# Maneuvering Flight
During maneuvering flight, a considerable amount of information is processed. Aircrews must be familiar with power margins, possible maneuvers to be performed, and escape plans. Mission rehearsals and continuous crew coordination allows the aircrew to anticipate maneuvers and decrease reaction time for unplanned events.
## Aerodynamics
Combat maneuvers require awareness of several key characteristics to be executed successfully.
### Best Rate-of-Climb/Maximum Endurance Airspeed
This airspeed offers the following benefits:
* Minimum total drag.
* Maximum excess power available.
* Lowest fuel flow during powered flight.
* Maximum single-engine gross weight that can be carried (for dual-engine aircraft). Aviators should always be mindful of their best rate-of-climb airspeed as it optimizes aircraft turning and climbing performance, maximizes available power margin, and minimizes fuel consumption.
### Bucket Speed
Bucket speed refers to the airspeed range that provides the best power margin for maneuvering flight. To determine the bucket speed:
* Use the cruise chart for current conditions.
* Enter at 50 percent of maximum torque available.
* Proceed to gross weight.
* Identify the lowest and highest airspeed intersecting the aircraft's gross weight.
* Note the speeds between which there is the greatest power margin for maneuvering flight. Lower speeds are more critical, as higher speeds may require trading airspeed energy to maintain altitude during maneuvering. When flying below the minimum bucket speed, reduce the bank angle to avoid potential altitude loss.
### Transient Torque
Transient torque is a phenomenon observed in single-rotor helicopters when lateral cyclic input is applied, caused by aerodynamic forces. In conventional American helicopters where the main rotor turns counterclockwise, left cyclic input results in a temporary rise in torque, while right cyclic input causes a temporary drop in torque.
At the rear half of the rotor disk, downwash is greater than seen at the forward half of the rotor disk. This effect is more pronounced for heavier aircraft which exhibit greater coning due to their weight, causing even greater downwash at the rear of the rotor disk. If a left cyclic input is made by the pilot, the following events occur leading to a temporary increase in torque:
* The swashplate commands an increased blade AOA as each blade passes over the tail.
* The increase in blade AOA causes the rotor disk to tilt left, which is felt as a left roll on the aircraft. With increased lift on the rotor blades passing over the tail, there is also increased drag (induced drag).
* The increased rotor drag due to the left turn will initially try to slow the rotor, but is sensed by the applicable engine computer. The engine responds by delivering more torque to the rotor system to maintain rotor speed.
The opposite holds true for right cyclic turns, but is less pronounced. Unlike the left hand turn, in right turns blade pitch is being changed at the front of the rotor disk where induced downwash is lower, so the drag penalty is lower. Transient torque is not as prevalent at slower airspeeds because the induced downwash distribution is nearly uniform across the rotor disk.
Five factors affect how much torque change occurs during transient torque:
* Torque transients are proportional with the amount of power applied. The higher the torque setting when lateral cyclic inputs are made, the higher or lower the transient.
* Rate of movement of the cyclic. The faster the rate of movement the higher resultant torque spike.
* Magnitude of cyclic displacement directly affects the torque transient. An example of worst-case scenario occurs when a pilot initiates a rapid right roll, then due to an unexpected event breaks left. The transition from right cyclic applied to left cyclic applied results in a large amount of pitch change in the advancing blade, resulting in large torque transients.
* Drag is increased or decreased by the factor of velocity squared. Thus, the higher the forward airspeed, the higher the torque transient results.
* High aircraft weight increases coning, which makes transient torque more pronounced.
Extreme caution must be used when maneuvering at near maximum torque available especially at high airspeeds. It is not uncommon to experience as much as 50 percent torque changes in uncompensated maneuvers with high power settings at high forward airspeeds. In these situations, the pilot must ensure collective is reduced as left lateral cyclic is applied and increased for right cyclic inputs. When recovering from these inputs, opposite collective inputs must be made so aircraft limitations are not exceeded.
As a good basic technique, imagine a piece of string tied between the cyclic and collective (right cyclic-collective increase/left cyclic-collective decrease). Also, inputs must be made to keep the aircraft from descending due to torque reductions (when recovering from left cyclic inputs with collective reduced).
{% hint style="info" %}
701C/D/DD/E equipped helicopters employ maximum torque rate attenuator which attempts to prevent transient torque related over-torques but may produce a rotor droop and loss of roll rate. Once the pilot has gained confidence in the ability of the maximum torque rate attenuator to prevent over-torques resulting from transient torque, aviators can aggressively maneuver the aircraft without closely monitoring engine torque.
{% endhint %}
### **Mushing**
Mushing is a temporary stall condition occurring in helicopters when rapid aft cyclic is applied at high forward airspeeds. Normally associated with dive recoveries, which result in a significant loss of altitude, this phenomenon can also occur in a steep turn resulting in an increased turn radius. Mushing results during high G-maneuvers when at high forward airspeeds aft cyclic is abruptly applied. This results in a change in the airflow pattern on the rotor exacerbated by total lift area reduction as a result of rotor disc coning. Instead of an induced flow down through the rotor system, an upflow is introduced which results in a stall condition on portions of the entire rotor system. While this is a temporary condition (because in due time the upflow will dissipate and the stall will abate), the situation may become critical during low altitude recoveries or when maneuvering engagements require precise, tight turning radii. High aircraft gross weight and high density altitude are conditions conducive to and can aggravate mushing.
