aerospace vehicles and systems unit 3a aircraft powerplants

AIRCRAFT SYSTEMS

Aerospace Vehicles and SystemsUnit 3A : Aircraft power plantsTopics to be discussedAircraft power plants, classification and principle of operation. Instruments and navigational aids. What is propulsion? The word is derived from two Latin words: pro meaning before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion system is a machine that produces thrust to push an object forward. On airplanes, thrust is mostly generated through some application of Newton's third law of action and reaction. A gas, or working fluid, is accelerated by a machine, and the reaction to this acceleration produces a force on the engine. For an aircraft is travelling through air in straight and level flight and at a constant true airspeed (TAS) ;The engines must produce a total thrust equal to the drag on the aircraft. If the engine thrust exceeds the drag, the aircraft will accelerate, and if the drag exceeds the thrust, the aircraft will slow down.For all engine types available for aircraft propulsion, the thrust force must always come from air or gas reaction forces normally acting on the engine or propeller surfaces.

PROPULSION - OVERVIEW

ARRANGEMENT OF THRUST AND DRAG FORCESThe engines must produce a total thrust equal to the drag on the aircraft. If the engine thrust exceeds the drag, the aircraft will accelerate, and if the drag exceeds the thrust, the aircraft will slow down.The two common methods of aircraft propulsion are:a. The propeller powered by piston or gas turbine engine.b. The jet engine.

AIRCRAFT POWER PLANTSPropeller EngineThe engine power produced drives a shaft which is connected to a propeller via a gearbox. The propeller cuts through the air accelerating it rearwards. The blade of a propeller behaves in the same way as the aerofoil of an aircraft: the air speeds up over the leading face of the propeller blade causing a reduced pressure with a corresponding increase of pressure on the rearward face .

In all cases of the jet engine, a high velocity exhaust gas is produced, the velocity of which, relative to the engine, is considerably greater than the aircraft speed (Va).Thrust is produced according to the following equation:T = m(Vj Va) where Vj is now the velocity of the gas stream at the propelling nozzle.

This is a simplified version of the full thrust equation as the majority of thrust produced is a result of the momentum change of the gas stream and is called the momentum thrust.

THE JET ENGINE

In the rocket engine, the gases which leave the engine are the products of the combustion of the rocket propellants carried; therefore no intake velocity term Va is required.The simplified version of the equation giving the thrust produced thus becomes: T = m x Vj, where m is the mass flow rate of the propellant and Vj is the exhaust velocity.

ROCKET ENGINE

Lecture notes by Wg Cdr B Prakash (Retd.)8Engine Operational Limits

INTERNAL COMBUSTION ENGINE FAMILY

PISTON/RECIPROCATING ENGINESIntroduction to Piston EnginesThe internal combustion piston engine or the reciprocating engine consists basically of:A cylinder which is closed at one end, a piston which slides up and down inside the cylinder, A connecting rod and crank by which reciprocating movement at the piston is converted to rotary movement of the crankshaft. In the closed end of the cylinder, known as the Cylinder Head, are inlet and exhaust valves and a sparking plug.

A Four-stroke Internal Combustion Engine

The Four Stroke Cycle

Constant Volume Diagram

The Four Stroke CycleThe sequence of operations by which the engine converts heat energy into mechanical energy is known as the four stroke or constant volume cycle. A mixture of petrol and air is introduced into the cylinder during the induction stroke and compressed during the compression stroke (1-2). At this point the fluid is ignited, the heat generated causing a rapid increase in pressure (2-3) which drives the piston down on its power stroke (3-4). Finally, the waste products of combustion are ejected during the exhaust stroke (4-1).

