Propeller Design PDF

BASIC HELICOPTER & PROPELLER DESIGN propeller Prepared By Engr. Shiara Denise M. Valencia Introduction...

BASIC HELICOPTER & PROPELLER DESIGN propeller Prepared By Engr. Shiara Denise M. Valencia Introduction to Propeller Engr SDVM 2  What is the purpose of a propeller? THE PURPOSE OF THE PROPELLER IS TO CONVERT THE ENGINE TORQUE INTO AXIAL THRUST OR PROPWASH. Engr SDVM 3 Engr SDVM 4  To provide the necessary force to propel the aircraft forwards, the propeller displaces a large volume of air rearwards. How well the propeller convert engine torque into thrust is measured by the propeller’s efficiency. Engr SDVM 5 Parts of a Propeller Engr SDVM 6  BASIC NOMENCLATURE OF PROPELLER As relative wind speed of the retreating blade decreases, the blade loses lift and begins to flap down. It reaches its maximum downflap velocity at the 9 o’clock position, where wind velocity is the least. This creates an upward flow of air and has the same effect as decreasing the induced flow velocity by imposing an upward velocity vertical vector to the relative wind which increases the AOA. Engr SDVM 7  BASIC NOMENCLATURE OF PROPELLER Hub – the central portion of the propeller that is fitted to the propeller shaft, securing the blades by their roots. Blade Shank – the thickened portion of the blade near the hub. Blade Tip – the portion of the blade farthest from the hub. Engr SDVM 8  BASIC NOMENCLATURE OF PROPELLER Leading Edge – the forward or “cutting edge” of the blade that leads in the direction of the propeller is turning. Spinner – the streamline fairing covering the hub area. Engr SDVM 9  BASIC NOMENCLATURE OF PROPELLER Blade face – relatively flat surface corresponding to the wing’s lower surface. Blade back – the curved, corresponding to the wing’s upper surface and this part of the propeller is viewed from in front of the aircraft. Chord line – the imaginary line drawn through the blade from LE to TE or sometimes the line tangent to the lower surface of the prop blade. Engr SDVM 10  PROPELLER NOMENCLATURE Engr SDVM 11 Propeller Aerodynamic Process Engr SDVM 12  Propeller Aerodynamic Process The work done by thrust is equal to the thrust times the distance it moves the aircraft. Work = Thrust x Distance The power expended by thrust is equal to the thrust times the velocity at which it moves the aircraft. Power = Thrust x Velocity If the power is measured in horsepower units, the power expended by the thrust is termed thrust horsepower. Engr SDVM 13  Propeller Aerodynamic Process The engine supplies brake horsepower through a rotating shaft, and the propeller converts it into thrust horsepower. In this conversion, some power is wasted. For maximum efficiency, the propeller must be designed to keep this waste as small as possible. Since the efficiency of any machine is the ratio of the useful pow er output to the pow er input , propeller efficiency is the ratio of thrust horsepower to brake horsepower. Engr SDVM 14  Propeller Aerodynamic Process Propeller efficiency is the ratio of thrust horsepower to brake horsepower The usual symbol for propeller efficiency is the Greek letter η (eta). Propeller efficiency varies from 50 percent to 87 percent, depending on how much the propeller slips. Engr SDVM 15 Propeller Engr SDVM 16 Engr SDVM 17  Pitch – refers to the distance a spiral threaded object moves forward in one revolution. Geometric Pitch – is the theoretical distance a propeller would advance in one revolution. No slippage. Effective Pitch (advance per rev) – is the actual distance a propeller advances in one revolution in the air. Slip – the difference between the GP and APR (EF). Engr SDVM 18  Thus, geometric or theoretical pitch is based on no slippage. Actual, or effective, pitch recognizes propeller slippage in the air. The relationship can be shown as: Geometric pitch - Effective pitch = slip Geometric pitch is usually expressed in pitch inches and calculated by using the following formula: GP = 2𝝅𝝅R x tangent of blade angle at 75 percent station Where: R = Radius at the 75 percent blade station Engr SDVM 19 Engr SDVM 20 Engr SDVM 21  Blade Angle – the angle between the blade’s chord line and plane of rotation. Angle of Attack – the angle between the blade chord and the relative airflow. Helix Angle – The angle that the actual path of the propeller makes to the plane of rotation. Engr SDVM 22 Engr SDVM 23 Engr SDVM 24  Blade Twist Sections near the tip of the propeller are at a greater distance from the propeller shaft and travel through a greater distance. Tip speed is therefore greater. The blade angle must be decreased towards the tip to give a constant geom etric pitch along the length of the blade. The blade angle determines the geometric pitch of the propeller. A small blade angle is called “fine pitch” A large blade angle is called “coarse pitch” Engr SDVM 25 Engr SDVM 26 Engr SDVM 27 Engr SDVM 28 Forces Acting On A Propeller Engr SDVM 39  FORCES ACTING ON A PROPELLER Centrifugal Force is a physical force that tends to throw the rotating propeller blades away from the hub. Engr SDVM 40  FORCES ACTING ON A PROPELLER Torque Bending Force tends to bend the propeller blades in the direction opposite that of rotation. Engr SDVM 41  FORCES ACTING ON A PROPELLER Thrust Bending Force is the thrust load that tends to bend propeller blades forward as the aircraft is pulled through the air. Engr SDVM 42  FORCES ACTING ON A PROPELLER Aerodynamic Twisting Force tends to turn the blades to a high blade angle. Engr SDVM 43  FORCES ACTING ON A PROPELLER Centrifugal Twisting Force tends to force the blades toward a low blade angle. Engr SDVM 44 Engr SDVM 45  In addition TO COMPUTE FOR THE HELIX ANGLE 𝐴𝐴𝐴𝐴𝐴𝐴 helix angle, tan ϴ = 