unique to helicopters
with thanks to the
helicopterpage
These
things are unique to helicopters.
Effective Translational Lift (ETL)
ETL is
a state of flight where the aircraft
leaves the cushion of ground effect, and
starts to fly forward into "Clean Air".
It is reached at approximately 15-20
knots of forward airspeed, and is
noticeable to the pilot as he/she must
change pedal inputs from heavy left
pedal inputs at a hover to a more
neutral setting. The pilot will also
feel a shuttering in the rotor system as
the aircraft begins to fly out of the
recycled rotor wash generated from the
aircraft rotor system. The cyclic will
try to move backward in the pilots hand
as the rotor system wants to "Blow
Back". The front of the rotor system
will try to rise and slow the aircraft
automatically. This happens since the
front of the rotor system is flying in
clean air first, and the aft portion of
the rotor system is still not in clean
air quite yet. It is then that the pilot
will further induce forward cyclic
inputs to keep the aircraft moving ahead
to gain airspeed. ETL can best be
characterized as the transition from a
hover to forward flight.
Nap Of
the Earth Flight
Nap of the
earth flight (NOE) is a mode of flight
in which the pilot must keep the
aircraft very close to the ground,
following the contours of hills,
streams, canyons, and all other land
features. Often, the aircraft is within
a few feet of power lines, trees, grass,
rocks, and other obstacles. This is
usually a form of combat manoeuvring
where you try to limit the enemy's view
of the aircraft. If they can't see it,
they usually can't shoot at it. It was
developed as an answer to the problem of
being shot down by heat seeking missiles
during the Viet Nam conflict. Pilots
used to fly very high so they were out
of the range of small arms fire. Once
the deployment of heat seeking shoulder
fired missiles had taken place, the
helicopter pilots needed to develop a
new tactic for routine flying to limit
the effectiveness of these portable and
very lethal weapons. By putting trees,
hills and other ground contours between
the person with the missile and the
aircraft, the pilots found themselves in
a lot safer environment. Also, small
arms fire was still not as much of a
problem since the aircraft was directly
over the shooter for a very short
period, and they could not sight in on
the target in so little time. By the
time a shooter could get his weapon
raised, the aircraft was usually out of
sight or there would be some obstacle
between the aircraft and the shooter. In
NOE flight the pilot must be in complete
control of the aircraft. All obstacles
must be avoided, and navigation must be
performed with fewer visual queues for
the pilot to work with. Things look very
different at such a low altitude. Ground
navigation techniques must be employed
as normal flight navigation is almost
useless at these low altitudes. Aircraft
limitations must not be exceeded, radio
calls must be performed, and time
schedules must be met. NOE requires an
instant division of attention, and quick
reflex actions. If you start taking fire
from the ground, an alternate route must
be established immediately. If obstacles
become a problem, evasive manoeuvres
must be initiated without hesitation. At
maximum airspeed, and minimal altitude,
instantaneous decisions must be made at
all times, or the aircraft could become
a smoking hole in the ground in a matter
of seconds. NOE flight can be the most
exciting, rewarding, and physically
exhausting flying anyone could do in a
helicopter.
WSPS
System
On the
front of most U.S. Army and many civil
helicopters you may notice a knife like
fixture on the top of the cockpit, and
one on the bottom of the aircraft near
the chin bubbles. These are not antennae
for radios like most people believe.
They are part of the Wire Strike
Protection System (WSPS). The WSPS is
made up of several components to protect
the helicopter from high wire strikes.
It was developed because of the
increased risk of wire strikes while
flying at NOE altitudes. If a helicopter
hits a power line (Telephone line,
electrical line, guy wire for a tower,
or any other wire obstacle), the rotor
system may become entangled with the
wire, and catastrophic failure of the
rotor system could lead to total
destruction of the aircraft. The WSPS
was developed to reduce the severity of
a wire obstacle collision by diverting
the wire into the cutter blade
assemblies. The cutter blades affixed to
the top and bottom of the frontal area
of the aircraft will usually cut the
wire and eliminate the hazard. On UH - 1
Huey helicopters, a set of bars will
carry the wire over external parts of
the windshield wipers. On OH-58
helicopters, the centre section of the
windshield has an abrasive cutting strip
(Built into the windshield deflector) to
score the wire and weaken it before it
comes in contact with the WSPS cutters.
