FIGURE 2-21. Fixed spoilers or stall strip.
arrangement keeps them out of the slipstream at cruise and high speeds.
• A fixed spoiler may be a small wedge affixed to the leading edge of the airfoil as shown in figure
2-21. This type spoiler causes the inboard portion of the wing to stall ahead of the outboard portion which results in aileron control right up to the occurrence of complete wing stall.
Direction of rudder movement Direction of airplane motion Direction of pedal movement.
Relative wind - FIGURE 2-22. Rudder action.
Use extreme accuracy in positioning leading edge spoilers when re-installing them after they have been removed for maintenance, Improper positioning may result in adverse stall characteristics. Always follow the manufacturers' instructions regarding location and method of attachment.
CONTROL AROUND THE VERTICAL AXIS Turning the nose of the aircraft causes the aircraft to rotate about its vertical axis. Rotation of the aircraft about the vertical axis is called yawing.
This motion is controlled by using the rudder as illustrated in figure 2-22.
The rudder is a movable control surface attached to the trailing edge of the vertical stabilizer. To turn the aircraft to the right, the rudder is moved to the right. The rudder protrudes into the airstream, causing a force to act upon it. This is the force necessary to give a turning movement about the center of gravity which turns the aircraft to the right. If the rudder is moved to the left, it induces a
- Bellcrank Pivot • - Pivot counterclockwise rotation and the aircraft similarly turns to the left. The rudder can also be used in controlling a bank or turn in fight.
The main function of the rudder is to turn the aircraft in flight, This turn is maintained by the side pressure of the air moving past the vertical surfaces. When an aircraft begins to slip or skid, rudder pressure is applied to keep the aircraft headed in the desired direction (balanced).
Slip or sideslipping refers to any motion of the aircraft to the side and downward toward the inside of a turn. Skid or skidding refers to any movement upward and outward away from the center of a turn.
CONTROL AROUND THE LATERAL AXIS When the nose of an aircraft is raised or lowered, it is rotated about its lateral axis. Elevators are the movable control surfaces that cause this rotation (figure 2-23). They are normally hinged to the trailing edge of the horizontal stabilizer.
Up
- — 0° Neutral Down FICURE 2-23. Elevator action.
The elevators are used to make the aircraft climb or dive and also to obtain sufficient lift from the wings to keep the aircraft in level flight at various speeds.
The elevators can be moved either up or down. If the elevator is rotated up, it decreases the lift force on the tail causing the tail to lower and the nose to rise. If the elevator is rotated downward, it increases the lift force on the tail causing it to rise and the nose to lower. Lowering the aircraft's nose increases forward speed, and raising the nose decreases forward speed.
Some aircraft use a movable horizontal surface called a stabilator (figure 2-24). The stabilator serves the same purpose as the horizontal stabilizer and elevator combined. When the cockpit control is moved, the complete stabilator is moved to raise or lower the leading edge, thus changing the angle of attack and the amount of lift on the tail surfaces.
Pivot FIcURE 2-24. Movable horizontal stabilator.
Aircraft empennages have been designed which combine the vertical and horizontal stabilizers. Such empennages have the stabilizers set at an angle as shown in figure 2-25. This arrangement is referred to as a butterfly or vee tail.
The control surfaces are hinged to the stabilizers at the trailing edges. The stabilizing portion of this arrangement is called a stabilator, and the control portion is called the ruddervator. The ruddervators can be operated both up or both down at the same Stabilizers time. When used in this manner, the result is the same as with any other type of elevator. This action is controlled by the stick or control column.
The ruddervators can be made to move opposite each other by pushing the left or right rudder pedal (figure 2-26). If the right rudder padel is pushed, the right ruddervator moves down and the left ruddervator moves up. This produces turning moments to move the nose of the aircraft to the right.
Ruddervator FIGURE 2-25. A butterfly or vee tail.
Direction of yaw Direction of surface travel
- Lateral axis Direction of pitch (A) (B) FIGURE 2-26. Ruddervator action. (A) functioning as an elevator; (B) functioning as a rudder.
