SIFT Principles of Helicopter Flight 4 — Questions and Answers
Question 1: What is 'vortex ring state' (settling with power)?
- A condition where rotor tip vortices reduce tail rotor effectiveness
- A dangerous condition where the helicopter descends into its own rotor downwash, causing loss of lift (Correct answer)
- Vibration from rotor blades entering ground-effect vortices
- Increased rotor efficiency from wingtip vortex recycling
Correct answer: A dangerous condition where the helicopter descends into its own rotor downwash, causing loss of lift
Vortex ring state occurs when the helicopter descends at low airspeed with power applied, causing rotor vortices to recirculate through the disc and destroy lift unpredictably.
Question 2: What are the three conditions that lead to vortex ring state?
- High altitude, heavy weight, and crosswind
- Low airspeed (under ~15 knots), high power setting, and high rate of descent (over ~300 fpm) (Correct answer)
- Engine failure, low rotor RPM, and aft cyclic
- Tail wind, low altitude, and high forward speed
Correct answer: Low airspeed (under ~15 knots), high power setting, and high rate of descent (over ~300 fpm)
Vortex ring state typically develops with low forward airspeed, an upward flow through the rotor from the descent, and sufficient power to sustain the vortex circulation.
Question 3: What is the primary recovery technique from vortex ring state?
- Apply full collective and pull aft cyclic
- Lower collective to reduce power and apply forward cyclic to gain airspeed (Correct answer)
- Reduce tail rotor thrust and yaw into the descent
- Increase engine RPM and maintain altitude with collective
Correct answer: Lower collective to reduce power and apply forward cyclic to gain airspeed
Recovery requires lowering collective to reduce power and applying forward cyclic to gain airspeed, moving the helicopter out of the recirculating vortex.
Question 4: What is 'dissymmetry of lift' and how does a fully articulated rotor correct for it?
- Uneven lift between fore and aft rotor disc; corrected by differential collective
- Uneven lift between advancing and retreating blades in forward flight; corrected by blade flapping (Correct answer)
- Lift difference between high and low rotor RPM; corrected by governor
- Asymmetric lift from wind gusts; corrected by cyclic input
Correct answer: Uneven lift between advancing and retreating blades in forward flight; corrected by blade flapping
Dissymmetry of lift is the unequal lift between the faster advancing blade and slower retreating blade; flapping hinges allow blades to rise and descend, equalizing angles of attack.
Question 5: What is the purpose of the freewheeling unit (sprag clutch) in a helicopter drivetrain?
- To limit maximum rotor RPM
- To allow the rotor to continue spinning freely if the engine fails or is decoupled (Correct answer)
- To engage the tail rotor at high airspeeds
- To regulate engine torque during power changes
Correct answer: To allow the rotor to continue spinning freely if the engine fails or is decoupled
The freewheeling unit automatically disconnects the engine from the rotor transmission when engine RPM drops below rotor RPM, allowing the rotor to autorotate freely.
Question 6: What is 'mast bumping' and why is it dangerous?
- Vibration from an unbalanced rotor causing the mast to oscillate
- Contact between the rotor hub and the rotor mast in semi-rigid rotor systems, which can cause blade separation (Correct answer)
- The main rotor striking the tail boom during extreme maneuvers
- Resonance between rotor frequency and airframe structure
Correct answer: Contact between the rotor hub and the rotor mast in semi-rigid rotor systems, which can cause blade separation
Mast bumping occurs in teetering rotor systems when the rotor hub contacts the mast, typically in low-G flight, and can rapidly lead to catastrophic blade separation.
Question 7: What is 'ground resonance' in helicopters?
- Vibration from rotor downwash reflecting off the ground surface
- A destructive self-reinforcing oscillation from unequal blade spacing amplified through landing gear during ground contact (Correct answer)
- Resonance between engine frequency and airframe during ground run-up
- Tail rotor resonance from crosswinds while on the ground
Correct answer: A destructive self-reinforcing oscillation from unequal blade spacing amplified through landing gear during ground contact
Ground resonance is a potentially destructive oscillation where rotor blade lead-lag imbalance couples with landing gear movement to create amplifying vibrations that can destroy the helicopter.
SIFT Principles of Helicopter Flight 4
What is 'vortex ring state' (settling with power)?