Aerodynamics of ahelicopter_pp


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Aerodynamics of ahelicopter_pp

  1. 1. Aerodynamics of a rotary wing type aircraft (Helicopter) Darshak Bhuptani Author affiliation: B.Tech Aerospace, Indian Institute for Aeronautical Engineering and Information Technology darshak2512@hotmail.comAbstract: The main effect of the rotating wing is that the aircraft tends to rotate in oppositeThe helicopter is a rotary wing type aircraft direction that of the rotors and this effect iswhich generates the main aerodynamic force known as torque. Description of torque andby rotating the rotor which hubs the wing and methods to overcome this is entitled below.rotates it at a very high speed. As a result ofthis rotation, the lift for an aircraft is Due to the motion of any system, there is aproduced at full throttle only. vibration associated with it. This tends to induce fatigue stress in the system which canThe blades which are used in helicopters are be fatal, so appropriate device should beof airfoil shape. The basic terminology and incorporated with the system so that vibrationpressure distribution over an airfoil is can be minimised. Various types of bladedescribed in detailed. As there is lift, there is setting, ground effect, hovering, effectivedrag force too. The description of the various translation lift, blade stall and its effect aretypes of drag force and the amount of the discussed.power required to overcome this ismentioned. Page | 1
  2. 2. Chapter 1 explains at least in part why an airfoil develops an aerodynamic force.Introduction to basic aerodynamics: All of the forces acting on a surface overAerodynamics concerns the motion of air and which there is a flow of air are the result ofother gaseous fluids and other forces acting skin friction or pressure. Friction forces areon objects in motion through the air (gases). the result of viscosity and are confined to aIn effect, Aerodynamics is concerned with the very thin layer of air near the surface. Theyobject (aircraft), the movement (Relative usually are not dominant and, from theWind), and the air (Atmosphere). aviators perspective, can be discounted.Newtons Laws of MotionNewtons three laws of motion are: As an aid in visualizing what happens to pressure as air flows over an airfoil, it isNewton’s first law: helpful to consider flow through a tubeInertia - A body at rest will remain at rest. (Please see Figure above). The concept ofAnd a body in motion will remain in motion conservation of mass states that mass cannotat the same speed and direction until be created or destroyed; so, what goes in oneaffected by some external force. Nothing end of the tube must come out the other end.starts or stops without an outside force tobring about or prevent motion. Hence, the If the flow through a tube is neitherforce with which a body offers resistance to accelerating nor decelerating at the input, thenchange is called the force of inertia. the mass of flow per unit of time at Station 1 must equal the mass of flow per unit of timeNewton’s second law: at Station 2, and so on through Station 3. TheAcceleration - The force required to produce mass of flow per unit area (cross-sectionala change in motion of a body is directly area of tube) is called the Mass Flow Rate.proportional to its mass and the rate ofchange in its velocity. Acceleration referseither to an increase or a decrease in velocity,although Deceleration is commonly used toindicate a decrease.Newton’s third law:Action / Reaction - For every action there isan equal and opposite reaction. If aninteraction occurs between two bodies, equalforces in opposite directions will be imparted At low flight speeds, air experiencesto each body. relatively small changes in pressure and negligible changes in density. This airflow isFluid flow and Airspeed measurement. termed incompressible since the air may(Bernoulli’s Principle) undergo changes in pressure without apparent changes in density. Such airflow is similar toDaniel Bernoulli, a Swiss mathematician, the flow of water, hydraulic fluid, or anystated a principle that describes the other incompressible fluid. This suggests thatrelationship between internal fluid pressure between any two points in the tube, theand fluid velocity. His principle, essentially a velocity varies inversely with the area.statement of the conversation of energy, Venturi effect is the name used to describe this phenomenon. Fluid flow speeds up Page | 2
  3. 3. through the restricted area of a venturi in because the air layers restrict the flow just asdirect proportion to the reduction in area. The did the top half of the venturi tube. As aFigure below suggests what happens to the result, acceleration causes decreased staticspeed of the flow through the tube discussed. pressure above the curved shape of the tube. A pressure differential force is generated by the local variation of static and dynamic pressures on the curved surface.The total energy in a given closed systemdoes not change, but the form of the energymay be altered. The pressure of the flowingair may be likened to energy in that the total A comparison can be made with waterpressure of flowing air will always remain flowing thru a garden hose. Water movingconstant unless energy is added or taken from through a hose of constant diameter exerts athe flow. In the previous examples there is no uniform pressure on the hose; but if theaddition or subtraction of energy; therefore diameter of a section of the hose in increasedthe total pressure will remain constant. or decreased, it is certain to change the pressure of the water at this point. SupposeFluid flow pressure is made up of two we were to pinch the hose, therebycomponents - Static pressure and dynamic constricting the area through which the waterpressure. The Static Pressure is that flows. Assuming that the same volume ofmeasured by an aneroid barometer placed in water flows through the constricted portion ofthe flow but not moving with the flow. The the hose in the same period of time as beforeDynamic Pressure of the flow is that the hose was pinched, it follows that thecomponent of total pressure due to motion of speed of flow must increase at that point. Ifthe air. It is difficult to measure directly, but a we constrict a portion of the hose, we notpitot-static tube measures it indirectly. The only increase the speed of the flow, but wesum of these two pressures is total pressure also decrease the pressure at that point. Weand is measured by allowing the flow to could achieve like results if we were toimpact against an open-end tube which is introduce streamlined solids (airfoils) at theVenter to an aneroid barometer. This is the same point in the hose. This principle is theincompressible or slow-speed form of the basis for measuring airspeed (fluid flow) andBernoulli equation. for analyzing the airfoils ability to produce lift.Static pressure decreases as the velocityincreases. This is what happens to air passingover the curved top of an aircrafts airfoil.Consider only the bottom half of a venturitube in the Figure below. Notice how theshape of the restricted area at Station 2resembles the top surface of an airfoil. Evenwhen the top half of the venturi tube is takenaway, the air still accelerates over the curvedshape of the bottom half. This happens Page | 3