THE DIRECTION OF MOTION OF THE CRANKSHAFT AND PISTON DURING THE FOUR STROKE CYCLE

One of the most noticeable differences between car and aero-engines is that, with the exception of those fitted to light aircraft, the latter possess more cylinders. This is because it is impracticable, for various reasons, to obtain much more than 74.5 kW per cylinder; consequently a high output would not be developed by a scaled-up version of a low-power engine.Even in engines of modest power it is often better to use a number of small cylinders in preference to fewer and larger, for not only does smoother operation result, but also, in many cases, a smaller frontal area can be obtained.Reciprocating Engine Working PrincipleThe piston moves inside a cylinder, into which a gas is inducted, then heated inside the cylinder itself by ignition of a fuel air mixture at high pressure (internal combustion engine). This hot, high pressure gases expand, pushing the piston to the bottom of the cylinder (BDC) creating the Power stroke. The piston is returned to the cylinder top (Top Dead Centre) either by a flywheel or the power from other pistons connected to the same shaft. In most types the "exhausted" gases are removed from the cylinder by this stroke.This completes the four strokes of a 4-stroke engine also representing 4 legs of a cycle. The linear motion of the piston is converted to a rotational motion by a connecting rod and a crankshaft. A flywheel is used to ensure continued smooth rotation (i.e. when there is no power stroke). Multiple cylinder power strokes also act as a flywheel.More cylinders in a reciprocating engine generally lead to a more vibration-free (smooth) operation. The total power output of a reciprocating engine is proportional to the volume of the combined pistons' displacement.

The figure shows a plot of pressure versus gas volume cycle.There are six numbered stagesbased on the mechanical operation of the engine. For the ideal four stroke engine, the intake stroke(1-2) andexhaust stroke (6-1)are done at constant pressure and do not contribute to the generation of power by the engine. During thecompression stroke (2-3), work is done on the gas by the piston. During thecombustion stroke (3-4), the volume is held constant and heat is released. During the power stroke (4-5), work is done by the gas on the piston. Between stage 5 and stage 6, residual heat is transferredto the surroundings so that the temperature and pressure return to the initial conditions of stage 1 (or 2). During the cycle, workis done on the gas by the piston between stages 2 and 3. Work is done by the gas on the piston between stages 4 and 5.IDEAL OTTO CYCLE

IDEAL OTTO CYCLETHE PROPELLERChanges in propeller blade angle from hub to tip.The propeller is a rotating airfoil, subject to induced drag, stalls, and other aerodynamic principles that apply to any airfoil. It provides the necessary thrust to pull, or in some cases push, the airplane through the air. The engine power is used to rotate the propeller, which in turn generates thrust very similar to the manner in which a wing produces lift. The amount of thrust produced depends on the shape of the airfoil, the angle of attack of the propeller blade, and the r.p.m. of the engine. The propeller itself is twisted so the blade angle changes from hub to tip. The greatest angle of incidence, or the highest pitch is at the hub while the smallest pitch is at the tip. (Caters for speed variation along the propeller length)

THE PROPELLERRelationship of travel distance and speed of various portions of propeller blade.The reason for the twist is to produce uniform lift from the hub to the tip. As the blade rotates, there is a difference in the actual speed of the various portions of the blade. The tip of the blade travels faster than that part near the hub, because the tip travels a greater distance than the hub in the same length of time. Changing the angle of incidence (pitch) from the hub to the tip to correspond with the speed produces uniform lift throughout the length of the blade.

IDEAL CARNOT CYCLE (p-V DIAGRAM)The Carnot Cycle is one of the fundamental thermodynamic cycles.A p-V diagram is used to plot the various processes in the Carnot Cycle. The cycle begins with a gas, colored yellow on the figure, which is confined in a cylinder, colored blue. The volume of the cylinder is changed by a moving red piston, and the pressure is changed by placing weights on the piston. We have two heat sources; the red one is at a nominal 300 degrees, and the purple one is at 200 degrees. Initially, the gas is in State 1 at high temperature, high pressure, and low volume. The first process performed on the gas is an isothermal expansion. The 300 degree heat source is brought into contact with the cylinder, and weight is removed, which lowers the pressure in the gas. The temperature remains constant, but the volume increases. During the process from State 1 to State 2 heat is transferred from the source to the gas to maintain the temperature. We will note the heat transfer by Q1 into the gas. IDEAL CARNOT CYCLE (p-V DIAGRAM)

The second process performed on the gas is an adiabatic expansion. During an adiabatic process no heat is transferred to the gas. Weight is removed, which lowers the pressure in the gas. The temperature decreases and the volume increases as the gas expands to fill the volume. During the process from State 2 to State 3 no heat is transferred.

