Module 17 - Propeller PDF

wi es S A a c s _ otr is s w e Module 17 c PROPELLER tr a is s i o Pag. w ...

wi es S A a c s _ otr is s w e Module 17 c PROPELLER tr a is s i o Pag. w  wi s A Module 17 – Propeller c e _S tr a is s o Copyright © 2020 by Aviotrace Swiss SA s w e All rights reserved. No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, c including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the publisher. tr a is s 01.04.2020 i o Ed2 Pag. 2 w  wi s A Module 17 – Propeller c e Table of Contents _S tr a 17.1 Fundamentals iss o w 17.2 Propeller construction s 17.3 Propeller pitch control ce 17.4 Propeller synchronizing a s 17.5 Propeller ice protection tr is 17.6 Propeller storage and preservation 01.04.2020 i o Ed2 Pag. 3 w  wi s A Module 17 – Propeller c e _S tr a iss o s w Chapter 17.01 ce tr FUNDAMENTALS a is s 01.04.2020 i o Ed2 Pag. 4 w  wi s A Module 17 – Propeller e S Blade element theory a c s _ tr is Propellers were installed on old airplanes in order to generate thrust; when the Wright brothers began their first flights, designers had developed the standard o two-bladed style. In the past, a propeller installation was the only way used in w order to produce trust in aircraft. s Today, propellers are still installed on aircraft ranging from small single engine e aircraft to large multi-engine transport category airplanes. c Moreover, turbine engine and propellers work together in the turboprop. a s All modern propellers consist of at least two blades that are connected to a r is central hub; each blade is essentially a rotating wing that produces lift to pull the t aircraft forward. 01.04.2020 i o Ed2 Pag. 5 w  wi s A Module 17 – Propeller e S Blade element theory a c s _ o tr Two-bladed is Three-bladed w propeller on Beagle propeller on Me- s Pub 109G ce r a is s Eight-bladed t Four-bladed contrarotating propeller on B-29 propellers i o w on Antonov AN-22 01.04.2020 Ed2 Pag. 6  wi s A Module 17 – Propeller e S Geometry a c s _ Propeller design has passed through many stages of development; through the years, r s increases in engines power output have request development of big propellers (four and six t i bladed propellers) of large diameter. Moreover, the introduction of new materials has produced thinner airfoil sections and greater strength. The geometric features of the blades o w determine the characteristics of the propeller propulsion. s The basic geometrical elements of propellers are: ce Blade root and tip Blade face and blade back a s Blade angle and pitch distribution r is Helix angle t Angle of attack of the blade o w Twist 01.04.2020 i Ed2 Pag. 7  wi s A Module 17 – Propeller e S Geometry a c s _ o tr is Propeller components s w ce tr a is s 01.04.2020 i o Ed2 Blade airfoil Pag. 8 w Propeller structure  wi s A Module 17 – Propeller e S Geometry a c s _ To describe the propeller pitch, a very important feature of aerodynamic propeller, r s first it is necessary study the geometric helix, a line wrapping around a cylinder with t i a constant angle. o w In a helix, there are two basic parameters: the geometric pitch and the angle. The pitch is the distance between two consecutive points on the cylinder surface, after a s rotation. The two features are strictly related: pitch high values involves high angle e values. c According to the previous mentioned helix features, in a propeller, the propeller s pitch(or geometric pitch) shows as every point of the section chord describes, after a a blade rotation, a helical motion, that moves the point forward. tr is The propeller pitch is the theoretical distance that a propeller advances longitudinally in one revolution. 01.04.2020 i o Ed2 Pag. 9 w  wi s A Module 17 – Propeller e S Propeller pitch a c s _ o tr is s w ce tr a is s i o w Propeller pitch as the path of a screw Geometric pitch on the geometric helix that moves through a solid 01.04.2020 Ed2 Pag. 10  wi s A Module 17 – Propeller e S Propeller pitch a c s _ Propeller pitch and blade angle describe two different concepts, but they are closely r s related and the two terms are commonly (but not sometimes correctly) used in an t i interchangeable way. An increase or decrease in one is usually associated with an increase or decrease of the other. In fact, the pitch is proportional to blade angle: o s w ce where P is the pitch, β is the blade angle (in degree). r a is s The effective pitch is the real distance that propeller covers forward in one revolution; t it describes how really the propeller advances in the air. o w For example, a propeller designated as a “74-48” has 74 inches in length and has an i effective pitch of 48 inches. 