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descent established in order to drive the rotor. Time is
also needed to stabilize, then increase the r.p.m. to the
normal operating range. The rate of descent must reach
a value that is normal for the airspeed at the moment.
Since altitude is insufficient for this sequence, you end
up with decaying r.p.m., an increasing sink rate, no
deceleration lift, little translational lift, and little
response to the application of collective pitch to cushion
the landing.
It should be noted that, once a steady state autorotation
has been established, the H/V diagram no longer
applies. An engine failure while descending through
section Aof the diagram, is less critical, provided a safe
landing area is available.
11-5
You should avoid the low altitude, high airspeed portion
of the diagram (section B), because your recognition of an
engine failure will most likely coincide with, or shortly
occur after, ground contact. Even if you detect an engine
failure, there may not be sufficient time to rotate the
helicopter from a nose low, high airspeed attitude to one
suitable for slowing, then landing. Additionally, the
altitude loss that occurs during recognition of engine failure
and rotation to a landing attitude, may not leave
enough altitude to prevent the tail skid from hitting the
ground during the landing maneuver.
Basically, if the helicopter represented by this H/V diagram
is above 445 feet AGL, you have enough time and
altitude to enter a steady state autorotation, regardless
of your airspeed. If the helicopter is hovering at 5 feet
AGL(or less) in normal conditions and the engine fails,
a safe hovering autorotation can be made. Between
approximately 5 feet and 445 feet AGL, however, the
transition to autorotation depends on the altitude and
airspeed of the helicopter. Therefore, you should
always be familiar with the height/velocity diagram for
the particular model of helicopter you are flying.
THE EFFECT OF WEIGHT VERSUS
DENSITY ALTITUDE
The height/velocity diagram depicts altitude and airspeed
situations from which a successful autorotation
can be made. The time required, and therefore, altitude
necessary to attain a steady state autorotative descent,
is dependent on the weight of the helicopter and the
density altitude. For this reason, the H/V diagram for
some helicopter models is valid only when the helicopter
is operated in accordance with the gross weight vs.
density altitude chart. Where appropriate, this chart is
found in the rotorcraft flight manual for the particular
helicopter. [Figure 11-3]
Figure 11-3. Assuming a density altitude of 5,500 feet, the
height/velocity diagram in figure 11-2 would be valid up to a
gross weight of approximately 1,700 pounds. This is found by
entering the graph at a density altitude of 5,500 feet (point A),
then moving horizontally to the solid line (point B). Moving vertically
to the bottom of the graph (point C), you find that with the
existing density altitude, the maximum gross weight under
which the height/velocity diagram is applicable is 1,700 pounds.
The gross weight vs. density altitude chart is not
intended as a restriction to gross weight, but as an advisory
to the autorotative capability of the helicopter
during takeoff and climb. You must realize, however,
that at gross weights above those recommended by the
gross weight vs. density altitude chart, the H/V diagram
is not restrictive enough.
VORTEX RING STATE (SETTLING WITH
POWER)
Vortex ring state describes an aerodynamic condition
where a helicopter may be in a vertical descent with up
to maximum power applied, and little or no cyclic
authority. The term “settling with power” comes from
the fact that helicopter keeps settling even though full
engine power is applied.
In a normal out-of-ground-effect hover, the helicopter
is able to remain stationary by propelling a large mass
of air down through the main rotor. Some of the air is
recirculated near the tips of the blades, curling up from
the bottom of the rotor system and rejoining the air
500
450
400
350
300
250
200
150
100
50
0
A
B
Smooth Hard Surface.
Avoid Operation in
Shaded Areas.
INDICATED AIRSPEED KNOTS
(CORRECTED FOR INSTRUMENT ERROR)
HEIGHT ABOVE SURFACE - FEET
0 10 20 30 40 50 60 70 80 90 100 110 120
A B
C
7,000
6,000
5,000
4,000
3,000
1,500 1,600 1,700 1,800 1,900
GROSS WEIGHT – POUNDS
DENSITY ALTITUDE – FEET
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