Monoacoque construction is economical and has su! Tt is extremely important that all repairs to monocoque structures restore the original shape, rigidity and strength to any area that has been damaged.
For enhanced strength, a substructure of formers and stringers is built and the skin is riveted to it. Former rings and bulkheads, which are formers that also serve as compartment walls, are made of rela tively thin sheet metal that have been formed in hydroprosses, and stringers are made of extruded aluminum alloy.
Stringers usually have a bulb on one of their sides to provide added strength to oppose bending loads. Longerons are also made of extruded aluminum alloy, but are heavier than the stringers in order to carry a large amount of the structural loads in the fuselage. One of the icant improvements was achieved by pressurizing the interior of the fuselage, With increased cabin and cockpit air pressure, the occu pants could be assured of receiving enough oxygen so that supplemental breathing equipment would not be required under normal conditions.
Cabins were pressurized to a pressure differential of only about two psi. Low pressurization created no major structural prob: ns, but when the first jet transports, the British Comets, were put into service with pressurization of 8 psi, significant problems did arise.
Continual flexing of the structure caused by the pressuriza tion and depressurization cycles fatigued the metal. For a number of these aircraft, a crack devel- oped at square comer of a cutout in the structure. When the cause of the structural failure was determined, new emphasis was placed on fail- safe design of aircralt structures. Stress risers or portions of the structure where the cross section changes abruptly were eliminated.
Joints and con- nections were carefully prestressed to minimize the cyelic stresses from the eyclie pressurization loads and, most important, the structure was designed with more than one load path for the stresses.
Inventors later turned to bicycle wheels to support the airframe on the ground. To land on water, some designers equipped their flying machines with floats, Upper Boor Latchea With Lower Door Latch Safety Pin nstates Locking Pin Ingpoction Holes Locking Pin Inspection Holes Figur Pressurized aircraft require greater structut strength especially in windows and doors to accommodate the cyclic pressurization stresses.
Two main wheels are attached to the airhame, ahead of the renter of gravity, to support most of the aircraft weight, and a small tail-skid or wheol at the ve back of the fuselage provides a third point of sup- porl. This arrangement allows adequate ground clearance for a long propeller and provides the lightest-weight landing gear available. Before hard- surfaced runways became commonplace, a steel- shoed tuil-skid provided adequate braking action for airplanes that had no regular wheel brakes.
Taxiing, these airplanes required a high degrve of skill, and a two-wheel dolly was placed under the tail-skid to maneuver the airplane by hand for ground maneu- vering. The main drawback of the conventional landing gear is that the airplane's center of gravity is behind the point of contact of the main wheels.
This makes it easy for the airplane to ground looy pilot allows the airplane to swerve slightly rolling on the ground. If the speed is below that which the rudder has sufficient control to counter- act the motion, the conter of gravity attempts to move ahead of the point of contact. With this configuration, a nose wheel is installed in the front of the airplane, and the two main wheels are moved behind the center of gravity.
The natural tendency for a nosewheel airplane is to move straight down the sunway, rather than attempting to spin around With current production airplanes, the tailwheel landing gear is found only on those airplanes used for special purposes such as agricultural operations.
A conventional landing gear is preferred for rough field applications where rugged or muddy sod sur- faces make taking off and landing difficult, Without the heavy structure required for a nose wheel and the reduced whee! Parasite drag is most felt on an airplane at high speed, and for low-speed airplanes, the simplicity ight weight of a fixed landing gear make the fixed-tricycle gear configuration a logical choice.
The decrease in drag provided by these streamlined fairings more than compensates for the additional weight. Generally, retractable landing gear fold up into the wing or fuselage. Retractic ms may be actuated with hydraulic eylinders or electrie motors, but some lighter airplanes employ mechan: ical linkage to pull the wheels up. However, the excessive size of the support structure required to hold these airplanes up in the water pro- duced so much drag that the aireraft were unable to carry profitable payload with the amount of engine power then available.
Most land-planes today can be fitted with twin floats that support them on the water. Due to the compromises required to make an airplane suitable for water operations, an efficient amphib- ian airplane is a challenge to designers. Amphibious planes must be designed with both land and water operations in mind.
Aircraft Structural Assembly and Rigging One successful approach to land and water opera: tions is to use amphibious floats, Amphibious floats are installed in the same manner as normal floats, but they have built-in retractable wheels that may be extended for operations on land or retracted for water operations.
