HELICOPTER AERODYNAMICS WORKBOOK
Compared to the vortex ring state, vertical autorotation state is a stable condition where
collective pitch settings will vary the rate of descent and rotor speed. Higher rotor speeds are
attained with lower pitch settings, lower rotor speeds with higher settings. This leads to the next
logical assumption, a desired range of rotor speed must exist. An excessively high rotor speed
produces overstressful centrifugal loads on hubs and blade roots, which can in turn overstress the
tail rotor. Rotor blades will stall at a very low rotor speed. 75% to 110% of normal rotor speed
is generally safe, and in this range, rate of descent is approximately twice the hover induced
velocity. This rate of descent is comparable to a helicopter descending under a parachute.
Autorotation, however, does not usually occur after entering vortex ring state. It usually
follows an engine failure if the pilot initiates corrective action in a timely manner. This action
centers on meticulous energy management focusing on rotor RPM and forward airspeed.
Once the engine selects the most convenient time and place to cease working, the power
required for flight, now autorotative flight, must come from another source. This energy comes
from the rate of decrease in potential energy as the helicopter loses altitude. The rotor will
initially slow down, feeding on its own energy due to the power loss. Lowering the collective
with little or no delay will stop this decay. If Nr is allowed to decay too much, the rotor will
stall, allowing the helicopter to assume flying qualities of a brick. The increasing upflow of air
through the rotor system effectively reverses the airflow, tilts the lift vector forward, increasing
thrust, which can now be managed by the pilot through small pitch changes through the
collective by controlling Nr (in-plane drag). Throughout this procedure, potential energy in the
form of loss in altitude is traded off to place kinetic energy in the rotor system.
Now that steady state autorotation has been achieved, the pilot has the option of stretching
his glide to a distant landing zone or increasing his loiter time in the air, provided sufficient
altitude exists. Just suppose the engine failed and there wasn't a suitable landing site
immediately in front of you, but there was one further away. What should one do? Luckily, for
pilots in a somewhat stress-inducing situation, the solution is fairly logical and in line with
normal reaction -- fly at optimum cruise speed (fast). This is called maximum glide range
airspeed. It is found at a point tangent to the power required curve from a line extending from
the origin. Again, there are tradeoffs, and in this case, higher speed and distance over the ground
reduces time aloft and rotor speed.
Another alternative on the other end of the spectrum is minimum rate of descent. This
occurs at the speed of minimum power required on the power required curve. If there is an
available field immediately in front of you, you may use this speed for extra time aloft to ensure
crew readiness for landing or make a prudent radio transmission, but there are other factors
which enter the ball game as the helicopter approaches the ground.
CUSHIONING THE TOUCHDOWN
As the ground becomes more in focus, the range of safe airspeed/rotor RPM combinations
narrows, and precise management of kinetic energy is necessary. At this point, your new goal is