Army Aviation Information: Rotorcraft Basics
What is being tested
Basic helicopter aerodynamics, flight controls, and Army aviation terminology. This is knowledge recall plus simple application, not engineering mathematics.
Aerodynamics
A rotor blade is an airfoil. Lift depends on angle of attack, airspeed, and air density. Relative wind is the airflow opposite the blade's path; angle of attack is the angle between the blade's chord line and the relative wind. Exceeding the critical angle produces a stall regardless of airspeed. Two standard explanations of lift are the pressure difference described by Bernoulli's principle and the downward deflection of air (Newton's third law).
Flight controls (know these cold)
- Cyclic tilts the rotor disc; it controls pitch (nose up or down) and roll (bank). Forward cyclic lowers the nose and accelerates.
- Collective changes the pitch of all blades together; it controls total thrust and therefore climb or descent. Raising collective demands more power.
- Anti-torque pedals change tail rotor thrust and control yaw and heading, not bank.
- Throttle governs engine power and is often correlated with the collective on modern helicopters.
Performance concepts
Translational lift (and effective translational lift) is the efficiency gained as forward airspeed gives the rotor cleaner air. Ground effect: near the surface downwash is restricted, induced flow drops, and less power is needed to hover. Dissymmetry of lift is the difference in airspeed between advancing and retreating blades, compensated by blade flapping. Retreating blade stall occurs at high airspeed and high loading. Autorotation is an unpowered descent in which air flows upward through the rotor, keeping it turning.
Trap and rule
A common trap is assigning collective to turns and pedals to climbs. Fix the pairing: cyclic controls where the nose points and how the disc tilts, collective controls up and down, pedals control yaw. To climb from a hover, smoothly raise collective and add power while holding heading with the pedals.