FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
A NOTAR (No Tail Rotor) helicopter produces anti-torque and directional control without an exposed tail rotor. Which principle allows low-pressure air exiting slots along the tailboom to generate a sideward anti-torque force?
- AThe Coandă effect, causing airflow to follow the curved tailboom surface
- BTranslating tendency created by main rotor downwash
- CBernoulli's principle acting directly on the main rotor blades
- DGyroscopic precession acting 90 degrees later in the rotation
Show answer & explanationAnswer & explanation
Correct answer: A. The Coandă effect, causing airflow to follow the curved tailboom surface
NOTAR systems use a variable-pitch fan to blow air into the tailboom, which is expelled through longitudinal slots. This airflow adheres to the curved surface of the boom (the Coandă effect), creating a low-pressure area that combines with main rotor downwash to produce a sideward anti-torque force, supplemented by a direct jet thruster for additional yaw control.
Why the other options are wrong
- B. Translating tendency describes tail-rotor-thrust-induced drift, not the NOTAR airflow principle.
- C. Bernoulli's principle relates to lift generation, not the boundary-layer control used by NOTAR.
- D. Gyroscopic precession is unrelated to how NOTAR generates yaw control.
NOTAR Anti-Torque System
A tail rotor replacement system that uses a fan-driven airflow exiting tailboom slots (via the Coandă effect) plus a direct jet thruster to provide anti-torque and yaw control.
- Variable-pitch fan blows air into the hollow tailboom
- Slots release air that follows the boom surface via Coandă effect
- A direct jet thruster at the tail provides additional directional control
- Eliminates exposed tail rotor blades, reducing certain hazards
Memory trick: No blades, just a boom that 'hugs' the air (Coandă) to push sideways.