Retractable skis are far more useful than the wheel-replacement type, since they allow the airplane to land on either a hard-surface runway or on snow. This type of ski is installed on the land ing gear with the wheel in place. For landing on a hard-surfaced ramway, the ski is pulled up so that the wheel sticks ont below the ski where the weight of the airplane can be supported by the tire rather than the ski.
For landing on snow, the ski is low- cred, making contact with the ground first and sup: porting the airplane. Figure ] that brought poor results, Most of tho earliest aire planes had the engine mounted behind the pilot or on the wing. In the case of the Wright Flyer, the ied engines drove the propellers by Ked chains.
As aizplane development progeossed. While increasing airflow, cowlings also minimized drag for engines that have large frontal areas, [Figure ] Figure Attaching skis to an airplane further extends its capability by allowing the pilot to take-off and land on snow and ice. Engines must bo started, cooled, controlled, and mounted in places whore they can efficiently provide thrust. Because there was so little knowledge of the requirements for flight in the early development of aviation, air- planes often went through many evolutionary steps Figure Cowlings on radial engines are specifically designed to increase airflow around the cylinders and to reduee dag, Today, almost all piston-powered airplanes use hor- izontally-opposed engines enclosed inside a pres- sure cowling, The cooling air for these engines centers the cowling from the front, above the engine, and then passes through baffles and fins to remove the heat.
A low-pressure area below the ated by air flowing over the bottom of the draws air through the engine to increase the amount of cooling.
They are far smaller and lighter than piston engines of the same power, while also producing far less vibration. As experience Increased with these engines, designers realized that space and weight could be saved by mounting the engines oulside the fuselage in self-contained pods.
The two most common locations for turbojet installations are beneath the wing and at the rear end of the fuselage. Engines on modern turbine-ongine alreratt aro attached tothe aircraft main structure through pylons under the wings or on the rear section of the fuselage. The mounts can be con- structed from welded alloy steel tubing, formed sheet metal, forged alloy fittings, or a combination, of all three.
Engine mounts are required to absorb not only the thrust, but also the vibrations pro- duced by the particular engine, or engine-propeller combination. There are several types of vibration isolators used and their inspection, repair and roplacomont is a regular part of any structural maintenance program.
Electrical wiring, fuel and oil lines, air ducting and many other items also require openings for safe routing. The complexity of the structure and its strength roquirements determine how an open- ing is fabricated. For instance, a passenger doar opening reinforcement on a training airplane has, the samo parts as ono on a transport catogory air plane, but the strength differs considerably.
Differences are also apparent in other parts of the airplane. As an example, the rear bulkhead panel on a pressurized airplane is not the same as on a non-pressurized one, though both serve the same function. A pilot expects any aiscraft to respond to control inputs in a consistent and predictable manner. An aircraft thal is owb-of-rig will be difficult or impossi: ble to trim for stable flight and will require constant attention by the pilot.
For example, if the elevator has insulfi- cient upward travel, the pilot may not be able to raise the nose sufficiently during to make a safe landing, he landing flare If the flight control system is improperly assem- bled, the potential for disaster is even greater Imagine the confusion that would result if the pilot commands a left turn and the airplane responds by turning right!
The three tis free to rotate about three axes, axes pass through a common reference point called ity CG , which is the theoretical point where the entire weight of the aircealt is eons the center of gy sidered to be concentrated. Since all theee axes pass through the CG, the aixplane always moves abut its CG, regardless of which axis is involved. Motion about the longitudinal axis is called roll, and this axis is often referred to as the roll axis, [Figure ] LATERAL AXIS The lateral axis is a straight line extending parallel to the wing span at right angles to the longitudinal axis.
You can think of it as extending from wingtip to wingtip, Motion about the lateral axis is called pitch and this axis is reforred to as the pitch axis. The allerons cause an airplane to roll about the longitudinal axis.
The elevators cause an airplane to pitch about the lateral axis. The rudder causes an airplane to yaw about the vertical axis. The primary purpose of the rudder is to coun- teract alleron drag and keep the fuselage streamlined with the relative wind. This improves the quality of tums and reduces drag, Stability is the characteristic of an airplane in flight that causes it to return to a condition of equilibrium, or stoady flight, after it is disturhed, Maneuverability is the characteristic of an airplane that permits the pilot to easily move the airplane about its axes and.
