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I read about convertible engines. I want to know if there is any engine existing to meet up hover and cruise together.

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The closest one is the P&W F-135, used in the F-35B VTOL-capable fighter. However, it switches between turbofan and turbofan+turboshaft modes. The fan's still running, but its flow is redirected down through thrust vectoring for hovering.

Here it is shown driving a lift fan, which includes the gearbox. A similar but larger gearbox can be used to drive a helicopter rotor, if necessary.

This particular engine is too large for a practical-sized helicopter. But it might be possible to convert smaller engines in a similar way.

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    $\begingroup$ To be precise, the F-135 is capable of switching between turbofan and turbofan/turboshaft hybrid mode. It never operates in turboshaft only -mode. $\endgroup$
    – Jpe61
    Mar 31 at 12:42
  • $\begingroup$ You could also state that it is a turbofan engine, period. One with a large power off-take in STOVL configuration. Even in STOVL mode the exhaust needs to provide a relative large airflow. $\endgroup$
    – 0scar
    Apr 2 at 18:29
  • $\begingroup$ Is there any internal modification of turbofan engine other than connecting the drive shaft required to use a turbofan engine both in hover and cruise condition like F-35B's engine ? $\endgroup$
    – Eaza
    Apr 6 at 8:24
  • $\begingroup$ @NahidaIslam For optimum performance or FADEC, perhaps. But generally almost any turbofan should be able to drive a gearbox instead of the fan or in addition to it. $\endgroup$
    – Therac
    Apr 6 at 10:35
  • $\begingroup$ In this case how to find the shaft power if turbofan is used? $\endgroup$
    – Eaza
    Apr 7 at 6:59

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