01.04.2020 Ed2 Pag. 11  wi s A Module 17 – Propeller e S Propeller pitch a c s _ The propeller slip is the difference between geometric pitch and effective pitch. When r s the propeller travels into air, inefficiencies prevent a propeller from moving forward at a t i rate equal to its geometric pitch; so, the geometric pitch and effective pitch are different from each other. o s w ce tr a is s 01.04.2020 i o Ed2 Pag. 12 w  wi s A Module 17 – Propeller e S Propeller pitch a c The basic function of a propeller is to convert the power produced by fuel’s s _ r s combustion into propulsive power. In order to do this, propeller accelerates the air in t i the opposite direction of the aircraft motion, developing thrust that acts in forward o direction. The higher pressure is on backward surface of the blade while the lower is on w the forward one. In fact the blade has a shape very similar to that one of a wing and has the same behaviour. e s c THRUST AIR tr a is s 01.04.2020 i o Ed2 Pag. 13 w  wi s A Module 17 – Propeller e S Propeller pitch a c s _ r s Another important parameter is the propulsive efficiency (η), defined as the ratio t i between propulsive power and engine power. o The propulsive power is the thrust times the velocity that moves the airplane; it is w defined as the power products by the propeller. The engine power is the torque times s the angular velocity; it’s the power absorbed from the propeller. The propulsive efficiency shows how much of the power produced by the engine shaft, e is really used for thrust generation. c Typically, propulsive efficiency varies from 50% to 85%, depending on how much the a s propeller slips. tr is This efficiency is zero when the aircraft has zero forward velocity, even if the propeller develops a lot of thrust. The highest efficiency is obtained in cruise airspeed and is about o 80-85%. 01.04.2020 i Ed2 Pag. 14 w  wi s A Module 17 – Propeller e S Rankine theory a c s _ The blade element theory let to calculate the total force produced by a propeller. It is based r s on a theory developed by the Scottish engineer and physicist Rankine, together with the t i basic aerodynamic airfoil theory that involves a 2D airfoil. The blade element theory basically states that the total force produced by a propeller blade is o w determined by analyzing the forces on each blade element. s The method used in this theory consists in divide a blade into infinitesimal parts, and then e determining the forces on each of these small blade elements. These forces are then evaluated along the entire blade and over one revolution, in order to obtain the total forces c and momentums produced by the propeller. a s The blade element may be used to perform a fairly detailed local analysis of the propeller. It is r is important to underline that this theory has two main restrictions: t 1. It disregards the interference of turbulent flow between the propeller blades o w 2. It provides an overestimate of the efficiency’s value 01.04.2020 i Ed2 Pag. 15  wi s A Module 17 – Propeller e S Blade element theory a c s _ tr is Unlike Rankine theory, the blade element theory considers a two-dimension airfoil; so it evaluates both linear velocity and rotational velocity of the blade. It is possible to o demonstrate that the basic forces of blade element are related to the following factors: s w Density of the air Relative wind e Aerodynamic chord Distance between the hub’s center and the propeller’s section c Aerodynamic coefficient (related to the angle of attack). a s The blade element theory sums up the Renard equations, which let the analytical r is calculation of the forces and the torque generated by the propeller. 01.04.2020 i o t Ed2 Pag. 16 w  wi s A Module 17 – Propeller e S Blade element theory c s _ The equation for thrust calculation is: tr a The equation for torque calculation is: is o s w As stated in the previous equations, it is possible to note a relationship of thrust and e torque with: c The number of revolutions for second (N) a s The propeller diameter (D) r is The density of air (ρ) t A coefficient of thrust (CT) and of torque (CQ). o There is another way to evaluate thrust in dependance of the area of the propeller and i w the difference between forward and backward pressure 01.04.2020 Ed2 Pag. 17  wi s A Module 17 – Propeller e S Blade element theory a c s _ tr is The low blade angle is often used for fixed-pitch propellers installed on aircrafts that must get maximum performance during take-off and climb. Propellers with high blade o angle are more suitable for high speed cruise and high altitude flight. s w The reverse angle is the possibility, for the adjustable-pitch propellers, to rotate the blades of a negative angle in order to produce the reverse thrust. e Several factors may cause the change of angle of attack; some are controlled by pilot and others occur automatically due to the propeller design. c s When the pilot controls are stationary, the angle of attack constantly changes while the a blade moves around the circumference of the rotor disk. tr is This angle is one of the primary factors that determine the value of thrust and drag. 01.04.2020 i o Ed2 Pag. 18 w  wi s A Module 17 – Propeller e S Operating regimes a c s _ r s With the variation in advance ratio, forward speed, coefficient of thrust (CT) and of t i torque (CQ), it is possible to define the operating regimes of the propeller. The change of these parameters, for fixed-pitch propellers, involves several different cases: o w Brake propeller; this situation cannot be realized during the flight; it is possible when s the aircraft is on the ground, with the engine in running condition and the airplane