Geared gas turbine engine
A gas turbine engine for an aircraft and a method of operating a gas turbine engine on an aircraft. Embodiments disclosed include a gas turbine engine for an aircraft including: an engine core has a turbine, a compressor, and a core shaft; a fan located upstream of the engine core, the fan has a plurality of fan blades; a nacelle surrounding the engine core and defining a bypass duct and bypass exhaust nozzle; and a gearbox that receives an input from the core shaft and outputs drive to the fan wherein the gas turbine engine is configured such that a jet velocity ratio of a first jet velocity exiting from the bypass exhaust nozzle to a second jet velocity exiting from an exhaust nozzle of the engine core at idle conditions is greater by a factor of 2 or more than the jet velocity ratio at maximum take-off conditions.
1. A method of operating a gas turbine engine on an aircraft, the gas turbine engine comprising:
an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;
a fan located upstream of the engine core, the fan comprising a plurality of fan blades:
a nacelle surrounding the engine core and defining a bypass duct and bypass exhaust nozzle; and
a gearbox that receives an input from the core shaft to drive the fan at a lower rotational speed than the core shaft,
wherein the method comprises operating the gas turbine engine to provide propulsion such that a jet velocity ratio of a first jet velocity exiting from the bypass exhaust nozzle to a second jet velocity exiting from an exhaust nozzle of the engine core at idle conditions is greater by a factor of around 2 or more than the jet velocity ratio at maximum take-off conditions
and wherein the jet velocity ratio, Rj, is defined as:
R
J
=
V
B
C
B
V
C
C
C
η
L
P
T
η
F
where V is the fully expanded first jet velocity, C is a thrust coefficient of the bypass exhaust nozzle, Vc is the fully expanded second jet velocity, Cc is a thrust coefficient of the engine core exhaust nozzle, η LPT is an isentropic efficiency of a lowest pressure turbine of the engine core and η F is an isentropic efficiency of the fan tip.
2. The method of claim 1 wherein the gearbox has a gear ratio of between around 2.5 and around 5.
3. The method of claim 1 wherein the jet velocity ratio is within a range from around 0.75 to around 1.3 at cruise conditions.
4. The method of claim 1 wherein the jet velocity ratio at idle conditions is between around 2 and 3.
5. The method of claim 1 wherein the jet velocity ratio at maximum take-off conditions is between around 0.75 and 1.3.
6. The method of claim 1 , wherein the fan has an outer diameter of between around 220 cm and around 290 cm.
7. The method of claim 1 , wherein the fan has an outer diameter of between around 320 cm and around 420 cm.
8. The method according to claim 1 wherein the factor is within a range of between around 2 and around 3.5.
9. The method according to claim 8 wherein the factor is within a range of between around 2.1 and around 3.16.
10. The method of claim 1 , wherein:
the turbine is a first turbine the compressor is a first compressor, and the core shaft is a first core shaft;
the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and
the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.