SIFT Principles of Helicopter Flight 2 — Questions and Answers
Question 1: What is translational lift in helicopter flight?
- Lift generated by forward flight that improves rotor efficiency (Correct answer)
- Lift created by tilting the rotor disc forward
- The vertical component of total rotor thrust
- Lift gained by increasing collective pitch
Correct answer: Lift generated by forward flight that improves rotor efficiency
Translational lift occurs when forward airspeed moves the rotor out of its own downwash, allowing each blade to encounter undisturbed air and generate more lift.
Question 2: What is the primary purpose of the tail rotor on a single-rotor helicopter?
- To provide forward thrust
- To counteract the torque produced by the main rotor (Correct answer)
- To control altitude
- To improve autorotation performance
Correct answer: To counteract the torque produced by the main rotor
The tail rotor produces thrust opposite to main rotor torque, preventing the fuselage from spinning in the opposite direction.
Question 3: During autorotation, how does the pilot control rate of descent?
- By increasing tail rotor thrust
- By adjusting collective pitch (Correct answer)
- By varying cyclic input
- By increasing engine throttle
Correct answer: By adjusting collective pitch
The pilot uses collective pitch to manage rotor RPM and control the rate of descent during autorotation.
Question 4: What is gyroscopic precession as it applies to helicopter rotors?
- The tendency of the rotor to resist changes in speed
- A force applied to a spinning rotor acts 90 degrees later in the direction of rotation (Correct answer)
- The wobble caused by unbalanced rotor blades
- The tendency of rotor blades to flap upward under lift
Correct answer: A force applied to a spinning rotor acts 90 degrees later in the direction of rotation
Gyroscopic precession means a force applied to a spinning disc takes effect 90 degrees in the direction of rotation from the point of application.
Question 5: What is blade flapping in a helicopter rotor system?
- The oscillation of blade pitch under vibration
- The up-and-down movement of blades to equalize lift between advancing and retreating sides (Correct answer)
- The flexing of blades due to centrifugal force
- The lag of blades behind the rotor hub during acceleration
Correct answer: The up-and-down movement of blades to equalize lift between advancing and retreating sides
Blade flapping allows rotor blades to rise on the advancing side and descend on the retreating side to compensate for dissymmetry of lift.
Question 6: What condition is known as 'retreating blade stall'?
- Engine failure causing all blades to lose lift
- Stall of the retreating blade when its angle of attack exceeds critical limits at high forward speeds (Correct answer)
- Stall caused by pulling the cyclic too far aft
- Loss of rotor RPM below the minimum safe threshold
Correct answer: Stall of the retreating blade when its angle of attack exceeds critical limits at high forward speeds
Retreating blade stall occurs at high forward speeds when the retreating blade's slowed relative airspeed requires excessive pitch to maintain lift, exceeding its critical angle of attack.
Question 7: What is the effect of increasing density altitude on helicopter performance?
- Increased lift and improved engine power
- Reduced lift, decreased engine power, and degraded performance (Correct answer)
- No change if outside air temperature is constant
- Increased rotor efficiency due to less air resistance
Correct answer: Reduced lift, decreased engine power, and degraded performance
Higher density altitude means thinner air, reducing rotor lift efficiency and engine power output, significantly degrading helicopter performance.
SIFT Principles of Helicopter Flight 2
What is translational lift in helicopter flight?