SIFT Principles of Helicopter Flight Questions and Answers 2 — Questions and Answers
Question 1: What is 'translational lift' in helicopter aerodynamics?
- The extra lift generated when forward airspeed improves rotor efficiency at around 16-24 knots (Correct answer)
- The lift provided by the horizontal stabilizer during cruise flight
- The increase in tail rotor lift needed for forward flight
- The lift generated by the fuselage in high-speed forward flight
Correct answer: The extra lift generated when forward airspeed improves rotor efficiency at around 16-24 knots
Translational lift is the improvement in rotor efficiency and increase in lift that occurs when the helicopter moves at approximately 16-24 knots forward and leaves its own rotor downwash behind.
In a hover, the rotor continuously recirculates its own downwash, which reduces efficiency. When the helicopter accelerates to translational lift speed (~16-24 knots), it outruns its downwash, the rotor begins working in cleaner air, efficiency improves significantly, and the helicopter can fly more efficiently. Pilots feel the aircraft 'come alive' at this speed.
Question 2: What is 'autorotation' in helicopter flight?
- Automatic stabilization of the rotor speed by the governor
- Flying the helicopter using only the tail rotor for propulsion
- An unpowered descent where aerodynamic forces keep the rotor spinning to enable a safe landing (Correct answer)
- The automatic rotation of the rotor during engine start
Correct answer: An unpowered descent where aerodynamic forces keep the rotor spinning to enable a safe landing
Autorotation is the emergency procedure where, following engine failure, the rotor is disengaged from the engine and kept spinning by aerodynamic forces during descent, allowing a safe landing.
During autorotation, the collective is lowered immediately to reduce blade pitch and allow the rotor to freewheel. Air flowing up through the rotor from below (caused by the descent) strikes the blades at a positive angle, generating rotor RPM. Just before touchdown, the pilot flares to slow descent rate and then uses stored rotor energy to cushion the landing.
Question 3: What causes 'torque roll' tendency in a single-rotor helicopter?
- Gyroscopic precession from the spinning rotor
- The main rotor's reaction torque trying to rotate the fuselage opposite to the rotor (Correct answer)
- Dissymmetry of lift in forward flight
- Center of gravity shifts during fuel burn
Correct answer: The main rotor's reaction torque trying to rotate the fuselage opposite to the rotor
By Newton's Third Law, if the engine torques the main rotor clockwise, the rotor torques the fuselage counterclockwise. This tendency is countered by the tail rotor.
In a conventional helicopter with a clockwise-rotating (from above) main rotor, the fuselage tends to rotate counterclockwise. The tail rotor produces a horizontal thrust to counteract this. Left pedal increases tail rotor thrust; right pedal decreases it. Loss of tail rotor effectiveness causes uncontrolled fuselage rotation.
Question 4: What is 'retreating blade stall' and when does it occur?
- Stall of all rotor blades during hovering in high density altitude
- Stall of the retreating blade at high forward airspeeds when it cannot generate sufficient lift (Correct answer)
- Stall caused by the pilot lowering the collective too rapidly
- Stall of the tail rotor blades during sharp turns
Correct answer: Stall of the retreating blade at high forward airspeeds when it cannot generate sufficient lift
At high forward airspeeds, the retreating blade's lower airspeed relative to the air may require such a high angle of attack to match lift that it stalls, causing loss of control.
In forward flight, the advancing blade has high airspeed (helicopter speed + blade rotational speed) while the retreating blade has lower airspeed (rotational speed - helicopter speed). To generate equal lift, the retreating blade must fly at a higher angle of attack. At very high speeds, this angle exceeds the stall angle. Retreating blade stall limits helicopter maximum airspeed.
Question 5: What is 'effective translational lift (ETL)' and how does it affect hover performance?
- A measure of lift efficiency that decreases in ground effect
- The speed at which a helicopter achieves maximum efficiency, approximately 16-24 knots, improving hover-out-of-ground-effect capability (Correct answer)
- The lift generated by the horizontal stabilizer during forward flight
- The maximum lift available at sea level standard conditions
Correct answer: The speed at which a helicopter achieves maximum efficiency, approximately 16-24 knots, improving hover-out-of-ground-effect capability
ETL (also called translational lift) occurs at approximately 16-24 knots, where rotor efficiency improves significantly as the helicopter transitions from hovering downwash to cleaner incoming air.
At ETL speed, the helicopter exits its recirculating downwash and the rotor begins operating in undisturbed air. Lift increases, power required decreases, and the helicopter can sustain flight at lower power settings. Climbing through ETL is sometimes called 'transitional lift' and produces a characteristic shudder and climb improvement.
Question 6: What is 'density altitude' and how does it affect helicopter performance?
- The altitude above the highest terrain feature in the area
- Pressure altitude corrected for temperature, representing air density; high density altitude reduces helicopter performance (Correct answer)
- The altitude above sea level as indicated on a standard altimeter
- The altitude calculated using only humidity corrections
Correct answer: Pressure altitude corrected for temperature, representing air density; high density altitude reduces helicopter performance
Density altitude is pressure altitude corrected for non-standard temperature. High temperature, low pressure, and high humidity all increase density altitude, reducing rotor efficiency and engine power.
At high density altitudes, air is less dense: there are fewer air molecules per cubic foot. This means the rotor must work harder to generate lift (blades must spin faster or pitch higher), and the engine produces less power. 'High, hot, and humid' conditions are dangerous for helicopter operations because the aircraft may be unable to hover or climb.
SIFT Principles of Helicopter Flight Questions and Answers 2
What is 'translational lift' in helicopter aerodynamics?