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Aerotécnica AC-14 was an early no tail rotor prototype which used the ducted exhaust from the turboshaft to counter the torque of the main rotor. The concept seems simple with few moving parts.

It didn't use a (seemingly complicated) enclosed variable pitch fan driven by the main rotor transmission, neither did it use Coandă effect (even if it may have), like modern NOTAR does.

Why has a much more complicated solution been adopted for modern no tail rotor systems, and why was AC-14 concept abandoned?

(Autorotation yaw behavior may be one safety issue, even if autogyro autorotation procedures are quite safe.)

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  • $\begingroup$ Well if your engine dies you lose the exhaust immediately. Also jetting the exhaust might hurt overall fuel economy. $\endgroup$ – user3528438 Aug 30 '17 at 16:53
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    $\begingroup$ @user3528438 Yes when the engine dies you lose the exhaust, but you also don't need anti-torque. $\endgroup$ – Koyovis Aug 30 '17 at 20:22
  • $\begingroup$ @user3528438 About fuel economy: I think turbomachines are all about compromises and very versatiles : optimisation may be done in a specific engine, providing efficient shaft + thrust power output. In addition, converting power via converting devices always ruin overall efficiency. ( for instance, 0.8 efficient engine running a 0.8 efficient rpm reducer running a 0.8 efficient fan, means 0.512 total efficiency.) $\endgroup$ – qq jkztd Aug 30 '17 at 22:48
  • $\begingroup$ It's a good question. My assupmtion was always that the hot gases make the idea unpractical, but you've found an actual application. $\endgroup$ – Koyovis Aug 31 '17 at 2:12
  • $\begingroup$ Modern NOTAR seem very similar to AC-14. So, is your question really boiling down to "why extra fan instead of the one that's already in the engine"? $\endgroup$ – Agent_L Aug 31 '17 at 10:47
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They use a shrouded fan because these use less power than a jet for generating the same thrust. I could find a reference in Principles of Helicopter Aerodynamics by J. Gordon Leishman: he states that using jet exhaust for anti-torque required high power and that directional sensitivity was poor.

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The reference to high power would refer to the generic issue with jets providing thrust: it costs less power to accelerate a higher volume of air to a lowish velocity (like a propeller does) than to accelerate a lowish volume of air to a higher velocity (like a jet exhaust does).

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