The WSPS system protects 90% of the
frontal area of the helicopter, and
reduces the hazard from most wire
strikes. With the WSPS, the pilot has a
95% chance of surviving a single wire
strike. The odds of survival decrease as
the number of wires increases. 2 wires
will reduce the chances to 75%, 3 wires
to 50%, and 4 wires to about 25%.
Although the WSPS system is quite
effective, care must still be used to
avoid all wire obstacles. Apache, Cobra,
and Blackhawk helicopters all have a
smaller, less noticeable WSPS system on
them. They can usually be seen just
above the cockpit, and near landing gear
struts. Chinooks do not have WSPS
systems.
Autorotation
Most
people think that a helicopter will fall
like a rock and the rotor system will
stop once the engine fails. This is a
totally false assumption. A helicopter
can continue to fly without any power
from the engine. "Autorotation" is the
term used for "Gliding" a helicopter
down after the engine fails or the
throttle is retarded to the idle
position.
If you look
at a rotor blade from the tip of the
blade toward the root, you will see it
will twist laterally. At the tip of the
blade, the leading edge may point down
while at the root of the blade, the
leading edge may point up. This allows
different regions of the blade to
perform different tasks, one of which is
Autorotation. The outer portion of the
blade, when the collective is lowered
all the way to what is called "Flat
Pitch", will drive the rotor system as
the aircraft glides downward, increasing
or maintaining the speed of the rotor
system. The rotor system is driven
normally by a centrifugal clutch which
is positively engaged while the engine
supplies power, but disengages when
power is removed. The rotor system "Free
Wheels", and continues to spin. The air
travelling upward through the rotor
system continues to drive the system and
maintain rotor RPM.
The
aircraft descends rather rapidly, but
with a high rotor RPM, the aircraft can
be cushioned to the ground effectively
and landed without incident. Additional
weights are housed in the tips of the
rotor blades to increase the inertia of
the rotor system, and aid in
autorotation. The procedure for
autorotation is to lower the collective
immediately and put in full right pedal,
and enter a steady state of
autorotation. Full right pedal must be
put in because the torque has stopped
from the lack of engine power, and the
tail rotor thrust is only necessary at
this point to control aircraft trim. (By
putting in right pedal, you effectively
neutralize the tail rotor, and it
provides no thrust). The pilot must find
a suitable landing area, and manoeuvre
as necessary to make the intended
landing area, making certain that the
rotor RPM is within limits. At
approximately 100 FT AGL (Above Ground
Level) start a progressive deceleration
to decrease forward airspeed, and about
15 FT AGL, lift the collective in a
quick jerking motion to cushion the
aircraft initially as a vertical brake.
The initial collective pitch pull should
be enough to retard the descent, and the
rest of the collective pitch should be
pulled in gradually and continually as
the aircraft settles to the ground. You
should land with little or no forward
airspeed, and the landing should be
relatively soft, depending on the
surface you are landing to. A safer
autorotational approach depends on where
you land. If you land in a field where
forward movement would be dangerous, you
should plan for a shorter landing run.
This requires a more vertical drop in
the last part of the autorotation. If
you have the room to slide, then a more
shallow approach can be made and a
longer ground run should be allowed. A
more vertical drop is harder to
accomplish and your timing needs to be a
lot more precise, where a shallower drop
is more forgiving and you need to be
less precise on your timing.
In a
Chinook, the rear wheel locks are
electrically operated. During
autorotation, If the rotor RPM decreases
below a certain value, the generators
will fall off line, and the wheel locks
will disengage. If this happens, the
aircraft will most likely land aft
wheels first, and without swivel locks
engaged on the rear wheels, it could
make for a very interesting ride.
|