TABS Even though an aircraft has inherent stability, it does not always tend to fly straight and level. The weight of the load and its distribution affect stability. Various speeds also affect its flight characteristics. If the fuel in one wing tank is used before that in the other wing tank, the aircraft tends to roll toward the full tank. All of these variations require constant exertion of pressure on the controls for correction. While climbing or gliding, it is necessary to apply pressure on the controls to keep the aircraft in the desired attitude.
To offset the forces that tend to unbalance an aircraft in flight, ailerons, elevators, and rudders are provided with auxiliary controls known as tabs.
These are small, hinged control surfaces (figure 2-27) attached to the trailing edge of the primary control surfaces. Tabs can be moved up or down by means of a crank or moved electrically from the cockpit. These tabs can be used to balance the 45
forces on the controls so that the aircraft flies straight and level, or may be set so that the aircraft maintains either a climbing or gliding attitude.
Control horn Control surface Fixed surface Tab A - TRIM TAB Variable linkage Hom free to
- pivot on hinge axis
B - SERVO TAB
- Control horn
C - BALANCE TAB — Control horn D - SPRING TAB Spring cartridge FIGURE 2-27. Flight control trim tab types.
Trim Tabs Trim tabs trim the aircraft in flight. To trim means to correct any tendency of the aircraft to move toward an undesirable flight attitude. Trim tabs control the balance of an aircraft so that it maintains straight and level flight without pressure on the control column, control wheel, or rudder pedals. Figure 2-27A illustrates a trim tab.
Note that the tab has a variable linkage which is adjustable from the cockpit. Movement of the tab in one direction causes a deflection of the control surface in the opposite direction. Most of the trim tabs installed on aircraft are mechanically operated from the cockpit through an individual cable system.
However, some aircraft have trim tabs that are operated by an electrical actuator. Trim tabs are either controlled from the cockpit or adjusted on the ground before taking off. Trim tabs are installed on elevators, rudders, and ailerons.
Servo Tabs Servo tabs (figure 2-27B) are very similar in operation and appearance to the trim tabs just diacussed. Servo tabs, sometimes referred to as flight tabs, are used primarily on the large main control surfaces. They aid in moving the control surface and holding it in the desired position. Only the servo tab moves in response to movement of the cockpit control. (The servo tab horn is free to pivot to the main control surface hinge axis.) The force of the airflow on the servo tab then moves the primary control surface. With the use of a servo tab less force is needed to move the main control surface.
Balance Tabs A balance tab is shown in figure 2-27C. The linkage is designed in such a way that when the main control surface is moved, the tab moves in the opposite direction. Thus, aerodynamic forces, acting on the tab, assist in moving the main control surface.
Spring Tabs Spring tabs (figure 2-27D) are similar in appearance to trim tabs, but serve an entirely different purpose. Spring tabs are used for the same purpose as hydraulic actuators; that is, to aid in moving a primary control surface. There are various spring arrangements used in the linkage of the spring tab.
On some aircraft, a spring tab is hinged to the trailing edge of each aileron and is actuated by a spring-loaded push-pull rod assembly which is also linked to the aileron control linkage. The linkage is connected in such a way that movement of the aileron in one direction causes the spring tab to be deflected in the opposite direction. This provides a balanced condition, thus reducing the amount of force required to move the ailerons.
The deflection of the spring tabs is directly proportional to the aerodynamic load imposed upon the aileron; therefore, at low speeds the spring tab remains in a neutral position and the aileron is a direct manually controlled surface. At high speeds, however, where the aerodynamic load is great, the tab functions as an aid in moving the primary control surface.
To lessen the force required to operate the control surfaces they are usually balanced statically and aerodynamically. Aerodynamic balance is usually achieved by extending a portion of the con.
Hinge line Overhang Handley-Page Friese FIGURE 2-28. Three forms of aerodynamic balance.
trol surface ahead of the hinge line. This utilizes the airflow about the aircraft to aid in moving the surface. The various methods of achieving aerodynamic balance are shown in figure 2-28.
Static balance is accomplished by adding weight to the section forward of the hinge line until it weighs the same as the section aft of it. When repairing a control surface use care to prevent upsetting or disturbing the static balance. An unbalanced surface has a tendency to flutter as air passes over it.
High-Lift Devices High-lift devices are used in combination with airfoils in order to reduce the takeoff or landing speed by changing the lift characteristics of an airfoil during the landing or takeoff phases. When these devices are no longer needed they are returned to a position within the wing to regain the normal characteristics of the airfoil.
