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downwind unless corrected.
With proper correction, airplane
stays on intended course.
WIND WIND
Figure 6-1. Wind drift.
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6-3
govern the time it takes for the airplane to progress
through any part of a turn. This is due to the constantly
changing groundspeed. When the airplane is headed
into the wind, the groundspeed is decreased; when
headed downwind, the groundspeed is increased.
Through the crosswind portion of a turn, the airplane
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 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 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 6-2]
To complete the turn, the rollout should be started at a
point where the wings will become level as the airplane
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 6-3] The airplane 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 airplane
back to the starting position.
20 Knot Wind
Intended ground path
Actual ground path
No Wind
Figure 6-2. Effect of wind during a turn.
Figure 6-3. Effect of wind during turns.
No Wind
Start & Finish
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A similar 360° turn may be started at a specific point
over the reference line, with the airplane headed
directly downwind. In this demonstration, the effect of
wind during the constant banked turn will drift the airplane
to a point where the line is reintercepted, 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 airplane
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, it can be seen 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
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AIRPLANE FLYING HANDBOOK 飞机飞行手册上(64)
