Chapter 11. HELICOPTER FLIGHT MANEUVERS (continued)

AUTOROTATIONS

In helicopter flying, an autorotation is a maneuver that can be performed by the pilot whenever the engine is no longer supplying power to the main rotor blades. A helicopter transmission is designed to allow the main rotor to rotate freely in its original direction if the engine stops. At the instant of engine failure, the blades will be producing lift and thrust from their angle of attack and velocity. By immediately lowering collective pitch (which must be done in case of engine failure), lift and drag will be reduced, and the helicopter will begin an immediate descent, thus producing an upward flow of air through the rotor system. The impact of this upward flow of air on the rotor blades produces a "ram" effect which provides sufficient thrust to maintain rotor RPM throughout the descent. Since the tall rotor is driven by the main rotor during autorotation, heading control can be maintained as in normal flight.

Several factors affect the rate of descent in autorotation - air density (density altitude), gross weight, rotor RPM, and airspeed. The pilot's primary control of the rate of descent is the airspeed. Higher or lower airspeed is obtained with the cyclic control just as in normal flight. A pilot has a choice in angle of descent varying from vertical descent to maximum range (minimum angle of descent). Rate of descent is high at zero airspeed and decreases to a minimum somewhere in the neighborhood of 50 to 60 miles per hour, depending upon the particular helicopter and the factors just mentioned. As the airspeed increases beyond that which gives minimum rate of descent, the rate of descent increases again. When an autorotative landing is to be made, the energy stored in the rotating blades can be used by the pilot to decrease the rate of descent and make a safe landing. A greater amount of rotor energy is required to stop a helicopter with a high rate of descent than is required to stop a helicopter that is descending more slowly. It follows then that autorotative descents at very low or very high airspeeds are more critical than those performed at the proper airspeed for the minimum rate of descent.

Each type of helicopter has a specific airspeed at which a power-off glide is most efficient. The best airspeed is the one which combines the most desirable (greatest) glide range with the most desirable (slowest) rate of descent. The specific airspeed is somewhat different for each type of helicopter, yet certain factors affect all configurations in the same manner. For specific autorotation airspeeds for a particular helicopter refer to the helicopter flight manual.

The specific airspeed for autorotations is established for each type of helicopter on the basis of average weather and wind conditions, and normal loading. When the helicopter is operated with excessive loads in high density altitude or strong gusty wind conditions, best performance is achieved from a slightly increased airspeed in the descent. For autorotations in light winds, low density altitude, and light loading, best performance is achieved from a slight decrease in normal airspeed. Following this general procedure of fitting airspeed to existing conditions, a pilot can achieve approximately the same glide angle in any set of circumstances and estimate the touchdown point.

When making turns during an autorotative descent, generally use cyclic control only. Use of antitorque pedals to assist or speed the turn causes loss of airspeed and downward pitching of the nose - especially when left pedal is used. When the autorotation is initiated, sufficient right pedal pressure should be used to maintain straight flight and prevent yawing to the left. This pressure should not be changed to assist the turn.

If rotor RPM becomes too high during an autorotative approach, collective pitch should be raised sufficiently to decrease RPM to the normal operating range, then lowered all the way again. This procedure may be repeated as necessary.

If the throttle has not been closed in practice autorotations, it will be necessary to reduce it as collective pitch is raised; otherwise, the engine and rotor will reengage. After the collective is lowered, the throttle should be readjusted to maintain a safe engine RPM.

RPM is most likely to increase above the maximum limit during a turn due to the increased back cyclic stick pressure which induces a greater airflow through the rotor system. The tighter the turn and the heavier the gross weight, the higher the RPM will be.

HOVERING AUTOROTATION

As the name implies, hovering autorotations are made from a hover. They are practiced so that a pilot will automatically make the correct response when confronted with engine stoppage or certain other inflight emergencies while hovering.

