Turbojet

History

The first turbojets were designed about simultaneously by Sir Frank Whittle in England and by Dr. Hans von Ohain in Germany in the Années 1930.

The first turbojet was built by Whittle in 1935 and turned to the bench in 1936. But it was von Ohain which made steal the first turbojet on a Heinkel He 178 in 1939.

The first Avion S with reaction built in series were fighter-bombers (Messerschmitt Me262 Schwalbe) used at the end of the Second world war.

After war, the turbojets spread, as well in the military aviation as civil, as well as the turbopropellers but only in the civilian.

The turbojets of today are machines of an extreme complexity. The development of a new engine requires means human, technological and financial considerable that only some rare Entreprise S has in the world.

If the Output and the Fiabilité of these engines improved considerably since their beginnings, their cost is very important, and in general represents for a civil aircraft one the third of the total costs of the Appareil.

In fact, the invention of the turbojet seems to be former to work of Whitle and von Ohain. Indeed, the French Maxime Guillaume deposited in May 1921 an patent application concerning a " engine by reaction on the air" and delivered in January 1922.

Various types of turbojets

Compressor centrifuges (or radial)

The first turbojets, designed starting from the prototype developped at the point by Whittle, were provided with a simple compressor centrifuges mû by a turbine. They had the merit of simplicity (only one stage of compression, only one tree connecting turbine and compressor) but their low length was accompanied by a strong diameter. The first English engines (De Havilland Goblin of the Vampire, Rolls-Royce Welland of the Gloster Meteor) were designed thus. German models (Heinkel - He S3) were carried out starting from work of von Ohain but because of delays of development left the place to the models with axial compressor.

The majority of the turbines for helicopters remain conceived on this principle which allows compact engines.

Compressing axial

With a better control of the metallurgy axial compressors combined with fixed bladings were introduced (stators). Because of less effectiveness, they required several stages turning at the same speed but could support number of revolutions definitely higher. The first of this type, was also the first built in great series, JUMO 004 of Junkers - Motoren which equipped the Me-262.

Simple and twin spool

In the first turbojets, turbine and compressor formed a single kinematic unit. One then spoke about compressors (and engines) simple body (or simple attachment ).

to still increase the effectiveness, the compressor was divided into two successive parts, with low and high pressure, driven from now on by two successive turbines high and low pressure. One then speaks about turbojet twin spool (or double attachment ). The number of revolutions of the two bodies is different, requiring two concentric trees and thus longer and heavier engines. N the other hand the output is clearly improved.

Moreover two trees turn in different directions not to cumulate the gyroscopic moments.

All the engines of new generation are with twin spool, even with triple body for those atvery strong by-pass ratio (very rare).
Certain engines, such as the M-53 equipping the Mirage 2000, have only one turbine per compressor.

Simple and double flow

The engines are said to simple flow when the totality of the allowed air crosses the engine and actuates the turbines.

the turbojets with simple flow are very noisy and reach their best output only beyond Mach 1.

Much more economic at the subsonic and less noisy speeds, the turbojets with double flow appeared in the Sixties.

In these engines, the first stages of the low pressure compressor (often reduced to only one which one calls Soufflante, English fan) are large-sized to aspire great quantities of air. The air precompressed by the blower does not pass completely by the engine, but a part (cold flow) circumvents it by its periphery to the conduit where it is ejected with hot gases (hot flow).

That allows, for moderate speeds (in lower part of Mach approximately 1,5) to increase the push by increase in the gas output and to reduce the noise level considerably.

The proportion of air circumventing the engine is variable according to the engines. It is all the more high as the engine is intended to fly at low speeds. This proportion is expressed by the ratio by-pass , equal to the report/ratio of mass cold flow (known as secondary) on mass hot flow (known as primary education).

The military engines optimized for the supersonic flight can have a by-pass ratio in lower part of 1, whereas the civil engines for airliners, optimized for cruisings around Mach 0,8, have by-pass ratios between 5 and 10. Such engines draw the essence of their push from cold flow (80%), hot flow representing only one weak share of thorough (20%), and approach Turbine S coupled to ducted Hélice S (Turbopropulseur S).

NB : There exist also Turbojets triples flow, but they are used little. Indeed the increase in the push is very weak compared to a double flow, whereas the obstruction strongly increases.

Turbopropeller S

It is the unit made up of a propeller driven by a turboshaft engine which converts the essence of the energy of gases produced into mechanical energy. It is not strictly speaking a reaction engine but a turbopropeller is very similar to a turbojet doubles flow atvery strong by-pass ratio whose blower would not be ducted. The aerodynamic limitations of the propellers limit its use to speeds lower than Mach about 0,7.

