|
| |
Ground Effect - What is it, and how
does it work?
I had a friend
who failed his CFI oral in part because he stated that ground effect was a cushion of
air beneath the helicopter. I am sure there were some other issues, however this
was one reason noted for his failure. You will hear this cushion explanation
many times in your career, and although the sensation of ground effect seems
like
a cushion, the cushion statement in and of itself is
not definitive enough for a CFI applicant...
In helicopters,
ground effect results for three basic reasons (actually number 3 below is the
result of number 2):
-
An
increase in the angle of attack of the rotor blades due to: In close
proximity to the ground (most noticeably within 1/2 rotor diameter), the
ground interferes with the down-flow of air beneath the rotor system (the
air begins to stack up); this
results in an increase in the angle of attack on the main rotor blades which
results in a necessary
decrease in collective pitch to maintain a given hover height above ground.
-
A
reduction in the size of rotor tip vortice: Due to ground
interference with the rotor down-wash,the vortices that encompasses the rotor
blade tip are reduced in size relative to the helicopter height above
ground. If you are hovering 10-feet above ground, the vortice will be larger
than if you were hovering 3-feet above ground. The consequence of this
decrease in vortice size is number 3 below.
-
An
increase in the effective area of the rotor disk: As the size of the
vortice is decreased, the portion of the blade producing lift is increased
which results in a necessary decrease in collective pitch to maintain a
given hover height.
|