TECHNIQUE:

1. To practice hovering autorotations, establish a normal hovering altitude for the particular helicopter being used, considering its load and the atmospheric conditions, and keep the helicopter headed into the wind. Hold maximum allowable RPM.
2. To enter autorotation, close the throttle quickly to ensure a clean split of the needles. This disengages the driving force of the engine from the rotor, thus eliminating torque effect. As the throttle is closed, right pedal must be applied to maintain heading. Usually, a slight amount of right cyclic stick will be necessary to keep the helicopter from drifting to the left, but use cyclic control as required to ensure a vertical descent and a level attitude. Leave the collective pitch where it is on entry.
3. In helicopters with low inertia rotor systems, the aircraft will begin to settle immediately. Keep a level attitude and ensure a vertical descent with cyclic control, heading with pedals, and apply upward collective pitch as necessary (generally the full amount is required) to slow the descent and cushion the landing. As upward collective pitch is applied, the throttle will have to be held in the closed position to prevent the rotor from re-engaging.
4. In helicopters with high inertia rotor systems, the aircraft will maintain altitude momentarily after the throttle is closed. Then, as the rotor RPM decreases, the helicopter will start to settle. As it settles, apply upward collective pitch (while holding the throttle in the closed position) to slow the descent and cushion the landing. The timing of this collective pitch ap- plication, and the rate at which it should be applied, will depend upon the particular helicopter being used, its gross weight, and the existing atmospheric conditions. Cyclic control is used to maintain a level attitude and to ensure a vertical descent. Heading is maintained with pedals.
5. When the weight of the helicopter is entirely on the skids, the application of upward collective pitch should cease. When the helicopter has come to a complete stop, lower the collective pitch to the full down position.
6. The timing of the collective pitch is a most important consideration. If it is applied too soon, the remaining RPM may not be sufficient to make a smooth landing. On the other hand, if collective pitch is applied too late, surface contact may be made before sufficient blade pitch is available to cushion the landing.
7. When entering the autorotation, the throttle should be rotated to the closed or override position to prevent the engine from re-engaging during the collective pitch application.

COMMON ERRORS:

1. Failing to use sufficient right pedal when power is reduced.
2. Failing to stop all sideward or backward movement prior to touchdown.
3. Failing to apply up-collective pitch properly, resulting in a hard touchdown.
4. Failing to touch down in a level attitude.

NO-FLARE AUTOROTATION

A no-flare autorotation can be used when the selected landing area is sufficiently long and smooth to permit a surface run. Practice no-flare autorotations may be made as follows.

TECHNIQUE:

1. When the desired position to begin the autorotation has been reached, place the collective pitch stick in the full-down position, maintaining cruising RPM with throttle. Decrease throttle to ensure a clean split of the needles and apply sufficient right pedal to maintain the desired heading. After splitting the needles, readjust the throttle so as to keep engine RPM well above normal idling speed but not high enough to cause rejoining of the needles. (The manufacturer will often recommend the RPM to use.)
2. Adjust attitude with cyclic control to obtain the best gliding speed (slowest rate of descent). Be sure to hold collective pitch in the full-down position. If it is permitted to rise, rotor RPM will decrease due to the increased drag from the increased pitch angle of the blades. At approximately 50 feet above the surface (check the manufacturer's recommendation for each helicopter), raise the nose slightly to obtain the desired landing speed and to slow the rate of descent.
3. If a landing is to be made from the autorotative approach, the throttle should be rotated to the closed or override position at this time and held in this position as collective pitch is raised so that the rotor will not re-engage. As the helicopter approaches normal hovering altitude, maintain a landing attitude with cyclic control, maintain heading with pedals, apply sufficient collective pitch (while holding the throttle in the closed position) to cushion the touchdown, and be sure the helicopter is landing parallel to its direction of motion upon contact with the surface. Avoid landing on the heels of the skid gear. The timing of the collective pitch application and the amount applied will be dependent on the rate of descent.
4. After surface contact is made, collective pitch may be increased (while holding the throttle in the closed position) to keep the helicopter light on the skids and allow it to slow down gradually; or it may be held stationary resulting in a shorter ground run; or it may be lowered cautiously for additional braking action, if required, due to a fast touchdown and limited landing area. Cyclic control stick should be held slightly forward of neutral and used to maintain groundtrack if landing is made in a crosswind. Pedals should be used to maintain heading. In the event insufficient pedal is available to maintain heading control as the rotor RPM decreases after touchdown, cyclic control should be applied in the direction of the turn.
5. After the helicopter has stopped, lower the collective pitch to the full-down position.
6. If a power recovery is to be made from practice autorotative approaches, the procedures in (3), (4), and (5) should be replaced with the procedures given under "Power Recovery From Practice Autorotations."

COMMON ERRORS:

1. Failing to use sufficient right pedal when power is reduced.
2. Lowering the nose too abruptly when power is reduced, thus placing the helicopter in a dive.
3. Failing to maintain full-down-collective pitch during the descent.
4. Application of up-collective pitch at an excessive altitude resulting in a hard landing, loss of heading control, and possible damage to the tail rotor and to the main rotor blade stops.
5. Pulling the nose up just prior to touchdown.

