TD 31

Question 1

Effective Translational Lift (ETL)

Answer 1

ETL is when the rotor completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air and occurs at about 16-2 kts.
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.

Indications: As translational lift becomes more effective, the combined effects of gyroscopic precession, dissymmetry of lift, and transverse flow effect cause both the nose to pitch up (blowback) and the aircraft to roll to the right.

Corrective action:
Aviators must correct with additional forward and left lateral cyclic input to maintain a constant rotor-disk attitude.
Four types of drag are induced drag, profile drag, parasite drag, and total drag

Question 2

Dissymmetry of Lift

Answer 2

Definition: differential lift between the advancing and retreating halves of the rotor disk caused by different wind velocities across each half. On the advancing half, add rotational velocity to rotational relative wind. On the retreating half, subtract rotational velocity from rotational relative wind.

Indications: pitch up (blowback) dying takeoff

Corrective action:
Blade flapping (aerodynamic)
Upflap - increases induced flow/drag, decreases lift
Downflap - decreases induced flow/drag, increases lift

Cyclic feathering (mechanical) FWD cyclic applied when blades are right and left, manifested forward and backward

No lift areas produced from dissymmetry of lift (from the hub to tip)

1. Reverse flow
2. Negative stall
3. Negative lift
   * tail rotor also experiences dissymmetry of lift in forward flight, but it compensated by automatic blade flapping by a delta hinge

Question 3

Airflow IGE/OGE

Answer 3

1. Hover occurs when lift produced by downward airflow from the rotor systems equals total weight of the helicopter.
2. Two types of airflow at a hover: induced flow and wing tip vortices.
3. Ground Effect - increased efficiency of the rotor system caused by interference of the airflow when near the ground.

IGE - ground surface to one rotor diameter height
- airflow is smaller due to ground interference pushing air out for recirculation, less induced flow and drag, less power

OGE - one rotor diameter height and above
- airflow is larger, due to no ground interference providing the recirculation, more induced flow and drag, more power

AoA will remain the same, but AoI larger for OGE