FOOT-NOTE (Etymology):

the French term turbopropeller is in fact derived from the English word turboprop composed of turbo and propeller (propeller) and which means driving with propeller literally driven by a turbine . French name is thus unsuitable since any turboshaft engine ensuring a propulsion should be called thus. As for saying " a turbopulseur" to indicate the plane been driven by such engines, it is a heresy in a pure state. As in many cases (example: transistor = receiving radio operator!) French created a perfectly unsuitable term starting from an approximate translation.

Around the turbojet

Order and regulation

The turbojets were ordered by a " lever of the gaz" until the introduction on Rolls-Royce/SNECMA Olympus-593 of the Harmony of the first analogical system of regulation. Since, the turbojets little by little were equipped with an electronic system of regulation full authority (FADEC), now numerical, starting from the Pratt & Whitney PW2000 and CFMI/CFM56-A in particular. This system spread on the commercial aircraft of any size. It is also the case of the totality of the recent military aircrafts.

Post-combustion

Post-combustion or " réchauffe" is a system allowing to increase the power provided by a turbojet by injecting fuel in ejected gases. This system is mainly used on the fast fighters, where it is in general used with parsimony because it increases much fuel consumption.

Air intake

To ensure the good performance of an engine, it is essential that the air flow upstream of the compressor is regular. The air intake is thus designed so that, whatever the operation carried out by the plane, they are not masked by the fuselage or the aerofoil, because in the contrary case, the ambient air not arriving more with sufficient pressure or having become turbulent, the engine would have tendency to " to miss air" (one speaks then about pumping ) with the risk of extinction of the flame in the combustion chamber that comprises. In addition, the pressure in entry having to be appreciably equal during all the phases of the flight (i.e. since speed zero until maximum speed) the geometry of the entry could be of variable section (see photo): full opening to starting and in particular in the figure called Cobra (mini speed, elevating 70°) and minimal opening at supersonic speed to slow down and reduce the flow.

Thrust reverser

The thrust reverser is a system making it possible to slow down a plane by using the push produced by the engine.

Vectorial push

Specificity of the military aircrafts (interceptors) most powerful, the conduit of (or of) the engine is prolonged by a directional device making it possible to deviate the jet and thus the direction of the push to increase the maneuverability of the apparatus. It is the case of the Russian prototypes Soukhoï SU-37 and Mig-29 OVT and of the recent American hunters F-22 and JSF. The most recent development (2005) is the Rockwell-MBB X-31.

The turbojet, generator of power

On civilian, plane board or soldier, the turbojet is not only one propelling body. It provides also all the energy supplied on board in electric, hydraulic and pneumatic form and feeds the system of pressurization and air conditioning. The power plant is thus often called “generator of power” or “powerplant”.

The Group Auxiliare de Puissance (GAP)

The turbojets require sometimes an auxiliary engine to be started: GAP ( Auxiliary Group of Power ) or APU ( auxiliary power links ).

It is about a " mini" turbomotor, often derived from a turboshaft engine of helicopter, and generally located in the tail of the apparatus, which provides the compressed air to feed the air starters of the turbojets, as well as electrical energy before startings. GAP can sometimes be used for the hydraulic power in help. GAP is started by the electric battery of the plane, or by an external group of power. GAP can also be used out of electric generator of help, when all the generators of all the turbojets are inoperative.

Calculation of the push

The push (NR) of a turbojet can be calculated roughly starting from the equation:

$F_ {pouss \ acute {E} E} = \ dowry {m} \ times (V_ {left} - V_ {entr \ acute {E} E})$

with:

$\ dowry {m} =$ Mass throughput of the air passing in the engine, the flow of the fuel being negligible (kg/s)

$V_ {left} =$ Velocity emission gases of conduit (m/s)

$V_ {entr \ acute {E} E} =$ Speed of entry of gases in compressor (m/s)

$\ dowry {m} \ times V_ {left} \,$ represents the push of the conduit, while $\ dowry {m} \ times V_ {entr \ acute {E} E} \,$ corresponds to the force of trail of the air intake. Thus so that the turbojet creates a push forwards, it is necessary naturally that the speed of exhaust fumes is higher than that of the aircraft.

Field of application

the turbojets are used on all the civil aircrafts means and large carrying, because they are the only ones with being able to reach transonic speeds (between Mach 0,8 and Mach 1) in an economic way.
the turbopropellers equip in general with the planes slower (around 500 km/h) and with lower capacity (less than 80 passengers).
Only the small private planes and ULM are still equipped with spark-ignition engine with Piston S.
In the military aviation, the use of the turbojet spread. It made it possible to exceed the wall of the sound. The turbopropellers equip with the planes of drive and some apparatuses with Transport Logistique not very fast, but able to do without true landing strips.

See too

Effectiveness of a turbojet
Turbojet of plane
Turbopropeller
List of the aeronautical motor mechanics

External bonds

See a detailed diagram of a Turbojet
Official site of the company Aviadvigatel {{in}}
Articles on the propulsions with reaction in “Avion.legendaire.Free.fr”

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