PROPELLER ENGINE

INTRODUCTION:  Turboprop engine is the turbine engine that drives an aircraft propeller through reduction gear box .Propeller engine develops thrust by moving large mass of air through a small change in velocity.  Simple type of turboprop consists of intake, compressor, combustion, turbine and a propelling nozzle etc.

 When the turboprop are flown at correct altitude and air speed they are extremely efficient and burn less fuel than most light jet. The turboprop engine has turbine section that extracts as much as 75 to 85 percent of the total power output to drive propeller. Turbine section of the turboprop usually has more stage than that of the turbo jet engine. The Gas Generator ( Compression, Combustion section and compression turbine) performs only one function converting fuel energy into high speed rotational energy. Turboprop engine can be categorised per the method used to achieved propeller drive. These categories are: 

i) Coupled power turbine / Fixed shaft engine:  In this case gas generator has additional turbine wheels. This additional turbine capability utilised the excess hot gas energy to drive the propeller and the shaft is mechanically connected to the gearbox. 

 fig: 2 Coupled power engine 

ii) Free turbine engine: A gas turbine act simply as a gas generator to supply high energy gas as to an independent free powered turbine. The gas are expanded across the free turbine which is connected to propeller drive shaft via  reduction gearbox 

Fig:3 Free turbine engine 

ii) Compound engine :  This engine arrangement feature two spool engine with the propeller drive directly connected to the low pressure spool. 

Fig: 4 Compounded engine

Turboprop can produce more thrust at take off speed which result in a shorter take off roll and they also have better reverse thrust so a shorter landing roll allow them into the smaller airport which is the main reason turboprop are predominated with regional airlines.  

The propeller is mounted on a propeller shaft driven by the turbine through the reduction gear box. The propeller transform the rotary power of the engine to the thrust.  If the engine running at a steady speed the  propeller  absorbing all the engine shaft horse power. The propeller is considered as the rotating wing. 

Propeller terminology  :  

1.  THRUST : The force produced by the propeller as it forces the mass of air to the rear of the aircraft . 
2. TORQUE : A fore that produces or try to produce rotation.  Propeller torque act in the plane of rotation and is the resistance to rotation offered by the propeller and oppose engine torque. 
3. AIR DENSITY: As increase in the air density increase the thrust and torque. 
4. PLANE OF ROTATION:  The plane in which propeller blade rotates and is perpendicular to the propeller shaft. 
5. PROPELLER SPEED : Thrust and the torque increased in direct proportion to the propeller speed. 
6. HORSE POWER : It the  unit of the mechanical power. 
7. SHAFT HORSEPOWER : The horsepower produced by rotating shaft. 
8. THRUST HORSEPOWER : The horse power equivalent of the produced by a jet engine.
9. ANGLE OF ATTACK:  The angle at which the aerofoil strikes the air is called angle of attack. the best angle of attack is obtained at 2 to 4 degree.  The optimum angle of attack will give the best thrust to torque ratio. 
10. BLADE STATION : A  reference position on a blade that is specified distance from the centre of the hub. 
11.  GEOMETRIC PITCH :  The theoretical distance a propeller would advance in one rotation if it were rotating in solid. Geometric pitch = TAN PITCH ANGLE X 2 x pi x r
12. EFFECTIVE PITCH : The theoretical distance a propeller advanced in one rotation through the air. 
13. PITCH DISTRIBUTION :  The gradual twist in the propeller blade from root to tip is called pitch distribution.The change in the shape of the aerofoil from the root to tip is necessary  to equalise the angle of attack from root to tip.  
14. SLIP : The difference  between the geometric pitch and effective pitch of the propeller. It is expressed in percentage of the mean geometric pitch or mean linear dimension.  

The amount of useful thrust produced by the propeller depends upon the amount of air that the propeller can move and increase in velocity that it can to move air mass. 

Thrust produced by the propeller is determined by 5 things : 

a) The shape of the aerofoil section 

b) The area of the aerofoil section

c) The angle of attack

d) The density of the air

e) The speed of the aerofoil moving through the air

Although the terms blade angle and pitch are often used to express the same thing, pitch will vary relative to the forward speed of the aircraft , whereas the blade angle can be locked in any position regardless of the forward speed.  An increase in forward velocity of the aircraft decreased the angle of attack and an increase in rotational velocity increased the angle of attack. 

The propeller efficiency vary from approx. 50 to 90 % depending on how much the propeller slip. 

 The propulsive efficiency is the ratio of the horse power to the shaft horse power. 

POWER ABSORPTION :  For the  maximum efficiency, the propeller must be able to absorb all the engine power available . 

POWER ABSORBED BY                                     LIMITATION 

• Increasing blade angle                            Reduction in thrust/ torque ratio,                                                                                 blade stall at low engine speed .
• Blade length increased                            High tip speed -reduced efficiency,                                                                                 propeller clearance of ground and                                                                                               aircraft structure.
• Higher propeller speed                           High tip speed - reduced efficiency 
• Altering the camber                                  Reduced aerodynamic efficiency 
• Increasing blade chord                           increased weight, increased turning                                                                             moment load 
• Increasing the number of the blades    Increased weight, structural                                                                                           difficulties at propeller hub 
• Contra rotating propeller                         Complicated pitch change                                                                                         mechanism, expense and maintenance 

SOLIDITY :  It is calculated at the blade master station which is about 0.7 of the blade length from root to tip. The greater the solidity, the greater the power absorbed by the propeller. 

