In a typical single main rotor helicopter, what is the primary purpose of the tail rotor?
ACS code: AM.II.N
Correct answer: To counteract the torque produced by the main rotor and provide directional control
Rationale: The spinning main rotor applies a torque to the fuselage that tends to rotate it in the opposite direction. The tail (anti-torque) rotor produces sideward thrust to counteract this torque and, through pedal inputs, allows the pilot to control the helicopter's heading about the vertical axis. Lift is produced by the main rotor, not the tail rotor.
Which control changes the pitch of all main rotor blades simultaneously and by an equal amount?
ACS code: AM.II.N
Correct answer: The collective pitch control
Rationale: The collective changes the pitch angle of all main rotor blades at the same time and by the same amount, which changes total rotor thrust and governs the helicopter's climb and descent. The cyclic changes blade pitch individually as the blades rotate, tilting the rotor disc; the pedals control tail rotor thrust.
On a fully articulated rotor head, which hinge allows each blade to move forward and aft in the plane of rotation?
ACS code: AM.II.N
Correct answer: The lead-lag (drag) hinge
Rationale: A fully articulated rotor provides three independent blade motions. The lead-lag (drag) hinge permits each blade to move fore and aft in the plane of rotation to accommodate the acceleration and deceleration caused by Coriolis effect. The flapping hinge allows up-and-down motion, and the feathering hinge allows the blade to change pitch.
The up-and-down movement of a rotor blade about a horizontal hinge is known as what?
ACS code: AM.II.N
Correct answer: Flapping
Rationale: Flapping is the vertical movement of a blade about a flapping hinge, which compensates for the dissymmetry of lift that occurs in forward flight between the advancing and retreating blades. Feathering is the pitch-change motion about the feathering hinge, and lead-lag is the fore-and-aft motion in the plane of rotation.
A two-bladed rotor system in which the blades flap as a single unit on a central hinge is referred to as which type of rotor?
ACS code: AM.II.N
Correct answer: Semirigid
Rationale: A semirigid rotor is usually a two-bladed system in which the blades are rigidly attached to the hub but the hub is free to teeter; as one blade flaps up the other flaps down about a common teetering hinge. A fully articulated rotor has separate flapping, lead-lag, and feathering hinges for each blade; a rigid rotor has no flapping or lead-lag hinges.
Dissymmetry of lift on a helicopter main rotor is caused by what?
ACS code: AM.II.N
Correct answer: The difference in airspeed between the advancing and retreating blades in forward flight
Rationale: In forward flight the advancing blade has the helicopter's forward speed added to its rotational speed while the retreating blade has that speed subtracted, so the two sides of the disc develop unequal lift. This is dissymmetry of lift, which is compensated for by blade flapping and cyclic feathering. It is not caused by altitude density changes or torque reaction.
In a single main rotor helicopter, the anti-torque pedals primarily control movement about which axis?
ACS code: AM.II.N
Correct answer: The vertical (yaw) axis
Rationale: The anti-torque pedals change the pitch and thrust of the tail rotor, which controls the helicopter's heading by rotating the nose left or right about the vertical axis (yaw). Pitch and roll about the lateral and longitudinal axes are controlled by the cyclic, which tilts the main rotor disc.
On a helicopter rotor blade, the angle of attack is the angle between the blade chord line and the
ACS code: AM.II.N
Correct answer: relative wind meeting the blade.
Rationale: Angle of attack is the aerodynamic angle between the blade chord line and the relative wind actually meeting the blade, and it changes with induced flow and blade flapping. The angle between the chord line and the plane of rotation is the pitch (blade) angle, which is set mechanically and is a different quantity. (FAA-H-8083-31, Rotorcraft Fundamentals — rotor blade aerodynamics.)
A helicopter rotor blade is an airfoil. It produces lift primarily by
ACS code: AM.II.N
Correct answer: creating a region of low pressure above the blade
Rationale: A rotor blade is an airfoil, and as it moves through the air the cambered upper surface accelerates the airflow and lowers the static pressure above the blade while the lower surface stays at higher pressure. This pressure differential is the source of rotor lift. Although the blade does deflect air downward, the lift mechanism is the low-pressure region created above the blade, not a screw-like action.