IDEAL CARNOT CYCLE (p-V DIAGRAM)

The third process performed on the gas is an isothermal compression. The 200 degree heat source is brought into contact with the cylinder, and weight is added, which raises the pressure in the gas. The temperature remains constant, but the volume decreases. During the process from State 3 to State 4 heat is transferred from the gas to heat source to maintain the temperature. We will note the heat transfer by Q2 away from the gas.


The fourth process performed on the gas is an adiabatic compression. Weight is added, which raises the pressure in the gas. The temperature increases and the volume decreases as the gas is compressed. During the process from State 4 to State 1 no heat is transferred.


At the end of the fourth process, the state of the gas has returned to its original state and the cycle can be repeated as often as you wish. During the cycle, work W has been produced by the gas, and the amount of work is equal to the area enclosed by the process curves. From the first law of thermodynamics, the amount of work produced is equal to the net heat transferred during the process: W = Q1 - Q2 The Carnot cycle describes the operation of refrigerators, the Otto cycle describes the operation of internal combustion engines, and the Brayton cycle describes the operation of gas turbine engines. P-V and T-s diagrams are often used to visualize the processes in a thermodynamic cycle and help us better understand the thermodynamics of engines.IDEAL CARNOT CYCLE (p-V DIAGRAM

FUNDAMENTALS OF GAS TURBINE ENGINES

INTRODUCTIONA gas turbine is essentially a heat engine using air as a fluid to produce thrust. The working cycle of the gas turbine is similar to that of a piston engine and both engine cycles have induction, compression, combustion and exhaust phases. However a gas turbine is able to deal with much larger amounts of energy for a given size and weight, and it has the added advantage that the mechanical motion is continuous and entirely rotational. In consequence the gas turbine runs more smoothly.Lecture notes by Wg Cdr B Prakash (Retd.)29

INTRODUCTIONAlthough the gas turbine engine differs radically in construction from the conventional four-stroke, five-event cycle reciprocating engine, both involve the same basic principle of operation. In the piston (reciprocating) engine, the functions of intake, compression, ignition, combustion, and exhaust all take place in the same cylinder and, therefore, each must completely occupy the chamber during its respective part of the combustion cycle. In the gas turbine engine, a separate section is devoted to each function, and all functions are performed at the same time without interruption. The gas turbine functions as an open cycle process.

How does a jet engine work ?The jet engine or, more correctly, the gas turbine is an internal combustion engine which produces power by the controlled burning of fuel. In both the gas turbine and the motor car engine, air is compressed, fuel is added and the mixture is ignited. The resulting hot gas expands rapidly and is used to produce the power. In the motor car engine, the burning is intermittent and the expanding gas moves a piston and crank to produce rotary or shaft power which is transmitted to the road wheels. Lecture notes by Wg Cdr B Prakash (Retd.)31

How does a jet engine work ?In a typical engine: We can think of the engine as being stationary and the cold air moving towards it. A compressorsqueezes the air (increases its pressure and temperature). This slows the air down by about 60 percent. ATF/Kerosene (liquid fuel) is pumped into the engine from afuel tankin the plane's wing/body.In thecombustion chamber, just behind the compressor, the kerosene mixes with the compressed air and burns fiercely, giving off hot exhaust gases. The burning mixture reaches a temperature of around 900C.The exhaust gases rush past a set of turbine blades, spinning them like a windmill. The turbine blades are connected to a long axle that runs the length of the engine. The compressor and the fan are also connected to this axle. So, as the turbine blades spin, they also turn the compressor. 8.The hot exhaust gases exit the engine through a taperingexhaust nozzle. The tapering design helps to accelerate the gases to high speeds. The hot exhaust air leaving the engine at the back is travelling much faster than the cold air entering it at the frontand that's what powers the plane. Military jets often have anafter burnerthat adds fuel into the exhaust jet to produce extra thrust. The backward-moving exhaust gases power the jet forward.

Lecture notes by Wg Cdr B Prakash (Retd.)32Layout of the gas turbineThe gas turbine engine is basically a machine designed to accelerate a stream of gas which is used to produce the reactive thrust necessary to propel the aircraft.