A aircraft that is stable in pitch will return to the angle of attack for which it is trimmed any time it is disturbed from this angle. Dynamic stability is concerned with the way the restorative forces act with regard to time.
At times, turbulence or erratic movement causes buf feting in an airplane. If an aircraft is designed with positive stability, it will return to its original flight condition when turbulence ceases.
Positive stability is desirable for most aircraft but advanced fighter aircraft with computer augmented flight controls may employ neutral or negative stability to enha combat maneuverability POSITIVE STABILITY Positive stability can be illustrated by considering the action of 4 ball in a U-shaped trough as shown in figure , If the ball is rolled up to one edge of the trough and released, positive static stability will cause it to roll back down towards its original posi- tion When the ball returns to its original position at the bottom of the trough, it will probably overshoot its position of equilibrium and start up the opposite side, As soon as it starts up the opposite slope, pos- itive static stability will tend to return it to the bot- tom.
The ball will rock back and forth, each time moving shorter distance up the slope, until it finally stops at the bottom — a demonstration of positive dynamic stability. In this condition, the balll is said to have nogative static st Dility, or to be statically unstable. If the corrective forces inerease with time, the body has negative dynamic stability. Negative stability, as lustrated by a ball rolling off the crest of a hill,is an undesirable characteristic in a planes. A pilot would be very likely to lose control of an air- plane with negative stability.
This illustrates noutral static stability The corrective forces of a body with neutral dynamic stability neither increase or decrease with time, Energy is not added to the body or taken away by any form of damping. An abject that has neutral stability cemains dis- placed from its original state whenever a force is applied.
A neutrally stable airplane would be difficult to control and would probably require computer-augmented flight controls. Because the wing's center of Lift is behind the center of gravity, the wing produces a nose-down pitching moment, This pitching moment is counteracted by a down load proxtuced by the horizontal tail surfaces.
Elevator trim can be adjusted by the pilot to produce the required down Joad at any speed. Figure ] Lateral, or roll, stability is provided primarily by dihedral in the wings. Dihedral is the upward angle between the wing and the lateral axis of the air- plane, The dihedral angle of most airplanes is usu- ally just a few degrees. When an airplane enters a downward sideslip toward the low wing, the direc tion of relative wind changes, The low wing experi- ences a higher angle of attack while the angle of attack on the high wing is reduced.
The changes im angle of attack cause the low wing to generate more Lift, at the same time the high wing generates less lift, and the combined forces tend to roll the aircraft back toa wings-level attitude. With the center of pressure ait of the center of gravity, an airplane wing produces nose-down pitching moment. Figure Wing dihedral is a major contributor to lateral stability. Airplanes generally have less roll stability than pitch stability. Aircraft Structural Assembly and Rigging Figure Because they are inherently more stable laterally, high-wing aircraft such as the Cessna on the left are designed more fuselage surface area behind the CG than for- ward of it When the airplane enters a sideslip, the relative wind strikes the side of the fuselage and the vertical tail.
Since the force exerted an the airplane aft of the CG tends to cause the nose to turn towards the sidoslip, this aligns the fuselage with the relative wind, [Figure ] CONTROL SYSTEMS The primary flight controls of an aircraft do no more than modify the camber, or aerodynamic: shape, of the surface to which they are attached.
This change in camber creates a change in the lift and drag pro- duced by the surface, with the immediate result of rotating the airplane about one of ils axes. This rata- tion produces the desired changes in the flight pa of the aircraft less dihedral than the typical low-wing aircraft, as exhibited by the Beechcraft Bonanza on the right.
The trailing edge of the elevator may have a trim tab to adjust the down load of the tail for hands-off flying at any desired airspeed.
Another means of providing the nacessary trimming force is to adjust the entire horizontal stabilizer by rotating it about a pivot point The elevator is connected to the control wheel or yoke in the cockpit with steel control cables, and moves up or down as the wheel is moved forwards or backwards, Pulling back on the wheel pulls the ele- vator-up cable and rotates the top of the elevator bell crank forward, The control horn on the bottom of the elevator torque tube is attached to the bell crank with, 1 push-pull rod, and as the bottom of the bell crank Figure A typical elevator-control system consists of cables connecting the control wheel with a bell erank in the rear fus lage.