2𝜋𝜋𝑟𝑟 TO COMPUTE FOR THE GEOMETRIC PITCH GP = 2𝜋𝜋r tan ϴ APR = effective pitch Pitches are measured in INCHES R = radius of the propeller Angles are measured in DEGREES GP = Geometric pitch Engr SDVM 46 Effects of Propeller on Aircraft Stability Engr SDVM 47  Due to its rotation, a propeller generates yawing, rolling, and pitching moments. These are due to several different causes: Torque reaction. Gyroscopic precession. Spiral (asymmetric) slipstream effect. Asymmetric blade effect. Engr SDVM 48  CRITICAL ENGINE In the case of a propeller engine aircraft the length of the thrust arm is determined by the asymmetric effect of the propeller. I f both engines rotate clockw ise, the starboard (right) engine w ill have a longer thrust arm than the port (left) engine. The critical engine is the engine, the failure of w hich w ould give the biggest yaw ing m om ent. Engr SDVM 49  CRITICAL ENGINE Engr SDVM 50  TORQUE REACTION Because the propeller rotates clockwise, the equal and opposite reaction (torque) will give the aircraft an anti- clockwise rolling moment about the longitudinal axis. Engr SDVM 51  TORQUE REACTION During take-off this will apply a greater down load to the left wheel, causing more rolling resistance on the left wheel making the aircraft want to yaw to the left. In flight, torque reaction will also make the aircraft want to roll to the left. Engr SDVM 52  TORQUE REACTION Torque reaction will be greatest during high power, low airspeed (IAS) flight conditions. Torque reaction can be eliminated by fitting contra-rotating propellers. Engr SDVM 53  GYROSCOPIC EFFECT When a force is applied to the rim of a propeller, the reaction occurs 90° ahead in the direction of rotation and in the same direction as the applied force. As the aircraft is pitched up or down or yawed left or right, a force is applied to the rim of the spinning propeller disc. Engr SDVM 54  GYROSCOPIC EFFECT 1. Pitch down - forward force on the top, force emerges 90° clockwise, left yaw. 2. Left yaw - forward force on the right, force emerges 90° clockwise, pitch up. 3. Right yaw - forward force on the left, force emerges 90° clockwise, pitch down. Engr SDVM 56  SPIRAL SLIPSTREAM EFFECT As the propeller rotates it produces a backward flow of air, or slipstream, which rotates around the aircraft. The spiral slipstream causes a change in airflow around the fin (vertical stabilizer). Due to the direction of propeller rotation (clockwise) the spiral slipstream meets the fin at an angle from the left, producing a sideways force on the fin to the right. Engr SDVM 57  SPIRAL SLIPSTREAM EFFECT The amount of rotation given to the air will depend on the throttle and RPM setting. Spiral slipstream effect can be reduced by: the use of contra or counter- rotating propellers. a small fixed tab on the rudder. the engine thrust line inclined slightly to the right. offsetting the fin slightly. Engr SDVM 58  ASYMMETRIC BLADE EFFECT SAME PRINCIPLE OF HELICOPTER’S ROTOR BLADE, ADVANCING AND RETREATING. In general, the propeller shaft will be The difference in thrust on the two sides of the propeller inclined upwards from the direction disc will give a yawing moment to the left with a clockwise of flight due to the angle of attack of rotating propeller in a nose-up attitude. the aircraft. Asymmetric blade effect will be greatest at full power and low airspeed (high angle of attack). This gives the down-going propeller blade a greater effective angle of attack than the up-going blade. The down-going (right) blade will generate more thrust. Engr SDVM 59 Classification of Propellers Engr SDVM 60  CLASSIFICATION Engr SDVM 61  INSTALLATION Pusher Type Tractor Type Pusher propellers are those mounted Tractor propellers are those mounted on the downstream end of a drive on the upstream end of a drive shaft shaft behind the supporting in front of the supporting structure. structure. Most aircraft are equipped with this Pusher propellers are constructed as type of propeller. fixed- or variable-pitch propellers. A major advantage of the tractor Seaplanes and amphibious aircraft propeller is that lower stresses are have used a greater percentage of included in the propeller as it rotates pusher propellers than other kinds of in relatively undisturbed air. aircraft. Engr SDVM 62  INSTALLATION Engr SDVM 63  INSTALLATION CONTRA-ROTATING COUNTER-ROTATING PROPELLERS PROPELLERS – mounted on two concentric shafts – Eliminate the torque effect of the that rotate in opposite directions. spinning propeller mass. Engr SDVM 64  PITCH FIXED-PITCH PROPELLER –a fixed-pitch propeller has the blade pitch, or blade angle, built into the propeller. The blade angle cannot be changed after the propeller is built. Fixed-pitch prop are design for best efficiency at one rotational and forward speed. Engr SDVM 65  PITCH VARIABLE PITCH PROPELLERS VARIABLE PITCH PROPELLERS ADJUSTABLE PITCH PROPELLER CONTROLLABLE PITCH PROPELLER These are propellers which can have their pitch adjusted on the ground The controllable-pitch propeller by mechanically resetting the blades permits a change of blade pitch, or in the hub. angle, while the propeller is rotating. In flight they act as fixed pitch This permits the propeller to assume propellers. a blade angle that will give the best performance for particular flight conditions. Engr SDVM 66  PITCH Assuming we will take off, which is better? Engr SDVM 67 Engr SDVM 68  PITCH VARIABLE PITCH PROPELLERS CONSTANT SPEED PROPELLER Controlled automatically to vary their pitch so as to maintain a selected RPM. By using propeller governors to increase or decrease propeller pitch, the engine speed is held constant. https://www.youtube.com/watch?v=QcxLottMghg&ab_channel =FlightInsight Engr SDVM 69