Turbine engine having two unducted propellers

Abstract

A turbine engine including two outer coaxial counter-rotating unducted propellers, of an upstream propeller and a downstream propeller, respectively. The blades of the downstream propeller are retractable in the lengthwise direction thereof so as to reduce the diameter of the propeller. The reduction in the diameter of the downstream propeller makes it possible to reduce noise caused by vortices generated by the upstream propeller.

1 - 9 . (canceled) 
     
     
         10 . A turbine engine comprising:
 two coaxial and contra-rotating unducted external fans, respectively of an upstream and a downstream fan,   wherein a length of blades of at least one of the two fans can be varied such that the diameter of the downstream fan is less than the diameter of the upstream fan.   
     
     
         11 . The turbine engine as claimed in  claim 10 , in which blades of the downstream fan are retractable along their longitudinal axis, so as to reduce the diameter of the downstream fan. 
     
     
         12 . The turbine engine as claimed in  claim 10 , in which the variable-length blades are telescopic with at least two blade elements which nest one inside the other and can slide one relative to the other along the axis of the blade. 
     
     
         13 . The turbine engine as claimed in  claim 12 , in which the two blade elements mounted so as to slide one relative to the other are formed at the tip of the blades. 
     
     
         14 . The turbine engine as claimed in  claim 12 , in which the two blade elements are formed at a root end of the blades. 
     
     
         15 . The turbine engine as claimed in  claim 10 , in which at least the blades of the downstream fan are variable-pitch blades. 
     
     
         16 . A method for reducing noise emitted by a turbine engine including two coaxial and contra-rotating unducted external fans, respectively of an upstream and a downstream fan, during a phase of operation of this engine, the method comprising:
 modifying a length of the blades of at least one fan so that the diameter of the downstream fan is less than that of the upstream fan during the phase of operation.   
     
     
         17 . The method as claimed in  claim 16 , the turbine engine being mounted on an aircraft, the phases of operation corresponding to take-off and landing of the aircraft. 
     
     
         18 . The method as claimed in  claim 16 , whereby the reduction in blade length is compensated for by increasing loading on the blade, by varying a blade pitch angle.