The angle of attack of a rotor blade is the angle between the
ACS code: AM.II.N
Correct answer: chord line and the relative wind.
Rationale: Angle of attack is the angle between the blade chord line and the relative wind (relative airflow) meeting the blade. The angle between the chord line and the plane of rotation is the blade pitch (feathering) angle, and the angle to the rotor spin axis is not a recognized aerodynamic definition.
On a helicopter rotor, blade dragging (lead-lag) refers to the
ACS code: AM.II.N
Correct answer: horizontal movement of each blade about its vertical hinge.
Rationale: Blade dragging, or lead-lag, is the in-plane fore-and-aft movement of a blade about its vertical (drag) hinge, accommodating Coriolis effect and cyclic drag variations. Vertical movement of the blade about a horizontal hinge is flapping, which is a separate motion.
The lift generated by a rotor blade is proportional to the
ACS code: AM.II.N
Correct answer: relative wind and the angle of attack.
Rationale: Blade lift increases with the square of the relative wind (the velocity the blade actually sees) and with the angle of attack up to the stall. Aircraft airspeed alone is not the relevant velocity for a rotor, and blade pitch only affects lift indirectly through the resulting angle of attack.
What effect does ground effect have on a hovering helicopter?
ACS code: AM.II.N
Correct answer: It increases the lift produced by the rotor.
Rationale: When hovering close to the surface the downwash is restricted and a high-pressure air cushion forms beneath the rotor disc, so for a given collective setting the rotor produces more lift (or the same lift for less power). Ground effect therefore increases rotor lift rather than having no effect.
Which device is commonly used to dampen in-plane vibration at a helicopter rotor head?
ACS code: AM.II.N
Correct answer: A bifilar vibration absorber.
Rationale: A bifilar (pendulum) vibration absorber fitted to the rotor head tunes itself with rotor speed to cancel in-plane rotor vibration. The swashplate transmits pitch inputs and the scissors (drive) links turn the swashplate with the mast, so neither performs a vibration-damping function.
In forward flight, the relative wind acting on a helicopter main rotor
ACS code: AM.II.N
Correct answer: is greatest on the advancing blade.
Rationale: In forward flight the aircraft's airspeed adds to the rotational speed on the advancing blade and subtracts on the retreating blade, so the relative wind is greatest on the advancing blade. This dissymmetry of lift is the reason blade flapping is needed to equalize lift across the disc.
As outside air temperature (OAT) increases, the operating ceiling of a helicopter will
ACS code: AM.II.N
Correct answer: decrease.
Rationale: A rise in outside air temperature reduces air density, so the rotor produces less thrust for a given power and the engine also delivers less power. Both effects lower the maximum altitude at which the helicopter can sustain flight, so the operating ceiling decreases.
When a rotor blade moves about its flapping hinge,
ACS code: AM.II.N
Correct answer: the blade angle of attack changes and the drag forces change.
Rationale: Flapping alters the blade's vertical velocity, which changes the direction of the relative wind and therefore the angle of attack; the accompanying change in lift and the Coriolis effect also alter the in-plane drag. Blade pitch (feathering) angle is set by the controls and is not automatically reduced by flapping.
To maintain a hover when air density decreases, the pilot must increase
ACS code: AM.II.N
Correct answer: collective pitch.
Rationale: Lower air density gives less thrust for a given blade pitch, so to hold position the pilot raises the collective, increasing the pitch of all blades equally to restore total rotor thrust. Rotor RPM is held constant by the governor, and cyclic only tilts the disc to change direction rather than total lift.
The coning angle of a helicopter rotor is determined by the balance between
ACS code: AM.II.N
Correct answer: rotor lift and centrifugal force.
Rationale: The coning angle is the balance between rotor lift bending the blades upward and centrifugal force holding them out in the plane of rotation. A higher lift or lower rotor RPM increases coning; the controlling pair is lift and centrifugal force, not thrust.
As a single-rotor helicopter accelerates in level forward flight past about 15 knots, the pilot will need to
ACS code: AM.II.N
Correct answer: reduce power because of the additional lift from effective translational lift.
