Aircraft Propulsion Systems Final

Define the terms "bore" and "stroke"

"Bore" is the diameter of the inside of the cylinder of a reciprocating engine "Stroke" is the distance traveled by a piston from the top to the bottom of its cylinder.

What are the disadvantages of an axial flow compressor?

- Difficulty and high cost of manufacture - Relatively high weight - High starting power requirements - Low pressure rise per stage (approximately 1.3:1 maximum) - Good compression in the cruise to takeoff power setting only

In a turboprop engine, the propeller is turned via a fixed or a free turbine. What is the difference between the two? Which is more advantageous and why?

- Fixed turbine is connected directly to the compressor, reduction gearbox and propeller shaft - Free turbine is connected only to the gearbox and the propeller shaft - A free turbine is more efficient than a fixed turbine because it can seek optimum design speed

What are the advantages of an axial flow compressor?

- High peak efficiencies created by its straight through design - Higher peak efficiencies attainable by adding additional stages - Small frontal area means low drag

What are the advantages of a centrifugal-flow compressor?

- High pressure rise per stage (up t0 10:1 to 15:1 in a dual stage) - Good compression efficiency over a wide rotational speed range (idle to full power) - Simplicity of manufacture and low cost as compared to the axial flow compressor - Low weight - Low starting power requirements - Shorter in length than axial flow compressors

What are the disadvantages of a centrifugal-flow compressor?

- Large frontal area for a given air flow - More than two stages is not practical because of the energy losses between stages - Compressor tip speed reaches supersonic (Mach 1.3) whereas axial flow compressors remain subsonic

What is a piston and what is its function?

- The piston is a plunger that moves back and forth or up and down within an engine cylinder barrel - The piston transmits the force of the burning and expanding gases in the cylinder through the connecting rod to the engine crankshaft

What are the advantages to having more than two poles on a magnet?

- more flux lines - greater the current generated - a stronger spark

What are the three types of engines which were designed and used after WWI? Briefly describe them.

-Radial Engines: the cylinders are arranged around the crankshaft but the crankshaft rotates, not the cylinders -Multiple-Row Radial Engines: the largest and most powerful piston-type engines ever built and used -Opposed, Flat or O-type Engines: This is an engine in which the cylinders are double-ended, with a piston at each end and no cylinder-head

What are the three types of engines which were designed and used during WWI? Briefly describe them.

-Rotary-Type Radial: In this engine, the crankshaft is held stationary and cylinders rotate about the crankshaft -In-Line: The cylinders are arranged in a single row parallel to the crankshaft -V-type: The cylinders are arranged on the crankcase in two rows forming the letter V

What are the three main reasons that excessive heat is harmful to a reciprocating engine?

1. Adversely affects the combustion of the fuel-air mixture - Mixture preheats and combustion occurs prematurely 2. Weakens and shortens the life of the engine - Parts may also warp and the engine may seize 3. Impairs lubrication - The viscosity of the oil is reduced

What are the advantages to a dual magneto system?

1. If one magneto fails, the other magneto will furnish ignition 2. Two sparks, each igniting the charge from two different points, provide quicker and more complete combustion (increased power)

What are the principle functions of the lubricating oil in an engine?

1. It lubricates to reduce friction between moving parts 2. It cools various parts of the engine 3. It seals the combustion chamber by filling spaces between the cylinder walls and piston rings (prevents flow of combustion gases past the piston rings) 4. It cleans the engine by carrying sludge away 5. It aids in preventing corrosion by protecting the metal from oxygen, water and other corrosive agents 6. It serves as a cushion between parts where impact loads are involved

What are the four ways of classifying magnetos?

1. Low-tension or high-tension 2. Rotating-magnet or inductor-rotor 3. Single or double 4. Flange-mounted or base-mounted

Name and define the three types of efficiencies associated with reciprocating-engine theory.

1. Mechanical efficiency: measured by the ratio of the brake horsepower to the indicated horsepower (ratio of output to input) 2. Thermal efficiency: measure of the heat losses in converting heat energy to mechanical work; ratio of the heat developed into useful work to the heat energy of the fuel 3. Volumetric efficiency: ratio of the volume of the fuel/air charge burned by the engine at atmospheric pressure and temperature to the piston displacement; how effectively can the engine fill up the space inside the cylinder with fuel/air mixture (the more the better).