  4. 4. Chapter 2 The Horizontal Hinge Pin (4) is theRotary wing plan forms: axis which permits up and down movement of the blade independent ofCommon terms used to describe the the other blades in the system.helicopter rotor system are shown here.Although there is some variation in systems The Trunnion (5) is splined to the mast and has two bearings throughbetween different aircraft, the terms shown which it is secured to the yoke. Theare generally accepted by most blades are mounted to the yoke and aremanufacturers. free to teeter (flap) around the trunnion bearings.The system below is an example of a Fully The Yoke (6) is the structural memberArticulated rotor system: to which the blades are attached and which fastens the rotor blades to the mast through the trunnion and trunnion bearings. The Blade Grip Retainer Bearings (7) is the bearing which permits rotation of the blade about its span wise axis so blade pitch can be changed (bladeSemi rigid Rotor Systems do not have vertical feathering)./ horizontal hinge pins. Instead, the entirerotor is allowed to teeter or flap by a trunnion Blade Twist is a characteristic builtbearing that connects the yoke to the mast into the rotor blade so angle of(this method is commonly used on two blades incidence is less near the tip than at therotor systems): root. Blade twist helps distribute the lift evenly along the blade by an increased angle of incidence near the root where blade speed is slower. Outboard portions of the blade that travel faster normally have lower angles of incidence, so less lift is concentrated near the blade tip. The Chord (1) is the longitudinal dimension of an airfoil section, measured from the leading edge to the trailing edge. The Span (2) is the length of the rotor blade from the point of rotation to the tip of the blade. The Vertical Hinge Pin (3) (drag hinge) is the axis which permits fore and aft blade movement independent of the other blades in the system. Page | 4
  5. 5. Chapter 3 root to tip. However, the symmetrical airfoil produces less lift than a non symmetricalAirfoils in general: airfoil and also has relatively undesirable stall characteristics. The helicopter blade (airfoil)An Airfoil is a structure, piece, or body must adapt to a wide range of airspeeds and angles of attack during each revolution of thedesigned to obtain a useful reaction upon rotor. The symmetrical airfoil deliversitself in its motion through the air. An airfoil acceptable performance under thosemay be no more than a flat plate (those alternating conditions. Other benefits aredarned engineers!) but usually it has a cross lower cost and ease of construction assection carefully contoured in accordance compared to the non symmetrical airfoil.with its intended application or function.Airfoils are applied to aircraft, missiles, or Non symmetrical (cambered) airfoils may have a wide variety of upper and lowerother aerial vehicles for: surface designs. The advantages of the non Sustentation (A Wing or Rotor Blade) symmetrical airfoil are increased lift-drag For Stability (As a Fin) ratios and more desirable stall characteristics. For Control (A Flight Surface, such Non symmetrical airfoils were not used in as a Rudder) earlier helicopters because the centre of For Thrust (A Propeller or Rotor pressure location moved too much when Blade) angle of attack was changed. When centre of pressure moves, a twisting force is exerted onSome airfoils combine some of these the rotor blades. Rotor system componentsfunctions. had to be designed that would withstand the twisting forces. Recent design processes andA helicopter flies for the same basic reason new materials used to manufacture rotorthat any conventional aircraft flies, because systems have partially overcome the problemsaerodynamic forces necessary to keep it aloft associated with use of no symmetricalare produced when air passes about the airfoils.rotor blades. The rotor blade, or airfoil, is thestructure that makes flight possible. Its shape Airfoil Terminology:produces lift when it passes through the air.Helicopter blades have airfoil sections Rotary-wing airfoils operate under diversedesigned for a specific set of flight conditions, because their speeds are acharacteristics. Usually the designer must combination of blade rotation and forwardcompromise to obtain an airfoil section that movement of the helicopter. An intelligenthas the best flight characteristics for the discussion of the aerodynamic forcesmission the aircraft will perform. affecting rotor blade lift and drag requires knowledge of blade section geometry. RotorAirfoil sections are of two basic types, blades are designed with specific geometrysymmetrical and non symmetrical. that adapts them to the varying conditions of flight. Cross-section shapes of most rotorSymmetrical airfoils have identical upper and blades are not the same throughout the span.lower surfaces. They are suited to rotary-wing Shapes are varied along the blade radius toapplications because they have almost no take advantage of the particular airspeedcentre of pressure travel. Travel remains range experienced at each point on the blade,relatively constant under varying angles of and to help balance the load between the rootattack, affording the best lift-drag ratios for and tip. The blade may be built with a twist,the full range of velocities from rotor blade Page | 5
  6. 6. so an airfoil section near the root has a larger The airfoil shown in the graphic is a Positivepitch angle than a section near the tip. Cambered Airfoil because the mean camber line is located above the chord line. The term "Camber" refers to the curvature of an airfoil to its surfaces. The mean camber of an airfoil may be considered as the curvature of the median line (mean camber line) of the airfoil. Pressure patterns on the airfoil: Distribution of pressure over an airfoil section may be a source of an aerodynamic twisting force as well as lift. A typical example is The Chord Line (1) is a straight line illustrated by the pressure distribution pattern connecting the leading and trailing developed by this cambered (non edges of the airfoil. symmetrical) airfoil: The Chord (2) is the length of the The upper surface has pressures distributed chord line from leading edge to trailing which produce the upper surface lift. edge and is the characteristic longitudinal dimension of an airfoil. The lower surface has pressures distributed which produce the lower surface force. Net The Mean Camber Line (3) is a line lift produced by the airfoil is the difference drawn halfway between the upper and between lift on the upper surface and the lower surfaces. The chord line force on the lower surface. Net lift is connects the ends of the mean camber effectively concentrated at a point on the line. chord called the Centre of Pressure. The shape of the mean camber is important in determining the aerodynamic characteristics of an airfoil section. Maximum Camber (4) (displacement of the mean camber line from the chord line) and where it is located (expressed as fractions or percentages of the basic chord) help to define the shape of the mean camber line. The Maximum Thickness (5) of an airfoil and where it is located (expressed as a percentage of the chord) help define the airfoil shape, and hence its performance. The Leading Edge Radius (6) of the airfoil is the radius of curvature given the leading edge shape. Page | 6
  7. 7. When the angle of attack is increased:Upper surface lift increases relative to thelower surface force.Since the two vectors are not located at thesame point along the chord line, a twistingforce is exerted about the centre of pressure.Centre of pressure also moves along the chordline when angle of attack changes, becausethe two vectors are separated. Thischaracteristic of non symmetrical airfoilsresults in undesirable control forces that mustbe compensated for if the airfoil is used in When the angle of attack is increased torotary wing applications. develop positive lift, the vectors remain essentially opposite each other and the twisting force is not exerted. Centre of pressure remains relatively constant even when angle of attack is changed. This is a desirable characteristic for a rotor blade, because it changes angle of attack constantly during each revolution. Relative wind: Knowledge of relative wind is particularly essential for an understanding of aerodynamics of rotary-wing flight because relative wind may be composed of multiple components. Relative wind is defined as theThe pressure patterns for symmetrical airfoils airflow relative to an airfoil:are distributed differently than for nonsymmetrical airfoils: Relative wind is created by movement of an airfoil through the air. As an example, consider a person sitting in an automobile on a no-wind day with a hand extended out the window. There is no airflow about the hand since the automobile is not moving. However, if the automobile is driven at 50 miles perUpper surface lift and lower surface lift hour, the air will flow under and over thevectors are opposite each other instead of hand at 50 miles per hour. A relative wind hasbeing separated along the chord line as in the been created by moving the hand through thecambered airfoil. Page | 7
