to let the other wing catch up.
Directional stability is also aided by using a large dorsal fin and a long fuselage.
The high Mach numbers of supersonic flight reduce the contribution of the vertical tail to directional stability. To produce the required directional stability at high Mach numbers, a very large vertical tail area may be necessary. Ventral (belly) fins may be added as an additional contribution to directional stability.
Lateral Stability We have seen that pitching is motion about the aircraft's lateral axis and yawing is motion about its vertical axis. Motion about its longitudinal (fore and aft) axis is a lateral or rolling motion. The tendency to return to the original attitude from such motion is called lateral stability.
The lateral stability of an airplane involves consideration of rolling moments due to sideslip. A sideslip tends to produce both a rolling and a yawing motion. If an airplane has a favorable rolling moment, a sideslip will tend to return the airplane to a level flight attitude.
The principal surface contributing to the lateral stability of an airplane is the wing. The effect of the geometric dihedral (figure 2-14) of a wing is a powerful contribution to lateral stability. As shown in figure 2-14, a wing with dihedral develops stable rolling moments with sideslip. With the relative wind from the side, the wing into the wind is subject to an increase in angle of attack and develops an increase in lift. The wing away from the wind is subject to a decrease in angle of attack and develops less lift. The changes in lift effect a rolling moment tending to raise the windward wing.
When a wing is swept back, the effective dihedral increases rapidly with a change in the lift coefficient of the wing. Sweepback is the angle between a line perpendicular to the fuslage center line and the quarter chord of each wing airfoil section.
Sweepback in combination with dihedral causes the dihedral effect to be excessive. As shown in figure 2-15, the swept-wing aircraft in a sideslip has the wing that is into the wind operating with an effective decrease in sweepback, while the wing out of the wind is operating with an effective increase in 38
Plane of symmetry Line perpendicular to Dihedral angle Effective increase in lift plane of symmetry due to sideslip Relative wind Decrease in lift due to sideslip FicURE 2-14. Contribution of dihedral to lateral stability.
sweepback. The wing into the wind develops more lift, and the wing out of the wind develops less.
This tends to restore the aircraft to a level flight attitude.
The amount of effective dihedral necessary to produce satisfactory flying qualities varies greatly with the type and purpose of the aircraft. Generally, the effective dihedral is kept low, since high roll due to sideslip can create problems. Excessive dihedral effect can lead to Dutch Roll, difficult rudder coordination in rolling maneuvers, or place extreme demands for lateral control power during crosswind takeoff and landing.
CONTROL Control is the action taken to make the aircraft follow any desired flight path. When an aircraft is said to be controllable, it means that the craft responds easily and promptly to movement of the controls. Different control surfaces are used to control the aircraft about each of the three axes.
Moving the control surfaces on an aircraft changes the airfiow over the aircraft's surface. This, in turn, creates changes in the balance of forces acting to keep the aircraft flying straight and level.
Reduced sweep of wing into wind results in increased lift Increased sweep of wing out of wind results in decreased lift Relative wind Relative wind FIGURE 2-15. Effect of sweepback on lateral stability.
Rudder Secondary Group Included in the secondary group are the trim tabs and spring tabs. Trim tabs (figure 2-17) are small airfoils recessed into the trailing edges of the primary control surfaces. The purpose of trim tabs is to enable the pilot to trim out any unbalanced condition which may exist during flight, without exert- Elevator ing any pressure on the primary controls. Lach trim tab is hinged to its parent primary control surface, but is operated by an independent control.
Spring tabs are similar in appearance to trim Spoiler tabs, but serve an entirely different purpose. Spring aileron tabs are used to aid the pilot in moving the primary control surfaces.
FICURE 2-16. Primary flight controls.
FLIGHT CONTROL SURFACES The flight control surfaces are hinged or movable airfoils designed to change the attitude of the aircraft during flight. These surfaces may be divided into three groups, usually referred to as the primary group, secondary group, and auxiliary group.
Primary Group The primary group includes the ailerons, elevators, and rudder (figure 2-16). These surfaces are used for moving the aircraft about its three axes.
Trim tab Trim tabs FIGURE 2-17. Trim tabs.
The ailerons and elevators are generally operated from the cockpit by a control stick on single-engine aircraft and by a wheel and yoke assembly on multiengine aircraft. The rudder is operated by foot pedals on all types of aircraft.