How do Boyle's Law and Charles' Law explain the operation of an engine (5 steps)?

1. The mixture of fuel and air burns when it is ignited and produces heat (Temperature increases) 2. The heat is absorbed by the gases in the cylinder, and the gases expand (Volume increases) 3. As the volume increases (closed cylinder) the pressure increases 4. The increase in pressure on the piston head forces it to move 5. Movement of the piston is transmitted to the crankshaft through the connecting rod

Name and define the three losses in the realistic reciprocating engine cycle.

1. Time Loss: this is caused by the fact that combustion is not instantaneous 2. Heat Loss: this is a loss caused by the fact the some heat is lost the cylinder walls 3. Exhaust Blowdown Loss: this is caused by the fact that the exhaust valve actually opens a few degrees before the piston reached BC on the power stroke

In order to induce a voltage in a coil of wire, what must occur?

A change in the magnetic flux lines must occur

Name the principle sections of an induction system

A complete induction system includes three sections 1. Air scoop and ducting leading to the carburetor 2. The carburetor, or air control section, of an injection system 3. The intake manifold and pipes

Which of the four types of gas turbine engines (turbojet, turboshaft, turboprop, or turbofan) would most likely be installed in a late model business jet of the 500 mph class?

A turbofan

What are the advantages of an air cooled engine?

Advantages to air-cooling 1. Weight of the engine is less than if liquid is used 2. The engine is less affected by cold-weather operations 3. For military operations, this engine is less susceptible to damage from gunfire

What is a supercharger?

An air compressor used to force more air into the combustion chamber of an internal combustion engine than can be achieved with standard atmospheric pressure. Superchargers allow an engine to develop maximum power when operating at high altitudes

What is the effect of supercharging on the density of air? How does it affect air temperature and subsequently the performance of the supercharger, and the engine itself?

As air is compressed its density rises and its temperature rises. The rising temperature reduces supercharger efficiency because heated air also expands. The expansion increases the amount of power required to compress it. Overall engine efficiency also decreases with a warmer charge.

At what altitudes is the compressor most efficent and why?

At higher altitudes because the air molecules are more dense (closer together) due to the colder temperatures; the compressor does not have to work as hard to compress the air to the desired compression ratio.

Why are reciprocating engines for aircraft called heat engines?

Because they utilize heat energy to produce power for propulsion

What causes efficiencies less than 100% in reciprocating engines?

Causes of decreased efficiencies include 1. Improper valve timing 2. High engine rpm 3. High carburetor air temperature 4. Improper design of the induction system 5. High combustion chamber temperature

What are the principle factors governing the grade of fuel used in an aircraft?

Compression ratio and Manifold Absolute Pressure (MAP)

There are two types of piston rings. What are they and what is the difference between them?

Compression rings and oil rings. Oil rings are used to control the quantity of lubricant supplied to the cylinder walls and to prevent oil from getting into the combustion chamber.

What is detonation?

Detonation is when the temperature and pressure of the compressed mixture in the cylinder reach such levels that instantaneous burning of the fuel/air mixture occurs

What does the term "displacement" mean when used in reference to a piston engine?

Displacement is the total volume of air/fuel mixture an engine can draw in during one complete engine cycle. In a piston engine, displacement is the volume that is swept as the pistons are moved from top dead center (TDC) to bottom dead center (BDC)

. What is the geometry of a subsonic turbine engine inlet duct? Why?

Divergent; because of the divergent inlet duct, the engine receives air with little internal turbulence and a more uniform pressure

What happens if the intake valve is opened too early?

Hot exhaust gases may enter the intake manifold and ignite the incoming fuel/air mixture causing a backfire

Describe both the Ideal Otto Cycle and the Realistic Otto Cycle. Be sure to note differences between the two.