Rationale: At roughly 15 knots the rotor outruns its own recirculating downwash and meets cleaner air, gaining translational lift and working more efficiently, so power can be reduced for the same lift. Rotor RPM is held constant by the governor, so the reason is the efficiency gain from translational lift, not an RPM change.
A helicopter has a main rotor that turns counterclockwise as viewed from above and an anti-torque tail rotor. The helicopter will tend to drift sideways toward the
ACS code: AM.II.N
Correct answer: right, regardless of how the tail rotor is mounted.
Rationale: The tail rotor produces sideways thrust to counter main-rotor torque, and that thrust pushes the whole helicopter sideways (translating tendency, or tail-rotor drift). For a counterclockwise main rotor this drift is toward the right, and it occurs regardless of which side the tail rotor is mounted on.
The tail rotor of a single-main-rotor helicopter
ACS code: AM.II.N
Correct answer: produces a force that opposes the main-rotor torque reaction.
Rationale: Driving the main rotor produces a torque reaction on the fuselage that would spin it opposite to the rotor; the tail rotor produces a sideways thrust whose moment opposes this torque reaction and holds heading. A force in the same direction would aggravate the yaw, and the tail rotor, being a rotor in forward flow, does experience dissymmetry of lift.
With the anti-torque (tail rotor) pedals in the neutral position, the tail rotor blade pitch is normally
ACS code: AM.II.N
Correct answer: positive.
Rationale: With the pedals at neutral the tail rotor is rigged with positive pitch so it still produces thrust to counter the main-rotor torque present in normal powered flight. A neutral or negative setting would leave the torque reaction uncountered and the helicopter would yaw.
On some helicopters, the main rotor mast is tilted laterally to correct for tail rotor
ACS code: AM.II.N
Correct answer: drift.
Rationale: Translating tendency (tail rotor drift) pushes the helicopter sideways during a hover because the tail rotor produces a side thrust. Tilting the main rotor mast laterally a few degrees inclines the rotor thrust to oppose that sideways force, so the helicopter can hover level. It addresses drift, not torque (which the tail rotor counters) or roll.
Certain helicopters tend to tilt laterally during landing. This tendency can be overcome by locating the tail rotor thrust line
ACS code: AM.II.N
Correct answer: in line with the main rotor hub.
Rationale: If the tail rotor thrust acts at a different height from the main rotor hub, the two opposing side forces form a couple that rolls the aircraft, felt as a lateral tilt at touchdown. Locating the tail rotor thrust line in line with the main rotor hub removes the rolling couple.
Ground effect for a helicopter in a hover is produced by
ACS code: AM.II.N
Correct answer: increased pressure under the main rotor disc.
Rationale: Close to the ground the rotor downwash cannot escape freely, so a region of increased pressure builds up beneath the rotor disc that helps support the helicopter. This increased pressure under the disc, not recirculation or a density rise above the fuselage, provides the extra support known as ground effect.
A helicopter hovering close to a tall building will tend to
ACS code: AM.II.N
Correct answer: drift toward it.
Rationale: The rotor downwash striking the building is deflected, lowering the pressure in the gap between the helicopter and the structure, so the machine is drawn toward the building. The pilot must apply cyclic away from the wall to hold position, confirming the natural tendency is to drift toward it.
The drooping of helicopter rotor blades at rest is overcome in flight by
ACS code: AM.II.N
Correct answer: centrifugal force.
Rationale: On the ground or at low rotor speed the blades droop under their own weight, but once the rotor turns the centrifugal force acting outward along each blade greatly exceeds the weight and pulls the blades up toward the horizontal. This centrifugal effect counters static droop and sets the coning angle in flight.
During a power-on descent of a helicopter, which forces are acting on the aircraft?
ACS code: AM.II.N
Correct answer: Lift, drag, thrust, and weight.
Rationale: In a powered (power-on) descent the engine still drives the rotor, so thrust is present alongside the ever-present weight, the rotor lift, and the drag opposing motion. All four forces therefore act, unlike an autorotative descent where engine thrust would be absent.