Two high-lift devices commonly used on aircraft are shown in figure 2-29. One of these is known as a slot, and is used as a passageway through the leading edge of the wing. At high angles of attack the air flows through the slot and smooths out the airflow over the top surface of the wing. This enables the wing to pass beyond its normal stalling point without stalling. Greater lift is obtained with the wing operating at the higher angle of attack.
The other high-lift device is known as a flap. It is a hinged surface on the trailing edge of the wing. The flap is controlled from the cockpit, and when not in use fits smoothly into the lower surface of each wing. The use of flaps increases the camber of a wing and therefore the lift of the wing, making it possible for the speed of the aircraft to be decreased without stalling. This also permits a steeper gliding angle to be obtained as in the landing approach. Flaps are primarily used during takeoff and landing.
The types of flaps in use on aircraft include: (1) Plain, (2) split, (3) Fowler, and (4) slotted. The plain (figure 2-30) is simply, hinged to the wing and forms a part of the wing surface when raised.
The split flap (figure 2-30) gets its name from the hinge at the bottom part of the wing near the trailing edge permitting it to be lowered from the 47
PLAIN Slot Flap SPLIT ficient flaps Slots & Slots FLAPS ALONE alone G 2 @ alone, Plain wing Angle of attack 13°15° 20° 23° FOWLER FIGURE 2-29. High-lift devices.
fixed top surface. The Fowler flap (figure 2-30) fits into the lower part of the wing so that it is flush with the surface. When the flap is operated, it slides AIR backward on tracks and tilts downward at the same time. This increases wing camber, as do the other SLOTTED types of flaps. However, Fowler flaps also increase the wing area; thus, they provide added lift without unduly increasing drag. FICURE 2-30. Types of wing flaps.
The slotted flap (figure 2-30). is like the Fowler flap in operation, but in appearance it is similar to Laminar flow area the plain flap. This flap is equipped with either - Turbulent flow area tracks and rollers or hinges of a special design.
During operation, the flap moves downward and rearward away from the position of the wing. The "slot" thus opened allows a flow of air over the the airflow and to improve the efficiency of the flap. upper surface of the flap. The effect is to streamline Local free-stream velocity.
BOUNDARY LAYER CONTROL DEVICES The layer of air over the surface which is slower moving in relation to the rest of the slipstream is called the boundary layer. The initial airflow on a smooth surface (figure 2-31) gives evidence of a very thin boundary layer with the flow occurring in smooth laminations of air sliding smoothly over one another. Therefore, the term for this type of flow is the laminar boundary layer.
- Wing surface • Boundary layer thickness- FIcURE 2-31. Boundary layer characteristics.
As the flow continues back from the leading edge, friction forces in the boundary layer continue to 48
dissipate the energy of the airstream, slowing it down. The laminar boundary layer increases in thickness with increased distance from the wing leading edge. Some distance back from the leading edge, the laminar flow begins an oscillatory disturbance which is unstable. A waviness occurs in the laminar boundary layer which ultimately grows larger and more severe and destroys the smooth laminar flow. Thus, a transition takes place in which the laminar boundary layer decays into a turbulent boundary layer. The same sort of transition can be noticed in the smoke from a cigarette in still air. At first, the smoke ribbon is smooth and laminar, then develops a definite waviness, and decays into a random turbulent smoke pattern.
Boundary layer control devices are additional means of increasing the maximum lift coefficient of a section. The thin layer of air adjacent to the surface of an airfoil shows reduced local velocities from the effect of skin friction. At high angles of attack, the boundary layer on the upper surface tends to stagnate (come to a stop). When this happens, the airflow separates from the surface and stall occurs.
Boundary layer control devices for high-lift applications feature various devices to maintain high velocity in the boundary layer and delay separation of the airflow. Control of the boundary layer's kinetic energy can be accomplished using slats and the application of suction to draw off the stagnant air and replace it with high-velocity air from outside the boundary layer.
Slats (figure 2-32) are movable control surfaces attached to the leading edge of the wing. 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. Thus, high-energy air is introduced into the boundary layer over the top of the wing. This is known as "boundary layer control." At low airspeeds this improves handling characteristics, allowing the aircraft to be controlled laterally at airspeeds below the otherwise normal landing speed.
