SIFT Principles of Helicopter Flight 3 — Questions and Answers
Question 1: What is ground effect in helicopter operations?
- Turbulence caused by rotor wash hitting the ground
- Increased rotor efficiency when flying within one rotor diameter of the ground (Correct answer)
- The tendency of helicopters to drift near the ground
- Reduced rotor RPM when near the surface
Correct answer: Increased rotor efficiency when flying within one rotor diameter of the ground
Ground effect occurs when the rotor system is within approximately one rotor diameter of the surface, where interference with ground-reflected airflow reduces induced drag and improves efficiency.
Question 2: What does the term 'effective translational lift' (ETL) refer to?
- Maximum lift generated at cruise speed
- The airspeed (typically 16-24 knots) at which the rotor becomes significantly more efficient (Correct answer)
- Lift transferred from the main rotor to the tail rotor
- Vertical lift component during climbing flight
Correct answer: The airspeed (typically 16-24 knots) at which the rotor becomes significantly more efficient
ETL is achieved around 16-24 knots when the rotor fully leaves its own downwash, resulting in a noticeable increase in rotor efficiency and a vibration sometimes called the 'translational lift bump.'
Question 3: In a hover, what primarily causes the helicopter to require more power than in slow forward flight?
- Higher tail rotor demand
- The rotor must work in its own downwash, creating induced drag (Correct answer)
- Increased cyclic deflection causes more drag
- Collective pitch is higher in a hover
Correct answer: The rotor must work in its own downwash, creating induced drag
In a hover, the rotor recirculates its own downwash, increasing induced drag and requiring more power than in forward flight where fresh air is encountered.
Question 4: What is the 'deadman's curve' (height-velocity diagram) used for?
- Identifying wind conditions unsafe for hovering
- Showing combinations of airspeed and altitude that may not allow safe autorotation after engine failure (Correct answer)
- Depicting maximum airspeed limits at various altitudes
- Showing minimum rotor RPM requirements at different weights
Correct answer: Showing combinations of airspeed and altitude that may not allow safe autorotation after engine failure
The height-velocity diagram identifies dangerous airspeed-altitude combinations where, following engine failure, insufficient altitude or airspeed exists to successfully complete an autorotation.
Question 5: What is 'lead-lag' (hunting) motion in rotor blades?
- The fore-and-aft movement of blades in their plane of rotation to accommodate Coriolis effect (Correct answer)
- The vertical flapping of blades under high G-loads
- Blade pitch changes due to collective input lag
- The delay in rotor response after cyclic input
Correct answer: The fore-and-aft movement of blades in their plane of rotation to accommodate Coriolis effect
Lead-lag is the fore-and-aft movement of rotor blades in the plane of rotation, accommodating Coriolis forces as blades change their distance from the rotor hub during flapping.
Question 6: What is the main advantage of a semi-rigid (teetering) rotor system?
- It allows individual blade pitch control
- It is mechanically simple with fewer parts because blades teeter as a pair (Correct answer)
- It allows blades to flap and lead-lag independently
- It provides better performance at high altitudes
Correct answer: It is mechanically simple with fewer parts because blades teeter as a pair
Semi-rigid teetering systems are simpler and lighter because the two-blade assembly teeters as a unit, eliminating the need for individual flapping and lead-lag hinges.
Question 7: What is 'torque effect' in a single-rotor helicopter and how is it countered during a hover?
- Engine vibration transmitted through the airframe; countered by engine mounts
- The fuselage tends to rotate opposite to the main rotor; countered by tail rotor thrust via pedal inputs (Correct answer)
- Rotor tilting from gyroscopic forces; countered by cyclic input
- Increased drag in forward flight; countered by additional collective pitch
Correct answer: The fuselage tends to rotate opposite to the main rotor; countered by tail rotor thrust via pedal inputs
Torque reaction causes the fuselage to rotate opposite the main rotor's direction; the pilot uses anti-torque pedals to adjust tail rotor thrust and maintain heading.
SIFT Principles of Helicopter Flight 3
What is ground effect in helicopter operations?