SIFT Principles of Helicopter Flight 5 — Questions and Answers
Question 1: What happens to required tail rotor thrust as altitude increases?
- It decreases because torque decreases at altitude
- It increases because the tail rotor must work harder in thinner air to produce the same anti-torque force (Correct answer)
- It remains constant regardless of altitude
- It increases only when hovering out of ground effect
Correct answer: It increases because the tail rotor must work harder in thinner air to produce the same anti-torque force
At higher altitudes, thinner air reduces tail rotor thrust efficiency, requiring higher pitch angles or limiting the anti-torque authority available to the pilot.
Question 2: What is the difference between hovering in ground effect (IGE) and out of ground effect (OGE)?
- IGE requires more power because the helicopter is closer to obstacles
- OGE requires more power because the rotor cannot benefit from ground interference with downwash (Correct answer)
- IGE requires more power because rotor vortices are trapped under the helicopter
- There is no power difference between IGE and OGE at the same weight
Correct answer: OGE requires more power because the rotor cannot benefit from ground interference with downwash
OGE hovering requires significantly more power because the rotor operates without the efficiency benefit of ground effect, and density altitude OGE limits are lower than IGE limits.
Question 3: What is 'LTE' (Loss of Tail Rotor Effectiveness) and under what conditions does it most often occur?
- Loss of tail rotor drive shaft, most common in hard landings
- Reduced tail rotor authority during certain wind and power conditions, most often at low airspeeds in left crosswind or tailwind (for US helicopters) (Correct answer)
- Tail rotor blade erosion reducing thrust, common in sandy environments
- Gyroscopic loss of effectiveness above critical rotor RPM
Correct answer: Reduced tail rotor authority during certain wind and power conditions, most often at low airspeeds in left crosswind or tailwind (for US helicopters)
LTE occurs when wind interferes with tail rotor effectiveness, particularly from certain wind azimuths during low-speed flight, reducing the pilot's ability to control yaw.
Question 4: Why is it dangerous to reduce collective during a high-speed autorotation just before touchdown?
- It prevents flare effectiveness and causes a hard landing
- Reducing collective too early depletes rotor energy needed for the flare and cushioned touchdown (Correct answer)
- It causes the tail rotor to produce excessive thrust
- It causes the rotor to overspeed and risk blade separation
Correct answer: Reducing collective too early depletes rotor energy needed for the flare and cushioned touchdown
Rotor RPM stores inertial energy; reducing collective prematurely in autorotation bleeds this energy, leaving insufficient rotor kinetic energy for the flare and cushioned landing.
Question 5: What is the purpose of the collective pitch control?
- To tilt the rotor disc forward and backward for directional control
- To simultaneously increase or decrease the pitch angle of all main rotor blades to control altitude and power (Correct answer)
- To control engine throttle independently of rotor pitch
- To change the pitch of the tail rotor blades for anti-torque control
Correct answer: To simultaneously increase or decrease the pitch angle of all main rotor blades to control altitude and power
The collective changes the pitch of all main rotor blades equally and simultaneously, controlling the total lift generated and the helicopter's altitude.
Question 6: What is 'compressibility effect' and at what rotor condition does it become significant?
- Air compressing under the fuselage during high-speed descent
- Shock wave formation on advancing rotor blade tips at high forward airspeeds, causing drag rise and vibration (Correct answer)
- Increased air density in ground effect compressing the rotor disc
- Blade deformation from centrifugal force at high RPM
Correct answer: Shock wave formation on advancing rotor blade tips at high forward airspeeds, causing drag rise and vibration
Compressibility effects occur when the advancing blade tip speed approaches the speed of sound, forming shock waves that dramatically increase drag and cause airframe vibration, limiting maximum airspeed.
Question 7: In a coordinated turn, what control inputs does a helicopter pilot typically use?
- Collective only, using pitch change for bank angle
- Cyclic to bank, collective to maintain altitude, and pedals to coordinate yaw (Correct answer)
- Pedals to turn and collective to maintain rotor RPM
- Cyclic for bank and pedals only, with no collective change needed
Correct answer: Cyclic to bank, collective to maintain altitude, and pedals to coordinate yaw
A coordinated turn requires cyclic to establish the bank, increased collective to compensate for the reduced vertical lift component, and pedals to maintain coordinated (slip-free) flight.
SIFT Principles of Helicopter Flight 5
What happens to required tail rotor thrust as altitude increases?