pulled rearward ce Fixed-point propeller; this condition occurs when the forward speed is zero, and the aircraft is on the ground when the engine is operating. In this situation, efficiency is a s zero tr is Tractor propeller; it represent the normal operating condition of a propeller. In this case, the efficiency is positive, and the ratio between thrust and absorbed power o w decrease with the increase of forward speed 01.04.2020 i Ed2 Pag. 19  wi s A Module 17 – Propeller e S Operating regimes a c s _ r s Zero-thrust propeller; this situation occurs when the blades rotate in the air with a t i specific angle of attack that doesn’t allow the thrust generation. The propeller requires however a certain value of power to the engine, but this is lost to win the drag to the o w blades rotation. In this situation, efficiency is zero s Braking propeller; in this case, the propeller absorbs a certain quantity of power from the engine, that is spent to win the drag and to slow down the airflow. This situation e takes place when the aircraft flies with limited power on a slightly downward trajectory, c or during a sunk with high power setting a s Auto-rotating propeller; this condition can occur during downward flight. The propeller r is absorbs from the airflow the required power to win the drag to the blade rotation, and t so it decelerates the airflow; so, propeller generate a low thrust without using engine power 01.04.2020 i o Ed2 Pag. 20 w  wi s A Module 17 – Propeller e S Operating regimes a c s _ tr is Windmill-propeller; in this case the propeller works as a windmill. It absorbs power from the airflow that goes through the propeller’s section and slows down it. A o w part of power is used to win the drag to the blades rotation, the other part is used to provide the power to the engine shaft. This condition must be avoided, because s it can be dangerous for the engine. In emergency condition, the windmill- propeller setting can be used to start an engine switched off (failure condition) e c Blocked propeller; this situation occurs when the engine stops working, for any reason. Generally the blocked propeller is characterized by a torque that rotates a s the blades. In a particular case, called feathering propeller, the torque is zero. o tr is 01.04.2020 i Ed2 Pag. 21 w  wi s A Module 17 – Propeller e S Relative airflow on blade angle of attack a c s _ r To understand the action of a propeller, it is necessary to study first its motion which is both t is rotational and forward. The direction of relative airflow(or relative wind) on each blade is related to the aircraft forward velocity and the rotational speed of the propeller. o s w ce r a is s First of all, let us consider a propeller that rotates on a stationary aircraft in an airport apron. t In this case, there is not forward velocity, and the direction of the relative wind is parallel to the rotational movement of the propeller. So, the angle of attack of the blade is the same as the i o w blade angle. In this condition, the angle of attack has the maximum value. 01.04.2020 Ed2 Pag. 22  wi s A Module 17 – Propeller e S Relative airflow on blade angle of attack a c s _ When the aircraft begins to move in forward direction, the relative wind vector rotates and r s changes its direction. In this case, the propeller is rotating and it is also moving forward, so t i the combination of rotation and forward motion generates a resultant relative wind. In this o case, the angle of attack is less than the blade angle. w If propeller speed increases, the trailing edge of the propeller blade travels with a greater s speed for a given value of forward movement; the relative wind strikes the propeller blade e at a greater angle and the angle of attack increases. a c s o tr is 01.04.2020 i Ed2 Pag. 23 w  wi s A Module 17 – Propeller e S Vibration and resonance a c s _ When a propeller produces thrust, blade vibration occurs due to the several and r s elevated aerodynamic and mechanical forces. The aerodynamic forces tend to bend the t i propeller blades forward on the tip producing buffeting and vibrations. o w Often during takeoff, the tip speeds approaches the speed of sound; therefore, tip speed control has great importance in noise reduction. On the other hand, mechanical s vibrations are caused by power pulse in a piston engine. Mechanical vibrations are more e destructive than aerodynamic vibrations; the reason for this is that the engine power pulses tend to create standing waves, which can cause metal fatigue and structural c failure, with consequence for flight safety. a s It is possible to damp the effect of vibrations through a careful design of engine- r is propeller coupling. Anyway, some engine-propeller combinations have a critical RPM t range at which the vibration forces are too great for allowing normal operation. These vibrations may produce harmonic stresses at specific combinations of airspeed and o w engine RPM; this could lead to fatigue damages and to propeller failure. 