  8. 8. air. Relative wind flows in the opposite In this graphic, angle of attack is reduced bydirection that the hand is moving. The induced flow, causing the airfoil to producevelocity of airflow around the hand in motion less lift:is the hands airspeed.When the helicopter is stationary on a no-wind day, Resultant Relative Wind isproduced by rotation of the rotor blades.Since the rotor is moving horizontally, theeffect is to displace some of the airdownward. The blades travel along the samepath and pass a given point in rapidsuccession (a three-bladed system rotating at320 revolutions per minute passes a givenpoint in the tip-path plane 16 times persecond). When the helicopter has horizontal motion,The graphic illustrates how still air is changed the resultant relative wind discussed above isto a column of descending air by rotor blade further changed by the helicopter airspeed.action: Airspeed component of relative wind results from the helicopter moving through the air. It is added to or subtracted from the rotational relative wind, depending on whether the blade is advancing or retreating in relation to the helicopter movement. Induced flow is also modified by introduction of airspeed relative wind. The pattern of air circulation through the disk changes when the aircraft has movement. Generally the downward velocityThis flow of air is called an Induced Flow of induced flow is reduced. The helicopter(downwash). It is most predominant at a moves continually into an undisturbed airhover under still wind conditions. Because the mass, resulting in less time to develop arotor system circulates the airflow down vertical airflow pattern. As a result, additionalthrough the rotor disk, the rotational relative lift is produced from a given blade pitchwind is modified by the induced flow. setting.Airflow from rotation, modified by inducedflow, produces the Resultant Relative Wind. Page | 8
  9. 9. Chapter 4 flight. If RPM is held constant, coning increases as gross weight and G-forceCentrifugal force: increase. If gross weight and G-forces are constant, decreasing RPM will causeHelicopter rotor systems depend primarily on increased coning. Excessive coning can occurrotation to produce relative wind which if RPM gets too low, gross weight is too high,develops the aerodynamic force required for or if excessive G-forces are experienced.flight. Because of its rotation and weight, the Excessive coning can cause undesirablerotor system is subject to forces and moments stresses on the blade and a decrease of totalpeculiar to all rotating masses. One of the lift because of a decrease in effective diskforces produced is Centrifugal Force. area:It is defined as the force that tends to makerotating bodies move away from the centre ofrotation. Another force produced in the rotorsystem is Centripetal Force. It is the forcethat counteracts centrifugal force by keepingan object a certain radius from the axis ofrotation.The rotating blades of a helicopter producevery high centrifugal loads on the rotor headand blade attachment assemblies. As a matterof interest, centrifugal loads may be from 6 to12 tons at the blade root of two to fourpassenger helicopters. Larger helicopters may Notice that the effective diameter of the rotordevelop up to 40 tons of centrifugal load on disk with increased coning is less than theeach blade root. In rotary-wing aircraft, diameter of the other disk with less coning. Acentrifugal force is the dominant force smaller disk diameter has less potential toaffecting the rotor system. All other forces act produce modify this force. Centrifugal force and lift effects on the bladeWhen the rotor blades are at rest, they droop can be illustrated best by a vector. First lookdue to their weight and span. In fully at a rotor shaft and blade just rotating:articulated systems, they rest against a staticor droop stop which prevents the blade fromdescending so low it will strike the aircraft (orground!). When the rotor system begins toturn, the blade starts to rise from the staticposition because of the centrifugal force. Atoperating speed, the blades extend straight outeven though they are at flat pitch and are not Now look at the same rotor shaft and bladeproducing lift. when a vertical force is pushing up on the tip of the blade:As the helicopter develops lift during takeoffand flight, the blades rise above the "straightout" position and assume a coned position.Amount of coning depends on RPM, grossweight, and G-Forces experienced during Page | 9
  10. 10. Forces applied to a spinning rotor disk by control input or by wind gusts will react as follows: This behaviour explains some of the fundamental effects occurring during various helicopter manoeuvres.The vertical force is lift produced when theblades assume a positive angle of attack. Thehorizontal force is caused by the centrifugalforce due to rotation. Since one end of theblade is attached to the rotor shaft, it is notfree to move. The other end can move andwill assume a position that is the resultant ofthe forces acting on it:The blade position is now "coned" and itsposition is a resultant of the two forces, liftand centrifugal force, acting on it. For example:Gyroscopic Precession: The helicopter behaves differently when rolling into a right turn than when rolling intoGyroscopic precession is a phenomenon a left turn.occurring in rotating bodies in which an During the roll into a left turn, the pilot willapplied force is manifested 90 degrees later in have to correct for a nose down tendency inthe direction of rotation from where the force order to maintain altitude. This correction iswas applied. required because precession causes a noseAlthough precession is not a dominant force down tendency and because the tilted diskin rotary-wing aerodynamics, it must be produces less vertical lift to counteractreckoned with because turning rotor systems gravity.exhibit some of the characteristics of a gyro. Conversely, during the roll into a right turn,The graphic shows how precession affects precession will cause a nose up tendencythe rotor disk when force is applied at a while the tilted disk will produce less verticalgiven point: lift.A downward force applied to the disk at Pilot input required to maintain altitude ispoint A results in a downward change in disk significantly different during a right turn thanattitude at point B, and an upward force during a left turn, because gyroscopicapplied at Point C results in an upward precession acts in opposite directions forchange in disk attitude at point D. each. Page | 10