Controlling boundary layer air by surface suction allows the wing to operate at higher angles of attack. The effect on lift characteristics is similar to that of a slot, because the slot is essentially a boundary layer control device ducting high-energy air to the upper surface.
Boundary layer control can also be accomplished by directing high-pressure engine bleed air through a narrow orifice located just forward of the wing Fixed slot Automatic slot
• Slat Boundary layer control by upper surface suction Boundary layer control by flap augmentation FIGURE 2-32. Methods of controlling boundary layer air.
flap leading edge. This directs a laminar flow (air in layers) of air over the wing and flaps when the flaps have opened sufficiently to expose the orifice.
The high-temperature, high-velocity laminar air passing over the wing and flaps delays flow separation (the airstream over an airfoil no longer follows the contour of the airfoil), hence reduces turbulence and drag (see figure 2-29). This results in a lower stall speed and allows slower landing speeds.
FORCES ACTING ON A HELICOPTER One of the differences between a helicopter and a fixed-wing aircraft is the main source of lift. The fixed-wing aircraft derives its lift from a fixed airfoil surface while the helicopter derives lift from a rotating airfoil called the rotor. Aircraft are classified as either fixed-wing or rotating wing. The word helicopter comes from a Greek word meaning "helical wing" or "rotating wing."
During any kind of horizontal or vertical flight, there are four forces acting on the helicopter-lift, 49
thrust, weight, and drag. Lift is the force required to support the weight of the helicopter. Thrust is the force required to overcome the drag on the fuselage and other helicopter components.
During hovering flight in a no-wind condition, the tip-path plane is horizontal, that is, parallel to the ground. Lift and thrust act straight up; weight and drag act straight down. The sum of the lift and thrust forces must equal the sum of the weight and drag forces in order for the helicopter to hover.
During vertical flight in a no-wind condition, the lift and thrust forces both act vertically upward.
Weight and drag both act vertically downward.
When lift and thrust equal weight and drag, the helicopter hovers; if lift and thrust are less than weight and drag, the helicopter descends vertically;
if lift and thrust are greater than weight and drag, the helicopter rises vertically.
For forward flight, the tip-path plane is tilted forward, thus tilting the total lift-thrust force forward from the vertical. This resultant lift-thrust force can be resolved into two components--lift acting vertically upward, and thrust acting horizontally in the direction of flight. In addition to lift and thrust, there are weight, the downward acting force, and drag, the rearward acting or retarding force of inertia and wind resistance.
In straight-and-level, unaccelerated forward flight, lift equals weight and thrust equals drag.
(Straight-and-level flight is flight with a constant heading and at a constant altitude.) If lift exceeds weight, the helicopter climbs; if the lift is less than weight, the helicopter descends. If thrust exceeds drag, the helicopter speeds up; if thrust is less than drag, it slows down.
In sideward flight, the tip-path plane is tilted sideward in the direction that flight is desired thus tilting the total lift-thrust vector sideward. In this case, the vertical or lift component is still straight up, weight straight down, but the horizontal or thrust component now acts sideward with drag acting to the opposite side.
For rearward flight, the tip-path plane is tilted rearward tilting the lift-thrust vector rearward. The thrust component is rearward and drag forward, just the opposite to forward fight. The lift component is straight up and weight straight down.
Torque Newton's third law of motion states, "To every action there is an equal and opposite reaction." As the main rotor of a helicopter turns in one direction, the fuselage tends to rotate in the opposite direction. This tendency for the fuselage to rotate is called torque. Since torque effect on the fuselage is a direct result of engine power supplied to the main rotor, any change in engine power brings about a corresponding change in torque effect. The greater the engine power, the greater the torque effect.
Since there is no engine power being supplied to the main rotor during autorotation, there is no torque reaction during autorotation.
The force that compensates for torque and provides for directional control can be produced by means of an auxiliary rotor located on the end of the tail boom. This auxiliary rotor, generally referred to as a tail rotor, or anti-torque rotor, produces thrust in the direction opposite to torque reaction developed by the main rotor (figure 2-33).
Foot pedals in the cockpit permit the pilot to increase or decrease tail-rotor thrust, as needed, to neutralize torque effect.