01.04.2020 i Ed2 Pag. 24  wi s A Module 17 – Propeller c e _S tr a iss o s w Chapter 17.02 ce tr a PROPELLER CONSTRUCTION is s 01.04.2020 i o Ed2 Pag. 25 w  wi s A Module 17 – Propeller e S Materials - wood a c Construction materials s _ tr is The materials typically used for propeller construction are: o w Wood Steel s Aluminum alloys e Composite material c For many years, wood was the most used material in propeller manufacturing. In fact, a s in the early year of aircraft development, all propellers were made from wood. Wood r has a specific molecular structure, that allows the damping of a wide range of engine t is vibrations and let to avoid the working near the resonance frequency 01.04.2020 i o Ed2 Pag. 26 w  wi s A Module 17 – Propeller e S Wood a c s _ Almost all fixed-pitch propellers produced are made from wood or aluminum. However, there r s are still several aircraft (old single engine aircraft and small utility airplane) that have installed t i wood propellers. o w The wooden fixed-pitch propellers have low weight, ease of replacement and low production s cost. Types of woods used in propeller manufacturing are hardwoods, such as ash and birch. e A wooden propeller is made up by a minimum of five layers of wood, that are dried and c laminated, with waterproof resin glue. Each layer has normally the same thickness and the same wood type, but in some cases it is possible to use different types of wood. The use of laminated a s wood instead of a solid block permit to obtain a laminated structure, that is less subjected to r is warp. 01.04.2020 i o t Ed2 Pag. 27 w  wi s A Module 17 – Propeller e S Wood a c Once the layers of wood are laminated together, they form a propeller blank. s _ o tr is s w e Propeller blank is roughed to the blade shape and then it is left to rest for a period of time. At this point, the rough-shaped blank is refinished to the exact airfoil and pitch dimension c required. In addition, the center bore and bolt holes are drilled, and a metal hub assembly is a s placed through the hub bore to accommodate the bolts and the face plate. In this part of the r process the propeller is called “white propeller”. 01.04.2020 i o t Ed2 Pag. 28 w is  wi s A Module 17 – Propeller e S Wood a c s _ r s When the white propeller is finished, it is sanded smooth; t i Sometimes, wooden propeller may be finished with a plastic coating (grey or black) which o w provides additional protection against chipping. In this case the propeller is defined “armor coated”. It is finally completed with a clear varnish coating. e s a c s o tr is 01.04.2020 i Ed2 Pag. 29 w  wi s A Module 17 – Propeller e S Alluminium a c s _ r s Nowadays, the most widely material used in propeller manufacturing is the aluminum, in t i particular aluminum alloys. Compared to wood, aluminum alloy requires less o maintenance and lets to construct thinner and more efficient blades, with the same w structural strength. s The aluminum alloy is used instead of aluminum for its high strength and weight. Once e forged, the propeller is shaped by machine and manual grinding. Then, the final phase is set by twisting the blades to the desired angle; at last, the propeller is heat treated. a c s The main disadvantage of aluminum propeller is their susceptibility to damages caused r is by resonant vibrations. Because of this, aluminum propellers must be longer tested during t the certification process. For this reason, aluminum propellers balancing have a great importance in this case. 01.04.2020 i o Ed2 Pag. 30 w  wi s A Module 17 – Propeller e S Requirements a c s _ r s The aviation authorities require that all propellers must be identified with the builder t i name, model designation, serial number, type certificate number, production certificate number and times in which the propeller has been reconditioned. Most of o w aluminum fixed-pitch propeller manufacturers stamp all the required information on the hub. e s a c s o tr is 01.04.2020 i Ed2 Pag. 31 w  wi s A Module 17 – Propeller e S Composite a c s _ r s In the last years, composite materials are introduced in propellers manufacturing. t i Composite materials are lightweight, resilient, and extremely durable. In addition, composites can absorb vibration and are resistant to damage and corrosion and can o w reduce centrifugal forces because less mass is accelerated radially. s A composite material is a structured combination of very strong fibers and resin binder, e and it is characterized by limited density. The individual elements that make up composites are called constituents: the matrix (resin) and the reinforcement (fibers). a c s If the fibers are mono-oriented, composite supports the load above all in the direction of r the fibers (reduce in this way centrifugal force); because of this, composites are usually t is constructed of several layers of fibers, oriented in different directions. Mono-oriented fiber composite allows obtaining a specific resistance higher than that of all the materials. o The main limitation of composite materials is the cost of the manufacturing technology. 