IDEAL: Intake Stroke: intake valve opens at TC; exhaust valve is closed Compression Stroke: intake valve closes at BC Ignition Event: occurs at TC Power Stroke: both valves initially closed, exhaust valve opens at BC Exhaust Stroke: exhaust valve open, intake valve closed, both valves reverse position at TC Heat increase (combustions) is instantaneous The cycle is adiabatic (occurring without gain or loss of heat) REALISTIC: Intake Stroke: intake valve is already open, exhaust valve is also open Compression Stroke: intake valve closes as much as 60° after BC Ignition Event: occurs before TC Power Stroke: exhaust valve opens 55-60° before BC Exhaust Stroke: exhaust valve open, intake valve opens ~ 15° before TC, exhaust valve closed after TC Heat increase (combustion) is gradual There is heat lost to the cylinder walls

How does turbine inlet temperature affect an aircraft's thermal efficiency

If the engine is designed to withstand higher temperature, the cooling air required decreases If the cooling requirement decreases, the compressor can be made smaller If the compressor is made smaller, the turbine which drives the compressor can also be made smaller The thermal efficiency goes up because less energy is extracted from the turbine and more is available in the tailpipe

Why might excessive temperatures associated with supercharging a reciprocating engine be problematic?

If the temperature of the charge gets too high, pre-ignition and detonation may take place (this may result in power loss or complete mechanical failure).

Why does ignition occur just before TC of the compression stroke? What would be the associated problem if ignition occurred after TC?

Ignition is timed to occur before TDC so that combustion is complete just after TDC, producing maximum pressure (smallest volume). If the fuel is burned as the piston is moving downward, there may be excessive heating of the cylinder walls (pre-ignition)

What is the primary purpose of the compressor in a gas turbine engine?

It is to increase the pressure of the air entering the engine inlet and to discharge it to the diffuser (and then combustor) at the proper velocity, temperature, and pressure.

What is added to aviation fuel to improve its anti-knock properties above 100 octane? What is a disadvantage to this additive?

Lead. Will leave a residue unless another additive is used to prevent it from forming. Even with the second additive, the lead/additive mix will cause excessive rusting of the steel and cast iron when combined with water.

List the disadvantages of the in-line engine

Low horsepower-to-weight ratio Requires either a cooling fluid to remove heat or complicated baffles to route cooling air (the rear-most cylinders receive little airflow)

What is the difference between a high tension and a low tension magneto?

Low-tension: delivers current at a low voltage by means of the rotation of an electric motor in the field of a permanent magnet. High-tension: delivers a high-voltage directly to the spark plug.

Lubricants can be classified into what categories? Which is used for lubricating aircraft engines and why?

Lubricants are classified according to their origins - Animal - Vegetable - Mineral - Synthetic Mineral are used to a large extent on aircraft engines because they are chemically stable at high temperatures - They also perform well at low temperatures (unlike animal and vegetable lubricants) - May be classified as solid, semisolid, or fluid

Why is magneto ignition superior to battery ignition?

Magneto ignition is superior to battery ignition because tt produces a hotter spark at high engine speeds and it is a self-contained unit (not dependent on any external source of electric energy).

What is a magneto?

Magneto is a special type of alternating-current (ac) generator that produces electric pulses of high voltages for purposes of ignition

What is Manifold Absolute Pressure and explain the significance of MAP in the operation of an aircraft engine.

Manifold Absolute Pressure (MAP) is the pressure in the intake manifold of an engine. In a normally aspirated engine, MAP is less than atmospheric pressure (this is due to friction losses within the induction system). In a supercharged engine, MAP may be higher than atmospheric pressure.

What is the manifold pressure and why is it important?

Manifold Pressure: the absolute pressure of the fuel/air mixture immediately before it enters the intake port of the cylinder - Excessive manifold absolute pressures (map) will result on excessive cylinder pressures and temperatures (unsafe conditions) -The above leads to pre-ignition and loss of power -It may also lead to permanent engine damage

What are the principle properties of aircraft engine oil?

Most important properties for aviation purposes are - Flash point - Viscosity - Pour point - Chemical stability

Gas turbine engines can be lumped into what two general categories? Which of the four engine types of turbine-jet engines fall into the two categories?

Power producers are the turboshaft and turboprop Thrust producers are the turbojet and turbofan

What is the relationship between constant speed propellers, feathering propellers, and reverse pitch propellers?

Practically all feathering and reverse-pitch propellers are constant speed propellers, but not all constant speed propellers can feather or reverse

What is pre-ignition?

Pre-ignition is when there is a hot spot in the engine that ignites the fuel/air mixture before the spark plug fires (burning carbon particles, red-hot spark plug electrodes)

List the advantages of the in-line engine.