FLARE AUTOROTATION

A flare autorotation (see figure 77 to the right) enables the pilot to land a helicopter at any speed between that resulting in little or no landing run, up to that of a no-flare autorotation; that is, anywhere between a zero groundspeed and the speed of touchdown resulting from a no-flare autorotation. The speed at touchdown and the resulting ground run will depend on the rate and amount of the flare - the greater the degree of flare and the longer it is held, the slower the touchdown speed and the shorter the ground run. The slower the speed desired at touchdown, the more accurate must be the timing and speed of the flare, especially in helicopters with low inertia rotor systems.

TECHNIQUE:

1. Enter the autorotation in the same manner as the no-flare autorotation. The technique is the same to the point where the flare is to begin. This point is slightly lower than the point at which the nose is raised in the no-flare autorotation.
2. At approximately 35 to 60 feet above the surface, depending on the helicopter (check the manufacturer's recommendation), initiate the flare by moving the cyclic stick to the rear. Heading is maintained by the pedals. Care must be taken in the execution of the flare so that the cyclic control is not moved rearward so abruptly as to cause the helicopter to climb, nor should it be moved so slowly as to allow the helicopter to settle so rapidly that the tail rotor might strike the surface. As forward motion decreases to the desired groundspeed, move the cyclic control forward to place the helicopter in preparation for landing attitude. (If a landing is to be made, the throttle should be rotated to the closed or override position at this time; if a power recovery is to be made, it should be made as the helicopter reaches the landing attitude.)
3. The altitude at this time should be approximately 3 to 10 feet, depending upon the helicopter being used. If a landing is to be made, allow the helicopter to descend vertically. Apply collective pitch, as necessary, to check the descent and cushion the landing. As collective pitch is increased, hold the throttle in the closed position so the rotor will not re-engage. Additional right pedal is required to maintain heading as collective pitch is raised due to the reduction in rotor RPM and the resulting reduced effect of the tail rotor.
4. After touchdown and after the helicopter has come to a complete stop, lower the collective pitch to the full-down position.

COMMON ERRORS:

1. Failing to use sufficient right pedal when power is reduced.
2. Lowering the nose too abruptly when power is reduced, thus placing the helicopter in a dive.
3. Failing to maintain desired rotor RPM.
4. Application of up-collective pitch at an excessive altitude resulting in a hard landing, loss of heading control, and possible damage to the tail rotor and to the main rotor blade stops.
5. Pulling the nose up just prior to touchdown on full autorotation.

POWER RECOVERY FROM PRACTICE AUTOROTATIONS

A power recovery is used to terminate practice autorotations at a point prior to actual touchdown. After the power recovery, a landing can be made or a go-around initiated.

TECHNIQUE:

1. To make a power recovery after the flare or level-off from an autorotation, coordinate upward collective pitch and increase throttle to join the needles at operating RPM. The throttle and collective pitch must be coordinated properly. If the throttle is increased too fast or too much, an engine overspeed will occur; if throttle is increased too slowly or too little in proportion to the increase in collective pitch, a loss of rotor RPM will result. Use sufficient collective pitch to check the descent and coordinate left pedal pressure with the increase in collective pitch to maintain heading.
2. If a go-around is to be made, the cyclic control should be moved forward to re-enter forward flight. If a landing is to be made following the power recovery, the helicopter can be brought to a hover at normal hovering altitude.
3. In transitioning from a practice autorotation to a go-around, care must be exercised to avoid an altitude-airspeed combination which would place the helicopter in an unsafe area of the height-velocity chart for that particular helicopter.

COMMON ERRORS:

1. Initiating recovery too late requiring a rapid application of controls, resulting in overcontrolling.
2. Failing to obtain and maintain a level attitude near the surface.
3. Adding throttle before collective pitch is applied.
4. Failing to coordinate throttle and collective pitch properly, resulting in either an engine overspeed or a loss of RPM.
5. Failing to coordinate left pedal with the increase in power.

RAPID DECELERATION OR QUICK STOP

Although used primarily for coordination practice, decelerations (see figure 78 to the right) can be used to make a quick stop in the air. The purpose of the maneuver is to maintain a constant altitude, heading, and RPM while slowing the helicopter to a desired groundspeed. The maneuver requires a high degree of coordination of all controls. It is practiced at an altitude that will permit a safe clearance between the tail rotor and the surface throughout the maneuver, especially at the point where the pitch attitude is highest. The altitude at completion should be no higher than the maximum safe hovering altitude prescribed by the manufacturer. In selecting an altitude at which to begin the maneuver, the overall length of the helicopter and the height-velocity chart must be considered.