01.04.2020 i Ed2 Pag. 32 w  wi s A Module 17 – Propeller e _S Blade cuff a c s r s Some types of propellers have t i removable blades secured to a hub o assembly by a set of clamping w rings. Each blade root has a flanged root, which mates with grooves in s the hub assembly. e In this case, a device called blade c cuff is installed on the blade shank. a s A blade cuff is an airfoil-shaped element, made of metal, plastic or tr is composite material thin sheet. 01.04.2020 i o Ed2 Pag. 33 w  wi s A Module 17 – Propeller e S Propeller tip types a c s _ The blade tip works as a wing-tip, with the addition that the propeller tip r approaches the speed of sound on every takeoff. Because of this, the same tip t is vortices and aerodynamic drag that complicate a wing design are present in choice of tip shape. o For many years, propeller tips were only rounded and squared-shaped; in the w last years, there have been some attempts to improve this classic design. s Nowadays, the main used tip designs are the following. e Round tip a c s Round tips; this type of tip was used in r is the past for all-wood propellers design. t The degree of roundness depends mainly on the width of the blade; None o w of round-tip types offers any i improvement in airflow disturbance 01.04.2020 Ed2 Pag. 34  wi s A Module 17 – Propeller e S Propeller tip types a c s _ r Square tip t is An increase of 5-10% in blade area let to reduce the o propeller RPM at a fixed-pitch, and allows to have w the same performance with a decreased propeller s diameter ce r a is s Q-tip t This specific design introduces many aerodynamic o w improvements, as a reduction of propeller diameter i and a decrease of tip speed, that let to reduce tip vortices 01.04.2020 Ed2 Pag. 35  wi s A Module 17 – Propeller e S Propeller tip types a c s _ o tr is w Scimitar-tip s Let to decrease the tip disturbance and the induced e drag, delaying the separation until the airflow has reached the trailing edge. Moreover, this tip shape c allows higher tip speed without reaching the speed a s of sound o tr is 01.04.2020 i Ed2 Pag. 36 w  wi s A Module 17 – Propeller e S Propellers classification a c s _ tr is The basic classification of propellers is related to the installation on the aircraft. Propellers can be divided in: o w 1. Tractor propellers s 2. Pusher-type propellers ce tr a is s 01.04.2020 i o Ed2 Pag. 37 w  wi s A Module 17 – Propeller e S Propellers classification a c s _ o tr is w Tractor propellers e s a c s Tractor propellers are mounted on the front of the drive shaft, and pull the aircraft through the air; most of pistons-engine and turboprop aircraft have this propeller configuration. The r is main advantage of tractor propellers is the lower stresses introduced in the propeller, while t blades rotate in relatively undisturbed air. 01.04.2020 i o Ed2 Pag. 38 w  wi s A Module 17 – Propeller e S Propellers classification a c s _ o tr is w Pusher-Type propellers e s c s In the pusher-type propeller configuration, propeller is mounted on the aft end of the a aircraft, and it pushes the airplane. Pusher-type propeller configuration is used on some r is twin-engine executive transport turboprops, on amphibious aircrafts and on some t unmanned aerial vehicles. i o w Pusher-type airplanes have an increased wing efficiency due to the absence of prop-wash over wing section, but typically they are structurally more complicated than equivalent tractor types, especially as a result of efforts to mount the tail plane behind the rear mounted propeller. 01.04.2020 Ed2 Pag. 39  wi s A Module 17 – Propeller e S Spinner a c s _ The spinner is the aerodynamic cone at the hub of an aircraft propeller that protects the hub r assembly and pitch change mechanism. Spinner is usually mounted on tractor propeller t is installations, in order to split the incoming airflow; in the pusher-type case, spinner is optional. o w The spinner design can increase the airplane speed with an increase in propeller efficiency, a s reduction in fuel and consequently an increasing in range. On some engine/propeller combinations, spinners direct airflow into engine cowl area for engine cooling purposes. ce tr a is s 01.04.2020 i o Ed2 Pag. 40 w  wi s A Module 17 – Propeller c e _S tr a iss o s w Chapter 17.03 ce tr a PROPELLER PITCH CONTROL is s 01.04.2020 i o Ed2 Pag. 41 w  wi s A Module 17 – Propeller e S Pitch Control a c s _ r s In the early days of aviation the first propellers built were with fixed pitch, with the t i performance limitations that derive from a blade pitch angle that cannot be changed in flight. o w Very soon, manufacturers developed variable pitch propellers, which offered high performance at all flight speeds. Lastly, the modern propellers with constant-speed pitch were created. This s evolution derived from the need to relieve the pilot from constantly being obliged to work on e the pitch control and throttle lever by hand. In fact, on variable-pitch, the pitch change system on which the operation of variable-pitch propellers is based, may be developed through c different methods: a s Mechanical method tr is Electro-hydraulic method propellers, this situation arose continuously in order to keep suitable engine speeds. 