SIFT Principles of Helicopter Flight Questions and Answers — Questions and Answers
Question 1: The tendency of a single-rotor helicopter's fuselage to rotate in the opposite direction of the main rotor is known as the torque effect. What is the primary component that counteracts this force?
- The cyclic pitch control
- The horizontal stabilizer
- The anti-torque system (tail rotor) (Correct answer)
- The main rotor's flapping hinges
Correct answer: The anti-torque system (tail rotor)
According to Newton's Third Law, the engine turning the main rotor creates an equal and opposite reaction, causing the fuselage to spin in the opposite direction. The tail rotor, or anti-torque system, produces thrust to counteract this torque and provide directional control.
Question 2: A helicopter pilot accelerating from a hover experiences a noticeable vibration and a tendency for the aircraft to roll to the right (in a counter-clockwise rotor system) between 10-20 knots. This phenomenon, which precedes Effective Translational Lift, is known as:
- Transverse Flow Effect (Correct answer)
- Retreating Blade Stall
- Translating Tendency
- Ground Effect
Correct answer: Transverse Flow Effect
Transverse flow effect occurs as the helicopter begins to move forward, causing a difference in airflow velocity across the rotor disc. The air flows more horizontally over the front portion and more vertically through the rear portion, creating a lift differential that, due to gyroscopic precession, results in a right roll.
Question 3: A helicopter requires significantly less power to hover at five feet above a runway compared to hovering at 100 feet. Which principle explains this increase in performance near the surface?
- Dissymmetry of lift
- Effective Translational Lift
- Gyroscopic Precession
- In-Ground Effect (IGE) (Correct answer)
Correct answer: In-Ground Effect (IGE)
When a helicopter is close to the ground (typically within one rotor diameter), the rotor's downwash is restricted by the surface. This creates a cushion of higher-pressure air, which reduces induced flow and rotor tip vortices, making the rotor system more efficient and requiring less power to produce the same amount of lift.
Question 4: During the transition from a hover into forward flight, at approximately 16-24 knots, a helicopter experiences a significant increase in lift and efficiency, often resulting in a slight climb without any increase in power. This aerodynamic phenomenon is called:
- Autorotation
- Effective Translational Lift (ETL) (Correct answer)
- Translating Tendency
- Coning
Correct answer: Effective Translational Lift (ETL)
Effective Translational Lift (ETL) occurs when the main rotor moves into undisturbed air, outrunning the recirculation of its own downwash. This cleaner airflow makes the rotor system much more efficient, providing a noticeable increase in lift.
Question 5: Due to the thrust produced by the tail rotor to counteract torque, a single-rotor helicopter in a hover has a natural tendency to drift sideways. This is known as:
- Retreating blade stall
- Transverse flow effect
- Translating tendency (Correct answer)
- Coriolis effect
Correct answer: Translating tendency
Translating tendency, or tail rotor drift, is the helicopter's inclination to drift laterally in the direction of tail rotor thrust. To counter the main rotor's torque (which would spin the nose right in a CCW system), the tail rotor pushes the tail to the right, causing the entire aircraft to want to move to the left. Pilots compensate with cyclic input. Note: The question describes the drift direction for a CW rotor system, but the principle is the same. The key is that tail rotor thrust causes a sideways drift. A better phrasing is 'drift in the direction of tail rotor thrust'. Let's assume a CW rotor to match the answer. Correction: For a standard US helicopter (CCW rotor), the torque is to the right. The tail rotor pushes the tail to the right to move the nose left, which creates a tendency for the entire helicopter to drift to the right. The pilot compensates with left cyclic.
Question 6: The principle that a force applied to a rotating object will manifest its effect approximately 90 degrees later in the direction of rotation is fundamental to helicopter flight controls. What is this principle called?
- Gyroscopic Precession (Correct answer)
- Bernoulli's Principle
- Venturi Effect
- Newton's Third Law
Correct answer: Gyroscopic Precession
Gyroscopic precession dictates that a force applied to a spinning disc (like a helicopter's main rotor) will have its maximum effect about 90 degrees later in the direction of rotation. Helicopter swashplate and pitch link designs are engineered to account for this, applying control inputs 90 degrees ahead of the desired blade action.
SIFT Principles of Helicopter Flight Questions and Answers
The tendency of a single-rotor helicopter's fuselage to rotate in the opposite direction of the main rotor is known as the torque effect.
What is the primary component that counteracts this force?