Chapter 9 — Ground Reference Maneuvers
Drift and Ground Track Control
Whenever any object is free from the ground, it is
affected by the medium with which it is surrounded.
This means that a free object will continue to move in
its current direction and speed unless acted upon by
another force. For example, if a powerboat is crossing
a river and the river is still, the boat could head
directly to a point on the opposite shore and travel on a straight course to that point without drifting. However,
if the river were flowing swiftly, the water current
would have to be considered. That is, as the boat
progresses forward with its own power, it must also
move upstream at the same rate the river is moving
it downstream. This is accomplished by angling the
boat upstream sufficiently to counteract the downstream
flow. If this is done, the boat will follow the
desired track across the river from the departure point
directly to the intended destination point. Should the
boat not be headed sufficiently upstream, it would
drift with the current and run aground at some point
downstream on the opposite bank. [Figure 9-1]
As soon as a powered parachute becomes airborne,
it is free of ground friction. Its path is then affected
by the air mass in which it is flying; therefore, the
powered parachute (like the boat) will not always
track along the ground in the exact direction that it is
headed. When flying with the longitudinal axis of the
powered parachute aligned with a road, the powered
parachute may get closer to or farther from the road
without any turn having been made. This would indicate
the air mass is moving sideward in relation to the
powered parachute. Since the powered parachute is
flying within this moving body of air (wind), it moves
or drifts with the air in the same direction and speed,
just like the boat moved with the river current. [See
Figure 9-1]
When flying straight and level and following a selected
ground track, the preferred method of correcting
for wind drift is to head the powered parachute sufficiently
into the wind to cause the powered parachute
to move forward into the wind at the same rate the
wind is moving it sideways. Depending on the wind
velocity, this may require a large wind correction angle
or one of only a few degrees. When the drift has
been neutralized, the powered parachute will follow
the desired ground track.
To understand the need for drift correction during
flight, consider a flight with a wind velocity of 30
knots from the left and 90° to the direction the powered
parachute is headed. After 1 hour, the body of air
in which the powered parachute is flying will have
moved 30 NM to the right. Since the powered parachute
is moving with this body of air, it too will have
drifted 30 NM to the right. In relation to the air, the
powered parachute moved forward, but in relation to
the ground, it moved forward as well as 30 NM to the
right.
There are times when the pilot needs to correct for
drift while in a turn. [Figure 9-2] Throughout the turn
the wind will be acting on the powered parachute from
constantly changing angles. The relative wind angle
and speed govern the time it takes for the powered
parachute to progress through any part of a turn. This
is due to the constantly changing groundspeed. When
the powered parachute is headed into the wind, the
groundspeed is decreased; when headed downwind,
the groundspeed is increased. Through the crosswind
portion of a turn, the powered parachute must be
turned sufficiently into the wind to counteract drift.
To follow a desired circular ground track, the wind
correction angle must be varied in a timely manner
because of the varying groundspeed as the turn progresses.
The faster the groundspeed, the faster the wind correction angle must be established; the slower
the groundspeed, the slower the wind correction
angle must be established. You will see then that the
PPC should have the steepest bank and fastest rate of
turn on the downwind portion of the turn and have
the shallowest bank and slowest rate of turn on the
upwind portion.
The principles and techniques of varying the angle
of bank to change the rate of turn and wind correction
angle for controlling wind drift during a turn are
the same for all ground track maneuvers involving
changes in direction of flight.
When there is no wind, it should be simple to fly along
a ground track with an arc of exactly 180° and a constant
radius because the flightpath and ground track
would be identical. This can be demonstrated by approaching
a road at a 90° angle and, when directly
over the road, rolling into a medium-banked turn, then
maintaining the same angle of bank throughout the
180° of turn. [Figure 9-2]
To complete the turn, the rollout should be started at a
point where the canopy will become level as the powered
parachute again reaches the road at a 90° angle
and will be directly over the road just as the turn is
completed. This would be possible only if there were
absolutely no wind and if the angle of bank and the
rate of turn remained constant throughout the entire
maneuver.
If the turn were made with a constant angle of bank
and a wind blowing directly across the road, it would
result in a constant radius turn through the air. However,
the wind effects would cause the ground track
to be distorted from a constant radius turn or semicircular
path. The greater the wind velocity, the greater
would be the difference between the desired ground
track and the flightpath. To counteract this drift, the flightpath can be controlled by the pilot in such a manner
as to neutralize the effect of the wind, and cause
the ground track to be a constant radius semicircle.
The effects of wind during turns can be demonstrated
after selecting a road, railroad, or other ground reference
that forms a straight line parallel to the wind. Fly
into the wind directly over and along the line and then
make a turn with a constant medium angle of bank
for 360° of turn. [Figure 9-3] The powered parachute
will return to a point directly over the line but slightly
downwind from the starting point, the amount depending
on the wind velocity and the time required to
complete the turn. The path over the ground will be an
elongated circle, although in reference to the air, it is
a perfect circle. Straight flight during the upwind segment
after completion of the turn is necessary to bring
the powered parachute back to the starting position.
A similar 360° turn may be started at a specific point
over the reference line, with the powered parachute
headed directly downwind. In this demonstration, the
effect of wind during the constant banked turn will
drift the powered parachute to a point where the line
is re-intercepted, but the 360° turn will be completed
at a point downwind from the starting point.
Another reference line which lies directly crosswind
may be selected and the same procedure repeated,
showing that if wind drift is not corrected the powered
parachute will, at the completion of the 360°
turn, be headed in the original direction but will have
drifted away from the line a distance dependent on the
amount of wind.
From these demonstrations, you will see where and
why it is necessary to increase or decrease the angle
of bank and the rate of turn to achieve a desired track
over the ground. The principles and techniques involved can be practiced and evaluated by the performance
of the ground track maneuvers discussed in
this chapter.
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