01.04.2020 i o Ed2 Pag. 42 w  wi s A Module 17 – Propeller e S Pitch Control a c s _ r s In modern constant-speed propellers, pitch changing is carried out through an t i electrohydraulic device. o In constant-speed propellers, the system allows the pilot to choose the most suitable w speed for a specific flight condition. A mechanism automatically changes the pitch when s necessary to adapt it to the engine rpm chosen and keeps it constant regardless of the engine load. In actual fact, the propeller speed control is obtained through the control of e the blade pitch angle. c This makes it possible to optimize performance, because with this type of propellers the a s main advantage is that of converting a high percentage of the power produced by the engine into useful traction, for a broad range of flight speeds. tr is In conclusion the changing in the pitch angle is due to the combination of centrifugal forces and hydraulic forces 01.04.2020 i o Ed2 Pag. 43 w  wi s A Module 17 – Propeller e S Speed Governor a c s _ r s The pitch change system for propellers with constant rpm takes place through the speed t i governor that operates a hydraulic mechanism, which makes it possible to change the propeller pitch. o w The governor is fitted on the engine, coupled with the shaft on which the propeller is installed. s This position allows it to "perceive” the engine rpm. The mechanism for changing the pitch is installed on the front part of the propeller hub, covered by the spinner. ce In the cockpit, next to the throttle lever and mixture control, the pilot has a blue lever, called propeller pitch control. This is used to set the governor to keep any rpm required, within the a s rpm range allowed by the manufacturer. tr is To sum up how it works: the required engine rpm is selected from the cockpit through the control, the governor “feels” the actual rpm of the propeller and operates the pitch change o mechanism which alters the blade pitch angle according to the rpm set. 01.04.2020 i Ed2 Pag. 44 w  wi s A Module 17 – Propeller e S Speed Governor a c s _ r s When the rpm tends to lower, falling below the rate the pilot has set for the governor (decrease t i that can be caused by a reduction of the flight speed or engine power), the governor itself activates the pitch control so that it reduces the blade pitch angle and, consequently, the o w propeller pitch. The reduction of the pitch translates into a lowering of the blade incidence angle, which in turn reduces both the traction and the air drag. This results in an increase of the s rpm, which moves to a rate pre-established by the governor. The opposite situation occurs when the rpm tends to increase. ce The propeller speed governor, called Constant Speed Unit (CSU), is also commonly called governor. In order to bring about its pitch control function according to the set rpm, the a s governor must perform three fundamental actions: r is 1. pressurizing the oil before it goes into the propeller hub t 2. controlling the quantity of oil that flows inside the hub 3. measuring the engine rpm 01.04.2020 i o Ed2 Pag. 45 w  wi s A Module 17 – Propeller e S Speed Governor a c s _ tr is A speed governor comprises three main parts: o The supply pump w The pitch control s The counterweights (speed sensitive flyweight assembly) e To protect the seals and other parts of the system from high pressure, a relief valve is installed. c The valve has a spring preloaded to the limit pressure rate; when the pressure s increases to the point of overcoming the pressure of the spring acting on the a valve plunger, the latter opens discharging the excess oil that returns to the r is pump. 01.04.2020 i o t Ed2 Pag. 46 w  wi s A Module 17 – Propeller e S Speed Governor a c s _ tr is The pitch control essentially comprises a o pilot valve that has the task of directing w the high pressure oil in and out of the propeller hub, and a series of rods and s mechanisms that connect it to the blades. e The pilot valve is located inside the propeller shaft. This is, in fact a hydraulic c actuator, which, alternately, depending on s the pressure acting on it, covers or a uncovers the oil duct allowing the oil to r is flow inside the hub and return to the t engine sump. 01.04.2020 i o Ed2 Pag. 47 w  wi s A Module 17 – Propeller e S Speed Governor a c s _ tr is With regard to the pitch range used by variable pitch propellers, there are two distinct o w operating fields: s 1. Alpha range (positive pitch range) e 2. Beta range (negative pitch range) a c s o tr is 01.04.2020 i Ed2 Pag. 48 w  wi s A Module 17 – Propeller e S Pitch Configuration a c s _ r s The positions taken by the blades during operation of variable pitch propellers define a t i series of pitch configurations: o Ground fine pitch; this corresponds with a pitch angle of approximately 0°, reducing w the drag on the propeller and consequently facilitating engine ignition. It is helpful s above all in turbo-propellers with direct coupling of the propeller, as the compressor and the turbine form a considerable inertial mass that can make difficult the starting of e the two coupled systems c Flight fine pitch; this corresponds with the minimum positive pitch that the propeller a s can take in flight developing the maximum engine power and reaching the possible rpm. Basically used during takeoff