FORCES ACTING ON THE PROPELLER :  The propeller is the one of the most highly stressed components, and 5 basic forces act on a propeller turning at high speed . these are : 

1. CENTRIFUGAL FORCE : It puts the greatest stress on a propeller as it pull the blades out of the hub. 
2. THRUST BENDING FORCE :  It is caused by the aerodynamic lift produced by the aerofoil shape of the blade as it move through the air. It tries to bend the blade forward and the force greatest near the tip.
3.  TORQUE BENDING FORCE : It tries to bend the propeller blade in its plane of rotation opposite to the direction to the rotation. 
4. AERODYNAMIC TURNING MOMENT : It tries to increase the blade angle making it coarser.
5. CENTRIFUGAL TURNING MOMENT : It tries to decrease the blade angle by making the blade at low pitch angle. CTM is always greater than the ATM. 

PROPELLER CONSTRUCTION AND MATERIALS : 

1. METAL PROPELLER:  Metal propeller are forged from high strength aluminium alloy and after being ground to their finished dimension and pitch are anodised to prevent corrosion .
2. COMPOSITE PROPELLER BLADE : composite material used in the propeller consists of two constitute; the fibre and the matrix. fibre must generally used are glass fibre and kevlar and the matrix is thermosetting epoxy resin. The strength and stiffness of the blade is determined by the materials, diameter and orientation of fibres. 
3. HARTZELL BLADE CONSTRUCTION : The typical Hartzell composite propeller has a machined aluminium alloy shank and moulded into this shank is a low density foam core.  It have a stainless steel mesh under the final layer of kevlar to protect against abrasion and nickel leading edge erosion shield is bonded in place. 
4. HEMILTON STANDARD BLADE CONSTRUCTION : It has the tremendous strength and fatigue resistance because of its solid aluminium alloy spar enclosed in a glass fibre shell . 
5. DOWTY ROTOL BLADE CONSTRUCTION :  It has two carbon fibres that run from the length of blade of both face and back and come smoothly together at the blade root. Polyurethane foam is injected into the inside of the blade and the entire unit is cured under heat and pressure. It is secured in a hub by expanding the carbon fibre spar with tapered glass fibre wedge and locking them between the inner and outer sleeves. 

PROPELLER ATTACHMENT METHODS : 

1. FLANGE  ATTACHMENT  : Some flange have index pin in the propeller flange so the propeller can be adjust in only one position relative to the shaft. This is done for synchronising and synchrophasing purpose.
2. SPLINED ATTACHMENT :  The most popular type of  propeller shaft  on the larger turboprop engine is the splined  shaft. The size of the splined shaft are identified by the SAE ( society of Automotive Engineers) number. Splines are longitudinal grooves cut in the periphery of the shaft. The grooves and lands are the same sizes. The purpose of the master splined is the same as the index pin. 

HUB CONES : The inside of the propeller hub is splined to match the shaft and the hub is correctly position of the shaft with two cones.  Two halves are marked with the same serial number to insure that only one matched set is used. 

PROPELLER SPINNER : These spinner have the dual aerodynamic function of streamlining the engine installation and directing cool air into the opening in the cowling. The propeller spinner and bulk head are critical components and cracks in either one can be repaired only if they do not exceed the allowable limits. 

PROPELLER PITCH CONTROL : Any movement of the blade is control by a propeller control unit that sends hydraulic pressure to turn the blade position.  

1. REVERSE PITCH :  It is used to obtain to get negative thrust to provide very efficient aerodynamic brake on landing and to reverse the aircraft during ground manoeuvres. Thrust reverse is only available when the aircraft is on the ground. 
2. GROUND FINE OR SUPER FINE : This position is used to off load the engine during starting and taxing when the power available from the turbine is insufficient to drive the propeller effectively. It also act as the effective brake because the propeller discs in the air flow are producing drag. This blade position is only available when the aircraft is on the ground. 
3. FLIGHT FINE PITCH : This position is the minimum blade angle allowed in flight and in this position the angle of attack is small and so accelerate small mass of air per revolution. 
4. COARSE PITCH : When coarse pitch is selected more torque is produced and the mass of air is accelerated greater for a low engine RPM , So saving fuel and engine wear in the cruise phase of the flight. 
5. FEATHERING :The primary purpose of the feathering propeller is to eliminate the drag created by the windmilling propeller when the engine fails and reduce the disturbance in the flow of air over the wings and tail of the aircraft. A feather blade is approximately a line of flight position, streamline with the line of sight position . The feathering position allow the propeller blade leading and trailing edge to the position parallel with the air flow thus produced the drag. 

ALPHA AND BETA MODE OF OPERATION : 

1. ALPHA :  It is the flight mode includes all operations from take off through to landing. 
2. BETA MODE : It is the ground operation