Presents a small frontal area (low-drag nacelle configuration). When the cylinders are mounted inverted, better pilot visibility and shorter landing gear is possible.

What are the basic components of a lubrication system?

Pressure Oil Pump Oil Pressure Relief Valve Oil Reservoir (separate or integral) Oil Pressure Gage Oil Temperature Gage Oil Filter Piping and Connections

What are the two most common types of aircraft engine used for the propulsion of almost all powered flights?

Reciprocating engine and gas-turbine engine

When designing an aircraft engine, which parameter is most important, performance or reliability? How is this parameter accounted for in engine design?

Reliability; often the engine has redundant parts.

How does the volumetric efficiency differ between non-supercharged and supercharged engines?

Supercharged engines often have volumetric efficiencies of over 100% because the air is compressed before it enters the cylinder

What happens to the Angle of Attack of a propeller blade as the aircraft flies faster?

The AOA decreases

What is meant by magnetic poles? How are they important to inducing a voltage in a coil of wire?

The North and South poles of a magnet (where the lines of flux flow into and out of); they determine the polarity of the magnet, and thus the direction of induced current flow.

Define the term "blade angle"

The angle between the chord of a particular blade section and the plane in which the propeller blades rotate.

Discuss the term "Angle of Attack in terms of a propeller blade.

The angle of attack of a propeller blade section is the angle between the chord of the blade section and the relative wind. The direction of the relative airstream depends upon the direction that the blade moves through undisturbed air and the velocity of the forward movement (the aircraft itself).

What is the function of the camshaft in a reciprocating engine?

The camshaft of an engine will operate the intake and exhaust valves for all the cylinders of the engine.

What is considered the main power section of a reciprocating engine?

The cylinder

What are the three types of circuits found in a magneto? Describe each

The magneto consists of three circuits 1. Magnetic Circuit: the permanent magnet, the coil core, the pole shoes, and the pole shoe extensions 2. Primary Circuit: the primary winding of the coil (few turns), the breaker points or contacts, and the capacitor or condenser 3. Secondary Circuit: The secondary winding of the coil (several thousand turns), the distributor and rotor, the high-tension ignition lead, and the spark plug

What can be said regarding the number of cylinders a reciprocating engine has in reference to its power production?

The more cylinders an engine has, the more power it is capable of producing.

The more volatile a fuel, the more easily it does what? This causes what?

The more easily it vaporizes. This causes icing

What type of reciprocating engine design provides the best power-to-weight ratio of all the different types of piston engines?

The multiple-row radial engine has the lowest weight-to-horsepower ratio of all the different types of piston engines

Explain the meaning of an octane number

The octane number of a fuel determines its antiknock value -The higher the octane, the better the anti- knocking capabilities

What inspection should be made with respect to the engine when an oil filter is serviced? Why?

The oil filter is one of the best places to screen for internal engine wear or damage

How is the movement of a propeller blade when it is developing "lift" (in a forward direction) different from that of a wing that develops lift (in an upward direction)?

The propeller blade is not only moving forward but also rotating clockwise.

What is the compression ratio of a cylinder?

The ratio of the volume of space in the cylinder when the piston is at the bottom of its stroke to the volume when the piston is at the top of its stroke.

The capacity of a supercharger (how much it will supercharge the air) depends upon what two things?

The size of the impeller and the speed at which it is rotated

What effect does the rotational speed of the rotor blades have on the overall compression ratio of a gas turbine engine?

The speed of the rotor determines the velocity present in each stage and thus the dynamic pressure

Define the term "flashpoint" in reference to oil.

The temperature to which oil must be heated in order to give off enough vapor to form a combustible mixture above the surface that will momentarily flash or burn when the vapor is brought into contact with a very small flame

What does the term "pitch" mean?

The theoretical distance through which the propeller (or the aircraft itself) would travel in one revolution. It is measured in inches.

How does a gas turbine engine work?

The turbine engine increases the potential energy of an airmass. It then converts all of that potential energy into kinetic energy. The velocity associated with the KE is exhausted out the rear of the engine. Using Newton's 3rd Law, the force of air leaving the engine causes the aircraft to propel forward.

What is the practical limit on compression ratio? Why?

