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The MD-900 is a helicopter which seems to be quite popular with law enforcement agencies.

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As you can see, instead of an anti-torque tail rotor, a fan exhaust is directed out slots in the tail boom. I was wondering if this works in regards to auto rotation, should the aircraft lose its engines.

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During an auto-rotation, you don't need torque compensation at all. Those vertical stabilizers are more than enough. Autogyros fly in auto-rotation all the time, and they don't need any torque compensating device, because in this type of aircraft the rotor is not powered by an onboard engine, but rotates freely, powered by the relative wind. In the absence of a torque applied by the engine to the rotor, there is no opposite reaction; hence, no compensation is necessary... A helicopter descending in autorotation is exactly like an autogyro gliding with the engine off...

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    $\begingroup$ i think your answer is true. but maybe youll need to add some details to answer. i think you should add these, why you dont need torque compensation when engine is not running. why you normally need it at a helicopter. whats the difference between helis and gyros and what are their design pricibles to need or not to need torqu compensating. why auto gyros dont need torque compensation. $\endgroup$ – hfc Mar 1 '17 at 10:42
  • $\begingroup$ You're suggesting it could autorotate on a straight glidepath. But unless those vertical stabilizers happen to have rudders then there would be no means to effect a controlled turn. $\endgroup$ – feetwet Jun 11 '17 at 16:12
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According to https://en.wikipedia.org/wiki/NOTAR the NOTAR fan is driven by the main rotor transmission. This will ensure maneuverability during autorotation.

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    $\begingroup$ This, plus the fact that in autorotation the rotor is not being driven by the engine, so there is far less torque acting on the fuselage. That's not the reason it works, but it's an alleviating factor $\endgroup$ – Jon Story Feb 9 '15 at 20:10
  • $\begingroup$ The flow through the rotor plane in a helicopter in autorotation is upwards, hence there is no rotor downwash blowing on the boom, and the NOTAR system probably won't work well –if at all– under those conditions... $\endgroup$ – xxavier Mar 2 '17 at 13:18
  • $\begingroup$ @xxavier good point. Please feel free to edit / add. It might be that the rotation of the exit can be adjusted to compensate for the flow change. $\endgroup$ – yankeekilo Mar 2 '17 at 14:38
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    $\begingroup$ In the highly respected book 'The Art of the Helicopter', by John Watkinson, chapter 5-11 (NOTAR) the following can be read: 'In the case of autorotation, the boom will be in upwash and the boundary layer control is no longer effective, but nor is it needed. The fan is driven from the main rotor and yaw control is by rotating the tail sleeve as usual'. In my opinion, this closes the discussion... $\endgroup$ – xxavier Mar 6 '17 at 11:16

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