# Rotor in Translation Aircrews should strive to understand the relationship between their rotor systems and different stages of flight starting from a stagnant hover. Such knowledge becomes critical during limited power situations. Understanding the forces acting on the rotor system as it transitions from a hover allows the aviators to anticipate control inputs for precise aircraft handling. *** ## **Airflow in Forward Flight** Airflow across the rotor system in forward flight varies from airflow at a hover. In forward flight, air flows opposite the aircraft’s flight path. The velocity of this air flow equals the helicopter’s forward speed. Because the blades turn in a circular pattern, the velocity of airflow across a blade depends on the position of the blade in the plane of rotation at a given instant, its rotational velocity, and airspeed of the helicopter. Therefore, the airflow meeting each blade varies continuously as the blade rotates. The highest velocity of airflow occurs over the right side (3 o’clock position) of the helicopter (advancing blade in a rotor system that turns counterclockwise) and decreases to rotational velocity over the nose. It continues to decrease until the lowest velocity of airflow occurs over the left side (9-o’clock position) of the helicopter (retreating blade). As the blade continues to rotate, velocity of the airflow then increases to rotational velocity over the tail. It continues to increase until the blade is back at the 3 o’clock position. The advancing blade (figure 1-53, blade A) moves in the same direction as the helicopter. The velocity of the air meeting this blade equals rotational velocity of the blade plus wind velocity resulting from forward airspeed. The retreating blade (blade C) moves in a flow of air moving in the opposite direction of the helicopter. The velocity of airflow meeting this blade equals rotational velocity of the blade minus wind velocity resulting from forward airspeed. The blades (B and D) over the nose and tail move essentially at right angles to the airflow created by forward airspeed; the velocity of airflow meeting these blades equals the rotational velocity. This results in a change to velocity of airflow all across the rotor disk and a change to the lift pattern of the rotor system. Figure 1-54, depicts force vectors acting on various blade areas in forward flight.
### **No-Lift Areas** In helicopter rotor systems, certain regions on the blades experience no lift due to specific airflow conditions. These areas include reverse flow, negative stall, and negative lift. ### **Reverse Flow** This occurs at the root of the retreating blade (Part A of Figure 1-54), where air flows backward from the trailing edge to the leading edge. It happens because the forward airspeed exceeds the rotational velocity at this point on the blade. ### **Negative Stall** Part B of Figure 1-54 illustrates negative stall. Here, rotational velocity surpasses forward flight velocity, causing the resultant relative wind to approach the leading edge. This results in a negative angle of attack (AOA) above the critical AOA, leading to blade stall. ### **Negative Lift** Part C of Figure 1-54 represents negative lift. In this area, a combination of rotational velocity, induced flow, and blade flapping reduces the AOA to a point where the blade produces negative lift. ### **Positive Lift and Positive Stall** * **Positive Lift**: Parts D and E of Figure 1-54 show positive lift areas where the blade produces lift. Here, the resultant relative wind creates a positive AOA, leading to lift generation. * **Positive Stall**: Under certain conditions, there can be a positive stall area near the blade tip. This occurs when the AOA exceeds the critical AOA, resulting in a stall despite producing positive lift. Retreating blade stall is covered in Emergencies.
### Dissymmetry of Lift Dissymmetry of lift refers to the unequal lift across the rotor disk resulting from differences in the velocity of air over the advancing and retreating blade halves (FAA-H-8083-21B). This imbalance in lift would render the helicopter uncontrollable in any situation other than hovering in a calm wind. Compensating for, correcting, or eliminating this unequal lift is essential to achieve symmetry of lift. In forward flight, two factors in the lift equation, namely blade area and air density, remain the same for the advancing and retreating blades. Airfoil shape is fixed for a given blade, and air density cannot be altered. Therefore, the only variables remaining are blade speed and angle of attack (AOA). Since rotor RPM must remain relatively constant, blade speed also remains constant. Consequently, AOA becomes the variable that compensates for dissymmetry of lift. This compensation is achieved through a combination of blade flapping and cyclic feathering. ### Blade Flapping Blade flapping compensates for dissymmetry of lift by inducing upward and downward flapping motion, which alters induced flow velocity and changes the AOA on the advancing and retreating blades. #### *Advancing Blade* As the relative wind speed of the advancing blade increases, the blade gains lift and begins to flap up (see Figure 1-55). It reaches its maximum upflap velocity at the 3-o’clock position, where the wind velocity is greatest. This upward flapping creates a downward flow of air, effectively increasing the induced flow velocity by imposing a downward vertical velocity vector to the relative wind, thereby decreasing the AOA.
