SIFT

The main rotor provides lift and thrust, while the tail rotor counteracts torque in single-main-rotor helicopters. Rotor systems are classified by how the blades are allowed to move.

System	HowItWorks	Characteristics
Fullyarticulated	Hingesorbearingsleteachbladeflapupanddown leadand ,	Threeormoreblades;smooth
	lagintheplaneofrotation andfeather(changepitch) ,	operationandgoodhandling
Semi -rigid (seesaw)	Twobladesrigidlyjoinedonateeteringhingeflapasaunit;	Simplerandlighter;limitationsatlow
	whenonebladeflapsup theotherflapsdown ,	rotor speed; common on two-blade
		helicopters
Rigid	No flapping or lead -lag hinges; blade flexibility absorbs the	Excellentcontrolresponse;common
Bearingless	movement Flexingcompositematerialsreplacemechanicalbearingsand hinges	inmodernmilitaryhelicopters 1 Reducedmaintenanceandweight

Mainrotorsystemtypes(Continued)

System	HowItWorks	Characteristics
Tandem	Twomainrotorsmountedforeandaft turninginopposite ,	Counter-rotation cancels torque; no
	directions (e .g CH-47 Chinook) .,	tailrotorneeded
Coaxial	Twomainrotorsonthesameshaft turninginopposite ,	Notailrotorneeded;compact
	directions	dimensions

Torque and Anti-Torque

Newton's third law explains torque reaction. When the engine turns the main rotor in one direction, an equal and opposite force tries to rotate the fuselage the other way; without compensation the helicopter would spin opposite to rotor rotation. Single-main-rotor helicopters use a tail rotor, mounted vertically on the tail boom, to produce thrust perpendicular to the longitudinal axis. By varying tail rotor thrust, the pilot controls yaw (rotation about the vertical axis).

In American helicopters the main rotor typically turns counterclockwise when viewed from above. The torque reaction would turn the fuselage clockwise, so the tail rotor produces thrust to the right to counteract it. High-power operations such as hovering and climbing produce more torque and require more tail rotor thrust; in autorotation, with the engine no longer driving the rotor, torque is minimal and tail rotor thrust requirements drop significantly.

Tail rotor failure is an emergency requiring immediate action. The pilot reduces power to minimize torque and enters autorotation or, if possible, establishes forward flight so the vertical stabilizer can help maintain directional control.

Translating Tendency (Drift)

Translating tendency, also called tail rotor drift, is the tendency of a single-rotor helicopter to drift laterally in the direction of tail rotor thrust while hovering. Because the tail rotor's sideward force also pushes the entire helicopter sideways, a helicopter with a counterclockwise main rotor drifts right in a hover. Pilots compensate by tilting the rotor disc slightly left with left cyclic. Some designs tilt the mast or transmission slightly to compensate automatically, reducing the cyclic input required.

Ground Effect

Ground effect is the increased lift and reduced power requirement experienced when hovering close to the ground, within about one rotor diameter of the surface. The ground restricts the downward velocity of the induced airflow and disrupts blade tip vortices, reducing induced drag. As a result, less power is required to hover in ground effect (IGE) than out of ground effect (OGE). Ground effect is greatest very close to the surface and becomes negligible at about one rotor diameter.

A helicopter may be able to hover IGE but not OGE because of power limitations. Pilots must account for this when operating from elevated platforms or sloped terrain and when transitioning to forward flight.

Translational Lift

Translational lift is the additional lift gained when the helicopter moves fast enough to leave the disturbed air mass created by its own downwash, typically at about 16 to 24 knots. In a hover the rotor recirculates the same turbulent air; with forward speed it moves into undisturbed air and produces lift more efficiently. The onset of effective translational lift (ETL) is felt as a sudden increase in performance, and the helicopter may tend to pitch up. Translational lift is available in any direction of flight (forward, sideward, or rearward) as long as the helicopter moves fast enough through the air mass.

Dissymmetry of Lift

Dissymmetry of lift is the unequal lift produced by the advancing and retreating halves of the rotor disc in forward flight. The advancing blade's relative wind is its rotational speed plus the helicopter's forward speed; the retreating blade's is its rotational speed minus forward speed. At constant pitch, the advancing side would produce more lift and roll the helicopter toward the retreating side.

Rotor systems compensate through blade flapping. The advancing blade flaps up, which reduces its angle of attack and lift; the retreating blade flaps down, which increases its angle of attack and lift. This automatic compensation equalizes lift across the disc. Flapping also tilts the disc rearward in forward flight, an effect called blowback that grows with airspeed; the pilot applies forward cyclic to counter it.

Gyroscopic Precession

Gyroscopic precession is a property of rotating masses: a force applied to a spinning object takes effect 90 degrees later in the direction of rotation. Main rotors behave like gyroscopes, so the blade flapping produced by a cyclic input occurs about 90 degrees after the point where pitch is changed.

In a counterclockwise rotor viewed from above, forward cyclic produces the greatest pitch decrease on the right side of the disc (the 3 o'clock position). Because of precession, the greatest downward flapping occurs 90 degrees later, at the 12 o'clock position, tilting the disc forward as intended. Control linkages are designed with this offset, so pilot inputs produce the expected response.

Coriolis Effect (Conservation of Angular Momentum)

When a rotor blade flaps upward, its center of mass moves closer to the axis of rotation, and the blade speeds up (leads) to conserve angular momentum. When it flaps downward, its center of mass moves outward and the blade slows (lags). Lead-lag hinges in fully articulated systems absorb these changes; without them, the constantly changing blade speeds would place destructive stresses on the rotor hub. The effect mirrors a spinning ice skater, who turns faster when pulling the arms in and slower when extending them.

Vortex Ring State (Settling with Power)

Vortex ring state is a dangerous condition that can develop when a helicopter descends into its own downwash at low airspeed with moderate to high power applied. The rotor vortices recirculate around the disc, the rotor operates partly in its own disturbed air, and lift efficiency falls sharply; the helicopter may descend rapidly even with high power.

+ +

Below effective translational lift

Rate of about 300 fpm or more

20 to 100 percent of available power

Recovery requires increasing forward airspeed to fly out of the disturbed air, or lowering collective and entering autorotation. Simply adding collective (power) will not arrest the descent and may make it worse. Vortex ring state is most likely during steep approaches, downwind operations, and attempts to descend vertically from a hover.

A question describing a helicopter that keeps sinking despite added power during a slow, steep descent points to vortex ring state. The correct recovery increases airspeed or reduces collective; it never adds more power.

Autorotation

Autorotation is rotor rotation sustained by air flowing up through the rotor from below rather than by engine power. It is the primary emergency procedure for engine failure and certain tail rotor malfunctions. When power is lost, the pilot immediately lowers the collective to reduce blade pitch so the rotor keeps turning, driven by the upward airflow as the helicopter descends. The spinning rotor stores kinetic energy that will cushion the landing.

Only the driving region produces the net forward force in the plane of rotation that keeps the rotor turning. Proper autorotative airspeed (typically about 60 to 80 knots in most helicopters) gives the best descent rate and keeps rotor RPM within limits. Near the ground the pilot flares to reduce forward and vertical speed, then raises the collective (the "cushion") to use the stored rotor energy for a safe touchdown.