The bell crank is connected to the elevator control horn by a push rod, moves back, it pushes the elevator up. Pushing in on the control wheel has the opposite result. The eleva- tordown cable is pulled and the bottom of the bell crank moves forward, causing the push-pull rod to pull the elevator down.
For such airplanes, an elevator down spring automatically lowers the nose to prevent an approach-to-landing stall caused by the excessi far-aft center of gravity. If an airplane with the ter of gravity too far aft is slowed down for landing, the trim tab will be working hard to hold the nose down, and the elevators will actually be in a slightly down position. If the airplane in this unstable con- dition encounters turbulence and slows down fur- ther, the elevator will streamline, and at this slow speed the trim tab cannot force it back down.
The nose of the airplane will pitch up, aggravating the situation and possibly causing a stall at this critical altitude. The elevator down spring holds a mechanical load on the elevator, forcing it down. This mechanical force is balanced by the aerodynamic force of the trim tab, and the airplane may be trimmed for its approach speed in the normal way. If the airplane encounters turbulence that slows it down, the trim tab will lose its effectiveness.
The down spring will pull the elevator down, lowering the nose so the air- speed will build up and prevent a stall. A typical stabllator control system Is litle different from an elevator control system. Airplanes with stabilators have essentially the same type of control system as those with elevators When the control yoke is pulled back, it pivots and pulls on the stabilato-up cable.
This pulls down on the balance arm of the stabilator and raises its trail- ing edge, rotating the airplane's nose up. Pushing the yoke in lifts the stabilator balance arm and low- ers the trailing edge of the stabilator. In many modern air- planes there is some form of mechanical intercon- nection between these two systems, usually not a positive one, but one that can be overridden if it is necessary to slip the airplane.
Rotation of the control wheel turns the drum to which the aileron control cables are attached. If the wheel is rotated to the right, the right cable is pulled and the left one is relaxed.
The cable rotates the right aileron bell crank, and the push-pull tube con- nected to it raises the right aileron. A balance cable connects both aileron bell cranks, and as the right aileron is raised, the balance cable pulls the left bell crank and its push-pull tube lowers the left aileron.
Figure ] Aileron drag is a big problem caused by the dis- placement of the ailerons The aileron that moves downward is the one that causes the problem, as it creates both more lift and drag, and this drag way out near the wing tip pulls the nose of the airplane around in the direction opposite to the way the airplane should turn.
The geometry of the bell cranks is such that the aileron moving upward travels a greater distance than the one moving down, and it produces enough parasite drag to counteract some or all of the induced drag on the opposite wing.
A typical aileron control system also consists of cables, bell eranks and push rods. The Frise aileron is the type most com- monly used today, and it minimizes aileron drag because of the location of its hinge point.
These ailerons have their hinge point some distance back from the leading edge. When the aileron is raised, its nose sticks out below the lower surface of the wing and produces enough parasite drag to counter the induced drag from the down aileron. The hinge line of a Frise alleron Is aft of the leading edge, allowing its nose to protrude slightly from the lower wing surface to counteract alleron drag Since aileron drag is produced each time the control wheel deflects the ailerons, many manufacturers connect the control wheel to the rudder control sys tem through an interconnecting spring.
When the wheel is moved to produce a right roll, the inter- connect cable and spring pulls forward on the right rudder pedal just enough to provent the nose of the airplane yawing to the left. Airplanes whose rudder pedals are connected rigidly to the nosewheo!
Forward movement of the right rudder pedal will deflect the rudder to the right. Trim tabs on the trailing edge of control sur- faces can be adjusted to provide an aerodynamic force to hold the surface in a desired position. If only one tab is used, it is nor- mally on the elevator, to permit adjustment of the tail load so the airplane can be flown hands-off at any given airspood. FAR This tab is located in the same place as a trim tab.
In many installations, one tab serves both functions. The basic difference is that the control rod for the balance tab is con- nected to the fixed surface on the same side as the horn on the tab.
If the control surface is deflected upward, the connecting linkage will pull the tab down. When the tab moves in the direction opposite control surface, it will create an aorod: that aids the movement of the surface.
FLAPS Perhaps the most universal lift-modifying devi used on moder airplanes are flaps on the trailing edge of the wing. These surfaces change the camber of the wing, increasing both lift and drag for any given angle of attack. The basic airfoil section, at 15 degrees angle of attack, has a lift coefficient of 1. If plain flaps are hinged to the trailing edge of this airfoil, we get a maximum lift coefficient of 2. Slotted flaps are even better, giving a maximum lift coefficient of 2.