Although the maneuver is called a rapid deceleration or quick stop, this does not mean that it should be rushed to completion. The rate of deceleration is at the discretion of the pilot. A quick stop is completed when the helicopter comes to a hover during the recovery.

TECHNIQUE:

1. Begin the maneuver at a fast hover speed headed into the wind at an altitude high enough to avoid danger to the tail rotor during the flare but low enough to stay out of the height-velocity chart shaded area throughout the performance. This altitude should be low enough that the helicopter can be brought to a hover during the recovery.
2. The deceleration is initiated by applying aft cyclic to reduce forward speed. Simultaneously, the collective pitch should be lowered as necessary to counteract any climbing tendency. The timing must be exact. If too little down-collective is applied for the amount of aft cyclic applied, a climb will result. If too much down-collective is applied for the amount of aft cyclic applied, a descent will result. A rapid application of aft cyclic requires an equally rapid application of down-collective. As collective pitch is lowered, right pedal should be increased to maintain heading and throttle should be adjusted to maintain RPM.
3. After speed has been reduced to the desired amount, recovery is initiated by lowering the nose and allowing the helicopter to descend to a normal hovering altitude in level flight and zero groundspeed. During the recovery, collective pitch should be increased as necessary to stop the helicopter at normal hovering altitude; throttle should be adjusted to maintain RPM; and left pedal should be applied as necessary to maintain heading.

COMMON ERRORS:

1. Initiating the maneuver by applying down-collective.
2. Applying aft cyclic stick too rapidly initially, causing the helicopter to "balloon" (a sudden gain in altitude).
3. Failing to effectively control the rate of deceleration to accomplish the desired results.
4. Allowing the helicopter to stop forward motion in a tail-low attitude.
5. Failing to maintain proper RPM.

SLOPE OPERATIONS

The approach to a slope is similar to the approach to any other landing area. During slope operations, allowance must be made for wind, barriers, and forced landing sites in case of engine failure. Since the slope may constitute an obstruction to wind passage, turbulence and downdrafts must be anticipated.

Slope landing

Usually, a helicopter should be landed on a cross-slope rather than on an upslope. Landing downslope or downhill is not recommended because of the possibility of striking the tail rotor on the surface.

TECHNIQUE:

1. At the termination of the approach, move slowly toward the slope, being careful not to turn the tail upslope. The helicopter should be hovered in position cross-slope over the spot of intended landing (fig. 79).
2. A downward pressure on the collective pitch will start the helicopter descending. As the upslope skid touches the ground, apply cyclic stick in the direction of the slope. This will hold the skid against the slope while the downslope skid is continuing to be let down with the collective pitch.
3. As collective pitch is lowered, continue to move the cyclic stick toward the slope to maintain a fixed position, and use cyclic as necessary to stop forward or aft movement of the helicopter. The slope must be shallow enough to allow the pilot to hold the helicopter against it with the cyclic stick during the entire landing. (A slope of 5° is considered maximum for normal operation of most helicopters. Each make of helicopter will generally have its own peculiar way of indicating to the pilot when lateral cyclic stick travel is about to run out; i.e., the rotor hub hitting the rotor mast, vibrations felt through the cyclic stick, and others. A landing should not be made in these instances since this indicates to the pilot that the slope is too steep.)
4. After the downslope skid is on the surface, continue to lower the collective pitch full down. Normal operating RPM should be maintained until the full weight of the helicopter is on the skids. This will assure adequate RPM for immediate takeoff in case the helicopter should start to slide down the slope. Pedals should be used as necessary throughout the landing to maintain heading. Before reducing the RPM, move the cyclic stick as necessary to check the security of the helicopter.

COMMON ERRORS:

1. Failure to maintain proper RPM throughout the entire maneuver.
2. Lowering the downslope skid too rapidly.
3. Applying excessive cyclic control into the slope, causing mast bumping.

SLOPE TAKEOFF

The procedure for a slope takeoff is almost the exact reverse of that for a slope landing (see figure 79 to the right).

