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# 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
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
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
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