o tr is 01.04.2020 i Ed2 Pag. 49 w  wi s A Module 17 – Propeller e S Pitch configuration a c s _ r s i Flight coarse pitch; this corresponds with a high positive pitch and a fairly low t rpm developed by the engine. It is generally used during cruising at high flight o speed w Feathered pitch; this corresponds with the position of the feathered blades, i.e. s with an arrangement of the profile directed at 90° in relation to the axis of e rotation which makes it possible to minimize drag. It is used on multi-engine aircraft in situations with an engine in a failure condition c s Reverse pitch; this corresponds with an aiming of the blades with reverse blade a pitch angle and thus with a reverse angle of attack, which in fact generates a r is braking action. It is used during taxiing when landing to reduce the runway t distance. 01.04.2020 i o Ed2 Pag. 50 w  wi s A Module 17 – Propeller e S Speed Governor a c s _ r s In the Alpha mode, the speed governor is controlled by the condition lever for t i controlling the system rpm, in the same way in which it is used for constant speed o propellers. w With the fixed setting of the power lever, the lever works with the fuel control unit to s control the flow of fuel to be supplied to the engine. Moving the lever forward will e increase the flow of fuel causing an increase of the engine power. c Lastly, the propeller will have to increase the pitch angle to absorb the power s increase and be able to keep the engine rpm constant. Vice versa, with the power a lever pulled backward the engine power will be lowered, which will cause a reduction r is of the pitch. 01.04.2020 i o t Ed2 Pag. 51 w  wi s A Module 17 – Propeller e S Cut-Off Valve a c s _ The controls in the cockpit consist in a power lever, a propeller control lever and a fuel cut- r s off lever. The former is connected to the cam assembly on the engine end; from here, the t i mechanical connections lead to the fuel control unit and Beta valve on the main governor. o w In the Alpha mode, the power lever controls the flow of fuel to the engine, adjusting the power at the output. In the Beta range, the lever controls both the fuel control unit and the s blade pitch angle. e The propeller control lever is connected to the main governor and controls the tension applied to the speeder spring like a conventional governor for constant-speed propellers. c Pulling the lever completely, the oil pressure is relieved from the piston, thereby allowing a s the propeller to be feathered. tr is The cut-off lever is generally used on propellers in reverse and has two functions. Firstly, it adjusts the flow of fuel to the control unit. Secondly, it adjusts the system rpm. The two o possible positions for the lever are: low idle, which supplies approximately 50% of the power i w and is used during ground operations, and high idle, with which about 70% can be obtained, which is used in flight. 01.04.2020 Ed2 Pag. 52  wi s A Module 17 – Propeller c e _S tr a iss o s w Chapter 17.04 ce tr a PROPELLER SYNCHRONIZING is s 01.04.2020 i o Ed2 Pag. 53 w  wi s A Module 17 – Propeller e S Synchronization Equipment a c s _ tr is The multi-engine airplanes incorporate an auxiliary system designed to improve the propulsive performance of the propellers, to reduce noise and reduce vibration levels. This o interaction amplifies noise and makes vibrations even more damaging to the structural w integrity. s A method to reduce the level of noise and vibration consists precisely in sync or match the e speed settings for each rotation-engine propeller group. c Synchronization systems control the engine speed and limit the vibrations through the s regulation of all engine-propeller groups at the same speed of rotation. Such systems can be a used for any flight operation except takeoff and landing. tr is Synchrophasing systems further reduce the level of noise and vibration from the propellers o of the multi-engine aircrafts, through phase shift between the various props and by adapting i w this phase shift to different flight conditions. 01.04.2020 Ed2 Pag. 54  wi s A Module 17 – Propeller e S Simultaneous Control System a c s _ r s The simultaneous control system allows control of all constant speed units of installed t i propellers. This device is controlled by a main control system that allows the pilot to modify to the same extent the speed of all engines using a single command (master o w control lever). s When the control lever is moved forward, all engines will increase their speed of rotation of the same value. In fact, once the sequence has been activated and the generator e rotates, the governors of the various propellers are set to a higher speed. The rotation of c the support arm of the generator is proportional to the movement of the control lever in cockpit, so also the change in speed of governors will be proportional to the displacement a s of the lever. Thus, a large movement of the lever will cause a significant increase in speed r is of rotation, before the support arm moves away from the rotating contact. t On the contrary, a limited motion of the lever will be followed by a small change in number o w of laps. When the main control lever is moved back, governors will diminish propellers i speed to the same extent, proportionally to lever movement. 