TECHNIQUE:

1. Adjust throttle to obtain takeoff RPM and move the cyclic stick in the direction of the slope so that the rotor rotation is parallel to the true horizontal rather than the slope.
2. Apply up-collective pitch. As the helicopter becomes light on the skids, apply pedal as needed to maintain heading.
3. As the downslope skid is rising and the helicopter approaches a level attitude, move the cyclic stick back to the neutral position, keeping the rotor disc parallel to the true horizon. Continue to apply up-collective pitch and take the helicopter straight up to a hover before moving away from the slope. In moving away from the slope, the tail should not be turned upslope because of the danger of the tail rotor striking the surface.

COMMON ERRORS:

1. Failure to adjust cyclic stick to keep the helicopter from sliding downslope.
2. Failure to maintain proper RPM.
3. Holding excessive "upslope" cyclic stick as the downslope skid is raised.

RECOVERY FROM LOW ROTOR RPM

Recovery from low rotor RPM is a procedure used to return to the normal rotor operating RPM. This recovery procedure, if performed properly, will normally regain lost rotor RPM while still maintaining flight. A low rotor RPM condition is the result of having an angle of attack on the main rotor blades (induced by too much upward collective pitch) that has created a drag so great that engine power available, or being utilized, is not sufficient to maintain normal rotor operating RPM.

When a low rotor RPM condition is realized, immediately lower the collective pitch. This action will decrease the angle of attack of the main rotor blades which, because of reduced rotor drag, will momentarily relieve excessive engine load. This action will also cause the helicopter to settle because some lift will be lost. As the helicopter begins to settle, smoothly raise the collective pitch just enough to stop the settling motion. This procedure, under critical conditions, may have to be repeated to regain normal rotor RPM or, if terrain permits, a landing may be necessary to restore the RPM. When operating at sufficient altitudes above the surface, however, it may be necessary to lower the collective pitch only once to regain sufficient rotor RPM. The amount that the collective pitch can be lowered will depend on the altitude available at the time the low rotor RPM condition occurs. When hovering near the surface, the collective pitch should be lowered cautiously to preclude hard contact with the surface. When the RPM begins to increase and attains approximately normal rotor operating RPM, anticipate decreasing the throttle slowly to prevent the engine from overspeeding.

If recovery from a low rotor RPM condition is not made soon enough, lifting power of the main rotor blades will be lost, including pedal effectiveness. Pedal ineffectiveness occurs as a result of the loss of tail rotor RPM because the tail rotor RPM is directly proportional to the main rotor RPM. If pedal effectiveness is lost, and the altitude is such that a landing can be made before the turning rate increases dangerously, decrease collective pitch slowly, maintain a level attitude with cyclic control, and land.

"S" TURNS

"S" turns present one of the most elementary problems in the practical application of the turn and in the correction for wind drift in turns. The reference line used, whether a road, railroad, or fence, 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 (see figure 80 to the right). The maneuver should be started at an altitude of 500 feet above the terrain and a constant altitude maintained throughout the maneuver. "S" turns may be started at any point. However, during early training, it may be beneficial to start on a downwind heading. Entering downwind permits the immediate selection of the steepest bank that is desired throughout the maneuver. The discussion that follows is based on choosing a reference line that is perpendicular to the wind and starting the maneuver on a downwind heading.

As the helicopter crosses the reference line, a bank is immediately established. This initial bank will be the steepest used throughout the maneuver since the helicopter is headed directly downwind. The bank should be gradually reduced as necessary to describe a ground track of a half circle. The turn should be timed so that, as the rollout is completed, the helicopter is crossing the reference line perpendicular to it and headed directly upwind. A bank is immediately entered in the opposite direction to begin the second half of the "S." Since the helicopter is on an upwind heading, this bank (and the one just completed before crossing the reference line) will be the shallowest in the maneuver. It should be gradually increased as necessary to describe a ground track which is a half circle identical in size to the one previously completed on the other side of the reference line. The steepest bank in this turn should be attained just prior to rollout (when the helicopter is approaching the reference line nearest to a downwind heading).

This bank, along with the initial bank entered at the beginning of the maneuver, will be the steepest bank used in "S" turns. The turn should be timed so that, as the rollout is completed, the helicopter is crossing the reference line perpendicular to it and again headed directly downwind.

As a summary, the angle of bank required at any given point in the maneuver is dependent on the groundspeed - the faster the groundspeed, the steeper the bank; the slower the groundspeed, the shallower the bank. Or, 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), the nose of the helicopter will be crabbed 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), the nose of the helicopter will be crabbed 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 upon the wind velocity and how nearly the helicopter is to a crosswind position. The stronger the wind, the greater the crab angle 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. This radius will depend on the airspeed of the helicopter, the velocity of the wind, and the initial bank chosen for entry.