  11. 11. Chapter 5Drag forces:Drag is simply force that opposes the motionof an aircraft through the air. However it doeshave separate components that comprise it.Total Drag produced by an aircraft is the sumof the Profile drag, Induced drag, andParasite drag. Total drag is primarily afunction of airspeed. The airspeed thatproduces the lowest total drag normallydetermines the aircraft best-rate-of-climbspeed, minimum rate-of-descent speed forautorotation, and maximum endurance speed. Curve "A" shows that parasite drag is very low at slow airspeeds andProfile Drag is the drag incurred from increases with higher airspeeds.frictional resistance of the blades passing Parasite drag goes up at an increasingthrough the air. It does not change rate at airspeeds above the midrange.significantly with angle of attack of the airfoilsection, but increases moderately as airspeed Curve "B" shows how induced dragincreases. decreases as aircraft airspeed increases. At a hover, or at lowerInduced Drag is the drag incurred as a result airspeeds, induced drag is highest. Itof production of lift. Higher angles of attack decreases as airspeed increases and thewhich produce more lift also produce helicopter moves into undisturbed air.increased induced drag. In rotary-wingaircraft, induced drag decreases with Curve "C" shows the profile dragincreased aircraft airspeed. The induced drag curve. Profile drag remains relativelyis the portion of the Total Aerodynamic constant throughout the speed rangeForce which is oriented in the direction with some increase at the higheropposing the movement of the airfoil. airspeeds.Parasite Drag is the drag incurred from the Curve "D" shows total drag andnon lifting portions of the aircraft. It includes represents the sum of the other threethe form drag and skin friction associated curves. It identifies the airspeed range,with the fuselage, cockpit, engine cowlings, line "E", at which total drag is lowest.rotor hub, landing gear, and tail boom to That airspeed is the best airspeed formention a few. Parasite drag increases with maximum endurance, best rate ofairspeed. climb, and minimum rate of descent inThe graphic illustrates the different forms of autorotation.drag versus airspeed: Page | 11
  12. 12. Chapter 6 needed to drive the tail rotor depending on helicopter size and design. Normally, largerTorque: helicopters use a higher percent of engine power to counteract torque than do smallerIn accordance with Newtons law of action aircraft. A helicopter with 9,500 horsepowerand reaction, the helicopter fuselage tends to might require 1,200 horsepower to drive therotate in the direction opposite to the rotor tail rotor, while a 200 horsepower aircraftblades. This effect is called torque. Torque might require only 10 horsepower for torquemust be counteracted and or controlled before correction.flight is possible. In tandem rotor and coaxialhelicopter designs, the rotors turn in opposite Heading Controldirections to neutralize or eliminate torqueeffects. In tip-jet helicopters, power originates In addition to counteracting torque, the tailat the blade tip and equal and opposite rotor and its control linkage also permitreaction is against the air; there is no torque control of the helicopter heading duringbetween the rotor and the fuselage. However, flight. Application of more control than isthe torque problem is especially important in necessary to counteract torque will cause thesingle main rotor helicopters with a fuselage nose of the helicopter to swing in themounted power source. The torque effect on direction of pedal movement. To maintain athe fuselage is a direct result of the constant heading at a hover or during takeoffwork/resistance of the main rotor. Therefore or approach, the pilot must use anti-torquetorque is at the geometric centre of the main pedals to apply just enough pitch on the tailrotor. Torque results from the rotor being rotor to neutralize torque and hold a slip ifdriven by the engine power output. Any necessary (keeping the aircraft in trim, the tailchange in engine power output brings about a is not used to turn the helicopter IN forwardcorresponding change in torque effect. flight. Heading control in forward trimmedFurthermore, power varies with the flight flight is normally accomplished with cyclicmanoeuvre and results in a variable torque control, using a coordinated bank and turn toeffect that must be continually corrected. the desired heading. Application of anti- torque pedals will be required when powerThe Anti-torque Rotor changes are made.Compensation for torque in the single main In an autorotation, some degree of right pedalrotor helicopter is accomplished by means of is required to maintain correct trim. Whena variable pitch anti-torque rotor (tail rotor) torque is not present, mast thrust bearinglocated on the end of a tail boom extension at friction tends to turn the fuselage in the samethe rear of the fuselage. Driven by the main direction as main rotor rotation. To counteractrotor at a constant ratio, the tail rotor this friction, the tail rotor thrust is applied inproduces thrust in a horizontal plane opposite an opposite direction to counter the frictionalto torque reaction developed by the main forces.rotor. Since torque effect varies during flightwhen power changes are made, it is necessary Translating Tendencyto vary the thrust of the tail rotor. Anti-torquepedals enable the pilot to compensate for During hovering flight, the single rotortorque variance. A significant part of the helicopter has a tendency to drift laterally toengine power is required to drive the tail the right due to the lateral thrust beingrotor, especially during operations when supplied by the tail rotor. The pilot maymaximum power is used. From 5 to 30 prevent right lateral drift of the helicopter bypercent of the available engine power may be tilting the main rotor disk to the left. This Page | 12
  13. 13. lateral tilt results in a main rotor force to the Angle of attack:left that compensates for the tail rotor thrustto the right. cHelicopter design usually includes one ormore features which help the pilotcompensate for translating tendency:Flight control rigging may be designed so therotor disk is tilted slightly left when thecyclic control is cantered. ANY Airfoils Angle Of Attack or AOA (4) isThe collective pitch control system may be an aerodynamic one.designed so that the rotor disk tilts slightlyleft as collective pitch is increased to hover It is: The angle between the airfoil chordthe aircraft. line and its direction of motion relative to the air (the resulting Relative Wind).The main transmission may be mounted sothat the mast is tilted slightly to the left when Several factors will affect rotor blade AOA.the helicopter fuselage is laterally level. Some are controlled by the pilot and some occur automatically due to the rotor system design. Pilots are able to adjust AOA by moving the cyclic and collective pitch controls. However, even when these controls are held stationary, the AOA constantly changes as the blade moves around the circumference of the rotor disk. Other factors affecting AOA, over which the pilot has little control, are: Blade Flapping Blade Flexing Wind Gusts / Turbulence AOA is one of the primary factors that determines amount of lift and drag produced by an airfoil. Angle of attack should not be confused with the Angle Of Incidence. Angle of Incidence (or AOI) is the angle between the blade chord line and the plane of rotation of the rotor system. It is a mechanical angle rather than an aerodynamic angle: Page | 13
  14. 14. main rotor shaft. An extreme airspeed differential between the blade tip and root is the result. The lift differential between the blade root and tip is even larger because lift varies as the square of the speed.In the absence of induced flow and/or aircraft Therefore, when speed is doubled, lift isairspeed, AOA and AOI are equal. increased four times.Whenever the relative wind is modified (byinduced flow / aircraft airspeed), then AOA This means that the lift at point "A" would beand AOI diverge becoming unequal. only one-fourth as much as lift at the blade tip (assuming the rotor airfoil has no bladeRotational velocities in the rotor twist along the span).system: Because of the potential lift differential alongDuring hovering, airflow over the rotor blades the blade resulting primarily from speedis produced by rotation of the rotor system. variation, blades are designed with a twist.The Graphic shows a two bladed system Blade twist provides a higher pitch angle atcommonly found: the root where speed is low and lower pitch angles nearer the tip where speed is higher. This design helps distribute the lift more evenly along the blade. It increases both the induced air velocity and the blade loading near the inboard section of the blade. This graphic compares a twisted versus an untwisted blades lift:Blade speed near the main rotor shaft ismuch less because the distance travelled atthe smaller radius is relatively small. The twisted blade generates more lift nearAt point "A", half way from the rotor shaft to the root and less lift at the tip than thethe blade tip, the blade speed is only 198 untwisted blade.knots which is one-half the tip speed.Speed at any point on the blades varies withthe radius or distance from the centre of the Page | 14
