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track along the ground in the exact direction that it is headed.
When fl ying with the longitudinal axis of the aircraft aligned
with a road, it may be noted that the aircraft gets closer to or
farther from the road without any turn having been initiated
by the pilot. This would indicate that the air mass is moving
sideward in relation to the aircraft. Since the aircraft is fl ying
9-3
Wind Wind
Current Current
Figure 9-1. Wind drift and wind correction angle (crab angle).
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.
When fl ying straight and level and following a selected ground
track, the preferred method of correcting for wind drift is to
head the aircraft (wind correction angle) suffi ciently into the
wind to cause the aircraft 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. This wind correction angle is also
commonly known as the crab angle. When the drift has been
neutralized, the aircraft follows the desired ground track.
To understand the need for drift correction during fl ight,
consider a fl ight with a wind velocity of 20 knots from the
left and 90° to the direction the aircraft is headed. After
1 hour, the body of air in which the aircraft is fl ying has
moved 20 nautical miles (NM) to the right. Since the aircraft
is moving with this body of air, it too has drifted 20 NM to
the right. In relation to the air, the aircraft moved forward;
but in relation to the ground, it moved forward as well as 20
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
is acting on the aircraft from constantly changing angles. The
relative wind angle and speed govern the time it takes for the
aircraft to progress through any part of a turn. This is due to
the constantly changing groundspeed. When the aircraft is
headed into the wind, the groundspeed is decreased; when
headed downwind, the groundspeed is increased. Through
the crosswind portion of a turn, the aircraft must be turned
suffi ciently 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 may be established. It can be seen then that
the steepest bank and fastest rate of turn should be made on
the downwind portion of the turn and 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 fl ight.
When there is no wind, it should be simple to fl y along a
ground track with an arc of exactly 180° and a constant
radius because the fl ightpath 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 wings become level as the aircraft again reaches
the road at a 90° angle and is directly over the road just as the
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Intended ground path
Actual ground path
No wind
20 knot wind
Figure 9-2. Effect of wind during a turn.
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 the difference between the desired ground track and
the fl ightpath. To counteract this drift, the fl ightpath 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
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Weight-Shift Control Aircraft Flying Handbook(103)
