Flare and Touchdown
From either of the two extreme airspeed range examples previously discussed (max glide/min rate of descent), we will assume a suitable landing zone is now easily within range. If we were at max glide at a high forward speed and associated high rate of descent, it is only logical we slow down.
How slow? Minimum rate of descent sounds logical. But, even at this airspeed, the helicopter's landing gear cannot absorb the amount of energy the helicopter is carrying at ground contact. Therefore, it is advantageous to carry 5 - 10kts extra airspeed over minimum rate of descent airspeed at flare altitude, banking on another trade-off - extra forward airspeed for high rotor RPM.
A nose-up cyclic flare (see figure) at 75 - 100ft AGL increases induced flow. The resulting increase in Angle of Attack (AOA) creates more lift, which decreases rate of descent.
Moreover, the downward shift in relative wind tilts the left vector at blade element more forward, resulting in a larger pro-autorotative force; this increases rotor RPM. Finally, the net rotor thrust is tilted aft, and this decreases ground speed. The flare should be maintained in an effort to reach a point to where forward speed is 5 - 10kts at close proximity to the ground.
At this point, increasing collective, increases thrust and augments braking action, using up part of the stored rotational energy. All that is left is to put in a little forward cyclic to level the aircraft and use that last rotational energy by pulling collective to cushion the landing.
If one chose to arrive at flare altitude at minimum rate of descent airspeed or less, there is little or no forward speed to trade off for this advantageous high rotor RPM. Forward speed is already low, and if too much flare is combined with an improperly timed flare (too high), forward speed may reduce to zero at a high altitude. This condition is known as becoming “vertical,” and since the rotor system already has little stored energy, there will not be enough thrust available with collective increase to slow rate of descent at touchdown to a non-destructive level.