Mushing can be recognized by the aircraft failing to respond immediately but continuing on the same flight path as before the application of aft cyclic. Slight feedback and mushiness may be felt in the controls. When mushing occurs, the tendency is to pull more aft cyclic which prolongs stall and increases recovery times. Make a forward cyclic adjustment to recover from the mushing condition. This reduces the induced flow, improves the resultant AOA, and reduces rotor disc coning which increases the total lift area of the disc. The pilot will immediately feel a change in direction of the aircraft and increased forward momentum as the cyclic is moved forward. To avoid mushing, the pilot must use smooth and progressive application of the aft cyclic during high G-maneuvers such as dive recoveries and tight turns.
### **Conservation of Angular Momentum**
The law of conservation of angular momentum states the value of angular momentum of a rotating body will not change unless external torques are applied. In other words, a rotating body continues to rotate with the same rotational velocity until some external force is applied to change the speed of rotation. Angular momentum can be expressed as follows:
***
Law of Conservation of Angular Momentum: Mass x Angular Velocity x Radius Squared
***
Changes in angular velocity, known as angular acceleration or deceleration, take place if the mass of a rotating body is moved closer to or further from the axis of rotation. The speed of the rotating mass increases or decreases in proportion to the square of the radius.
An excellent example for this principle is when watching a figure skater on ice skates. The skater begins a rotation on one foot, with the other leg and both arms extended. The rotation of the skater’s body is relatively slow. When a skater draws both arms and one leg inward, the moment of inertia (mass times radius squared) becomes much smaller and the body is rotating almost faster than the eye can follow. Because the angular momentum must, by law of nature, remain the same (no external force applied), the angular velocity must increase.
The mathematician, Coriolis, was concerned with forces generated by such radial movements of mass on a rotating disc or plane. These forces cause acceleration and deceleration. It may be stated as a mass moving radically—
* Outward on a rotating disk exert a force on its surroundings opposite to rotation.
* Inward on a rotating disk exert a force on its surroundings in the direction of rotation.
The major rotating elements in the system are the rotor blades. As the rotor begins to cone due to G-loading maneuvers, the diameter of the disc shrinks. Due to conservation of angular momentum, the blades continue to travel the same speed even though the blade tips have a shorter distance to travel due to reduced disc diameter. This action results in an increase in rotor RPMs. Most pilots arrest this increase with an increase in collective pitch.
Conversely, as G-loading subsides and the rotor disc flattens out from the loss of G-load induced coning, the blade tips now have a longer distance to travel at the same tip speed. This action results in a reduction of rotor RPMs. However, if this droop in rotor continues to the point it attempts to decrease below normal operating RPM, the engine control system adds more fuel/power to maintain the specified engine RPM. If the pilot does not reduce collective pitch as the disc unloads, the combination of the engines compensating for the RPM slowdown and the additional pitch added as G-loading increased may result in exceeding the torque limitations or power the engines can produce. This problem is exacerbated by effects of the TAF encountered during maneuvering flight.
### **High Bank Angle Turns**
As the angle of bank increases, the amount of lift opposite the vertical weight decreases (figure 1-71). If adequate excess engine power is available, increasing collective pitch enables continued flight while maintaining airspeed and altitude. If sufficient excess power is not available, the result is altitude loss unless airspeed is traded (aft cyclic) to maintain altitude or altitude is traded to maintain airspeed.
At some point (airspeed/angle of bank) sufficient excess power is not available and the aviator must apply aft cyclic to maintain altitude (table 1-3). The percentages shown are not a direct torque percentage, but percentage of torque increase required based on aircraft torque to maintain straight and level flight. If indicated cruise torque is 48 percent and a turn to 60 degrees is initiated, a torque increase of 48 percent (96 percent torque indicated) is required to maintain airspeed and altitude.
**Table 1-3. Bank angle versus torque**
| Bank Angle (Degree) | Increase in TR (Percent) |
| ------------------- | ------------------------ |
| 0 | --- |
| 15 | 3.6 |
| 30 | 15.4 |
| 45 | 41.4 |
| 60 | 100.0 |
*TR=torque*
Additionally, rotor system capability may limit the maneuver as opposed to insufficient excess power. In high energy maneuvering, the rotor is normally a limiting factor. It is not unusual for a reduction in collective to be required to achieve maximum performance when maneuvering at increased G-loads, altitudes, or high weights.
Aviators must be familiar with this characteristic, anticipate cyclic input results, and apply appropriate control inputs to conduct combat maneuvers successfully. Aviators unfamiliar with this characteristic may be surprised at the rapid build of sink rates when turning the aircraft to bank angles approaching 60 degrees. When flying heavy aircraft in a high hot environment, sufficient time and altitude may not be available to arrest the resultant descent.
