100+ Free Module 12 Helicopter Systems Practice Questions

Explanation: Dissymmetry of lift occurs in forward flight due to the difference in relative wind speed between the advancing blade (rotational speed plus forward speed) and the retreating blade (rotational speed minus forward speed). If uncompensated by flapping, it causes an upward tilt on the advancing side.

Explanation: In a semi-rigid teetering rotor, the two blades are rigidly attached to a common hub that tilts (teeters) on a trunnion. As the advancing blade experiences higher relative airflow, it flaps up, decreasing its angle of attack, while the retreating blade flaps down, increasing its angle of attack.

Explanation: Due to gyroscopic precession and aerodynamic resonance in the rotor system, applying a force or pitch change at a specific point results in the maximum blade displacement occurring approximately 90 degrees later in the direction of rotation.

Explanation: During autorotation, the rotor disc is divided into three regions: the driven (propeller) region near the blade tip, the driving (autorotative) region in the mid-span where total aerodynamic force is tilted forward, and the stall region near the blade root where angle of attack exceeds critical angle.

Explanation: When a rotor blade flaps upward, its center of gravity shifts inward toward the axis of rotation. By conservation of angular momentum ($m v r = \text{constant}$), reducing radius $r$ causes the blade to accelerate forward (lead) in its plane of rotation.

Explanation: Vortex Ring State occurs when a helicopter descends into its own downwash at low forward speed (< ETL) with power applied and a descent rate usually exceeding 300-500 ft/min. Recirculation of air around the blade tips destroys lift and increases rate of descent.

Explanation: ETL occurs between 16 and 24 knots when the main rotor completely leaves its own recirculated hover downwash and moves into clean, undisturbed air. Induced flow is reduced, increasing overall rotor efficiency and lift for a given collective setting.

Explanation: In a semi-rigid teetering hub, placing the teetering hinge pin above the center of gravity of the blade assembly (underslung design) ensures that as the hub teeters, the CG of the combined blades stays at a nearly constant radial distance from the mast axis, minimizing Coriolis forces and eliminating the need for lead-lag hinges.

Explanation: In forward transition (10–20 kts), air passing over the rear portion of the rotor disc has been acted upon by the blades longer, acquiring a higher downwash (induced) velocity than air over the forward portion. This causes less lift at the rear, which due to 90° phase lag manifests as a lateral roll.

Explanation: Hovering near the surface (within ~1 rotor diameter) causes the ground to physically restrict the full development of rotor downwash and tip vortices. This reduces induced flow velocity, increases effective angle of attack, and reduces induced drag, lowering power required.