10.1 Rotorcraft Principles: Helicopters & Gyroplanes

Rotorcraft Principles: Helicopters & Gyroplanes

An Advanced Ground Instructor may give ground training for any certificate or rating except the instrument rating, including rotorcraft ratings, and 14 CFR 61.213(a)(4)(ii) pulls in the knowledge areas for every aircraft category. The FAA's AGI sample questions include helicopter performance, helicopter never-exceed speed, surface taxi technique, cyclic inputs and gyroscopic precession, blade flapping, and gyroplane rotor behavior. The primary reference is the Helicopter Flying Handbook (FAA-H-8083-21B), with gyroplane material in the older Rotorcraft Flying Handbook (FAA-H-8083-21).

Helicopter Flight Controls

In a hover the pedals control heading, the collective controls height, and the cyclic controls position. During a surface taxi in a wheeled helicopter, the Helicopter Flying Handbook states that the collective controls starting, stopping, and speed, the cyclic maintains ground track, and the pedals maintain heading. The cyclic is not used to control groundspeed.

Aerodynamics of the Rotor System

• Gyroscopic precession: A force applied to a spinning rotor takes effect about 90° later in the direction of rotation. A cyclic input that changes blade pitch at one point in the rotation therefore produces maximum flapping 90° later, and the designers rig the controls to account for it. Decreasing pitch on the blade at a position 90° ahead of where the change is needed tilts the disk in the desired direction.
• Dissymmetry of lift: In forward flight, the advancing blade meets relative wind at rotational speed plus forward speed, and the retreating blade at rotational speed minus forward speed. Without compensation, the advancing side would produce more lift.
• Blade flapping: Blade flapping is the upward and downward movement of the blades as they rotate. The advancing blade flaps up, which reduces its angle of attack, and the retreating blade flaps down, which increases its angle of attack. Flapping, helped by cyclic feathering, equalizes lift across the disk.
• Retreating blade stall: At high forward speed, the retreating blade needs so much angle of attack that it stalls. That is the main limit on a helicopter's forward speed. Warning signs are an abnormal low-frequency vibration, a nose-up pitch, and a roll toward the retreating side (left for the counterclockwise rotors on most U.S. helicopters). Recovery: reduce collective, reduce airspeed, ease the maneuver, and restore rotor RPM. Because stall comes earlier in thin air and at high weight, many VNE placards decrease as density altitude increases, and some also vary with gross weight.
• Coriolis effect (conservation of angular momentum): When a blade flaps up, its center of mass moves closer to the mast and it accelerates; when it flaps down, it slows. Lead-lag hinges or blade flexibility absorb the change.

Hovering, Transition & Low-Speed Hazards

• Translating tendency: Tail rotor thrust pushes the whole helicopter sideways in a hover (to the right in U.S. designs with counterclockwise main rotors). Designers tilt the mast or rig the cyclic to offset it.
• Ground effect: Within about one rotor diameter of a firm surface, rotor downwash is restricted and less power is needed to hover (in ground effect, IGE, versus out of ground effect, OGE).
• Effective translational lift (ETL): At roughly 16 to 24 knots of airspeed, the rotor moves into undisturbed air, efficiency rises noticeably, and the helicopter tends to climb; the transition is felt as a brief vibration.
• Settling with power (vortex ring state): The helicopter descends into its own downwash. The Helicopter Flying Handbook lists three conditions: a vertical or nearly vertical descent of at least 300 fpm, the rotor using some engine power (20 to 100 percent), and a horizontal speed slower than ETL. Adding collective makes it worse; recovery is to lower collective and fly forward (or, in some procedures, laterally) into clean air.
• Dynamic rollover: With a skid or wheel on the ground acting as a pivot, the helicopter can roll past its critical rollover angle in a second or two, even at a modest bank angle when the roll rate is high. Smoothly lowering the collective is the most effective recovery.
• Height-velocity diagram: The shaded areas show combinations of height and airspeed from which a safe autorotation after an engine failure may be impossible; avoid them.

Helicopter Performance & Autorotation

The Helicopter Flying Handbook groups the factors that determine performance as density altitude, gross weight, and wind. High density altitude reduces engine power and rotor efficiency, heavy weight raises the power needed to hover, and a headwind (especially above ETL speed) reduces the power required. Performance charts give hover ceilings IGE and OGE.

If the engine fails, a freewheeling unit lets the rotor keep turning. The pilot lowers the collective immediately to preserve rotor RPM, and the upward flow of air through the rotor keeps it turning in autorotation, much as a windmill turns. Near the ground, a flare reduces speed and descent rate, and the pilot cushions the landing with collective.

Gyroplanes

A gyroplane (autogyro) has an unpowered rotor that turns in autorotation for its entire flight, while a separate engine and propeller, usually in a pusher arrangement, provide thrust.

Rotorcraft Certification Notes

Rotorcraft appear throughout Part 61: student pilots may solo at 16; private helicopter applicants log at least 40 hours (61.109(c)); and recreational pilots are exempt from the 180-horsepower limit in rotorcraft (61.101(e)(1)(iii)). Cross-country time for rotorcraft ratings uses a landing point more than 25 NM from the original departure point (61.1).