Auxiliary Group Included in the auxiliary group of flight control surfaces are the wing flaps, spoilers, speed brakes, slats, leading edge flaps and slots.
The auxiliary groups may be divided into two sub-groups. Those whose primary purpose is lift augmenting and those whose primary purpose is lift decreasing. In the first group are the flaps, both trailing edge and leading edge (slats), and slots. The lift decreasing devices are speed brakes and spoilers.
The trailing edge airfoils (Haps) increase the wing area thereby increasing lift on takeof and decrease the speed during landing. These airfoils are retractable and fair into the wing contour.
Others are simply a portion of the lower skin which extends into the airstream thereby slowing the aircraft.
Leading edge flaps are airfoils extended from and retracted into the leading edge of the wing. Some installations create a slot (an opening between the extended airfoil and the leading edge). The flap (termed slat by some manufacturers) and slot create additional lift at the slower speeds of takeoff and landing. Other installations have permanent slots built in the leading edge of the wing. At cruising speeds, the trailing edge and leading edge flaps (slats) are retracted into the wing proper.
Lift decreasing devices are the speed brakes (spoilers). In some installations, there are two types of spoilers. The ground spoiler is extended only after the aircraft is on the ground thereby assisting in the braking action. The flight spoiler 40
assists in lateral control by being extended whenever the aileron on that wing is rotated up. When actuated as speed brakes, the spoiler panels on both wings raise up--the panel on the "up" aileron wing raising more than the panel on the down aileron side. This provides speed brake operation and later control simultaneously.
Slats are movable control surfaces attached to the leading edges of the wings. When the slat is closed, it forms the leading edge of the wing. When in the open position (extended forward), a slot is created between the slat and the wing leading edge. At low airspeeds this increases lift and improves handling characteristics, allowing the aircraft to be controlled at airspeeds below the otherwise normal landing speed.
CONTROL AROUND THE LONGITUDINAL AXIS The motion of the aircraft about the longitudinal axis is called rolling or banking. The ailerons (figure 2-18) are used to control this movement.
The ailerons form a part of the wing and are located in the trailing edge of the wing toward the tips. Ailerons are the movable surfaces of an otherwise fixed-surface wing. The aileron is in neutral position when it is streamlined with the trailing edge of the wing.
Ailerons respond to side pressure applied to the control stick. Pressure applied to move the stick toward the right raises the right aileron and lowers Ailerons Vertical axis Level flight FIGURE 2-18. Aileron action.
the left aileron, causing the aircraft to bank to the right. Ailerons are linked together by control cables so that when one aileron is down, the opposite aileron is up. The function of the lowered aile ron is to increase the lift by increasing the wing camber. At the same time, the down aileron also creates some additional drag since it is in the area of high pressure below the wing. The up aileron, on the opposite end of the wing, decreases lift on that end of the wing. The increased lift on the wing whose aileron is down, raises this wing. This causes the aircraft to roll about its longitudinal axis as shown in figure 2-19.
As a result of the increased lift on the wing with the lowered aileron, drag is also increased. This drag attempts to pull the nose in the direction of the high wing. Since the ailerons are used with the rudder when making turns, the increased drag tries to turn the aircraft in the direction opposite to that desired. To avoid this undesirable effect, aircraft are often designed with differential travel of the ailerons.
Differential aileron travel (figure 2-20) provides more aileron up travel than down travel for a given movement of the control stick or wheel in the cockpit.
The spoilers, or speed brakes as they are also called, are plates fitted to the upper surface of the wing. They are usually deflected upward by hydraulic actuators in response to control wheel movement in the cockpit. The purpose of the spoilers is to disturb the smooth airflow across the top of the airfoil thereby creating an increased amount of drag and a decreased amount of lift on that airfoil.
Spoilers are used primarily for lateral control, When banking the airplane, the spoilers function with the ailerons. The spoilers on the up aileron side raise with that aileron to further decrease the lift on that wing. The spoiler on the opposite side remains in the faired position. When the spoilers are used as a speed brake, they are all deflected upward simultaneously. A separate control lever is provided for operating the spoilers as speed brakes.
While we tend to think of a spoiler as being a fairly complicated, controlled device, we should keep in mind that some are not controllable. Some spoilers are automatic in operation in that they come into effect only at a high angle of attack. This 41
Longitudinal axis Roll direction