[FF-1] Understanding Main Rotor Aerodynamic Effects

Dissymmetry of Lift. There is a difference in lift between the advancing half of the rotor disk and the retreating half. In directional flight, the aircraft relative wind is added to the rotational relative wind on the advancing blade and subtracted on the retreating blade. The blade passing the tail and advancing around the right side of the helicopter has an increasing speed which reaches maximum at the 3 o'clock position. As the blade rotation continues, the speed reduces to essentially rotational speed over the nose of the helicopter. The blade speed then decreases progressively and reaches minimum airspeed at the 9 o'clock position. The blade speed then increases progressively and again reaches rotational speed as it passes over the tail.

 

Blade Flapping. The up and down movement of a rotor blade, in conjunction with cyclic feathering, causes dissymmetry of lift to be minimized. The advancing blade, upon meeting the progressively higher airspeeds brought about by the addition of forward flight airspeed to rotational airspeed, responds to the increase of speed by producing more lift. The blade climbs or flaps upward, and the change in relative wind and angle of attack reduces the amount of lift that would have been generated. In the case of the retreating blade, the opposite is true.

Cyclic Feathering. Blade flapping alone is not sufficient to compensate for dissymmetry of lift. During forward flight, the blade pitch angle is lower on the advancing side of the disk to compensate for increased blade speed on that side. Blade pitch angle is increased on the retreating blade side to compensate for decreased blade speed on that side. These changes in blade pitch are introduced by the blade feathering mechanism and are called cyclic feathering. Pitch changes are made to individual blades throughout their rotation independent of the others in the system, and they are controlled by the cyclic stick.

Gyroscopic Precession. This is a phenomena occurring in rotating bodies in which an applied force is manifested 90 degrees ahead in the direction of rotation from where the force was applied. The swashplate input to tip the rotor disk to the left (9 o'clock) must be applied at the 12 o'clock position. A loss of lift at the 9 o'clock position (e.g., retreating blade stall) will cause the disk to tip toward the 6 o'clock position, causing the helicopter nose to pitch up.

 

Translational Lift. The efficiency of the hovering rotor system is improved with each knot of
incoming wind gained by either horizontal movement or surface wind. As the incoming wind enters the rotor system, turbulence and vortices are left behind and the flow of air becomes more horizontal. This improved rotor efficiency resulting from directional flight is called translational lift. At approximately 1 to 5 knots, the downwind vortices begins to dissipate and induced flow down thorough the rear of the rotor disk is more horizontal than at a hover. At 10 to 15 knots, airflow is much more horizontal than at a hover. The leading edge of the down wash pattern is being overrun and is well back under the helicopter nose.

At approximately 16 to 24 knots, the rotor completely out runs the recirculation of old vortices 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 air passing through the rotor system is more horizontal, depending on helicopter forward airspeed. 

As the helicopter speed increases, translational lift becomes more effective, causing the nose to pitch up. This tendency is caused by the combined effects of dissymmetry of lift and gyroscopic precession. Pilots must correct for this tendency in order to maintain a constant rotor disk attitude that will move the helicopter through the speed range where effective translational lift (ETL) occurs. If the nose is permitted to pitch up while passing through this speed range, the aircraft may also tend to roll slightly to the right.

The tail rotor also becomes more aerodynamically efficient in forward flight. The tail rotor works in progressively less turbulent air as airspeed increases. As tail rotor efficiency improves, more thrust is produced for a given tail rotor position (angle-of-attack). This causes the aircraft nose to yaw left, requiring the application of right pedal as speed increases.

 

Transverse Flow Effect. Between approximately 10 to 20 knots, air passing through the rear
portion of the rotor disk has a greater induced flow angle than air passing through the forward portion. This downward flow at the rear of the rotor disk tends to cause a reduced angle-of-attack, requiring greater pitch to produce enough lift to keep the forward portion of the rotor disk lower than the rear. Less induced flow and more lift is produced at the front portion of the disk because airflow is more horizontal. These differences between the fore and aft portions of the rotor disk are called transverse flow effect. They cause unequal drag in the fore and aft parts of the disk, resulting in vibrations that are easily recognizable by the
pilot. Due to gyroscopic precession, the increased lift at the forward portion of the rotor system will result in a right rolling motion in the helicopter.