Another training maneuver to use is the S-turn, which helps correct for wind drift in turns. This maneuver requires turns to the left and right.
The pilot can choose to use a road, a fence, or a railroad for a reference line. Regardless of what is used, it should be straight for a considerable distance and should extend as nearly perpendicular to the wind as possible. The object of S-turns is to fly a pattern of two half circles of equal size on opposite sides of the reference line. [Figure 9-16] The maneuver should be performed at a constant altitude between 500 and 800 feet above the terrain. As mentioned previously, if the student pilot is having a difficult time maintaining the proper altitude and airspeed, have him or her attempt the S-turn at a lower altitude, providing better ground reference. The discussion that follows is based on choosing a reference line perpendicular to the wind and starting the maneuver with the helicopter facing downwind.
As the helicopter crosses the reference line, immediately establish a bank. This initial bank is the steepest used throughout the maneuver since the helicopter is headed directly downwind and the groundspeed is greatest (position 1). Gradually reduce the bank, as necessary, to describe a ground track of a half circle. Time the turn so that, as the rollout is completed, the helicopter is crossing the reference line perpendicular to it and heading directly upwind (position 2). Immediately enter a bank in the opposite direction to begin the second half of the “S” (position 3). Since the helicopter is now on an upwind heading, this bank (and the one just completed before crossing the reference line) is the shallowest in the maneuver. Gradually increase the bank, as necessary, to describe a ground track that is a half circle identical in size to the one previously completed on the other side of the reference line (position 4). The steepest bank in this turn should be attained just prior to rollout when the helicopter is approaching the reference line nearest the downwind heading. Time the turn so that as the rollout is complete, the helicopter is perpendicular to the reference line and is again heading directly downwind (position 5).
In summary, the angle of bank required at any given point in the maneuver is dependent on the groundspeed. The faster the groundspeed is, the steeper the bank is; the slower the groundspeed is, the shallower the bank is. To express it another way, the more nearly the helicopter is to a downwind heading, the steeper the bank; the more nearly it is to an upwind heading, the shallower the bank. In addition to varying the angle of bank to correct for drift in order to maintain the proper radius of turn, the helicopter must also be flown with a drift correction angle (crab) in relation to its ground track; except, of course, when it is on direct upwind or downwind headings or there is no wind.
One would normally think of the fore and aft axis of the helicopter as being tangent to the ground track pattern at each point. However, this is not the case. During the turn on the upwind side of the reference line (side from which the wind is blowing), crab the nose of the helicopter toward the outside of the circle. During the turn on the downwind side of the reference line (side of the reference line opposite to the direction from which the wind is blowing), crab the nose of the helicopter toward the inside of the circle. In either case, it is obvious that the helicopter is being crabbed into the wind just as it is when trying to maintain a straight ground track. The amount of crab depends on the wind velocity and how close the helicopter is to a crosswind position. The stronger the wind is, the greater the crab angle is at any given position for a turn of a given radius. The more nearly the helicopter is to a crosswind position, the greater the crab angle. The maximum crab angle should be at the point of each half circle farthest from the reference line.
A standard radius for S-turns cannot be specified, since the radius depends on the airspeed of the helicopter, the velocity of the wind, and the initial bank chosen for entry. The only standard is crossing the ground reference line straight and level, and having equal radius semi-circles on both sides.
- Using antitorque pedal pressures to assist turns.
- Slipping or skidding in the turn.
- An unsymmetrical ground track during S-turns across a road.
- Improper correction for wind drift.
- Failure to maintain selected altitude or airspeed.
- Excessive bank angles.