Hi Steve, I just got home from a couple weeks flying in India. Here are a few comments from me:
In a NOTAR helicopter you have 3 anti-tq producers, the fan and thruster at all times, the fan and circulation control tailboom at a hover and very low speed, and the vertical fins become effective as you gain forward airspeed. In autorotation, you are not countering torque, you are countering the friction that makes the fuselage tend to rotate the same direction as the main rotor is turning. A NOTAR helicopter counters this friction with the fan and thruster as well as the vertical fins in the descent. You will notice that if you drag the NR down to slow your descent or increase your glide, it will take more right pedal as the fan is slowed. The lower portion of the vertical fins is most effective in the descent and in a 520N they are fairly small, increasing the need for right pedal in the descent. As you begin your flare, you may notice a little right yaw if you get a good NR increase without adjusting the pedals to compensate. If you tend to hold the aircraft off the ground with the remaining NR, as it decays so will the fan RPM, this will cause a left yaw as the fuselage follows the NR without enough airflow out the thruster to counter that rotation. In a power-off autorotation to the ground, you will not see any anti-tq / anti-friction developed from the circulation control tailboom as there will not be enough downwash to develop any lift on the boom. If you perform a power recovery autorotation, you may get a good yaw in the flare (right), if you start with a low NR and drive the RPM up with the engine, your feet probably won’t be quick enough in coordination to completely counter that. When you terminate at a hover you will get a sudden increase in anti-tq (left yaw) as the MR downwash comes down across the tailboom requiring a sudden change in pedal position.
A couple other things to note: If you change NR in the descent, you will need to change the pedal position due to the change in airflow thru the thruster due to a change in fan speed, that will result in a change in vertical fin position requiring a change in pedal position, changing fan pitch, dragging down or speeding up NR ect………… If you make smooth and correct control inputs, it is easy, if you are chasing trim and NR, you will fight all the way to the ground. If you seem to be chasing trim, just freeze the pedals for a few seconds, check your trim, make an adjustment and your back in business. In the 600, if you get the groundspeed very near zero, and have no headwind, or even worse, a left crosswind, the nose will yaw to the left, you will apply right pedal, the NR will decay more, the fan will slow more, you will apply more right pedal ect……. A little trick here is just to put in a slight right of centerline nose, as you level and cushion this little yaw will give you time to get the right pedal in as you cushion and hopefully you will keep the nose pointing the right way. A big trick in NOTAR autorotation is the RATE of pedal application as well as the amount.
Cheers
Mark