Powered Parachute Flying Handbook

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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