The total effect of Fowler flaps is not seen in just the lift and drag coefficients, because they not only provide an excellent increase in the lift coeffi- cient, but they also increase the wing area. Increased wing area has an important effect on both lift and drag. They are about the same size as the aileron and are hinged so they can be deflected, usually in incre- ments of 10, 25, and 40 degrees.
Generally speaking, the effect of these flaps is minimal, and they are sel- dom found on modem airplanes. A tal rotor is designed to produce thrust in a direction opposite torque. The thrust produced by the tail rotor ls sufficient to move the helicopter laterally. The engine must mix a given mass of air with its fuel in order to release energy, and the less dense the air, the greater the volume needed to release the required energy.
Lift, as you remember from basic aerodynamics, is, determined by the lift coefficient, the area of the surface, and the dynamic pressure. Dynamic pres- sure is the product of one-half of the density of the air and the square of the velocity of the blade through the air. Air density has an important effect on helicopter performance, and the pilot is concerned with den- sity altitude, which is the eltitude in standard air that is the same as the existing pressure altitude.
This effect usually occurs less than one rotor diameter above the surface. As the surface friction restricts the induced airflow through the rotor, the lift vector increases. This allows a lower rotor blade angle for the same amount of lift, which reduces induced drag. Ground effect also restricts the generation of blade tip vortices due to the downward and out- ward airflow, producing lift from a larger portion of the blade. When the helicopter gains altitude vertically, with no forward airspeed, induced air- flow is no longer restricted and the blade tip vor- tices increase with the decrease in outward air- flow.
Asa result, a higher pitch angle is required to move more air down through the rotor, and more power is needed to compensate for the increased drag.
Specifications for helicopter performance provide the hover ceiling in two ways, IGE and OGE, These figures provide the maximum altitude at which the helicopter will hover at its rated gross weight in ground effect and out of ground effect. Figure ] Ground effect is at its maximum in a no-wind condi- tion over a firm, smooth surface. When pilots wish, to rise vertically, they increase the pitch angle of all of the rotor blades with the collective pitch, and at the same time, add engine power.
Moving one con- trol does all this, as we will see later. The additional power increases the amount of torque, and the pilot must adjust tail rotor pitch with the pedals to pre- vent the nose of the helicopter from turning, When the pilot wants to descend vertically, blade pitch angle and the power are reduced, and the pedals are adjusted again to correct for any tendency of the nose to rotate.
The helicopter will descend verti- cally when the thrust produced by the rotor system becomes loss than the weight. The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the relative wind speed act- ing on the retreating blade is reduced.
Therefore, as a result of the relative wind speed, the advancing blade side of the rotor disc produces more lift than tho retreating blade side. This hypothetical situa- tion is defined as dissymmetry of lift. The blade tip speed of this helicopter approximately knots. If the helicopter is moving forward at knots, the relative wind speed on the advancing side Is knots. On the retreating side, itis only knots.
This difference in speed causes a dissymmetry of It. If this condition were allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift.
In reality, the main rotor blades flap and feather automatically to equalize lift across the rotor disc. Articulated rotor systems, usually with three or more blades, incorporate a horizontal hinge to allow the individual rotor blades to move, or flap up and down as they rotate.
A somirigid rotor system two blades utilizes a teetering hinge, which allows the blades to flap as a unit. When one blade flaps up, the other flaps down, In figure , as the rotor blade reaches the advanc- ing side of the rotor disc 4 , the increase in lift causes it to flap up until itreaches its maximum upflap veloc ity, When the blade flaps upward, the angle between the chord line and the resultant relative wind decreases.
This decreases the angle of attack, which reduces the amount of lift produced by the blade. At position C , the decreased lift causes the blade to flap down until itis at its maximum downflapping veloc- ity. Due to downflapping, the angle between the chord line and the resultant relative wind increases. This increases the angle of attack and thus the amount of lift produced by the blade. At a high forward speed, the retreating blade stalls because of a high angle of attack and slow relative wind speed.
This situation is called retreating blade stall and is evidenced by a nose pitch up, vibration, and a rolling tendency usually to the left in heli- copters with counterclockwise blade rotation. The pilot can avoid retreating blade stall by not violating the never-exceed speed.
