Aerodynamics

Question 1

Translating Tendency

Answer 1

During hovering flight, the counterclockwise rotating, single-rotor helicopter has a tendency to drift laterally to the right. The translating tendency results from right lateral tail-rotor thrust exerted to compensate for main rotor torque.

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.

When blade flapping compensates for dissymmetry of lift, the upward and downward flapping motion changes induced flow velocity. This changes AOA on the advancing and retreating blades.

Advancing: flap up and speed up
Max up flap at 3 o’clock

Retreating: flap down and slow down
Max down flap at 9 o’clock

Question 3

Transverse Flow Effect

Answer 3

In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion.

The front portion of the disk produces an increased AOA and more lift because airflow is more horizontal. These differences in lift between the fore and aft portions of the rotor disk are called transverse flow effect.

This effect causes unequal drag in the fore and aft portions of the rotor disk and results in vibration easily recognizable by the aviator. It occurs between 10 and 20 knots.

Question 4

Effective Translational Lift - ETL

Answer 4

Occurs at about 16 to 24 knots, when the rotor completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air.

As single-rotor aircraft speed increases, translational lift becomes more effective, nose rises or pitches up, and aircraft rolls to the right.