Means of Propulsion

II) Means of propulsion

A. Propeller

In its debuts, in the XIXth century and during the first half of the XXth, the aviation uses essentially the propeller as the means of drive.... Propellers hardly changed since the time(period) the pioneers. Their shape is also decisive as that of the wings and a subtle evolution spectacularly improved their efficiency. We return in used materials there: the forged aluminum replaced the plywood of the debuts, propellers then won in force by adapting itself to the power always greater of the engines.

First of all, let us note that the plane rises in the air only if it is subjected to a push from the bottom up, superior to its weight. To create this strength, it is necessary to create an aerodynamic reaction applied to the wings of the plane and for it, fact of moving in the air with a certain speed.

The propoller aims at transforming the mechanical energy supplied by the engine with traction which communicates with the plane the translatory movement necessary for the flight.

A propoller works as one saw(lived) in the wood, in the fur and has measure which the helix turns(shoots), it advances(moves)? Principle of the Archimedes' screw

 The shape and the size of pales are the elements which influence the tractive strength of the propoller, these two characteristics are thus essential.

Thus so as to obtain a more important return on the tractive strength, the scientists increased the size of these last ones, only the size of these cannot be infinitely increased. Indeed, they were physically limited by the speed at the end of pales!
 

Explanations: as one augment the size of a propoller, the speed at the end of pales, for the same number of trs / min engine, also increases, but if it gets closer too much meadows to the speed of sound: mach 1, the sound bang risks to degrade very strongly the propoller ! To counter in this problem, propollers are thus generally constituted by pales 2, 3 or 4. So we can decrease the size of the pales of propollers without that there is of too big echoed on its propellent return.

The shape of porpollers also comes to play on their characteristics, we find 2 big different types :
1) The most classic, rounded off shape is the one which offers a good return
2) A little less classic, rectangular shape which has a less good return of has its shape which engenders of the trail but which favors the performances of rise
 The propoller was the first propulsion system in aviation and rest used by the light planes, the regional transport aircraft flying unless Mach 0.6, (-of 600km / hour) and the military transport aircraft.
The propfan is an evolution of the propoller higher speeds of which shape of the pales allows to reach(affect), until Mach 0.8. We find the propoller under a different shape in the blowing machines of reactors double flow of airliners (it is a propoller careened multipale placed upstream to the reactor; it assures the main part of the push). We still find it under the shape of dawns of compressors stacked in turbojet engines.
[ The propoller is also used by helicopters (main rotor and rotor of tail)]


B. Reactors


The reactor takes over!
  Whether it is of the type with turbine (turbojet engine) or without turbine ( stratoréacteur ) or with rocket, its essential characteristic is its enormous push with regard to its weight and its weak dimensions. Endowed with a practically unlimited power, the plane can then reach(affect) enormous speeds! The reactor allows the plane, 50-year-old hardly, to cross(exceed) the cape of 1000 kph.

The turbojet engine is the first means of belonging propulsion has the category of reactors.


How works a reactor? 4 stages!

In a turbojet engine, the air enters by a buzzard of entrance(entry) situated in front. It gets through a compressor, which has the shape of a paddle wheel to the multiple pales girls, and which plays the double role of drive of the classic propoller and the compressor of air.

The compressed air is brought in a combustion chamber, where it is carried(worn) in a temperature and in a pressure even higher, then it is thrown(planned) on the pales of the turbine, which it makes turn at high speed.

The turbine supplies, through a propeller shaft, the power necessary for the training of the compressor.

Finally, gases go out again relaxed by the turbine, and it escape by a nozzle. Their speed of ejection leads(infers) the strength of the reactor of the plane.

 

 

In brief : the propulsion with reaction is the stake in movement of a system provoked by a strength of push, created by reaction to the eviction towards the back of a gaseous flow with high speed.
Jet engines used on aeronautics use a lot of air. This one is compressed at first, then oxidizes the fuel, and is finally dilated through a buzzard to expel the fluid with high speed.


To make simple:
Imagine 2 standard springs. One of 2 millimeters in thickness, other one of the 4 millimeters.
That of the 4 millimeters is thus more stiff: that is that the strength necessary for its extension or its compression is more important.
It is thus easier to compress the least stiff spring (that of the 1 millimeter).
Imagine now that both springs are compressed and that you put an object on each of these springs. When to go to you to release(leave) the object, it is the most stiff spring which will produce most work or power...

 

Thus, to summarize, we compress the cold air), we warm it and we relax it .Thus to summarize, we compress the cold air, we warm it and we relax it.Thus to summarize, we compress the cold air, we warm it and we relax it.

Here are two engines of a fighter plane in flight (30 Sukhoi SU 30) : 


And one airliner : 



 The modern military reactors are in double flow also for the civil reactors but they do not answer the same requests.


Reactor doubles flows : this important perfection allowed to lower the consumption and to reduce the level of noise of reactors.


Postcombustion (warms): the principle of the postcombustion is to burn with some fuel in a part fitted out behind the engine. The fuel injected in the flow of warm gases wastes away by bringing a notable increase of push. This type of reactor is reserved for the military devices which need an increasing of power in certain cases (takeoff, laborers(operations)), he had only rarely used on civil planes (supersonics Concorde and Tu-144). The increase of push is important, on the engine SNECMA M-88 of the Rafale for example, it is normally 5.000 kgp, it reaches(affects) 7.500 kgp with warms it. The inconveniences are the engendered noise, the high consumption and the signature

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