Stalls on both airplanes and helicopters are caused by exactly the same thing: an excessive angle of attack. As the forward speed of the helicopter increases, the velocity of the air over the retreating blade decreases; and as it flaps downward, its angle of attack increases. At some given airspeed, the angle of attack becomes excessive and a stall occurs.
When a blade stalls, lift is lost and drag builds up. This increased flow is most noticeable when the airspeed reaches approxi- mately 16 to 24 knots. As the helicopter accelerates through 16 to 24 knots, it moves out of its own downwash and vortices into relatively undisturbed air. The airflow is now more horizontal, which reduces induced flow and drag with a correspond- ing increase in angle of attack. Effective translational lit is easily recognized in flight by a transient induced aerodynamic vibration and Increased performance of the helicopter.
It is the means by which a helicopter can be landed safely in the event of an engine failure. All helicopters must have this capability to be certified. Innormal powered flight, airis drawn into the main rotor system from above and exhausted downward. During autorotation, airflow onters the rotor disc from below as the helicopter descends, creating an aerodynamic force that drives the rotor and keeps it turning. This is known as tho autorotative force. Figure ] Figure demonstrates the way this autorotative force comes about.
Air flowing upward through the rotor produces an angle of attack such as wo see hore. From our study of basic aerodynamics, we know that the lift produced by an airfoil acts per- pendicularly to the relative wind, while the induced drag acts parailel, but in opposite direc- tion, to the relative wind. If we consider the com- ponents of lift and drag that act along the plane of rotation of the rotor, we see that there is a net force acting in the direction of rotor rotation along this, plane.
This part of the rotor disc is called the autorotation region. When the angle between the relative wind and, the axis of rotation is high, the resultant lift wil be ahead of the axis of rotation, and there will be an autorotative force that pulls the rotor in its direction of rotation.
The drag force that acts along the plane of rotation is greater than the component of the lift that acts along this plane. The result is a force that tries to deceler- ate the rotor, This is called an anti-autorotative Figure During autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. If the rotor disc is tilted forward, the aircraft will fly forward; if it is tilted back, it will fly backward; and tilting it to the side will cause it to fly sideways.
The early gyro- planes and some of the simplest rotary wing aircraft, mainly amatour-built, have a control bar that directly tilts the rotor head. This form of direct con- trol usually requires the control forces to be quite light. The swash plate is used to transmit control inputs from the cockpit controls to the main rotor blades.
It consists of two main parts: the stationary swash plate and the rotating swash plate. The sta- tionary swash plate is mounted to the main rotor mast and connected to the cyclic and collective con- trols in the cockpit by a series of pushrods.
It is restrained from rotating but is able to tilt in all directions and move vertically. The rotating swash plate is mounted to the stationary swash plate by means of a bearing and is allowed to rotate with the main rotor mast. Both swash plates tilt and move up Aircraft Structural Assembly and Rigging and down as a unit.
The rotating swash plate is con- nected to the main rotor grips by the pitch links. Control rods transmit collective and cyclic control inputs to the stationary swash plate , causing It to tilt or move vertically. The pitch links, attached from the rotating swash plate to the pitch arms on the rotor blades, transmit these movements to the blades.
This causes all the rotor blades to increase or decrease blade pitch angle by the same amount, or collectively as the name implies. As the collective pitch control is raised, there is a simultaneous and equal increase in pitch angle of all main rotor blades, and lift increases. As it is lowered, there is a simultaneous and equal decrease in pitch angle, and lift is decreased.
This is done through a series of mechenical linkages, and the amount of movement in the collective lever will determine the amount of blade pitch change. A friction control is adjusted by the pilot to prevent inadvertent collective pitch movement. Figure ] Changing the pitch angle on the blades changes the blade angle of attack and lift. With a change in angle of attack end lift comes a change in drag, and the speed or rp.
As the pitch angle of the blades is increased, the angle of attack and drag increase, while the rotor rp. Decreasing pitch angle decreases both angle of attack and drag, and rotor p. To maintain a constant rotor r. This system maintains rp. Once the rotor p. Governors are common on all turbine helicopters Some helicopters do not use correlators or governors and require the pilot to coordinate all collective and throttle movements together.
When the collective is raised, the throttle must be increased; when the col: loctive is lowered, the throttle must be decreased. Some tur ine helicopters have the throttles mounted on the overhead pane!