Other methods of compensating for torque and providing directional control are illustrated in figure 2-33.
The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of gyroscopic action, one of which is precession. Gyroscopic precession is the resultant action or deflection of a spinning object when a force is applied to this object. This action occurs approximately 90° in the direction of rotation from the point where the force is applied (figure 2-34). Through the use of this principle, the tip-path plane of the main rotor may be tilted from the horizontal.
The movement of the cyclic pitch control in a two-bladed rotor system increases the angle of attack of one rotor blade with the result that a greater lifting force is applied at this point in the plane of rotation. This same control movement simultaneously decreases the angle of attack of the other blade a like amount thus decreasing the lifting force applied at this point in the plane of rotation. The blade with the increased angle of attack tends to rise; the blade with the decreased angle of attack tends to lower. However, because of the gyroscopic precession property, the blades do not rise or lower to maximum deflection until a point approximately 90° later in the plane of rotation.
As shown in figure 2-35, the retreating blade angle of attack is increased and the advancing blade angle of attack is decreased resulting in a tipping forward of the tip-path plane, since maxi- 50
Vane in rotor slipstream Anti-torque rotor Differential torque between two rotors Differential tilt of rotor thrusts FIcURE 2-33. Methods for achieving directional control.
mum deflection takes place 90° later when the blades are at the rear and front respectively.
In a three-bladed rotor, the movement of the cyclic pitch control changes the angle of attack of each blade an appropriate amount so that the end result is the same, a tipping forward of the tip-path plane when the maximum change in angle of attack is made as each blade passes the same points at which the maximum increase and decrease are made for the two-bladed rotor as shown in figure 2-35.
As each blade passes the 90° position on the left, the maximum increase in angle of attack occurs. As each blade passes the 90° position to the right, the maximum decrease in angle of attack occurs. Maximum deflection takes place 90° later, maximum upward deflection at the rear and maximum downward deflection at the front, and the tip-path plane tips forward.
Dissymmetry of Lift The area within the tip-path plane of the main rotor is known as the disk area or rotor disk. When hovering in still air, lift created by the rotor blades at all corresponding positions around the rotor disk is equal. Dissymmetry of lift is created by horizontal flight or by wind during hovering flight, and is the difference in lift that exists between the advancing blade half of the disk area and the retreating blade half.
At normal rotor operating r.p.m. and zero air- 51
As the helicopter moves into forward flight, the relative wind moving over each rotor blade becomes a combination of the rotational speed of the rotor and the forward movement of the helicopter. As Reaction occurs shown in figure 2-37, the advancing blade has the combined speed of the blade velocity plus the speed here of the helicopter. On the opposite side, the retreating blade speed is the blade velocity less the speed of the helicopter.
Force applied here It is apparent that the lift over the advancing New axis Old axis Gyro tips up here Gyro tips - down here FIGURE 2-34. Gyroscopic precession principle.
speed, the rotating blade tip speed of most helicopter main rotors is approximately 400 m.p.h. When hovering in a no-wind condition, the speed of the relative wind at the blade tips and at any specific point along the blade is the same throughout the tip-path plane (figure 2-36). However, the speed is reduced as this point moves closer to the rotor hub as indicated in figure 2-36 by the two inner circles.
blade half of the rotor disk will be greater than the lift over the retreating blade half during horizontal fight or when hovering in a wind.
Due to the greater lift of the advancing blade the helicopter would roll to the left unless something is done to equalize the lift of the blades on both sides of the helicopter.
Blade Flapping In a three-bladed rotor system, the rotor blades are attached to the rotor hub by a horizontal hinge which permits the blades to move in a vertical plane, t.e., flap up or down, as they rotate (figure 2-38). In forward flight and assuming that the blade-pitch angle remains constant, the increased lift on the advancing blade will cause the blade to flap up decreasing the angle of attack because the relative wind will change from a horizontal direction to more of a downward direction. The decreased lift on the retreating blade will cause the blade to flap down increasing the angle of attack because the relative wind changes from a horizontal direction to more of an upward direction. The com- Low pitch applied High flap result Low flap result High pitch applied FICURF. 2-35. Rotor disk acts like a gyro.
Direction of rotation ===== FIcURE 2-38. Blade flapping action (vertical plane).