  15. 15. Dissymmetry of lift: Since lift increases as the square of the airspeed, a potential lift variation existsDissymmetry of lift is the difference in lift between the advancing and retreating sides ofthat exists between the advancing half of the the rotor disk. This lift differential must berotor disk and the retreating half. It is caused compensated for, or the helicopter would notby the fact that in directional flight the be controllable.aircraft relative wind is added to the rotationalrelative wind on the advancing blade, and To compare the lift of the advancing half ofsubtracted on the retreating blade. The blade the disk area to the lift of the retreating half,passing the tail and advancing around the the lift equation can be used. In forwardright side of the helicopter has an increasing flight, two factors in the lift formula, densityairspeed which reaches maximum at the 3 ratio and blade area are the same for both theoclock position. As the blade continues, the advancing and retreating blades. The airfoilairspeed reduces to essentially rotational shape is fixed for a given blade. The onlyairspeed over the nose of the helicopter. remaining variables are changes in bladeLeaving the nose, the blade airspeed angle of attack and blade airspeed. These twoprogressively decreases and reaches variables must compensate for each otherminimum airspeed at the 9 oclock position. during forward flight to overcomeThe blade airspeed then increases dissymmetry of lift.progressively and again reaches rotationalairspeed as it passes over the tail. Two factors, Rotor RPM and Aircraft Airspeed, control blade airspeed during flight. Both factors are variable to some degree, but must remain within certain operating limits. Angle of attack remains as the one variable that may be used by the pilot to compensate for dissymmetry of lift. The pitch angle of the rotor blades can be varied throughout their range, from flat pitch to the stalling pitch angle, to change angle of attack and to compensate for lift differential. The next graphic shows the relationship between blade pitch angle and blade airspeed during forward flight:Blade airspeed at the outboard edge of theshaded circle is 0 knots. Within the reverseflow area, the air actually moves over theblade backwards from trailing edge toleading edge. From the reverse flow area outto the blade tip, the blade airspeedprogressively increases up to 294 knots.At an aircraft airspeed of 100 knots, a 200knot blade airspeed differential existsbetween the advancing and retreating blades. Page | 15
  16. 16. Dissymmetry of Lift and the Tail Rotor The tail rotor also experiences dissymmetry of lift during forward flight, because of its own advancing and retreating blades. Although the plane of rotation is vertical, the effects are the same as for the main rotor in the horizontal plane. Dissymmetry is usually corrected for by a flapping hinge action. Two basic types of flapping hinges, the Delta and Offset. Either can be found on helicopters in the fleet. Note that the delta hinge (b) is not oriented parallel to the blade chord, designed that way so that flapping automatically introduces cyclic feathering which corrects for dissymmetry of lift.Note that blade pitch angle is lower on theadvancing side of the disk to compensate forincreased blade airspeed on that side.Blade pitch angle is increased on theretreating blade side to compensate fordecreased blade airspeed on that side.These changes in blade pitch are introducedeither through the blade featheringmechanism or blade flapping.When made with the blade featheringmechanism, the changes are called CyclicFeathering.Pitch changes are made to individual bladesindependent of the others in the system andare controlled by the pilots cyclic pitchcontrol. The offset hinge is located outboard from the hub and uses centrifugal force to produce substantial forces that act on the hub itself. One important advantage of offset hinges is Page | 16
  17. 17. the presence of control regardless of lift understandable that the maximum upwardcondition, since centrifugal force is flapping velocity will take place directly overindependent of lift. the right side of the helicopter, and the maximum downward flapping velocity takesBlade flapping: place directly over the left side of the helicopter. (This discussion assumes counterBlade Flapping is the up and down clockwise blade rotation, for clockwisemovement of a rotor blade, which, in rotation, they are reversed)conjunction with cyclic feathering, causesDissymmetry of Lift to be eliminated. The flapping velocities are at maximum values directly over the right and left sides ofThe advancing blade, upon meeting the the helicopter, because at those locations theprogressively higher airspeeds brought about airspeed differential is at its the addition of forward flight velocity tothe rotational airspeed (of the rotor), responds In the study of cyclic pitch, in a dynamicto the increase of speed by producing more system such as a main rotor system withlift. inertia, there is a phase angle between the maximum applied force and the maximumThe blade flaps (or climbs) upward, and the displacement.change in relative wind and angle of attackreduces the amount that would have been The force-displacement phase is 90 degrees,generated. and is not affected by blade mass or any kind of air dampening. It then follows that if the maximum upward and flapping velocity is directly over the right side of the helicopter, the maximum displacement or actual flapping will take place over the nose of the aircraft. Conversely, if the maximum downwardIn the case of the retreating blade, the flapping velocity is directly over the left sideopposite is true: of the helicopter, the maximum displacement or actual flapping will take place over the tail of the aircraft. The following graphic illustrates this relationship:As it loses airspeed, reducing lift causes it toflap down (or settle), thus changing itsrelative wind and angle of attack. Theresulting larger angle of attack retains the liftthat would have been lost because of thereduced airspeed.Flapping VelocityFlapping Velocity, both upward anddownward, must be of such a value as toincrease or decrease the angle of attack sothat the lift will remain constant. It is Page | 17
  18. 18. The total result of this action is a rotor tilt tothe rear which is completely independent ofany additional cyclic stick action and whichcauses an angular separation between thecontrol axis and the thrust axis of the rotor.There is yet another periodic force with aphase-displacement angular separation of 90degrees. This one arises from periodiclongitudinal forces which result from rotorconing while the helicopter is in directionalflight and causes the rotor to tilt to the side. The above graphic shows that the higher angle of attack at the front of the rotor will cause the blade to flap up over the left side of the helicopter. The lower angle of attack over the rear of the rotor will cause the blade to flap down over the right side. The rotor will thus be tilted a little to the right. The sidewardFrom the above graphic it may be seen that tilt of the rotor is increased at low forwardthe relative wind created by the helicopters speeds when the induced velocities are large,forward flight causes angle of attack because the inflow not only approaches thedifferences between the front and rear of the rear of the rotor but, additionally, is bentrotor. The blade over the nose of the downward. This increases the angle of attackhelicopter experiences an increase in angle of differences.attack because the aircraft relative windapproaches the blade level with or below itsspan. The blade over the rear of the helicopterexperiences a reduced angle of attack becausethe aircraft relative wind approaches it fromabove. Page | 18