Some turbine helicopters have throttles mounted on the overhead panel or on the floor of the cockpit. The function of the throttle is to regulate engine p. If the correlator system does not maintain the desired r. However, if the governor fails, the pilot will have to uso the throttle to maintain np. When the main rotor disc is tilted, the horizontal compo- nent of lift will move the helicopter in the direction of tilt.
The cyclic can pivot in, all directions. An increase in pitch angle will increase angle of attack; a decrease in pitch angle will decrease angle of attack. For example, if the cyclic is moved forward, the angle of attack decreases as the rotor blade passes the right side of the helicopter and increases on the left side.
Because of gyroscopic procession, this results in. This assumes that the direction of rotor blade rote- tion is counterclockwise as viewed from above, which is the case of single rotor helicopters manu- factured in the United States.
Without this provision, the fuselage will assume an excessive nose-down pitch at high airspeeds, but the elevator holds it at a more level attitude. Helicopters that do not have synchronized elevators instead may havo a fixed horizontal stabilizer, that has a very pronounced airfoil section, mounted on.
This serves two main purposes: to overcome high control forces and to prevent vibrations from the rotor system from being fed back into the controls. The hydraulic sys- tem consists of actuators, also called servos, on each flight control, a pump that is usually driven by the main rotor gearbox, and a reservoir to store the hydraulic fluid.
A switch in the cockpit can turn the system off, although it is left on under normal con- ditions. A pressure indicator in the cockpit may be installed to monitor the system, [Figure ] When a control input is made, a valve inside the servo directs hydraulic fluid under pressure to the piston to change the rotor pitch. This type of servo uses a system of check valves to make the system irreversible; that is, to make it so the pilot can con- trol the pitch of the rotor blades, but vibration from.
This gives the pilot enough time to land the helicopter with normal control. When the engine drives the rotor, the same force that spins the rotor tries to spin the fuselage. If the helicopter is to be controlled about its vertical axis, there must be some way to counteract this torque.
Some designers have driven the rotor with a jet of air from the rotor blade tip, thus eliminating any torque from a fuselage-mounted transmission drive. Others used two rotors, either on coaxial shafts, or mounted side by side or fore and aft.
Despite all of the different attempts to com- Figure A typical hydraulic system for helicopters inthe light to medium range is shown here. The pilot changes the pitch of the tall rotor to correct for torque by moving the pedals in the cockpit. Some of the flapping hinges incorporate a Delta-three hinge, whose hinge line is angled with respect to the rotor span, When the blade flaps, it also changes its effective pitch angle and can correct for the dissymmetry of lift without a severe flap- ping angle.
The pilot varies the thrust to rotate the fuselage while hovering or to compensate for any changes in the torque as power and speed are changed.
This system uses a sories of rotating blades shrouded within a vertical tail. Because the blades are located within a circular duct, they are less likely to come in contact with people or objects. This system uses low- pressure air, which is forced into the tailboom by a fan mounted within the helicopter.
The air is forced Aircraft Structural Assembly and Rigging through horizontal slots, located on the right side of the tailboom, to a controllable rotating nozzle, pro- viding antitorque and directional control. Directing the thrust from the control- lable rotating nozzle creates the rest. An airplane has both positive static and dynamic stability about all three of its axes, but the helicopter is not so fortunate, One of the developments pioneered by Bell Helicopter was the stabilizer bar.
The control rods from the swash plate attach to the stabilizer bar, and pitch con- trol links connect the blade pitch arms to the stabi- lizer bar.
In flight, the stabilizer bar acts as a very effective gyro, having rigidity in spaco; that is it does not want to depart from its plane of rotation. If the helicopter tilts, the stabilizer bar remains in its origi- nal plane of rotation and an angular difference is formed between the bar and the mast. This is trans- mitted to the rotor blades as a pitch change in the cor- rect direction to right the helicopter.
If the rotor system in figure tilts to the right in flight, the angle of attack of the descending rotor blade will increase and the blade will flap up. As it flaps, the offset hinge will cause its pitch angle to increase and automatically produce lift in the direction needed to right the helicopter. It has the same effect as the pilot applying corrective use of the cyclic control, but this is automatic. The simplest of these systoms is a force trim system, which uses a mag- notic clutch and springs to hold the cyclic control in the position where it was released.