Main rotor blades commonly use a symmetrical airfoil section because it
ACS code: AM.II.N
Correct answer: keeps the center of pressure from shifting.
Rationale: A symmetrical airfoil keeps the center of pressure essentially fixed as angle of attack changes, avoiding large pitching (feathering) moments and twisting loads as each blade cycles. A highly cambered or reverse-cambered section would move the center of pressure and impose unwanted torsional loads on the blade.
Autorotative force is the
ACS code: AM.II.N
Correct answer: component of the total reaction acting forward in the plane of rotation, opposing drag.
Rationale: In autorotation the upflow tilts the total reaction of part of the blade ahead of the rotor axis; the in-plane component of that reaction acts forward, opposing profile drag and keeping the rotor turning. It is an aerodynamic driving force, not a control input the pilot applies.
Autorotation in a helicopter is
ACS code: AM.II.N
Correct answer: the production of lift by freely rotating rotor blades without engine power.
Rationale: Autorotation is the condition in which airflow up through a descending rotor keeps the blades turning and still generating lift without engine power, allowing a controlled power-off descent. Directional control is retained, and it is the normal response to a power loss rather than a cause of one.
A helicopter rotor blade derives its lift from the fact that
ACS code: AM.II.N
Correct answer: the blade creates a region of low pressure above it.
Rationale: Like any airfoil, the rotor blade accelerates air over its upper surface and creates a region of low pressure there; the pressure difference across the blade is what produces lift. Treating the rotor as a simple airscrew or as merely pushing air downward misstates the actual lift mechanism.
A two-bladed helicopter rotor head mounted on a central gimbal that allows the blades to flap as a unit is called a
ACS code: AM.II.N
Correct answer: semirigid rotor system.
Rationale: A two-bladed head mounted on a central gimbal (teetering or see-saw) lets the two blades flap as one unit without individual flapping or drag hinges; this is the semirigid rotor system. A fully articulated head has separate flap, drag, and feather hinges, while a rigid head allows no flapping at all.
Because of phase lag (gyroscopic precession), the maximum displacement of a rotor blade occurs
ACS code: AM.II.N
Correct answer: approximately 90 degrees of rotation after the pitch input is applied.
Rationale: A spinning rotor behaves like a gyroscope, so the maximum blade displacement occurs approximately 90 degrees of rotation after the point at which the pitch input is applied. This phase lag is why cyclic pitch changes are mechanically fed into the disc ahead of the direction in which the disc is intended to tilt.
Vortex ring state (settling with power) in a helicopter occurs when
ACS code: AM.II.N
Correct answer: the rotor settles into its own recirculating tip-vortex wake at low forward speed.
Rationale: Vortex ring state arises when the rotor settles into its own recirculating tip-vortex wake at low or zero forward speed, so the blades meet their own downwash and lift collapses. It is a low-speed recirculation phenomenon, not something that occurs at high forward speed or simply from proximity to the ground.
A helicopter climb with forward airspeed requires less power than a vertical climb because of
ACS code: AM.II.N
Correct answer: translational lift.
Rationale: As a helicopter gains forward speed it leaves its own downwash and the rotor meets cleaner, less disturbed air, providing extra lift for the same power. This efficiency gain, known as translational lift, is why a climb with forward airspeed needs less power than a pure vertical climb.
Horizontal (translational) flight in a helicopter is achieved by
ACS code: AM.II.N
Correct answer: tilting the rotor disc in the direction of flight with cyclic.
Rationale: Translational, or horizontal, flight is produced by tilting the rotor disc with cyclic so a component of rotor thrust acts in the direction of travel. Raising the collective changes total lift, not direction, so the disc tilt is what produces horizontal movement.
The most desirable rotor blade design is one in which the center of pressure
ACS code: AM.II.N
Correct answer: does not move along the chord of the blade.
Rationale: The best rotor blade design keeps the center of pressure essentially fixed (as with a symmetrical section), so changing angle of attack does not generate a varying pitching moment that would twist the blade. A wandering center of pressure imposes cyclic torsional loads and control difficulties.
When the cyclic stick is eased forward from a hover, what happens to the rotor's vertical lift?