The type of fuel used because each fuel has critical pressure and temperature and will eventually explode, vice burn evenly (at higher compression ratios)

The type of powerplant used in any particular aircraft is dependant upon what factors?

The type of powerplant used depends upon the mission of the aircraft and the aircraft's cruising speed.

What are connecting-rod assemblies? What is the difference between the large end and the small end?

These are the links which transmit the forces between the pistons and the crankshaft of the engine; they furnish the means of converting the reciprocating motion of the piston to a rotating movement of the crankshaft. The large end is the end of the connecting-rod that connects to the crankshaft; the small end is the end of the connecting-rod that connects to the piston.

What is the purpose of a piston ring?

These are used to ensure a tight fit between the cylinder walls and the piston skirt. This is necessary to prevent loss of gases and to minimize friction (these are not rubber, but cast iron)

What is the importance of understanding TDC and BDC?

These positions are used to set the timing and valve overlap

What is a reciprocating engine?

This is an engine that utilizes one or more pistons in order to convert pressure into a rotating motion.

What is a cam?

This is the device used for actuating the valve lifting mechanism for the inlet and exhaust valves.

Define viscosity

This is the fluid friction of oil (otherwise known as the oil's internal resistance to flow). High viscosity means heavy, slowly flowing oil (viscous). Low viscosity means freely flowing oil.

What is ram pressure recovery? When is it greatest? What effect does high ram pressure recovery have on a gas turbine engine?

This is the point at which pressure inside the inlet returns to ambient (generally occurs at M 0.1 to 0.2). Before this point, air inside the inlet is less than atmospheric. It is greatest at the aircraft's fastest speeds. The greater the ram pressure recovery, the higher the compression ratio, the greater the thrust, the less fuel burned.

What does it mean when we say that a turbofan engine is "fully ducted"? Which turbofan engines are fully ducted and why only those?

This means that the fan discharge air is ducted along the entire length of the engine. Low and medium bypass engines are "fully ducted. High bypass engines are not usually fully ducted because the required increase in weight would overcome the drag reduction benefit.

What is the limiting factor with respect to the rpm at which a propeller may be operated? Why?

Tip speed is the principle factor is determining the efficiency of high-performance airplane propellers. The speed at the tip is greater than at the root (rpm) and it is necessary to keep the tip speed below the speed of sound or excessive stresses will occur.

How may one increase the induced voltage in a magnet?

To increase the induced voltage - Increase the rate of the magnet movement - Increase the number of turns in the coil - Increase the strength of the magnet

What is the purpose of carburetion or fuel metering

To provide the combustible mixture of fuel and air necessary for the operation of an engine

What is the purpose of a lubrication system?

To supply oil to the engine at the correct pressure and volume to provide adequate lubrication and cooling for all parts of the engine which are subjected to friction

Describe the effect of each of the three principal forces acting on a propeller in flight

Torque bending force: in the form of air resistance, tends to bend the blades opposite to the direction of rotation Aerodynamic twisting force: tends to turn the blades to a higher blade angle Centrifugal twisting force: tends to turn the blades to a lower blade angle, stronger than the aerodynamic twisting force If the propeller's rpm is doubled, the latter two forces will quadruple

How are turbine rotor blades and stator vanes different than compressor rotor blades and stator vanes in a gas turbine engine?

Turbine blades and vanes vector the air tangentially, rather than axially Unlike compressor stator vanes, which are located AFT of the rotor blades, turbine stator vanes are located IN FRONT OF the rotor blades - The turbine vanes act as nozzles, rather than diffusers - Turbine vanes increase velocity and decrease static pressure

Explain vapor lock and its three primary causes

Vapor lock is when fuel vapor and air collects in various sections of the fuel system. Three primary causes are low atmospheric pressure at high altitudes, excessive fuel temperatures, turbulence (sloshing) of the fuel.

Explain the operation of a constant speed prop

When the pilot adjusts rpm with the throttle, the blade angle is automatically adjusted. The prop automatically changes pitch to maintain a constant engine speed. In straight and level flight, if engine power is increased via the throttle, the blade angle will be increased so the prop absorbs the additional power (rpm remains constant).

When there is a small blade angle on the propeller, how is the forward motion of an airplane affected?

When there is a small blade angle, there is low pitch and the aircraft does not move very far forward with one rotation of the blade.