Lecture notes by Wg Cdr B Prakash (Retd.)33

Gas Turbine Schematic Diagram

Lecture notes by Wg Cdr B Prakash (Retd.)34GAS TURBINE ENGINE WORKING PRINCIPLE0 to 1 : Free stream. (Air velocity same as that of the aircraft)1 to 2 : Inlet diffusion Velocity reduces, Pressure increases (Ram pressure increase as per Bernoullis equation P + V2 +gh =constant).2 to 3 : Isentropic compression3 to 4 : Heat addition at constant pressure. (Air and fuel mixed and burned at constant pressure)4 to 5 : Isentropic expansion through turbine by which work is developed. Work just sufficient to run the compressor.6 to 7 : Heat addition through afterburner if available.7 to 8 : Isentropic expansion through nozzle, leading to reaction thrust.Lecture notes by Wg Cdr B Prakash (Retd.)35

Brayton Cycle- Schematic Diagram


Thermodynamics of a jet engine are modeled approximately by a Brayton Cycle. Ideal Brayton cycle comprise of the following Thermodynamics Processes:Isentropic Compression Process. (1-2)Isobaric Heat Addition Process. (2-3)Isentropic Expansion Process. (3-4)Isobaric Heat Rejection Process. (4-1)BRAYTON CYCLEBRAYTON CYCLE - T-s DiagramThe gas turbine cycle can be represented by a temperature/entropy (T/S) diagram. (Entropy is a measure of disorder; the greater the entropy or degree of disorder in the gas, less work can be extracted from it.) Point 1 represents the entry to the compressor, the air undergoes adiabatic compression along the line 1-2. Heat is added to the air in the form of burning fuel which causes constant pressure heating along the line 2-3. Adiabatic expansion through the turbines, line 3-4, extracts energy from the gas stream to drive the compressor and possibly a propeller, fan or rotor system. The remainder of the gas stream is discharged through the exhaust system to provide thrust, line 4-1.Lecture notes by Wg Cdr B Prakash (Retd.)38

Typical gas flow through a gas turbineAs the gas turbine engine is reliant upon heat to expand the gases, the higher the temperature in the combustion phase the greater the expansion of the gases. However, the combustion temperature has to be limited to a level that can be safely accepted by the materials used in the turbine and exhaust components. Figure shows the gas flow through a typical gas turbine and also gives representative values for temperature, gas velocities and pressures.Lecture notes by Wg Cdr B Prakash (Retd.)39

Turbojets are the oldest kind of general-purpose jet engines. Turbojets are rotary engines that extracts energy from a flow of combustion gas. They produce thrust by increasing the velocity of the air flowing through the engine and operate on Newtons third law of motion " For every action there is an equal and opposite reaction.

TURBOJET ENGINES SUMMARY

Air intake

Air intake aims at bringing large amounts of surrounding air into the engine. A tube-shaped inlet, like one you would see on an airliner usually of cylindrical or conical design.Inlets come in many shapes and sizes depending on the aircraft.

The compressor rotates at very high speed, adding energy to the airflow and at the same time squeezing it into a smaller space. Compressing the air increases its pressure and temperature. The compressor is driven by the turbine. Compressors used in turbojet engines are mainly classified as:Axial Flow Compressors. Centrifugal Compressors.

1. Axial compressors are rotating, airfoil based compressors in which the working fluid principally flows parallel to the axis of rotation.2. Axial compressors consist of a shaft that drives a central drum which has a number of annular airfoil rows attached. 3.These rotate between a similar number of stationary airfoil rows attached to a stationary tubular casing. 4. A pair of rotating and stationary airfoils is called a stage. 5.The cross-sectional area between rotor drum and casing is reduced in the flow direction to maintain axial velocity as the fluid is compressed.

1. Centrifugal compressors are rotating, airfoil based compressors in which the working fluid principally flows perpendicular to the axis of rotation. 2. Centrifugal compressors consist of a shaft that drives a impeller which has a number of curved blades. 3. The impeller rotates in a casing which is designed to convert the kinetic energy of the fluid into pressure energy before leaving the compressor.

1. In a turbojet the air and fuel mixture passes unconfined through the combustion chamber. 2. As the mixture burns its temperature increases dramatically. 3. The combustion chamber is usually in the form of cans, which comprise the fuel injector and flame holder.