  19. 19. Chapter 6 You can recognize transverse flow effect because of increased vibrations of theTransverse flow effect: helicopter at airspeeds just below effective translational lift (ETL) on takeoff and justIn forward flight, air passing through the rear passing through ETL during landing.portion of the rotor disk has a greaterdownwash angle than air passing through the To counteract transverse flow effect, a cyclicforward portion. This is due to that air being input will be needed to correct the rollingaccelerated for a longer period of time as it tendency.travels to the rear of the rotor system.The downward flow at the rear of the rotordisk causes a reduced angle of attack,resulting in less lift. Increased angle of attackand more lift is produced at the front portionof the disk because airflow is morehorizontal. These differences between thefore and aft parts of the rotor disk are calledtransverse flow effect. They cause unequaldrag in the fore and aft parts of the diskresulting in vibrations that are easilyrecognizable by the pilot. The vibrations aremore noticeable for most helicopters between10 and 20 knots.So, what does this mean to us pilots? Well,the result is a tendency for the helicopter toroll slightly to the Right as it acceleratesthrough approximately 20 knots or if theheadwind is approximately 20 knots.(Assuming a counter clockwise main rotorrotation, reverse for a clockwise rotation). Page | 19
  20. 20. Chapter 7Ground effect: When operating in ground effect, the downward and outward airflow pattern tendsGround Effect is a condition of improved to restrict vortex generation. This makes theperformance encountered when operating outboard portion of the rotor blade morenear (within 1/2 rotor diameter) of the efficient and reduces overall systemground. It is due to the interference of the turbulence caused by ingestion andsurface with the airflow pattern of the rotor recirculation of the vortex swirls.system, and it is more pronounced the nearerthe ground is approached. Increased blade Rotor efficiency is increased by ground effectefficiency while operating in ground effect is up to a height of about one rotor diameter fordue to two separate and distinct phenomena. most helicopters. This graphic displays theThe high power requirement needed to hover percent increase in rotor thrust experienced atout of ground effect is reduced when various rotor heights:operating in ground effect.First and most important is the reduction ofthe velocity of the induced airflow. Since theground interrupts the airflow under thehelicopter, the entire flow is altered. Thisreduces downward velocity of the inducedflow. The result is less induced drag and amore vertical lift vector. The lift needed tosustain a hover can be produced with areduced angle of attack and less powerbecause of the more vertical lift vector: At a rotor height of one-half rotor diameter, the thrust is increased about 7 percent. At rotor heights above one rotor diameter, the thrust increase is small and decreases to zero at a height of about 1 1/4 rotor diameters. Maximum ground effect is accomplishedThe second phenomenon is a reduction of the when hovering over smooth paved surfaces.Rotor Tip Vortex: While hovering over tall grass, rough terrain, revetments, or water, ground effect may be seriously reduced. This phenomenon is due to the partial breakdown and cancellation of ground effect and the return of large vortex patterns with increased downwash angles. Two identical airfoils with equal blade pitch angles are compared graphically: Page | 20
  21. 21. The top airfoil is out-of-ground-effect while velocity, an increase of blade pitch (angle ofthe bottom airfoil is in-ground-effect. The attack) would induce the necessary lift for aairfoil that is in-ground-effect is more hover. The forces of lift and weight reach aefficient because it operates at a larger angle state of balance during a stationary hover.of attack and produces a more vertical liftvector. Its increased efficiency results from a Hovering is actually an element of verticalsmaller downward induced wind velocity flight. Assuming a no-wind condition, the tip-which increases angle of attack. The airfoil path plane of the blades will remainoperating out-of-ground-effect is less horizontal. If the angle of attack of the bladesefficient because of increased induced wind is increased while their velocity remainsvelocity which reduces angle of attack. constant, additional vertical thrust is obtained. Thus, by upsetting the vertical balance of forces, helicopters can climb or descend vertically. Airflow in the Hover At a hover, the rotor tip vortex (air swirling around the blade tip from above to below) reduces the effectiveness of the outer blade portions. Also, the vortexes of the preceding blade severely affect the lift of the following blades. If the vortex made by one passing blade remains a vicious swirl for some number of seconds, then two blades operating at 350 RPM create 700 long lastingIf a helicopter hovering out-of-ground-effectdescends into a ground-effect hover, bladeefficiency increases because of the morefavourable induced flow. As efficiency of therotor system increases, the pilot reducesblade pitch angle to remain in the ground-effect hover. Less power is required tomaintain however in-ground-effect than forthe out-of-ground-effect hover.The Hover:Hovering is the term applied when ahelicopter maintains a constant position at aselected point, usually a few feet above the Vortex patterns per minute. This continuousground (but not always, helicopters can hover creation of new vortexes and ingestion ofhigh in the air, given sufficient power). existing vortexes is a primary cause of high power requirements for hovering.For a helicopter to hover, the main rotor mustsupply lift equal to the total weight of the During hover, the rotor blades move largehelicopter. With the blades rotating at high volumes of air in a downward direction. This Page | 21
  22. 22. pumping process uses lots of horsepower and efficiency of the rotor system and improveaccelerates the air to relatively high aircraft performance.velocities. Air velocity under the helicoptermay reach 60 to 100 knots, depending on the Improved rotor efficiency resulting fromsize of the rotor and the gross weight of the these changes is termed Effectivehelicopter. Translational Lift (or ETL). The graphic shows an airflow pattern at airspeeds between 1-5 knots:This is the air flow around a hoveringhelicopter(Note it is out of ground effect): Note how the downwind vortex is beginning to dissipate and induced flow down through the rear of the rotor disk is more horizontal than at a hover.Note how the downwash (induced flow) of airhas introduced another element into the This graphic below shows the airflow patternrelative wind which alters the angle of attack at a speed of 10-15 knots. Airflow is muchof the airfoil. When there is no induced flow, more horizontal than at a hover. The leadingthe relative wind is opposite and parallel to edge of the downwash pattern is beingthe flight path of the airfoil. In the hovering overrun and is well back under the helicoptercase, the downward airflow alters the relative nose. At about 16 to 24 knots (dependingwind and changes the angle of attack so less upon the size, blade area, and RPM of theaerodynamic force is produced. This rotor system) the rotor completely outruns thecondition requires the pilot to increase recirculation of old vortexes, and begins tocollective pitch to produce enough work in relatively clean air:aerodynamic force to sustain a hover.Although this does increase the lift, it alsoincreases the induced drag, and so total powerrequired is higherEffective translation lift:The efficiency of the hovering rotor system isimproved with each knot of incoming windgained by horizontal movement or surfacewind. As the incoming wind enters the rotor The air passing through the rotor system issystem, turbulence and vortexes are left nearly horizontal, depending on helicopterbehind and the flow of air becomes more forward air speed.horizontal. All of these changes improve the Page | 22
