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HELICOPTER AERODYNAMICS WORKBOOK
CHAPTER 4
to reduce the kinetic energy along the flight path to zero at the same time ground contact is
made, while trading off the stored kinetic energy in rotor RPM for thrust to maintain power
requirements for flight before the blades reach a stalled condition. This may seem like a very
large chunk to swallow, but if taken in small bites, the process becomes much easier (see figure
4-4).
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 (low rate of descent at ground contact = less pain). 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 5-10 kts extra airspeed over minimum rate of descent airspeed at flare altitude, banking on
another tradeoff -- extra forward airspeed for high rotor RPM. Figure 4-4
Figure 4-4
A nose-up cyclic flare (see figure 4-5) at 75-100 feet AGL (for the TH-57) increases induced
flow. The resulting increase in 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 5-10 kts at close proximity to the ground (10-15 ft). At this
point, increasing collective, increases thrust and augments braking action, using up part of the
AUTOROTATION 4-5


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