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side is higher due to the forward speed of the
Figure 11-4. Vortex ring state.
11-7
helicopter, while the relative airflow on the retreating
side is lower. This dissymmetry of lift increases
as forward speed increases.
To generate the same amount of lift across the rotor
disc, the advancing blade flaps up while the retreating
blade flaps down. This causes the angle of attack
to decrease on the advancing blade, which reduces
lift, and increase on the retreating blade, which
increases lift. As the forward speed increases, at
some point the low blade speed on the retreating
blade, together with its high angle of attack, causes a
loss of lift (stall).
Retreating blade stall is a major factor in limiting a
helicopter’s top forward speed (VNE) and can be felt
developing by a low frequency vibration, pitching
up of the nose, and a roll in the direction of the
retreating blade. High weight, low rotor r.p.m., high
density altitude, turbulence and/or steep, abrupt
turns are all conducive to retreating blade stall at
high forward airspeeds. As altitude is increased,
higher blade angles are required to maintain lift at a
given airspeed. Thus, retreating blade stall is
encountered at a lower forward airspeed at altitude.
Most manufacturers publish charts and graphs showing
a VNE decrease with altitude.
When recovering from a retreating blade stall condition,
moving the cyclic aft only worsens the stall
as aft cyclic produces a flare effect, thus increasing
angles of attack. Pushing forward on the cyclic
also deepens the stall as the angle of attack on the
retreating blade is increased. Correct recovery from
retreating blade stall requires the collective to be
lowered first, which reduces blade angles and thus
angle of attack. Aft cyclic can then be used to slow
the helicopter.
GROUND RESONANCE
Ground resonance is an aerodynamic phenomenon
associated with fully-articulated rotor systems. It
develops when the rotor blades move out of phase
with each other and cause the rotor disc to become
unbalanced. This condition can cause a helicopter to
self-destruct in a matter of seconds. However, for
this condition to occur, the helicopter must be in
contact with the ground.
If you allow your helicopter to touch down firmly on
one corner (wheel type landing gear is most
conducive for this) the shock is transmitted to the
main rotor system. This may cause the blades to
move out of their normal relationship with each
other. This movement occurs along the drag hinge.
[Figure 11-5]
Figure 11-5. Hard contact with the ground can send a shock
wave to the main rotor head, resulting in the blades of a
three-bladed rotor system moving from their normal 120°
relationship to each other. This could result in something like
122°, 122°, and 116° between blades. When one of the other
landing gear strikes the surface, the unbalanced condition
could be further aggravated.
If the r.p.m. is low, the corrective action to stop ground
resonance is to close the throttle immediately and fully
lower the collective to place the blades in low pitch. If the
r.p.m. is in the normal operating range, you should fly the
helicopter off the ground, and allow the blades to automatically
realign themselves. You can then make a normal
touchdown. If you lift off and allow the helicopter to
firmly re-contact the surface before the blades are
realigned, a second shock could move the blades again
and aggravate the already unbalanced condition. This
could lead to a violent, uncontrollable oscillation.
This situation does not occur in rigid or semirigid rotor
systems, because there is no drag hinge. In addition,
skid type landing gear are not as prone to ground
resonance as wheel type gear.
DYNAMIC ROLLOVER
A helicopter is susceptible to a lateral rolling tendency,
called dynamic rollover, when lifting off the surface.
For dynamic rollover to occur, some factor has to first
cause the helicopter to roll or pivot around a skid, or
landing gear wheel, until its critical rollover angle is
reached. Then, beyond this point, main rotor thrust continues
the roll and recovery is impossible. If the critical
rollover angle is exceeded, the helicopter rolls on its
side regardless of the cyclic corrections made.
Dynamic rollover begins when the helicopter starts to
pivot around its skid or wheel. This can occur for a
variety of reasons, including the failure to remove a
tiedown or skid securing device, or if the skid or wheel
122° 116°
122°
　
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