  23. 23. As the helicopter speed increases, ETL As forward airspeed increases, the "no lift"becomes more effective and causes the nose areas move left of centre, covering more ofto rise, or pitch up (sometimes called the retreating blade sectors:blowback). This tendency is caused by thecombined effects of dissymmetry of lift and This requires more lift at the outer retreatingtransverse flow effect. Pilots must correct for blade portions to compensate for the loss ofthis tendency in order to maintain a constant lift of the inboard retreating sections. In therotor disk attitude that will move the area of reversed flow, the rotational velocityhelicopter through the speed range where of this blade section is slower than the aircraftblowback occurs. If the nose is permitted to airspeed; therefore, the air flows from thepitch up while passing through this speed trailing to leading edge of the airfoil. In therange, the aircraft may also tend to roll to the negative stall area, the rotational velocity ofright. the airfoil is faster than the aircraft airspeed; therefore air flows from leading to trailingWhen the single main rotor helicopter edge of the blade. However due to the relativetransitions from hover to forward flight, the arm and induced flow, blade flapping is nottail rotor becomes more aerodynamically sufficient to produce a positive angle ofefficient. Efficiency increases because the tail attack. Blade flapping androtor works in progressively less turbulent airas speed increases. As tail rotor efficiencyimproves, more thrust is produced. Thiscauses the aircraft nose to yaw left if the mainrotor turns counter clockwise. During atakeoff where power is constant, the pilotmust apply right pedal as speed increases tocorrect for the left yaw tendency.Retreating blade stall:A tendency for the retreating blade to stall in Rotational velocity in the negative lift area isforward flight is inherent in all present day sufficient to produce a positive angle ofhelicopters and is a major factor in limiting attack, but not to a degree that producestheir forward speed. Just as the stall of an appreciable lift.airplane wing limits the low speedpossibilities of the airplane, the stall of a rotor This graphic depicts a rotor disk that hasblade limits the high speed potential of a reached a stall condition on the retreatinghelicopter. The airspeed of the retreating side:blade (the blade moving away from thedirection of flight) slows down as forwardspeed increases. The retreating blade must,however, produce an amount of lift equal tothat of the advancing blade. Therefore, as theairspeed of the retreating blade decreases withforward aircraft speed, the blade angle ofattack must be increased to equalize liftthroughout the rotor disk area. As this angleincrease is continued, the blade will stall atsome high forward speed. Page | 23
  24. 24. The Helicopter Will Roll Into The Stalled Side, (Dependent Upon Rotor Direction Of Rotation. When operating at high forward airspeeds, the following conditions are most likely to produce blade stall: High Blade loading (high gross weight) Low Rotor RPM High Density Altitude Steep or Abrupt TurnsIt is assumed that the stall angle of attack for Turbulent Airthis rotor system is 14 degrees. Distributionof angle of attack along the blade is shown at When flight conditions are such that bladeeight positions in the rotor disk. Although the stall is likely, extreme caution should beblades are twisted and have less pitch at the exercised when manoeuvring. An abrupttip than at the root, angle of attack is higher manoeuvre such as a steep turn or pull upat the tip because of induced airflow. may result in dangerously severe blade stall. Aircraft control and structural limitations ofUpon entry into blade stall, the first effect is the helicopter would be threatened.generally a noticeable vibration of thehelicopter. This is followed by a rolling Blade stall normally occurs when airspeed istendency and a tendency for the nose to pitch high. To prevent blade stall, the pilot must flyup. The tendency to pitch up may be slower than normal when:relatively insignificant for helicopters withsemi rigid rotor systems due to pendulum The Density Altitude is much Higher thanaction. If the cyclic stick is held forward and Standardcollective pitch is not reduced or is increased,this condition becomes aggravated; the Carrying Maximum Weight Loadsvibration greatly increases, and control maybe lost. By being familiar with the conditions Flying high drag configurations such aswhich lead to blade stall, the pilot should floats, external stores, weapons, speakers,realize when his is flying under such floodlights, sling loads, etc.circumstances and should take correctiveaction. The Air is Turbulent When the pilot suspects blade stall, he canThe major warnings of approaching retreating possibly prevent it from occurring byblade stall conditions are: sequentially:Abnormal Vibration Reducing Power (collective pitch)Nose Pitch up Reducing Airspeed Page | 24
  25. 25. Reducing "G" Loads during Manoeuvring causes loss of rotor efficiency even though power is still supplied from the engine.Increasing Rotor RPM to Max AllowableLimit This graphic shows induced flow along the blade span during normal hovering flight:Checking Pedal TrimIn severe blade stall, the pilot loses control.The helicopter will pitch up violently and rollto the left. The only corrective action then isto accomplish procedures as indicatedpreviously to shorten the duration of the stalland regain control. Downward velocity is highest at the blade tip where blade airspeed is highest.Settling with power: As blade airspeed decreases nearer the disk centre, downward velocity is less.Settling with Power is a condition of poweredflight where the helicopter settles into its own This graphic show induced airflow velocitydownwash. pattern along the blade span during a descentIt is also known as Vortex Ring State. conducive to settling with power:Conditions conducive to settling with powerare a vertical or nearly vertical descent of atleast 300 feet per minute and low forwardairspeed. The rotor system must also be usingsome of the available engine power (from 20to 100 percent) with insufficient poweravailable to retard the sink rate. Theseconditions occur during approaches with a The descent is so rapid that induced flow attailwind or during formation approaches the inner portion of the blades is upwardwhen some aircraft are flying in turbulence rather than downward.from other aircraft. The up flow caused by the descent hasUnder the conditions described above, the overcome the down flow produced by bladehelicopter may descend at a high rate which rotation.exceeds the normal downward induced flowrate of the inner blade sections. As a result,the airflow of the inner blade sections is If the helicopter descends under theseupward relative to the disk. This produces a conditions, with insufficient power to slow orsecondary vortex ring in addition to the stop the descent, it will enter vortex ring state:normal tip vortex system. The secondaryvortex ring is generated about the point on theblade where airflow changes from up todown. The result is an unsteady turbulentflow over a large area of the disk which Page | 25