01.04.2020 Ed2 Pag. 55  wi s A Module 17 – Propeller e S Simultaneous Control System a c s _ Maintenance of a system of simultaneous synchronization includes inspections, r s interventions to control the cleanliness of the components, verification of equipment. t i Like any other aircraft cable, also the cable that connects the main lever must be inspected, adjusted, oiled and cleaned regularly. o w In case of malfunction of the main control system, the cause may be: e s Cable connecting the lever to the rest of the main control mechanism, which may be broken or slack c Lack of electricity to main control a s Failure of the DC generator r is A defect in the rotating contact 01.04.2020 i o t Ed2 Pag. 56 w  wi s A Module 17 – Propeller e S Synchroscope a c s _ tr is Some aircraft are equipped with a Synchroscope in the cockpit. o w This instrument shows the pilot the comparison of s speed between the slave and master engine, by rotating pointers. The number of pointers is equal to the number e of slave motors; each will rotate in one direction when c the slave engine speed exceeds that of master engine, in the opposite direction if the slave engine speed is a s smaller than the master engine. tr is The pilot should make a manual adjustment by pulling the handle forward or backward on each engine to o ensure that the pointers will stop in a still position and i w the slave engines are approaching the master. 01.04.2020 Ed2 Pag. 57  wi s A Module 17 – Propeller c e _S tr a iss o s w Chapter 17.05 ce tr a PROPELLER ICE PROTECTION is s 01.04.2020 i o Ed2 Pag. 58 w  wi s A Module 17 – Propeller e S Ice Protection a c s _ tr is In bad weather, one of the problems associated with the use of aircrafts is the accumulation of ice on the surface of the plane. In fact, to ensure a safe flight during bad o weather, auxiliary systems that prevent or remove ice formation from the plane must be w installed on the aircraft. In addition, these systems can improve the performance of the s aircraft in case of rainfall during the flight. e The two different methods used to protect the propellers from the ice are: c s Anti-icing systems (anti-icing), preventing the formation and adhesion of ice on the a blades tr is De-icing systems (de-icing), removing the ice after it has accumulated on the blades 01.04.2020 i o Ed2 Pag. 59 w  wi s A Module 17 – Propeller e S Anti-Icing System a c s _ All those devices that are intended to prevent ice forming on the propeller are called anti-icing. tr is A typical anti-icing system uses a anti-icing fluid (isopropyl alcohol) that is directed on the propeller blades, preventing moisture to condense, to adhere to the blades and form ice. This o system must be operating when atmospheric conditions could cause ice formation. s w A typical anti-icing fluid system for a multi-engine aircraft includes: e A common control unit, which includes a rheostat c A reservoir containing the anti-icing fluid a s A feed pump, to direct the liquid toward the propellers tr is A slinger ring for each propeller (C shape) 01.04.2020 i o Ed2 Pag. 60 w  wi s A Module 17 – Propeller e S Anti-Icing System a c Following the basic maintenance to ensure proper operation of the system: s _ tr is Check all the pipes to verify the conditions or obstructions caused by the presence of o insects w s The filter between the reservoir and the pump must be cleaned during the annual 100 hours inspection e Check the condition of the materials of the outer tubes providing fluid to the blades c to avoid the deterioration of the material, damage of various kinds and the s detachment of the tubing from the blades r a is Inspect the area around the tubes and the feet boots to control corrosion. If any is t found, the tubes must be removed, the corroded area treated, following the instructions in the manual to disassemble and assemble the tubes and then treat the i o w area in an overhaul centre when necessary. 01.04.2020 Ed2 Pag. 61  wi s A Module 17 – Propeller e S De-Icing System a c s _ tr is The de-icing systems allow a certain amount of ice build up on the propeller to proceed then to eliminate it. Usually, the de-icing system is electrical. This device leaves the ice o w accumulating on the blades and then at regular intervals of time, removes it. Electrical de-icing systems exploit the Joule. s Due to continuous electricity availability during flight an electrical de-icing method is e preferred. In addition, this system can be used on aircraft of any size. c On multi-engine aircrafts, the system allows de-icing of a propeller induced by electric a s heaters placed on the blades. o tr is 01.04.2020 i Ed2 Pag. 62 w  wi s A Module 17 – Propeller e S De-Icing System a c s _ r s An electrical de-icing system for a multi-engine aircraft includes as basic components: o t i Block brush and slip ring for each propeller s w Relay for each propeller e Electric heaters (heating elements) c s a Control system in cockpit tr is Single timer for the entire system 01.04.2020 i o Ed2 Pag. 63