Advancing blade half to equalize the lift over the two halves of the rotor disk.
The amount that a blade flaps up is a compromise 400 300 100 100 300 400 between centrifugal force, which tends to hold the
m.p.h. Tip speed blade straight out from the hub, and lift forces which tend to raise the blade on its flapping hinge.
As the blades flap up they leave their normal tip- Retreating blade half path plane momentarily. As a result, the tip of the blade which is flapping must travel a greater dis tance. Therefore, it has to travel at a greater speed for a fraction of a second, in order to keep up with the other blades.
The blade flapping action creates an unbalance FIGURE 2-36. Comparison of rotor blade speeds for the advancing blade and retreating blade during hover. condition with resulting vibration. To prevent this vibration, a drag hinge (figure 2-39) is incorpo.
rated which permits the blade to move back and forth in a horizontal plane.
Direction of flight Hub (100 m.p.h.)
- Drag hinge FIGURE 2-39. Action of drag hinge (horizontal plane).
500 tection of rotation FIcURE 2-37. Comparison of rotor blade speeds for the advancing and retreating blade during forward fight.
bination of decreased angle of attack on the advancing blade and increased angle of attack on the retreating blade through blade flapping action tends With the blades free to move back and forth on the drag hinges, an unbalanced condition is created since the c.g. (center of gravity) will not remain fixed but moves around the mast. This c.g. move ment causes excessive vibration. To dampen out the vibrations, hydraulic dampers limit the movement of the blades on the drag hinge. These dampers also tend to maintain the geometric relationship of the blades.
A main rotor which permits individual movement of the blades from the hub in both a vertical and
• horizontal plane is called an articulated rotor. The hinge points and direction of motion around each hinge are illustrated in figure 2-40.
In a two-bladed system, the blades flap as a unit.
As the advancing blade flaps up due to the increased lift, the retreating blade flaps down due to the decreased lift. The change in angle of attack on 53
Lead or lag Drag hinge Flap Flap hinge Pitch Axis of rotation FIGURE 2-40. Articulated rotor head.
each blade brought about by this flapping action tends to equalize the lift over the two halves of the rotor disk.
The position of the cyclic pitch control in forward flight also causes a decrease in angle of attack on the advancing blade and an increase in angle of attack on the retreating blade. This together with blade flapping equalizes lift over the two halves of the rotor disk.
Coning Coning (figure. 2-41) is the upward bending of the blades caused by the combined forces of lift and centrifugal force. Before takeoff, the blades rotate in a plane nearly perpendicular to the rotor mast, since centrifugal force is the major force acting on them.
As a vertical takeoff is made, two major forces are acting at the same time centrifugal force acting outward perpendicular to the rotor mast, and lift acting upward and parallel to the mast.. The result of these two forces is that the blades assume Before takeoff lift Centrifugal force During takeoff FICURE 2-41. Blade coning.
a conical path instead of remaining in the plane perpendicular to the mast.
Coning results in blade bending in a semirigid rotor; in an articulated rotor, the blades assume an upward angle through movement about the flapping hinges.
Ground Effect When a helicopter is in a hovering position close to the ground, the rotor blades will be displacing air downward through the disk faster than it can escape from beneath the helicopter. This builds up a cushion of dense air between the ground and the helicopter (figure 2-42). This cushion of more dense air, referred to as ground effect, aids in supporting the helicopter while hovering. It is usually effective to a height of approximately one-half the rotor disk diameter. At approximately 3 to 5 m.p.h. groundspeed, the helicopter will leave its ground cushion.
FICURE 2-42. Ground effect.
Autorotation Autorotation is the term used for the flight condition during which no engine power is supplied and the main rotor is driven only by the action of the relative wind. The helicopter transmission or power train is designed so that the engine, when it stops, is automatically disengaged from the main rotor system to allow the main rotor to rotate freely in its original direction.
When engine power is being supplied to the main rotor, the flow of air is downward through the rotor. When engine power is not being supplied to the main rotor, that is, when the helicopter is in autorotation, the flow of air is upward through the rotor. It is this upward flow of air that causes the rotor to continue turning after engine failure.
The portion of the rotor blade that produces the forces that cause the rotor to turn when the engine is no longer supplying power to the rotor, is that 54