## Monday, February 23, 2009

### 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 move in whatever
direction and speed that the medium moves.

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.

As soon as an airplane becomes airborne, it is free of
ground friction. Its path is then affected by the air mass
in which it is flying; therefore, the airplane (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 airplane aligned with a road, it
may be noted that the airplane gets closer to or farther

would indicate that the air mass is moving sideward in
relation to the airplane. Since the airplane 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.

When flying straight and level and following a
selected ground track, the preferred method of correcting for wind drift is to head the airplane (wind
correction angle) sufficiently into the wind to cause
the airplane 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
airplane 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 airplane
is headed. After 1 hour, the body of air in which the
airplane is flying will have moved 30 nautical miles
(NM) to the right. Since the airplane is moving with
this body of air, it too will have drifted 30 NM to the
right. In relation to the air, the airplane 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.
Throughout the turn the
wind will be acting on the airplane from constantly
changing angles. The relative wind angle and speed

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.

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.
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.

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
the ground. The principles and techniques involved can
be practiced and evaluated by the performance of the
ground track maneuvers discussed in this chapter.