#### *Retreating Blade* As the relative wind speed of the retreating blade decreases, the blade loses lift and begins to flap down (refer to Figure 1-56). It reaches its maximum downflap velocity at the 9 o'clock position, where wind velocity is the least. This downward flapping motion generates an upward flow of air, effectively reducing the induced flow velocity by imposing an upward velocity vector to the relative wind. Consequently, this upward flow increases the angle of attack (AOA) of the retreating blade.
#### *Over the Aircraft Nose and Tail* Blade flapping over the nose and tail of the helicopter are essentially equal. This results in an equalization, or symmetry, of lift across the rotor system. Upflapping and downflapping do not alter the total amount of lift produced by the rotor blades. Once blade flapping compensates for dissymmetry of lift, the rotor disk tilts to the rear, a phenomenon known as blowback. The maximum upflap occurring over the nose and the maximum downflap occurring over the tail contribute to blowback, which would naturally decrease airspeed. To counteract this effect, the aviator employs cyclic feathering to control the attitude of the rotor disk. ### Cyclic Feathering Cyclic feathering compensates for dissymmetry of lift by adjusting the angle of attack (AOA). In a hover, equal lift is generated across the rotor system with identical pitch and AOA on all blades and at all points in the rotor system (ignoring compensation for translating tendency). The rotor disk remains parallel to the horizon. To generate thrust, the rotor system must be tilted in the desired direction of movement. Cyclic feathering alters the angle of incidence differentially around the rotor system. Forward cyclic movements reduce the angle of incidence at one part of the rotor system while increasing it at another part. Maximum downflapping of the blade over the nose and maximum upflapping over the tail tilt the rotor disk and thrust vector forward. To prevent blowback, the aviator must continuously move the cyclic forward as the helicopter's velocity increases. Figure 1-57 depicts the changes in pitch angle as the cyclic is advanced at higher airspeeds. During a hover, the cyclic is centered, and the pitch angle on both the advancing and retreating blades is the same. At low forward speeds, advancing the cyclic forward decreases the pitch angle on the advancing blade while increasing it on the retreating blade, resulting in a slight rotor tilt. As forward speed increases, the aviator must further advance the cyclic forward, reducing the pitch angle on the advancing blade and increasing it on the retreating blade. Consequently, the rotor experiences even more tilt than at lower speeds.
This horizontal lift component (thrust) generates higher helicopter airspeed. The higher airspeed induces blade flapping to maintain symmetry of lift. The combination of flapping and cyclic feathering maintains symmetry of lift and desired attitude on the rotor system and helicopter. ### Tandem-Rotor Helicopter Dissymmetry of Lift In tandem-rotor helicopters, the aviator does not manually compensate for dissymmetry of lift when applying forward cyclic. Automatic cyclic-feathering systems are installed on tandem-rotor helicopters. These systems are activated through computer-generated commands at specified airspeeds, usually starting around 70 knots. At low airspeeds, blade flapping compensates for dissymmetry of lift. As airspeed increases, these systems program allowing a more level fuselage attitude and reduce stresses on the rotor driving mechanisms. If the cyclic-feathering system fails to properly feather the rotor system at higher airspeeds, greater blade- flapping angles and nose-low flight attitudes occur and induce increased stresses on the rotor-driving mechanisms. *** ## Translational Lift Improved rotor efficiency resulting from directional flight is known as translational lift. With each knot of incoming wind gained through horizontal movement or surface wind, the hovering rotor system's efficiency improves. As the incoming wind enters the rotor system, turbulence and vortexes are left behind, and the airflow becomes more horizontal. Additionally, during the transition from hover to forward flight, the tail rotor becomes more aerodynamically efficient. Working in progressively less turbulent air, the tail rotor produces more thrust, causing the nose of the aircraft to yaw left (with a main rotor turning counterclockwise), prompting the aviator to apply right pedal in response, decreasing the angle of attack in the tail rotor blades. Figure 1-58 illustrates the airflow pattern for 1 to 5 knots of forward airspeed. It shows the beginning of the dissipation of the downwind vortex and the induction flow down through the rear of the rotor system becoming more horizontal.
Figure 1-59 shows the airflow pattern at a speed of 10 to 15 knots. At this increased airspeed, the airflow continues to become more horizontal. The leading edge of the downwash pattern is being overrun and is well back under the nose of the helicopter.
*** ## Transverse Flow Effect In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion. This is due to the fact that the greater the distance air flows over the rotor disk, the longer the disk has to work on it and the greater the deflection on the aft portion. Downward flow at the rear of the rotor disk causes a reduced angle of attack (AOA), resulting in less lift. Conversely, the front portion of the disk produces an increased AOA and more lift because airflow is more horizontal. These differences in lift between the fore and aft portions of the rotor disk are called transverse flow effect (figure 1-60). This effect causes unequal drag in the fore and aft portions of the rotor disk and results in vibration easily recognizable by the aviator. It occurs between 10 and 20 knots. Transverse flow effect is most noticeable during takeoff and, to a lesser degree, during deceleration for landing. Gyroscopic precession causes the effects to be manifested 90 degrees in the direction of rotation, resulting in a right rolling motion.
