Which portion of the disc creates the least lift during TFF?

Which portion of the disc creates the least lift during TFF?

• A. Forward half
• B. Center portion
• C. Outer ring
• D. Aft half

Answer: (D)

In a translational flow field, the rotor disk experiences nonuniform air flow when the helicopter is moving forward. The forward half of the disk encounters relatively undisturbed, higher-energy air and tends to generate more lift because the local angle of attack is effectively increased and the blade sections see stronger relative wind. The rear half, however, sits in the rotor’s own wake and in downwash caused by the front portions. This disturbed flow reduces the effective angle of attack and the inflow seen by those blade sections, so they produce less lift. Because this wake and downwash affect the aft portion more, the aft half ends up contributing the least lift during translational forward flight. The center and outer portions don’t suffer as much from wake-induced loss and still benefit from higher local velocities, so they can produce more lift than the aft half in this regime.

In a translational flow field, the rotor disk experiences nonuniform air flow when the helicopter is moving forward. The forward half of the disk encounters relatively undisturbed, higher-energy air and tends to generate more lift because the local angle of attack is effectively increased and the blade sections see stronger relative wind. The rear half, however, sits in the rotor’s own wake and in downwash caused by the front portions. This disturbed flow reduces the effective angle of attack and the inflow seen by those blade sections, so they produce less lift. Because this wake and downwash affect the aft portion more, the aft half ends up contributing the least lift during translational forward flight. The center and outer portions don’t suffer as much from wake-induced loss and still benefit from higher local velocities, so they can produce more lift than the aft half in this regime.