ACS code: AM.II.N
Correct answer: Vertical lift is reduced.
Rationale: Easing the cyclic forward tilts the rotor disc forward so that total rotor thrust gains a forward (propulsive) component and a smaller vertical component. With the vertical lift reduced, the helicopter begins to descend and accelerate forward unless collective is increased.
The coning angle of a spinning main rotor is established by the balance between which two forces?
ACS code: AM.II.N
Correct answer: Centrifugal force and lift.
Rationale: Rotor coning results from the balance between lift, which bends the blades upward, and centrifugal force, which holds them outward in the plane of rotation. Centrifugal force alone would keep the blades flat, so both forces are required to form the cone.
The purpose of main rotor blade tracking is to bring the blade tips into a common plane of rotation, which is described as bringing them into what condition?
ACS code: AM.II.N
Correct answer: Aligned to a common tip path.
Rationale: Tracking adjusts the rotor so that every blade follows the same tip path, aligning all tips into one common plane of rotation. Proper alignment eliminates the once-per-revolution vibration caused by a blade flying high or low relative to the others.
Most of the lift produced by a rotor blade is the result of which pressure condition?
ACS code: AM.II.N
Correct answer: Low pressure above the blade.
Rationale: Lift comes mainly from the reduced (low) pressure created over the cambered upper surface of the blade as airflow accelerates there. The underside carries higher pressure, and downwash is a result of lift rather than its cause, so the low pressure above the blade is the key factor.
Static stability of a helicopter is best defined as which of the following?
ACS code: AM.II.N
Correct answer: The initial tendency to move back toward neutral after a disturbance.
Rationale: Static stability is the initial tendency of a disturbed helicopter to return toward its original (neutral) condition. A tendency to oscillate about neutral describes dynamic behavior, and stability while hovering is only one specific flight case rather than the general definition.
In forward flight, the increased relative airflow over the advancing blade causes it to do what?
ACS code: AM.II.N
Correct answer: Flap up, which reduces its angle of attack and lift.
Rationale: In forward flight the advancing blade meets a higher relative velocity and tends to generate more lift, so it flaps upward. Flapping up reduces its angle of attack and therefore its lift, which helps equalize lift across the rotor disc.
When transitioning from a hover to forward flight, what change in engine power is generally required?
ACS code: AM.II.N
Correct answer: Increase the engine power.
Rationale: Leaving the hover for forward flight requires tilting the disc with cyclic, which diverts some rotor thrust into propulsion. To keep vertical lift supporting the weight, collective and therefore engine power must be increased; holding or reducing power would let the helicopter sink as it accelerates.
The momentary rise and fall of rotor RPM that occurs immediately after a change in collective pitch is called what?
ACS code: AM.II.N
Correct answer: Transient droop.
Rationale: Immediately after a collective change the governor lags slightly, so rotor RPM momentarily overshoots or dips before settling; this temporary excursion is transient droop. Static droop is the small steady RPM difference that remains, which is a different effect.
Because a rotor system behaves gyroscopically, the maximum flapping response to a blade pitch change occurs approximately how far around the disc?
ACS code: AM.II.N
Correct answer: 90 degrees later in the direction of rotation.
Rationale: Owing to gyroscopic precession, the maximum response to a force applied to a rotor appears about 90 degrees of rotation later (phase lag). A blade given a pitch and lift change therefore reaches its maximum flap displacement roughly 90 degrees further around the disc.
The rotor disc of a helicopter is best described as which of the following?
ACS code: AM.II.N
Correct answer: The circular area swept by the rotating blade tips.
Rationale: The rotor disc is the circular area swept by the turning blades, whose diameter equals the blade tip-to-tip distance. The rotor head hub is the central mounting and ground cushion is a near-surface effect, so neither describes the disc.
A delta-three hinge in a rotor head is designed to achieve which result?
ACS code: AM.II.N
Correct answer: Pitch-flap coupling through an offset hinge axis.
Rationale: The delta-three hinge skews or offsets the flapping hinge axis so that blade flapping is mechanically coupled to a change in blade pitch. This pitch-flap coupling, produced by the offset hinge geometry, reduces flapping and helps stabilize the rotor.