Rotorcraft Fundamentals — FAA A&P Test Questions (ACS AM.II.N)

What ACS AM.II.N covers

Questions in this area come from the rotorcraft material in FAA-H-8083-31, and they cluster around four things. First, the controls: the collective changes the pitch of every main rotor blade simultaneously and by an equal amount, the cyclic varies blade pitch through each revolution so the tip-path plane tilts, and the anti-torque pedals change tail rotor pitch to control yaw, which is movement about the vertical axis. Second, the tail rotor itself, which on a single main rotor helicopter counteracts the torque reaction applied to the fuselage by the driven main rotor and also gives the pilot directional control. Third, rotor head types and their hinges. A fully articulated head gives each blade a flapping hinge for up-and-down motion, a lead-lag or drag hinge for fore-and-aft motion in the plane of rotation, and a feathering bearing for pitch change. A semirigid, or teetering, head carries two blades that flap together as a single unit about one central hinge. A rigid head has no flapping or lead-lag hinges and depends on blade flexing instead. Fourth, rotor aerodynamics: angle of attack is the angle between the blade chord line and the relative wind, not between the chord line and the plane of rotation, and dissymmetry of lift appears in forward flight because the advancing blade meets a higher relative airspeed than the retreating blade. Expect questions on how blade flapping and cyclic feathering equalize that difference, on the Coriolis effect and why lead-lag motion has to be allowed and damped, and on autorotation, ground resonance, and rotor track and balance terminology.

Where this sits on the test

ACS AM.II.N is tested on the FAA Airframe written test, one of 1,748 ACS-tagged questions in the Airframe bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

• FAA-H-8083-31

Three traps candidates fall into

1. Candidates reverse the hinges. A flapping hinge is oriented horizontally but permits vertical, up-and-down blade motion, while a lead-lag hinge is oriented vertically but permits horizontal, fore-and-aft motion in the plane of rotation. Name the hinge by the motion it allows, not by the axis it looks like.
2. Collective and cyclic get swapped. The collective changes every blade's pitch by the same amount at the same time and mainly changes total rotor thrust. The cyclic changes each blade's pitch as it travels around the disc, so pitch varies with position and the rotor disc tilts.
3. Dissymmetry of lift gets blamed on engine torque or on the tail rotor. It comes from the airspeed difference between the advancing and retreating blade in forward flight, and blade flapping is what compensates for that difference rather than what creates it.

7 free sample questions from ACS AM.II.N

1. AM.II.NTap an answer

   In a typical single main rotor helicopter, what is the primary purpose of the tail rotor?

   • ATo produce most of the lift that keeps the helicopter airborne in flight
   • BTo counteract the torque produced by the main rotor and provide directional control
   • CTo increase forward airspeed by adding thrust along the longitudinal axis
2. AM.II.NTap an answer

   Which control changes the pitch of all main rotor blades simultaneously and by an equal amount?

   • AThe cyclic pitch control
   • BThe anti-torque pedals
   • CThe collective pitch control
3. AM.II.NTap an answer

   On a fully articulated rotor head, which hinge allows each blade to move forward and aft in the plane of rotation?

   • AThe lead-lag (drag) hinge
   • BThe flapping hinge
   • CThe feathering hinge
4. AM.II.NTap an answer

   The up-and-down movement of a rotor blade about a horizontal hinge is known as what?

   • ALead-lag
   • BFeathering
   • CFlapping
5. AM.II.NTap an answer

   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?

   • ASemirigid
   • BFully articulated
   • CRigid
6. AM.II.NTap an answer

   Dissymmetry of lift on a helicopter main rotor is caused by what?

   • AThe difference in airspeed between the advancing and retreating blades in forward flight
   • BThe torque reaction of the main rotor acting on the fuselage
   • CThe reduction in air density as the helicopter climbs to higher altitudes
7. AM.II.NTap an answer

   In a single main rotor helicopter, the anti-torque pedals primarily control movement about which axis?

   • AThe vertical (yaw) axis
   • BThe longitudinal (roll) axis
   • CThe lateral (pitch) axis

Frequently asked questions

Is helicopter material really on the FAA Airframe written test?

Yes. Rotorcraft is one of the airframe subject areas, so helicopter questions can appear on your Airframe written test even if you never plan to work on helicopters. How many show up varies by test form. Treat it as required vocabulary: rotor system types, control functions, hinge names, and a handful of aerodynamic terms cover most of what is asked.

What is the difference between the collective and the cyclic?

The collective raises or lowers the pitch of all main rotor blades at once and by an equal amount, which changes total rotor thrust. The cyclic varies each blade's pitch as it travels around the disc, so lift is greater on one side than the other and the tip-path plane tilts. One control changes how much lift; the other changes where it acts.

What is the difference between fully articulated, semirigid, and rigid rotor systems?

A fully articulated system gives each blade its own flapping hinge, lead-lag hinge, and feathering bearing, and normally carries three or more blades. A semirigid or teetering system has two blades that flap as a single unit about one central hinge and has no lead-lag hinges. A rigid system has neither flapping nor lead-lag hinges, so the blades flex to absorb that motion.

What causes dissymmetry of lift on a helicopter rotor?

In forward flight the advancing blade adds the helicopter's airspeed to its own rotational speed while the retreating blade subtracts it, so the advancing side would generate more lift if nothing corrected it. Blade flapping, along with the cyclic feathering built into the control system, equalizes lift across the disc so the aircraft does not roll toward the retreating side.

What do the anti-torque pedals actually do?

On a single main rotor helicopter the pedals change the pitch of the tail rotor blades, which changes tail rotor thrust. That controls yaw, or movement about the vertical axis, and it also balances the torque reaction the main rotor transmits to the fuselage. The pedals do not change main rotor pitch and they do not tilt the rotor disc.

All 40 ACS areas

Primary sources

• FAA Mechanic Airman Certification Standards (FAA-S-ACS-1)
• 14 CFR Part 65 — Certification: Airmen Other Than Flight Crewmembers