Boxing the wake has no apparent practical use except as a training tool. It does allow the pilot to demonstrate the skill needed to maneuver the glider relative to the tow plane, and it is a required task on pilot practical tests. The wake created by the tow plane is caused primarily by its wing, although propwash contributes some to the wake. If it were visible it would appear much like horizontal tornadoes flowing back from the tow plane wing tips.

By the time the wake reaches the glider in tow, the disturbed air is somewhat below the tow plane's flight path and about a wide as the tow plane's wing span. Boxing the wake is simply maneuvering the glider in a vertical rectangular path around the disturbed air. The direction is optional, but the tow pilot will prefer that the box be made clockwise. That way the rising side of the box will occur on the left, where the increased load due to climbing will help offset the left turning tendency of the tow plane.

We begin the boxing maneuver with the glider in the high tow position in straight flight. The tow plane will appear on the horizon when viewed from the glider because they are at the same height. Both aircraft wings are level.

It is customary for the glider pilot to signal the tow pilot prior to boxing the wake by dropping down into low tow position by applying a slight increase in forward stick pressure. Then either box from there or return to the high tow position for boxing. Moving into low tow allows the glider pilot to review the visual clues that indicate that the glider is below the wake. For most tow planes the bottom side of the horizontal tail surfaces will be clearly visible and the bottom side of the wing will be barely visible.

Transition between high and low tow directly behind the tow plane exposes the glider to balanced vortices from both tow plane wing tips. Although not part of "boxing the wake", it is beneficial for students to explore the vortices separately by moving laterally while in the wake. Unless a student has some practice dealing with the rolling motion produced by a single vortex, inadvertent entry into one can be an unpleasant experience.

An observant pilot may notice that the wake appears to be higher, relative to the tow plane, when the glider is moving up compared with its position when descending through the wake. This phenomenon is the result of the difference in load on the tow plane imposed by the glider when it is ascending versus when it is descending. The climb path of the tow plane is shallower when the glider is climbing and steeper when it is descending.

Lateral movement on tow, necessary to box the wake, can be accomplished through use of either aileron or rudder or some combination of the two. It is probably easier to keep the glider wings level and move laterally with rudder alone. A more elegant method is to lower the wing slightly in the direction the pilot wants to move and keep the glider and tow plane longitudinal axes parallel with rudder. An additional benefit of this method is that it is consistent with other uses of aileron to effect lateral movement, both on tow and elsewhere, such as during a side slip. In our example here you will note that we use aileron for lateral movement and keep the glider longitudinal axis parallel to that of the tow plane.

It is again important to develop visual references to determine when the glider has moved out enough to clear the wake before it descends to the low tow position. A method that works well with most tail wheel tow planes is to observe the apparent motion of the tail wheel relative to the main gear of the tow plane. When the glider is directly behind the tow plane the tail wheel is centered between the main gear wheels. As the glider moves laterally, the tail wheel appears to move toward the opposite main gear wheel. When it has moved just outside that wheel the glider will be in position to clear the wake as it descends.

The load imposed on the tow plane by the glider is the sum of the conventional drag from pulling the glider through the air and the force necessary to pull the glider up a slope equal to its climb angle. When the glider climbs or descends relative to the tow plane it is modifying this slope and thus the load on the tow plane.
When the climb or descent is to one side, as when boxing the wake, the change in load is reflected in a change in lateral force due to the angle between the tow line and the glider longitudinal axis. If the pilot fails to make necessary adjustments with the aileron and rudder, the glider will move farther out when descending and move farther in during ascent. The result will be a trapezoidal path around the wake rather than the desired rectangular path.
This error is most likely to occur when moving rapidly up or down, so the pilot can minimize the risk by simply executing the box maneuver slowly. In all cases, the pilot should maintain the desired position by adjusting elevator for vertical position, aileron for lateral position and rudder for heading.

2000 Jim D. Burch

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