SIFT Principles of Helicopter Flight 4

Question 1

What is 'vortex ring state' (settling with power)?

Accepted Answer

A dangerous condition where the helicopter descends into its own rotor downwash, causing loss of lift

Answer

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?

Accepted Answer

Low airspeed (under ~15 knots), high power setting, and high rate of descent (over ~300 fpm)

Answer

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?

Accepted Answer

Lower collective to reduce power and apply forward cyclic to gain airspeed

Answer

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?

Accepted Answer

Uneven lift between advancing and retreating blades in forward flight; corrected by blade flapping

Answer

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?

Accepted Answer

To allow the rotor to continue spinning freely if the engine fails or is decoupled

Answer

The freewheeling unit automatically disconnects the engine from the rotor transmission when engine RPM drops below rotor RPM, allowing the rotor to autorotate freely.

SIFT Exam Practice Test

SIFT Exam Practice Test

SIFT Principles of Helicopter Flight 4