1.Hot gases leaving the combustor are allowed to expand through the turbine. Turbines are usually made up of high temperature metals such as inconel / Ni based alloys. 2.The turbine's rotational energy is used primarily to drive the compressor and other accessories, like fuel, oil, and hydraulic pumps.3.In a turbojet almost 60% of all the power generated by burning fuel is used by the compressor to compress the air for the engine.

1. After the turbine, the gases are allowed to expand through the exhaust nozzle to atmospheric pressure, producing a high velocity jet in the exhaust plume. This results in thrust production as per Newtons Third Law.2. In a convergent nozzle, the ducting narrows progressively to a throat.

An afterburner or "reheat jet-pipe" is a device added to the rear of the jet engine. It provides a means of spraying fuel directly into the hot exhaust, where it ignites and boosts available thrust significantly; a drawback is its very high fuel consumption rate.

The thrust reverser is, essentially, a pair of clamshell doors mounted at the rear of the engine which, when deployed, divert thrust normal to the jet engine flow to help slow an aircraft upon landing. They are often used in conjunction with spoilers.

Merits of Turbojet Engines:

Very high power-to-weight ratio.More compact than most reciprocating engines of the same power rating. Fewer moving parts than reciprocating engines. Low operating pressures. High operation speeds. Low lubricating oil cost and consumption.

Demerits of Turbojet Engines:

High cost Longer startup than reciprocating engines Less responsive to changes in power demand compared to reciprocating engines.

7 CYLINDER BMW 801 AIRCRAFT ENGINE

Gas turbine : Thrust EquationThe concept of momentum and pressure thrust give rise to the full thrust equation:

Thrust (N) = Momentum thrust + Pressure thrust= me x Ve - ma x Vo + Ae x (Pe - Po) - Ai x (Pl - Po)

Where ma = Mass flow of air in kg/sme = Mass Flow of combustion products in kg/s = ma + mf (mass flow of fuel in kg/s)Ve = Final Velocity of Gas Stream in m/sVo = Initial Velocity of Gas Stream in m/sAe = Area of Propelling Nozzle in mAi =Area of Intake in mPe = Exit Pressure from Propelling Nozzle in PaPo = Atmospheric Pressure in PaPl = Engine Inlet Pressure in PaLecture notes by Wg Cdr B Prakash (Retd.)52Calculation of ThrustTo illustrate the calculation of thrust, using the following data : Propelling Nozzle Outlet: Area (Ae) = 0.2150 m, Pressure (Pe) = 143.325 kPa ,Pressure (P0) = 101.325 kPa (ISA), Mass Flow (ma) = 70 kg/s, Velocity (Ve) = 590 m/sThrust (kN) = (70 x 590 + 0.215 x (143325 - 101325))/1000 = 50.33 kN

By fitting an afterburner to the engine, the thrust can be greatly increased.

The parameters of the afterburning nozzle are as follows: Afterburner Propelling Nozzle Outlet: Area (Ae) = 0.2900m, Pressure (Pe) = 136.325 kPa, Pressure ( Po) = 101.325 kPa, Mass Flow (ma) = 70 kg/s, Velocity (Ve) = 740 m/s,Thrust = (70 x 740 + 0.29 x (136325 - 101325) )/1000 = 61.950 kN

It can be seen that the increase in thrust is 11.62 kN or 23%. This increase is small compared to modern by-pass engines with afterburning which have thrust increases in the order of 80%. However, the use of this increased thrust results in a disproportionately high increase in fuel consumption.Lecture notes by Wg Cdr B Prakash (Retd.)53Turbojet The turbojet, the simplest and earliest type of gas turbine, is used principally in high-speed aircraft where its relatively small frontal area and high jet velocity are advantageous. The turbine extracts only sufficient energy from the gas stream to drive the compressor, leaving the remaining energy to provide the thrust. Lecture notes by Wg Cdr B Prakash (Retd.)54

Rolls-Royce/Snecma OlympusLecture notes by Wg Cdr B Prakash (Retd.)55

Rolls-Royce RB183 Mk 555Lecture notes by Wg Cdr B Prakash (Retd.)56

TurbofanThe turbofan is the most common type of gas turbine used for aircraft propulsion today. Part of the air entering the engine is compressed fully and passed into the combustion chamber, while the remainder, compressed to a lesser extent, bypasses the combustion section, to provide cold thrust. This bypass flow rejoins the hot flow downstream of the turbine.


