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ALC-104: Helicopter - General and Flight Aerodynamics
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Translational Flight

Fig5-1.JPG
While in forward flight, the tip-path plane is tilted forward, thus tilting the total lift-thrust force forward from the vertical. This resultant lift-thrust force can be resolved into two components—lift acting vertically upward and thrust acting horizontally in the direction of flight. In addition to lift and thrust, there is weight as the downward acting force, and drag as the rearward acting or retarding force. See figure 5-1.

As the helicopter moves forward, it begins to lose altitude because of the lift that is lost as thrust is diverted forward. However, as the helicopter begins to accelerate, the rotor system becomes more efficient due to the increased airflow. The result is excess power over that which is required to hover. Continued acceleration causes an even larger increase in airflow through the rotor disc and more excess power.

Forward flight produces several aerodynamic phenomena and effects:

 


 

Translational lift is present with any horizontal flow of air across the rotor. This increased flow is most noticeable when the airspeed reaches approximately 16 to 24 knots. As the helicopter accelerates through this speed, the rotor moves out of its vortices and is in relatively undisturbed air. The increase in horizontal reduces induced flow and drag with a corresponding increase in angle of attack and lift. The additional lift available at this speed is referred to as effective translational lift (ETL). See figure 5-2.Fig5-2.JPG


When a single-rotor helicopter flies through translational lift, the air flowing through the main rotor and over the tail rotor becomes less turbulent and more aerodynamically efficient. As the tail rotor efficiency improves, more thrust is produced causing the aircraft to yaw left in a counterclockwise rotor system. It will be necessary to use right torque pedal to correct for this tendency on takeoff. Also, if no corrections are made, the nose rises or pitches up, and rolls to the right. This is caused by combined effects of dissymmetry of lift and transverse flow effect, and is corrected with cyclic control. Translational lift is also present in a stationary hover if the wind speed is approximately 16 to 24 knots. In normal operations, always utilize the benefit of translational lift, especially if maximum performance is needed.

 


 

As the rotor blades rotate they generate what is called rotational relative wind. This airflow is characterized as flowing parallel and opposite the rotor’s plane of rotation and striking perpendicular to the rotor blades leading edge. This rotational relative wind is used to generate lift. As rotor blades produce lift, air is accelerated over the foil and projected downward. Anytime a helicopter is producing lift, it moves large masses of air downward through the rotor system. This downwash, referred to as induced flow can significantly change the efficiency of the rotor system. As shown by figure, rotational relative wind combines with induced flow to form the resultant relative wind. As induced flow increases when transitioning to forward flight, resultant relative wind becomes less horizontal. Since angle of attack is determined by measuring the difference between the chord line and the resultant relative wind, as the resultant relative wind becomes less horizontal, angle of attack decreases. See figure 5-3.

Fig5-3.JPG

 


 

As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disc area and increases at the aft disc area. This increases the angle of attack at the front disc area causing the rotor blade to flap up, and reduces angle of attack at the aft disc area causing the rotor blade to flap down. Because the rotor acts like a gyro, maximum displacement occurs 90° in the direction of rotation. The result is a tendency for the helicopter to roll slightly to the right as it accelerates through approximately 20 knots or if the headwind is approximately 20 knots. You can recognize transverse flow effect because of increased vibrations of the helicopter at airspeeds just below effective translational lift on takeoff and after passing through effective translational lift during landing. To counteract transverse flow effect, a cyclic input needs to be made.

 


Fig5-4.JPG
When the helicopter moves through the air, the relative airflow through the main rotor disc is different on the advancing side than on the retreating side. The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the relative wind speed acting on the retreating blade is reduced by the helicopter’s forward airspeed. Therefore, as a result of the relative wind speed, the advancing blade side of the rotor disc produces more lift than the retreating blade side. This condition is defined as dissymmetry of lift. See figure 5-4.


Left uncompensated, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. To prevent this, the main rotor blades flap and feather automatically to equalize lift across the rotor disc. Articulated rotor systems, usually with three or more blades, incorporate a horizontal hinge (flapping hinge) to allow the individual rotor blades to move, or flap up and down as they rotate. A semi-rigid (two-blade) rotor system utilizes a teetering hinge, which allows the blades to flap as a unit: when one blade flaps up, the other flaps down.


A more detailed explanation is illustrated in figure 5-5. The advancing rotor blade reaches its maximum “up-flap” velocity at point “A”. The upward flapping of the advancing blade reduces the angle between the blade chord line and the relative wind. This decreases the angle of attack, which reduces the amount of lift produced by the blade. At position “C” the retreating rotor blade is at its maximum “down-flapping” velocity. Due to downward flapping, the angle between the chord line and the resultant relative wind increases. This increases the angle of attack and thus the amount of lift produced by the blade. The combination of blade flapping and slow relative wind acting on the retreating blade normally limits the maximum forward speed of a helicopter. At a specific forward speed, the retreating blade stalls because the high angle of attack and the relative wind speed being below stall speed. This situation is called retreating blade stall and is evidenced by a nose pitch up, vibration, and a rolling tendency—usually to the left in helicopters with counterclockwise blade rotation.

Fig5-5.JPG


Retreating blade stall can be avoided by not exceeding the never-exceed speed, designated VNE, which is usually indicated on a placard and marked on the airspeed indicator by a red line. The advancing blade achieves maximum up-flapping displacement over the nose and maximum down-flapping displacement over the tail. This causes the tip-path plane to tilt to the rear and is referred to as blowback.

Fig5-6.JPG
Figure 5-6 shows how the rotor disc was originally oriented with the front down following the initial cyclic input, but as airspeed is gained and flapping eliminates dissymmetry of lift, the front of the disc comes up, and the back of the disc goes down. This reorientation of the rotor disc changes the direction in which total rotor thrust acts so that the helicopter’s forward speed slows, but can be corrected with cyclic input.


 



In sideward flight, the tip-path plane is tilted in the direction that flight is desired. This tilts the total lift-thrust vector sideward. In this case, the thrust component now acts sideward with drag acting to the opposite side. For rearward flight, the tip-path plane is tilted rearward, which in turn tilts the resultant lift/thrust vector rearward. Drag now acts forward. See figure 5-7.

Fig5-7.JPG

 


Fig5-8.JPG
In forward flight, the rotor disc is tilted forward, which also tilts the total lift-thrust force of the rotor disc forward. When the helicopter is banked during forward flight, the rotor disc has a sideward tilt, and a component of the resultant lift now acts horizontally towards the direction of turn (centripetal force) to oppose inertia (centrifugal force). See figure 5-8.


As the angle of bank increases, the total lift force is tilted more toward the horizontal, thus causing the rate of turn to increase because more lift is acting horizontally. Since the resultant lifting force acts more horizontally, the effect of lift acting vertically is deceased. To compensate for this decreased vertical lift, the angle of attack of the rotor blades must be increased in order to maintain altitude. The steeper the angle of bank, the greater the angle of attack of the rotor blades required to maintain altitude. Thus, with an increase in bank and a greater angle of attack, the resultant lifting force increases and the rate of turn is faster.