432
12 Thrust Gas Turbines
The main problem with the unducted fan and the prop-fan is the intense noise
emission, much higher than with a conventional propeller. This problem impedes
airliner use of these concepts. Further, the prop-fan is limited to medium size aircraft,
requiring a thrust per engine of about 100 kN. The reason is the power limitation
on the gearbox, currently to about10 MW. The benefit of reduced fuel consumption
would be more interesting with the larger class of engines with thrust of the 400 kN
order for large airliners. The problem of noise emission has been under intense study
for some time, but significant improvement is certainly still far away. The problem of
noise emission can be weakened by shrouding the propellers. This has been studied
in the past, but the concept is then not essentially different from a turbofan.
12.2.6 Geared Turbofan
A high bypass ratio implies a large fan diameter and a low tip speed of the blades
of the driving LP-turbine. The bypass ratio of the turbofan engines under development nowadays (2014) is about 10, as already mentioned. The LP turbine driving
the fan is large. The LP-turbine may be compacted by letting it run faster and driving the fan over a reduction gearbox, as with a propeller. The concept is called a
geared turbofan. Currently (2014), a series of such engines is under development
by Pratt and Whitney: the PW1000G series. The engine under test has an epicyclic
gearbox with a speed ratio 3:1. The LP turbine has three stages. The bypass ratio is
12 and the thrust at sea level is 100–150 kN. The power passing through the gearbox is about 13.5 MW. Further development aims at increasing the speed ratio of
the gearbox (larger bypass ratio) and the power passing through it (larger engine).
The technological challenges are development of light, high-strength materials and
reduction of the friction heat.
12.3 Performance Parameters of Aircraft Propulsion
Systems
We first define some concepts of power and efficiency for single-flow turbojets.
Afterwards, these concepts are extended to double-flow systems. With double-flow
systems, we treat turbo-propellers, bypass jet engines, turbofan engines, prop-fan
engines and unducted fan engines all with the same form of analysis.
12.3.1 Specific Thrust
For a single-jet, thrust is given by
T m v
= ∆ ,
12 Thrust Gas Turbines
The main problem with the unducted fan and the prop-fan is the intense noise
emission, much higher than with a conventional propeller. This problem impedes
airliner use of these concepts. Further, the prop-fan is limited to medium size aircraft,
requiring a thrust per engine of about 100 kN. The reason is the power limitation
on the gearbox, currently to about10 MW. The benefit of reduced fuel consumption
would be more interesting with the larger class of engines with thrust of the 400 kN
order for large airliners. The problem of noise emission has been under intense study
for some time, but significant improvement is certainly still far away. The problem of
noise emission can be weakened by shrouding the propellers. This has been studied
in the past, but the concept is then not essentially different from a turbofan.
12.2.6 Geared Turbofan
A high bypass ratio implies a large fan diameter and a low tip speed of the blades
of the driving LP-turbine. The bypass ratio of the turbofan engines under development nowadays (2014) is about 10, as already mentioned. The LP turbine driving
the fan is large. The LP-turbine may be compacted by letting it run faster and driving the fan over a reduction gearbox, as with a propeller. The concept is called a
geared turbofan. Currently (2014), a series of such engines is under development
by Pratt and Whitney: the PW1000G series. The engine under test has an epicyclic
gearbox with a speed ratio 3:1. The LP turbine has three stages. The bypass ratio is
12 and the thrust at sea level is 100–150 kN. The power passing through the gearbox is about 13.5 MW. Further development aims at increasing the speed ratio of
the gearbox (larger bypass ratio) and the power passing through it (larger engine).
The technological challenges are development of light, high-strength materials and
reduction of the friction heat.
12.3 Performance Parameters of Aircraft Propulsion
Systems
We first define some concepts of power and efficiency for single-flow turbojets.
Afterwards, these concepts are extended to double-flow systems. With double-flow
systems, we treat turbo-propellers, bypass jet engines, turbofan engines, prop-fan
engines and unducted fan engines all with the same form of analysis.
12.3.1 Specific Thrust
For a single-jet, thrust is given by
T m v
= ∆ ,