  26. 26. The vortex ring state can be completely excess power. During the early stages ofavoided by descending on flight paths power settling, the large amount of excessshallower than about 30 degrees (at any power may be sufficient to overcome the upspeed). flow near the centre of the rotor. If the sink rate reaches a higher rate, power will not beFor steeper approaches, vortex ring state can available to break this up flow, and thus alterbe avoided by using a speed either faster or the vortex ring state of flow.slower than the area of severe turbulence andthrust variation. Normal tendency is for pilots to recover from a descent by application of collective pitchAt very shallow angles of descent, the vortex and power. If insufficient power is availablering wake is shed behind the helicopter. for recovery, this action may aggravate power settling resulting in more turbulence and aAt steep angles, the vortex ring wake is higher rate of descent. Recovery can bebelow the helicopter at slow rates of descent accomplished by lowering collective pitchand above the helicopter at high rates of and increasing forward speed. Both of thesedescent. methods of recovery require altitude to be successful. Hazards of rotating helicopter’s rotor blades: It is particularly tragic that rotor blade (and tail rotor blade) strike mishaps, along with airmen, have included bystanders, passengers, and children among the injured persons. Rotor strike mishaps differ from other aircraft mishaps in that they usually result in fatal or serious injury. This is due to the fact that a rotor rotating under power, even at slow speed, has sufficient force to inflict serious injury. It should be remembered that aThis graphic shows the horizontal speed rotating rotor is extremely dangerous andversus vertical speed relationship for a typical should be treated with all due caution.helicopter in a descent. Straight linesemanating from the upper left corner are linesof constant descent angle. Superimposed onthis grid are flow state regions for the typicalhelicopter. From this, several conclusionsregarding vortex ring state can be made:Power settling is an unstable condition. Ifallowed to continue, the sink rate will reachsufficient proportions for the flow to beentirely up through the rotor system. Ifcontinued, the rate of descent will reachextremely high rates. Recovery may beinitiated during the early stages of powersettling by putting on a large amount of Page | 26
  27. 27. Chapter 8 considered to help prevent accidents on airport ramp areas:Conspicuity: 1. when the possibility of passengersThe rotor is difficult to see when in operation, wandering on the ramp exists, physicaland the nonprofessional public is often not barriers should be provided such as ropeaware of its danger. Even personnel familiar stanchions from the aircraft to the terminalwith the danger of a turning rotor are likely to doors.forget it. 2. Airport management personnel should be1. Some manufacturers of rotor blades use on the alert to keep unauthorized personspaint schemes to increase the conspicuity of from milling around on ramps among parkedthe blades. Owners should give strong aircraft. When spectators are permitted toconsideration to maintaining the conspicuity view and move among aircraft parked on apaint scheme of the original manufacturer. ramp, the airport management personnel should caution those persons to stay clear of2. In the event that the paint scheme does not all propellers and not touch or move them.lend itself to conspicuity, the owner shouldconsider having the blade repainted. A 3. Helicopter landing and ramp areas shouldcustomized paint scheme should not be used be marked and provided with safety barriersuntil an evaluation is made by a person to restrict access by unauthorized persons.qualified to determine that it will not interferewith the pilots visibility, promote vertigo, or 4. Tail rotor danger areas should be clearlycreate an unbalanced blade condition. marked on ramp areas. Helicopters should be parked with tail rotors within the marked3. In August of 1978, the FAA issued Report area.No. FAA-AM-78-29, ConspicuityAssessment of Selected Propeller and TailRotor Paint Schemes. The report summarizesthe evaluation of three paint schemes forairplane propellers and two for helicopter tailrotor blades. The document is available to thepublic through the National Technical Aircraft Service Personnel:Information Service, Springfield, Virginia22161. Persons directly involved with aircraft service are most vulnerable to injuries by rotors. Working around aircraft places them in theIn-Flight Crew Personnel: most likely position for possible rotor strike mishaps. Aircraft service personnel shouldPersons directly involved with enplaning or develop the following safety habits:deplaning passengers and aircraft servicingshould be instructed as to their specific duties 1. Treat all rotors as if they were turning,through proper training, with emphasis placed remain clear of the rotor arcs.on the dangers of rotating rotors. Rampattendants and passenger handling personnel 2. Remember when removing an externalshould be made aware of the proper power source from an aircraft, keep theprocedures and methods of directing equipment and yourself clear of the rotor.passengers to and from parked aircraft. Thefollowing safety measures should be 3. Always stand clear of rotor blade paths Page | 27
  28. 28. (rotor arcs), especially when moving the rotor blades. Safety through education is therotor. Particular caution should be practiced best and most positive means available foraround warm engines. reducing potential mishaps from blade strikes.4. Ground personnel should be given 5. The prestart portion of the checklist shouldrecurrent rotor safety lectures to keep them include an item to make sure the rotor bladesalert to dangers when working around are clear. The proper use of the aircrafthelicopters. checklist should be taught to all student pilots.5. Be sure all equipment and personnel areclear of an aircraft before giving the pilot thesignal to depart. In Summary: In reviewing rotor blade strike mishaps, theFlight Personnel / Flight Instructors most impressive fact is that every one of them(CFIs): was preventable. The danger of rotor blade strikes is universally recognized.Prior to starting an engine, flight personnelshould make certain that all personnel are The pilot can be most effective in ensuringclear of the rotor. that his or her passengers arrive and depart the vicinity of the helicopter safely by1. The engine of a helicopter should be shut stopping the engine / rotor system completelydown (and rotor stopped / rotor brake at the time of loading and unloading, or byengaged if equipped) before boarding or providing a definite means of keeping themdeplaning passengers. This is the simplest clear of the rotors if they are left in motion.method of avoiding mishaps. Prominent warning signs, placed in the2. Boarding or deplaning of passengers, with aircrafts interior near or on the inside face ofan engine running, should only be allowed the aircraft doors to alert passengers andunder close supervision. The pilot in crewmembers of rotor hazards, could becommand should have knowledge that either helpful in preventing a mishap.the company or the airport operator hasground attendants fully trained in theirspecific duties to board or deplane passengers References:from an aircraft with an engine(s) running /rotors spinning. The pilot should instruct, before they exit an aircraft with engine(s) running / rotors spinning, thepath to follow to avoid the rotor blades. When it is necessary to discharge a n=0521858607passenger from an aircraft on which theengine is running / rotors spinning, never the aircraft with the tail rotor in the pathof the passengers route from the aircraft. www.pruftechnik.com4. When flight and ground instructors areinstructing their students about rotors, they emphasize the dangers of rotating Page | 28