Examples of the turbofan are the RB211 in the Boeing 747, the 535 in the Boeing 757, the ADOUR in the Jaguar and Hawk, and the RB199 in the Tornado. Rolls-Royce Trent 800Lecture notes by Wg Cdr B Prakash (Retd.)58

Low-bypass TurbofanDescriptionOne- or two-stage fan added in front bypasses a proportion of the air through a bypass chamber surrounding the core. This is the engine of high-speed military aircraft, some smaller private jets, and older civilian airliners such as the Boeing 707, the McDonnell Douglas DC-8, and their derivativesAdvantagesAs with the turbojet, the design is aerodynamic, with only a modest increase in diameter over the turbojet required to accommodate the bypass fan and chamber. It is capable of supersonic speeds with minimal thrust drop-off at high speeds and altitudes yet still more efficient than the turbojet at subsonic operationDisadvantagesNoisier and less efficient than high-bypass turbofan, with less static (Mach 0) thrust. Added complexity to accommodate dual shaft designs. Lecture notes by Wg Cdr B Prakash (Retd.)59HF 20 Honda Low Bypass Turbofan Engine

High-bypass TurbofanDescriptionFirst stage compressor drastically enlarged to provide bypass airflow around engine core, and it provides significant amounts of thrust. Compared to the low-bypass turbofan and no-bypass turbojet, the high-bypass turbofan works on the principle of moving a great deal of air somewhat faster, rather than a small amount extremely fast. Most common form of jet engine in civilian use today- used in airliners like the Boeing 747, most 737s and all Airbus aircraftAdvantagesQuieter due to greater mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason, cooler exhaust temperature. Less noisy and exhibit much better efficiency than low bypass turbofansDisadvantagesGreater complexity (additional ducting, usually multiple shafts) and the need to contain heavy blades. Fan diameter can be extremely large, especially in high bypass turbofans such as the GE90.More subject to FOD and ice damage. Top speed is limited due to the potential for shockwaves to damage engine. Lecture notes by Wg Cdr B Prakash (Retd.)61High Bypass Turbojet Engine

High Bypass Turbojet Engine

Propfan / Unducted FanDescriptionTurbojet engine that also drives one or more propellers. Similar to a turbofan without the fan cowlingAdvantagesHigher fuel efficiency, potentially less noisy than turbofans, could lead to higher-speed commercial aircraft, popular in the 1980s during fuel shortagesDisadvantagesDevelopment of propfan engines has been very limited, typically noisier than turbofans, complexity

Lecture notes by Wg Cdr B Prakash (Retd.)64Propfan

Unducted Fan

Turbofan Thrust

Lecture notes by Wg Cdr B Prakash (Retd.)67TurbopropThe turboprop is a turbojet with an additional turbine which uses the energy remaining in the gas stream, after sufficient has been absorbed to drive the compressor, to drive a propeller. The additional turbine, called the power turbine, drives the propeller through a shaft and a reduction gear. A small amount of residual thrust remains in the exhaust gases during normal operation. The turboprop is a very efficient powerplant for relatively low-speed, low-altitude aircraft, (eg, 250 kmph/9000 m).

Lecture notes by Wg Cdr B Prakash (Retd.)68Examples of the turboprop are the DART in the British Aerospace 748 and the Fokker F27, and the TYNE in the Transall C-160 and Dassault-Breguet Atlantic. Turboprop power is measured in total equivalent horsepower (tehp) or kilowatts (kW), ie: the shaft horsepower plus the residual thrust.

TurbopropLecture notes by Wg Cdr B Prakash (Retd.)69

TurboshaftThe turboshaft is effectively a turboprop without a propeller, the power turbine in this case being coupled to a reduction gearbox or directly to an output shaft. In the same way as the turboprop, the power turbine absorbs as much of the remaining gas energy as possible and the residual thrust is very low. Turboshaft power is normally measured in shaft horsepower (shp) or kilowatts (kW).

