Method of taking off auxiliary power from an airplane turbojet, and a turbojet fitted to implement such a method

1. A method of taking auxiliary power from an airplane dual-spool turbojet comprising, from upstream to downstream, a fan, a low pressure compressor, a high pressure compressor, a combustion chamber, a high pressure turbine and a low pressure turbine, wherein said low pressure turbine is mechanically connected to a first axial shaft which drives said fan and said low pressure compressor such that said fan produces thrust, said high pressure turbine is mechanically connected to a second axial shaft which drives said high pressure compressor, the method comprising:

taking off said power via a radial shaft mechanically coupled to said second axial shaft and driven by the high pressure turbine; and

controlling degradation of efficiency of said low pressure turbine, within an idling operating range, to enable said high pressure turbine to operate at a speed that is sufficient for supplying the required auxiliary power,

wherein the efficiency is degraded by increasing radial clearance of moving blades of said low pressure turbine using a low pressure turbine axis clearance control system, such that a rotation speed of said high pressure turbine is increased and the thrust produced by said fan is compatible with said turbojet operating at idling speed.

2. A method according to claim 1 , wherein the variation in the efficiency of said low pressure turbine is servo-controlled to a measurement of the torque delivered by a mechanical transmission system arranged between said radial shaft driven by the high pressure turbine and an electricity generator.

3. A method according to claim 1 , wherein the thrust produced by said fan when said turbojet is operating at idling speed does not increase when the rotation speed of said high pressure turbine is increased and the efficiency of said low pressure turbine is degraded.

4. An airplane dual-spool turbojet comprising, from upstream to downstream:

a fan, a low pressure compressor, a high pressure compressor, a combustion chamber, a high pressure turbine and a low pressure turbine, said low pressure turbine being mechanically connected to a first axial shaft which drives said fan and said low pressure compressor such that said fan produces thrust, said high pressure turbine being mechanically connected to a second axial shaft which drives said high pressure compressor;

a radial shaft mechanically coupled to said second axial shaft and driven by said high pressure turbine;

a mechanical transmission system coupled to said radial shaft to take power therefrom;

efficiency-varying means for varying the efficiency of said low pressure turbine; and

control means for controlling said efficiency-varying means when the turbojet is operating within an idling range to enable said high pressure turbine to operate at a speed that is sufficient for supplying required auxiliary power, said control means being controlled by a sensor delivering a signal representative of the amount of auxiliary power being consumed,

wherein said efficiency-varying means comprises a system for controlling radial clearance of moving blades of said low pressure turbine using a low pressure turbine axis clearance control system so as to reduce the efficiency of said low pressure turbine by increasing the radial clearance of said moving blades of said low pressure turbine such that a rotation speed of said high pressure turbine is increased and the thrust produced by said fan is compatible with said turbojet operating at idling speed.

5. A turbojet according to claim 4 , wherein said efficiency-varying means comprise a computer controlled by a torque sensor associated with a mechanical transmission system arranged between said radial shaft driven by the high pressure turbine and an electricity generator.

6. A turbojet according to claim 4 , wherein the thrust produced by said fan when said turbojet is operating at idling speed does not increase when the rotation speed of said high pressure turbine is increased and the efficiency of said low pressure turbine is degraded.

7. A method of taking auxiliary power from an airplane dual-spool turbojet comprising, from upstream to downstream, a fan, a low pressure compressor, a high pressure compressor, a combustion chamber, a high pressure turbine and a low pressure turbine, wherein said low pressure turbine is mechanically connected to a first axial shaft which drives said fan and said low pressure compressor such that said fan produces thrust, said high pressure turbine is mechanically connected to a second axial shaft which drives said high pressure compressor, the method comprising:

taking off said power via a radial shaft mechanically coupled to said second axial shaft and driven by the high pressure turbine; and

controlling degradation of efficiency of said low pressure turbine, within an idling operating range, to enable said high pressure turbine to operate at a speed that is sufficient for supplying the required auxiliary power,

wherein the efficiency is degraded by controlling a variable pitch of nozzle vanes of said low pressure turbine using a pitch-varying system which is actuated from outside of a casing, such that a rotation speed of said high pressure turbine is increased and the thrust produced by said fan is compatible with said turbojet operating at idling speed.

8. A method according to claim 7 , wherein the variation in the efficiency of said low pressure turbine is servo-controlled to a measurement of the torque delivered by a mechanical transmission system arranged between said radial shaft driven by the high pressure turbine and an electricity generator.

9. A method according to claim 7 , wherein the thrust produced by said fan when said turbojet is operating at idling speed does not increase when the rotation speed of said high pressure turbine is increased and the efficiency of said low pressure turbine is degraded.

10. An airplane turbojet comprising, from upstream to downstream:

a fan, a low pressure compressor, a high pressure compressor, a combustion chamber, a high pressure turbine and a low pressure turbine, said low pressure turbine being mechanically connected to a first axial shaft which drives said fan and said low pressure compressor such that said fan produces thrust, said high pressure turbine being mechanically connected to a second axial shaft which drives said high pressure compressor;

a radial shaft mechanically coupled to said second axial shaft and driven by said high pressure turbine;

a mechanical transmission system coupled to said radial shaft to take power therefrom;

efficiency-varying means for varying the efficiency of said low pressure turbine; and

control means for controlling said efficiency-varying means when the turbojet is operating within an idling range to enable said high pressure turbine to operate at a speed that is sufficient for supplying required auxiliary power, said control means being controlled by a sensor delivering a signal representative of the amount of auxiliary power being consumed,

wherein said efficiency-varying means includes a system for controlling the pitch of the nozzle vanes of said low pressure turbine using a pitch-varying system which is actuated from outside of a casing, such that a rotation speed of said high pressure turbine is increased and the thrust produced by said fan is compatible with said turbojet operating at idling speed.

11. A turbojet according to claim 10 , wherein said efficiency-varying means includes a computer controlled by a torque sensor associated with a mechanical transmission system arranged between said radial shaft driven by the high pressure turbine and an electricity generator.

12. A turbojet according to claim 10 , wherein the thrust produced by said fan when said turbojet is operating at idling speed does not increase when the rotation speed of said high pressure turbine is increased and the efficiency of said low pressure turbine is degraded.