As DA increases, the rotor system becomes ______ effective.

• A. Less
• B. More
• C. The same
• D. Variable

Answer: (A)

As density altitude rises, the rotor system becomes less effective because air density falls. Lift generated by the rotor is proportional to air density; with thinner air, for the same rotor speed and blade pitch, there is less dynamic pressure on the blades and therefore less lift. In addition, engine power output also decreases with thinner air, so there’s less power available to drive the rotor and maintain rotor RPM and thrust. The combined effect is reduced rotor performance at higher density altitudes. The other options don’t fit because lift and overall rotor effectiveness do not increase or stay the same as air becomes thinner; they reliably trend downward with higher density altitude.