Lecture notes by Wg Cdr B Prakash (Retd.)70The most common application of the turboshaft is the helicopter, in which the engine drives both the main and tail rotors. Turboshafts are also widely used for industrial and marine installations, including power and pumping stations, hovercraft and ships.Examples of the turboshaft are the GEM in the Westland Lynx and the GNOME in the Westland Sea King helicopters.

Turboshaft Engines

RamjetA ramjet is properly shaped duct with no compressor or turbine. It is used for high speed propulsion and missiles. Compression is achieved by decelerating the high speed incoming air in the diffuser. The aircraft must already be in flight at a high speed. Ramjet is typically used in aircraft flying above Mach 1.


Ramjet Engine

RamjetDescriptionIntake air is compressed entirely by speed of oncoming air and duct shape (divergent), and then it goes through a burner section where it is heated and then passes through a propelling nozzleAdvantagesVery few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all air-breathing jets (thrust / weight ratio up to 30 at optimum speed), cooling much easier than turbojets as there are no turbine blades to coolDisadvantagesMust have a high initial speed to function, inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories, usually limited to a small range of speeds, intake flow must be slowed to subsonic speeds, noisy, fairly difficult to test, difficult to maintain combustion.

Lecture notes by Wg Cdr B Prakash (Retd.)74PulsejetDescriptionAir is compressed and combusted intermittently instead of continuously. Some designs use valvesAdvantagesVery simple design, commonly used on model aircraftDisadvantagesNoisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly.Lecture notes by Wg Cdr B Prakash (Retd.)75

PulsejetLecture notes by Wg Cdr B Prakash (Retd.)76

Scramjet

Lecture notes by Wg Cdr B Prakash (Retd.)77As the velocity increases the total temperature of the gas stream rises above the dissociation temperature of the combustion products. This prevents efficient burning if the gas stream is diffused to subsonic speeds. To solve this, fuels with high propagation velocities such as hydrogen are used while diffusing the intake air to supersonic speeds without having a large rise in temperature of the gas stream. The challenge is one of obtaining stable flames fronts.ScramjetDescriptionSimilar to a ramjet without a diffuser; airflow through the entire engine remains supersonicAdvantagesFew mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficienciesDisadvantagesStill in development stages, must have a very high initial speed to function (Mach >6), cooling difficulties, very poor thrust / weight ratio (~2), extreme aerodynamic complexity, airframe difficulties, testing difficulties / highly expensive

Lecture notes by Wg Cdr B Prakash (Retd.)78Scramjet Engine

Lecture notes by Wg Cdr B Prakash (Retd.)80ROCKETDescriptionCarries all propellants and oxidants on-board, emits jet for propulsionAdvantagesVery few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 10.0 or so), thrust / weight ratio over 100, no complex air inlet, high compression ratio, very high speed (hypersonic) exhaust, good cost / thrust ratio, easy to test, works in a vacuum-indeed works best exo-atmospheric which is kinder on vehicle structure at high speed, fairly small surface area to keep cool, no turbine in hot exhaust streamDisadvantagesNeeds lots of propellant- very low specific impulse typically 100-450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidizer on-board which increases risks. Extraordinarily noisy

The factors which affect the choice of powerplant for a particular aircraft include: a. Power output; b. Efficiency; c. Power/ weight and power/volume ratios; d. Cost; e. Reliability; f. Maintainability; g. Noise and pollution.For low speed application, propeller engines are often chosen because of their overall high efficiency. For higher speeds, the propeller is replaced by the turbofan or turbojet.Piston engines are used in small aircraft because of their advantages of efficiency and cost over the small gas turbine. For larger aircraft, turboprop engines are preferred as they have good power/weight ratios and are easily maintained. For air transport application, where fuel efficiency is extremely important, high by-pass ratio turbofans are being used by the majority of large aircraft, with lower by-pass ratio turbofans and turboprops being used in the smaller aircraft. Modern combat aircraft use low by-pass afterburning turbofans, which give a higher efficiency at subsonic speed and provide a greater thrust augmentation (>80%) in afterburning mode.

Choice of Aircraft Powerplant