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On the helicopter representation in this answer, the turbine is horizontal, with gear required to drive the vertical-axis propeller.

enter image description here

Are there helicopters where the turbine is vertical and directly driving the propeller, without gear? What are the advantages and drawbacks, or why isn't it possible if so?

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    $\begingroup$ Welcome to Av.SE! $\endgroup$ – Ralph J Jan 23 '18 at 14:10
  • $\begingroup$ @RalphJ Thanks! I will wait a bit before accepting one, but there are already pretty good answers $\endgroup$ – Eth Jan 23 '18 at 16:31
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    $\begingroup$ @Eth. There does appear to be at least 1 helicopter with an engine mounted vertically. The Bell 207 Sioux Scout: en.m.wikipedia.org/wiki/Bell_207_Sioux_Scout. But .... it has a piston engine, not a turbine. See picture of the engine installation here:goo.gl/images/BGVtsq $\endgroup$ – Penguin Jan 24 '18 at 12:01
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    $\begingroup$ @Eth. The Bell H13, and 47, used in "MASH", also had a vertically mounted Franklin or Lycoming pistion engine, stated here: en.m.wikipedia.org/wiki/Bell_47 $\endgroup$ – Penguin Jan 24 '18 at 12:12
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    $\begingroup$ No gearbox, assuming the rotors could be driven at nominal turbine speeds, would mean a typical helicopter propeller's tips would have instantaneous linear velocity on the order of around mach 30 and would apply something like 120 tons of centrifugal force to the rotor shaft. Clearly this won't work... $\endgroup$ – J... Jan 24 '18 at 14:34
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Discounting the V-22 Osprey and the other tiltrotors, the drawbacks are:

  • Wasted cabin space
  • L-duct inlet(s) needed to guide the air in
  • With that potential problems with getting the air in as the air prefers fewer turns
  • A reduction gearbox is still needed
  • Gearbox still needed for the tail rotor
  • Harder maintenance access as it will be buried into the fuselage
  • If the exhaust is channeled downwards: potential tarmac melting, fuselage weakening, and handling issues in low hover
  • If channeled upwards, well, that's where the rotor is
  • Otherwise, extra heat ducting needed, i.e., added complexity and weight and expensive materials
  • Any fire or uncontained failure will cause grave danger.

Below is the best illustration I could find, and it gives you an idea about the previous points:

enter image description here http://www.melm-md.com/images/N600_image_02.jpg

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    $\begingroup$ If channeled upwards, well, that's where the rotor is citation needed. $\endgroup$ – Grimm The Opiner Jan 23 '18 at 16:38
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    $\begingroup$ @ymb1 I think it was a joke... $\endgroup$ – Notts90 Jan 23 '18 at 17:32
  • $\begingroup$ @GrimmTheOpiner Not always qph.ec.quoracdn.net/… $\endgroup$ – Christian Jan 24 '18 at 13:37
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No a turboshaft cannot directly drive the rotor without a reduction gear, the rotor torque is too high for the ungeared turbine torque. The rotor blades are much longer than those of a prop or fan (relatively) and the rotor turns slower, a definite case for torque gearing.

Mounting the engine vertically saves a 90 deg gearing assembly, but places the engine where the payload is. Not good.

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