### **Maneuvering Flight and Total Aerodynamic Force**
The cyclic inputs and associated rotor disc pitch changes required to accomplish successful combat maneuvers have a substantial effect on total aerodynamic force (TAF). Large aft cyclic inputs increase inflow through the rotor system. Since lift is perpendicular to resultant relative wind, the TAF of each rotor blade may move to a point aligned with or forward of the axis of rotation (much like the driving and driven region of a blade during autorotational flight). While the engine control system reduces fuel flow to reduced load, the rotor system may still climb to transient ranges or attempt to overspeed.
Conversely, when the cyclic is rapidly repositioned to a more forward position, inflow through the rotor is rapidly reduced resulting in the blade TAF moving aft of the axis of rotation and a slowing of rotor RPM (see Figure 1-72). The engine control systems sense this and increase fuel flow to the engines to maintain rotor RPM causing torque to increase. As a general rule, when traveling at airspeeds above bucket speed, aft cyclic results in a reduction in torque and an increase in rotor RPMs. Recovery from an aft cyclic input (pushover or high G-turn recovery) results in torque increase as the engines compensate for the rotor system slow down. In aggressive maneuvers, this may result in an overtorque or overspeed if appropriate collective input is not made to keep torque and rotor consistent.
This phenomenon is exacerbated by high gross weight and also affected by ambient temperature and density altitude. Typically, cold dry air results in more rapid rotor RPM increase during aft cyclic input and a corresponding higher torque increase with a forward cyclic input. Hot temperatures and higher density altitudes result in more collective input required to arrest a climbing rotor.
### **Angular Momentum and Total Aerodynamic Force Combined Effects**
Angular momentum and total aerodynamic force combine during cyclic pitch changes. During aft cyclic or G-loading, the rotor increases, and torque goes down. During G-load recovery, torque increases as the engine control systems work to maintain a rotor RPM attempting to decrease. Aviators must be able to apply appropriate and timely collective inputs to maintain consistent torque and keep rotor RPM within limits.
### **Dig-In**
Dig-in is applying aft cyclic to such a point that causes the main rotor disk thrust vector to almost parallel the flight path of the helicopter. While making large aft cyclic movements, the pilot must be aware of the helicopter’s tendency to rapidly and unpredictably build G-forces. As the cyclic is moved aft, the rotor disk responds by tilting aft, which tilts the thrust vector aft and ultimately causes the aircraft to pitch nose-up. This rapid pitch-up also increases the length of the aircraft thrust vector, which increases the pitch-up rate. The rapid onset of the pitch-up motion due to this tilting and then lengthening of the thrust vector is considered destabilizing and countered by the helicopter’s horizontal tail or stabilizer, which tries to drive the nose back down. For large pitch-up rates, the tendency of the main rotor to continue pitching up overpowers the horizontal tail/stabilizer, and the aircraft digs in and slows down rapidly. Dig-in could be accompanied by airframe vibration and sometimes control feedback. Vibration and control feedback are normally canceled out by modern aircraft systems.
Aft cyclic movements give predictable increases in G-load up to the dig-in point; however, the dig-in occurs at different G-levels for each model of helicopter. The point at which dig-in occurs depends on several factors, but most important is the size of the horizontal tail/stabilizer and the amount of rotor offset. For most helicopters, this point is between 1.5 and 2.0 Gs. Pilots should be prepared for dig-in during aggressive aft cyclic inputs, especially during break turns.
***
## **Guidelines**
Below are good practices to follow during maneuvering flight:
* Never move the cyclic faster than trim, torque, and rotor can be maintained. When entering a maneuver and the trim, rotor, or torque reacts quicker than anticipated, pilot limitations have been exceeded. If continued, the aircraft limitation is exceeded. The maneuver is performed with less intensity until all aspects of the machine can be controlled.
* Changes are anticipated in aircraft performance due to loading or environmental condition. The normal collective increase to check rotor speed at sea level standard may not be sufficient at 4,000 feet pressure altitude (PA) and 95 degrees F (4K95).
* The following characteristics are anticipated during aggressive maneuvering flight and adjust or lead with collective as necessary to maintain trim and torque:
* Left turns, torque increases.
* Right turns, torque decreases.
* Application of aft cyclic, torque decreases and rotor climbs.
* Application of forward cyclic (especially when immediately following aft cyclic application), torque increases and rotor speed decreases.
* Always leave a way out.
* Know where the winds are.
* Most engine malfunctions occur during power changes.
* If combat maneuvers have not been performed in a while, start slowly to develop proficiency.
* Crew coordination is critical. Everyone needs to be fully aware of what is going on and each crewmember has a specific duty.
* In steep turns, the nose drops. In most cases, energy (airspeed) must be traded to maintain altitude as the required excess engine power may not be available (to maintain airspeed in a 2G/60 degree turn rotor thrust/engine power has to increase by 100 percent). Failure to anticipate this at low altitude endangers the crew and passengers. The rate of pitch change is proportional to gross weight and density altitude.
* Many maneuvering flight over-torques occur as the aircraft unloads Gs. This is due to insufficient collective reduction following the increase to maintain consistent torque and rotor as G-loading increased (dive recovery or recovery from high G-turn to the right).
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