*** ## Effective Translational Lift Effective translational lift (ETL) (figure 1-61) occurs with the helicopter at about 16 to 24 knots, when the rotor—depending on size, blade area, and RPM of the rotor system—completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air. The rotor no longer pumps the air in a circular pattern but continually flies into undisturbed air. The flow of air through the rotor system is more horizontal, therefore induced flow and induced drag are reduced. The angle of attack (AOA) is subsequently increased, which makes the rotor system operate more efficiently. This increased efficiency continues with increased airspeed until the best climb airspeed is reached, when total drag is at its lowest point. Greater airspeeds result in lower efficiency due to increased parasite drag.
As single-rotor aircraft speed increases, translational lift becomes more effective, nose rises or pitches up, and aircraft rolls to the right. The combined effects of dissymmetry of lift, gyroscopic precession, and transverse flow effect cause this tendency. Aviators must correct with additional forward and left lateral cyclic input to maintain a constant rotor-disk attitude. *** ## Autorotation In an emergency that results in an autorotation, the crew must work together as quickly as possible to prevent rotor decay beyond recoverable limits. The following section details the aerodynamics at work during an autorotational descent and landing. ### Aerodynamics of Vertical Autorotation During powered flight, rotor drag is overcome with engine power. When the engine fails or is deliberately disengaged from the rotor system, some other force must sustain rotor RPM so controlled flight can be continued to the ground. Adjusting the collective pitch to allow a controlled descent generates this force. Airflow during helicopter descent provides energy to overcome blade drag and turn the rotor. When the helicopter descends in this manner, it is in a state of autorotation. In effect, the aviator exchanges altitude at a controlled rate in return for energy to turn the rotor at an RPM that provides aircraft control and a safe landing. Helicopters have potential energy based on their altitude above the ground. As this altitude decreases, potential energy is converted into kinetic energy used in turning the rotor. Aviators use this kinetic energy to slow the rate of descent to a controlled rate and affect a smooth touchdown. Most autorotations are performed with forward airspeed. For simplicity, the following aerodynamic explanation is based on a vertical autorotative descent (no forward airspeed) in still air. Under these conditions, forces that cause the blades to turn are similar for all blades, regardless of their position in the plane of rotation. Therefore, dissymmetry of lift resulting from helicopter airspeed is not a factor. During autorotation, the rotor disk is divided into three regions—driven, driving, and stall (figure 1-62).
### Driven Region This region, also called the propeller region and nearest the blade tip, normally consists of nearly 30 percent of the disk radius. In the driven region, the Total Aerodynamic Force (TAF) acts above the blade and behind the axis of rotation. This region creates lift, which slows the rate of descent, and drag, which slows rotation of the blade. Region size varies with the blade pitch setting, rate of descent, and rotor RPM. Any change in these factors also changes the size of the regions along the blade span. ### Driving Region Extending from about the 25 to 70 percent radius of the blade, the driving region lies between the driven and stall regions. It can also be identified as the area of autorotative force because it is the region of the blade that produces the force necessary to turn the blades during autorotation. TAF in the driving region is inclined slightly forward of the axis of rotation and produces a continual acceleration force. This direction of force supplies thrust, which tends to accelerate the rotation of the blade. The size of the region varies with the blade pitch setting, rate of descent, and rotor RPM. Any change in these factors also changes the size of the regions along the blade span. ### Stall Region This region includes the inboard 25 percent of the blade radius and operates above the stall Angle of Attack (AOA), causing drag, which tends to slow the rotation of the blade. ### Blade Region Relationships Figure 1-63 illustrates the three regions along with force vectors and two equilibrium points on the blade span. The figure helps locate these regions/points and depicts the interplay of force vectors. Force vectors differ in each region due to slower rotational relative wind near the blade root and increasing speed toward the blade tip. Additionally, blade twist gives a more positive AOA in the driving region than in the driven region, resulting in different combinations of aerodynamic force at every point along the blade. Equilibrium Points There are two points of equilibrium on the blade—point B, between the driven and driving regions, and point D, between the driving and stall regions. At these points, TAF is aligned with the axis of rotation, resulting in neither acceleration nor deceleration force developed. The aviator manipulates these regions to control all aspects of the autorotative descent. For example, increasing the collective pitch causes changes in the pitch angle in all regions, moving points of equilibrium B and D along the blade span, thus adjusting the size of the driven, driving, and stall regions. ### Aerodynamics of Autorotation in Forward Flight Aerodynamic forces in forward flight are produced in the same manner as in vertical autorotation. However, forward speed changes the inflow of air up through the rotor disk, altering the location and size of the regions on the retreating and advancing sides of the rotor disk. The retreating side experiences an increased AOA, moving all three regions outboard along the blade span, with the stall region growing larger and an area near the hub experiencing reversed flow. Conversely, the advancing side experiences a decreased AOA, causing the driven region to occupy more of the blade span.
