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Hover Out Of Ground Effect (OGE)
Hovering out of ground effect is the same as hovering in ground effect
except that it will generally require more power due to not being in
ground effect.
Basic Concepts
There are many reasons a helicopter pilot may need to hover out of ground
effect. In some cases, he will be hovering fairly close to the ground, but
need to stay a little higher than 1/2 rotor diameter in altitude. A common
reason might be obstacles that prevent a landing such as tall grass, shrubs,
etc.
In other cases, he may be coming to a stop hundreds or thousands of feet above
the ground. Electronic News Gathering (ENG) helicopters do this all the time
when on station filming a breaking news story. So will helicopters involved
in external lift operations.
Performance Charts
Every helicopter Pilot Operating Handbook (POH) has both In Ground Effect (IGE)
and Out Of Ground Effect (OGE) hover charts. This allows the pilot to predict
whether the helicopter will be capable of hovering OGE or not. If the performance
chart indicates that the helicopter is not capable of OGE hovering at the particular
density altitude and weight, the pilot will either have to plan to use an IGE hover,
or will have to make some other change to enable OGE hover, such as reducing the
weight of the aircraft, or waiting until the temperature is lower.
Ring Vortex State
Ring Vortex State (or Settling with Power) is
never a problem when hovering IGE, but is always a possibility when attempting to
hover OGE. If the pilot zeroes his airspeed and attempts to hover out of ground
effect but does not have adequate performance, the aircraft will start to settle
into the rotor downwash. If the pilot allows this to reach around 300 feet/minute
or so, the aircraft can enter the ring vortex state. Depending on how high above
the ground the helicopter is, the pilot may or may not have enough altitude to
recover. Thus one of the things a pilot should do when attempting to hover OGE is
to assure himself that he has adequate power to maintain the hover. Generally he
would do this by checking the POH before the flight, or examine the power gauges
as he comes to an out of ground effect hover.
Holding Position
In a low IGE hover, a pilot generally holds position by just looking at the ground
in front of or next to the helicopter. If the helicopter starts to drift, the
pilot makes control inputs to maintain a zero ground speed.
When hovering OGE hundreds or thousands of feet in the air, this is a more difficult
task because it is not as apparent when the helicopter makes small movements over
the ground.
Besides just looking at the ground far below the helicopter, the pilot can line up
objects to determine if he is moving. For instance, if he sees a tree in front of
a building, and the tree is in front of a particular window, movement of the helicopter
will cause the tree to move against the building so that it no longer appears in
front of that particular window. A control input to move it back will help to restore
the helicopter position. With two or more well picked pairs of objects, the pilot
can hold position fairly accurately. In addition, he can use parts of the helicopter
such as the landing gear against the backdrop of the ground to help hold position.
If he moves his head, hover, that picture will change and he needs to account for that.
Paul Cantrell
paul at copters.com
(replace " at " with "@" to email me - this avoids SPAMMERS I hope)
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