## Fan Drive Gear System

A very commonly boasted off phrase by aircraft engine manufacturers these days is, "The Ultra High By-Pass Ratio". Have you ever wondered what do they mean by this ultra-high bypass ratio? What is the reason behind the ultra-high bypass ratio?

Before starting the discussion about the ultra-high bypass ratio and the fan drive gear system let us have a brief introduction to the basic composition and working of an aircraft engine. The engine consists of basically five sections
1. Fan
2. Compressor ( low pressure and high pressure)
3. Diffusion and combustion chamber
4. Turbine ( low pressure and high pressure)
5. Exhaust Nozzle

The ambient air is sucked into the engine by the fan, the air coming inside from the fan is divided into two parts the bypass air and the secondary air. As it can be seen from the image the air entering the core engine is called the secondary air and the part of the air that does not enter the core engine and goes through the engine is called the bypass air. The air which enters the core engine is initially compressed in the compressor stage of the aircraft engine which increases the pressure and temperature of the air. After compression, the air is sent to the diffusion and combustion chamber where the compressed air is burnt along with air and fuel mixture which further increases the pressure and temperature values to a very high value. After this, the high pressure and temperature air is sent in the turbine section where high pressure and temperature air is expanded which increases the velocity of air to very high values at the expense of its pressure. The expansion process also produces rotary motion of the N1 and N2 shafts which is used to drive the compressor and fan. After expansion, the high-velocity air is passed through the exhaust nozzle which further increases the velocity of the air which is leaving from the aft of the engine. Now the thrust is produced as according to Newton's Third law of motion that for every action there is an equal and opposite reaction. The secondary air coming out from the aft of the engine is the action for which we get a forward push which is the reaction. Thrust is not only produced by the air coming out of the exhaust nozzle but also due to the by pass air. To know more about the construction of an aircraft and thrust production in an aircraft engine you can read here

A very interesting fact which should be remembered is that the major amount of thrust produced by an aircraft engine ( approx 80% )  is by the by pass air and the rest 20 % is produced by the air passing through the core engine. The ratio of the f mass flow rate of the by pass air to the mass flow rate of the secondary air is called the by pass ratio. For ex- 10:1 by pass ratio means that 10 kg of air passes through the by pass duct for every 1 kg of air passing through the core engine.

As you all have understood about the by pass ratio, let us move our discussion towards ultra high by pass ratio. For a normal turbojet engine such as the V2500 engine the by pass ratio is around 5-6 :1 but in case of PW1100G-JM the by pass ratio increases to a value of 12:1, ye that is ultra high by pass ratio.

Advantages of ultra high by pass ratio

1. Lower fuel consumption ( upto 20% fuel consumption reduction )

2. Lesser noise created by the engine

Simple thrust equation of aircraft propulsion is

Here Pe and Po are the pressures at the exit and inlet of the engine.
Ve and Vo are the velocity of the air at the exit and inlet of the engine.
Me and Mo are the mass flow rate at the exit and inlet of the engine.

We can very clearly see that as the value of Me that is the mass of air leaving the engine increases the value of thrust force F provided by the engine increases. You will be easily be able to relate to the fact that increasing the mass of air coming inside the engine would surely increase the mass of air leaving from the engine hence a very high value of by pass ratio would increase the thrust value.

Now the other method of increasing thrust would have been increasing the velocity of air leaving the engine. The only method of doing that is to increase the mass of air fuel mixture burning inside the combustion chamber which would increase the fuel cost.

So we can say that having an ultra high by pass ratio is better than burning more amount of fuel both environmentally as well as economically.

After knowing about what is ultra high by pass ratio and what does it affects now let us examine the technology behind ultra high by pass ratio.

THE FAN DRIVE GEAR SYSTEM

Fan drive gear system was initially named as ATFI Advanced Technology Fan Integrator then the it again got a new name as Geared turbofan technology and now we know it as the Fan Drive Gear System. Though the name of technology has changed over time but the engineering behind the technology has always remained the same.

If moving the larger mass of the air is the solution then we can simply make the fan blades very large and make them spin very slowly or we can make more air to pass through the core engine. But both the situations have some shortcomings in the first case as we have the fan coupled directly to the low pressure compressor hence we can not rotate the fan at lower speeds as that would require higher compressor stages. Now if we move the fan at a higher speed then we would get a disadvantage in form of tip losses at the tip of the fan blade hence lowering the engine's efficiency. Now the next solution was to make more to pass through the core engine, that would create a great hike in the fuel requirement as well as a major increment in the noise produced by the engine due to the friction of air.

To counter these problems a person at Pratt and Whitney brought a new technology after working for around 20 years endlessly. A 3:1 ratio gearbox is used between fan and the low pressure compressor shaft, (assuming that you have read the above mentioned article at least once) now the fan is rotated at slow speeds as required and the LPC is made to rotate at high speed compared to the fan and that too in opposite direction. The high pressure spool connecting the high pressure compressor to the high pressure turbine and the fan rotate in the same direction but the fan rotates at a much lower speed as compared to the fan and the low pressure compressor along with the low-pressure turbine rotate in the opposite direction as compared to both of them hence apart from solving the first problem as we discussed earlier we also get a rotational balancing in the engine.

This ends up discussion about the fan drive gear system apart from creating the ultra high by pass ratio the FDGS have many other advantages such as reduction in compressor and turbine stages and more......