### Autorotative Phases Autorotations may be divided into three distinct phases—entry, steady-state descent, and deceleration and touchdown. Each phase is aerodynamically different from the others. ### Entry This phase is entered after loss of engine power. The loss of engine power and rotor RPM is more pronounced when the helicopter is at high gross weight, high forward speed, or in high-density altitude conditions. Any of these conditions demand increased power (high collective position) and a more abrupt reaction to loss of that power. In most helicopters, it takes only seconds for RPM decay to fall into a minimum safe range requiring a quick collective response from the aviator. Entry is a combination of figures 1-65 and 1-66. #### *Level-Powered Flight at High Speed* Figure 1-65 shows the airflow and force vectors for a blade in this configuration. Lift and drag vectors are large, and the Total Aerodynamic Force (TAF) is inclined well to the rear of the axis of rotation. An engine failure in this mode will cause rapid rotor RPM decay. To prevent this, an aviator must lower the collective quickly, reducing drag, and inclining the TAF vector forward, nearer the axis of rotation.
#### *Collective Pitch Reduction* Figure 1-66, shows airflow and force vectors for a blade immediately after power loss and subsequent collective reduction, yet before the aircraft has begun to descend. Lift and drag are reduced, with the Total Aerodynamic Force (TAF) vector inclined further forward than it is in powered flight. As the helicopter begins to descend, the airflow begins to flow upward and under the rotor system. This causes the TAF to incline further forward until it reaches an equilibrium that maintains a safe operating RPM.
### Steady-State Descent Figure 1-67 shows airflow and force vectors for a blade in steady-state autorotative descent. Airflow is now upward through the rotor disk because of the descent. This inflow of air creates a larger Angle of Attack (AOA) although blade pitch angle has not changed since the descent began. Total Aerodynamic Force (TAF) on the blade is increased and inclined further forward until equilibrium is established, rate of descent and rotor RPM are stabilized, and the helicopter is descending at a constant angle. Angle of descent is normally 17 to 20 degrees, depending on airspeed, density altitude, wind, and type of helicopter.
### Deceleration and Touchdown Figure 1-68 illustrates airflow and force vectors for a blade in autorotative deceleration. To execute an autorotative landing, aviators reduce airspeed and the rate of descent just before touchdown. They can partially achieve both actions by applying aft cyclic, which alters the attitude of the rotor disk relative to the relative wind. This change in attitude inclines the resultant lift of the rotor system to the rear, decelerating forward speed. It also increases the Angle of Attack (AOA) on all blades by altering the direction of airflow through the rotor system, thereby boosting rotor RPM. Consequently, the lifting force of the rotor system increases, and the rate of descent decreases. Once the aviator reduces forward speed to a safe landing speed, the helicopter is brought into a landing attitude while applying collective pitch to cushion the touchdown.
### Glide and Rate of Descent in Autorotation Helicopter airspeed and drag play significant roles in determining the rate of descent during autorotation. The rate of descent is highest at very low airspeeds, decreases to a minimum at some intermediate speed, and then increases again at faster speeds. The airspeeds for achieving the minimum rate of descent and maximum glide distance vary by helicopter type and can be found in individual operator manuals. ### Circle of Action The circle of action refers to a point on the ground that appears stationary in the pilot's field of view during a steady-state autorotation. It represents the point of impact if the pilot applied no deceleration, initial pitch, or cushioning pitch during the last 100 feet of autorotation. Typically, the circle of action is located two or three helicopter lengths short of the touchdown point, depending on factors such as wind conditions and the rate and amount of deceleration and collective application. ### Last 50 to 100 Feet Autorotation is considered to end at 50 to 100 feet above the ground, at which point landing procedures commence. To perform a power-off landing for rotary-wing aircraft, the aviator trades airspeed for lift by decelerating the aircraft during the final 100 feet of descent. Deceleration is timed and applied to minimize the rate of descent and forward airspeed just before touchdown. At approximately 10 to 15 feet above the ground, the energy exchange is nearly complete. Initial pitch application occurs at this stage to utilize some rotor energy to reduce the rate of descent before cushioning the touchdown. Collective pitch application remains the primary control input for cushioning the landing. Due to variations among helicopter types, aviator experience with a specific aircraft is crucial for accurately predicting the available energy exchange at 100 feet and determining the appropriate amount of deceleration and collective pitch required for a safe and successful landing.
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