TD 31

Question 1

Effective Translational Lift

ETL

Answer 1

Effective translational lift (ETL) occurs with the helicopter at about 16 to 24 knots, when the rotor—depending on size, blade area, and RPM of the rotor system—completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air. The rotor no longer pumps the air in a circular pattern but continually flies into undisturbed air. The flow of air through the rotor system is more horizontal, therefore induced flow and induced drag are reduced. The AOA is subsequently increased, which makes the rotor system operate more efficiently. This increased efficiency continues with increased airspeed until the best climb airspeed is reached, when total drag is at its’ lowest point. Greater airspeeds result in lower efficiency due to increased parasite drag.

Question 2

Dissymmetry of Lift

Answer 2

Dissymmetry of lift is the differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half. This difference in lift would cause the helicopter to be uncontrollable in any situation other than hovering in a calm wind. There must be a means of compensating, correcting, or eliminating this unequal lift to attain symmetry of lift.

Question 3

Airflow IGE/OGE

Answer 3

IGE
Rotor efficiency is increased by ground effect to a height of about one rotor diameter (measured from the ground to the rotor disk) for most helicopters. This increase in AOA requires a reduced blade pitch angle. This reduces the power required to hover IGE.

OGE
The benefit of placing the helicopter near the ground is lost above IGE altitude. Above this altitude, the power required to hover remains nearly constant, given similar conditions (such as wind). Induced flow velocity is increased causing a decrease in AOA. A higher blade pitch angle is required to maintain the same AOA as in IGE hover. The increased pitch angle also creates more drag. More power to hover OGE than IGE is required by this increased pitch angle and drag.

Question 4

Thunder Storm

Answer 4

To minimize the effects of thunderstorms encountered in flight, perform the following:

    1. Torque: Adjust torque to a value corresponding to maximum endurance airspeed.  
    2. Occupants: Check that all occupants are seated with seat belts and harnesses tightened.  
    3. PITOT HTR switch(es) - ON.  
    4. Avionics - Reduce volume on any equipment affected by static.  
    5. Interior lights - Adjust to full bright at night to minimize blinding effect of lightning.  

b. In the storm:
1. Maintain a level attitude and constant power setting. Airspeed fluctuations should be expected and disregarded.
2. Maintain original heading, turning only when necessary.
3. The altimeter is unreliable, due to differential barometric pressures within the storm. An indicated gain or loss of several hundred feet is not uncommon and should be allowed for in determining minimum safe altitude.