More advanced systoms use electric servos that actually move the flight controls. These servos receive control com- mands from a computer that senses helicopter alti- tude.
The SAS may be overridden or disconnected by the pilot at any time. Stability augmentation systems allow the pilot more time and concentration to accomplish other duties. It improves basic aircraft control harmony and reduces outside disturbances, thus reducing pilot workload. These systems are useful when pilots are required to perform other duties such as sling load- ing and search and rescue operations. The autopilot can actually fly the heli- copter and perform certain functions selected by the pilot.
The most advanced autopilots can fly an instrument approach to a hover without any additional pilot input once the initial functions have been selected. The autopilot system consists of electric actuators servos connected to the flight controls.
The num- ber and location of these servos depend on the type of system installed, A two-axis autopilot controls the helicopter in pitch and roll; one servo controls fore and aft cyclic and another controls left and right cyclic. A three-axis autopilot has an additional servo connected to the antitorque pedals and con- trols the helicopter in yaw. A four-axis system uses a fourth servo, which controls the collective power.
A control panel in the cockpit has a number of switches that allows the pilot to select the desired functions and engage the autopilot An automatic disengage feature is usually included for safety purposes, which disconnects the autopilot in heavy turbulence or when oxtreme flight atti tudes are reachod. Even though all autopilots can be overridden by the pilot, thoro is also an autopilot disengage button located on the cyclic or collective which allows you to completely disengage the autopilot without removing your hands from the controls.
Because autopilot systems and installa- tions differ from one helicopter to another, it is important that you refer to the autopilot operating procedures located in the Rotorcraft Flight Manual. Although not all vibration can ever be completely eliminated, irreversible con- trols and special vibration-absorbing engine and transmission mounts have minimized its effects.
To keep it simple and use- ful, we categorize them basically into two frequency ranges, two modes, and two conditions. The break between low and medium, and between medium and high is rather nebulous, so in this text we lump them all into either low or high frequencies. Those are normally associated with the main rotor systom. They may have a ratio of , , or with the main rotor, depending on the rotor configura- tion, and may be caused by either a static or a dynamic unbalance condition or by acrodynamic forces acting upon the rotor.
Any component that turns at a high p. Vibration Modes Vibrations can cause the helicopter to jump up and down in what is called a vertical vibration, or to shake sideways, which is called lateral vibration. Vertical vibrations are generally caused by some dis- parity in lift as the rotor spins and generally points to an out-of-track condition.
Lateral vibrations are most often caused by an out-of-balance condition of the rotor. Modes of helicopter vibration are shown here. Once an observation is made, it is plotted ona chart, which then directs us to make a move to correct the track or balance.
Generally, only one cor- rective move is made at a tim Some of the newer systems use infrared sensors together with computers that direct us to which cor- rective moves to make. In these systems, more than one move can be made each time, thus accelerating, the track and balance procedure. In almost all of these systems, an electronic pickup is used to key the system each time a rotor blade passes a reference point. An accelerometer or velometer measures the amplitude of the vibration and marks the position in the blade path where this vibration occurs.
Static bal- ance refers to balancing a blade while it is not mov- ing and is generally removed from the helicopter. Dynamic balance is accomplished with the blades mounted on the helicopter and turning. Before helicopter rotor is mounted on the mast, it mu be balanced both chordwise and spanwise.
After this aligument, or chordwise balanc is accomplished, balance the rotor spanwise by adding. Some helicopters, specifically those with three or more blades, may require the hub to be balanced without the blades, and then the blades are installed that have been balanced against a master blade.
Old methods made use of a marking stick, which was raised until it Figur sists of aligning the blades so that they are straight across the hub, just contacted the bottom of the rotor blade at the tip, or a flag that was moved into the tip plane until the tip just touched the flag. They left a mark of their respec- tive color when the blade tips touched the flag, If the rotor was in track, the colored marks were superim- posed.
The drawbacks to these two methods is that the helicopter can only be tracked on the ground and not in the air where it is most important, Modern tracking systems either use a strobe light held by the technician or an infra-red light mounted on the helicopter to determine blade track.
The rotor is turned at the proper p. A special reflector is installed on the tip of each blade, and they form a distinctive pattern as the strobe illu- minates each reflector. If the blades are in track, the images will be all in line, but if they are not in track, tho images will be staggered up and down.
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