Which statement best describes the effect of increasing density altitude on hover power and rotor efficiency?

Which statement best describes the effect of increasing density altitude on hover power and rotor efficiency?

• A. Hover power decreases as density altitude increases
• B. Hover power remains constant
• C. Hover power increases as density altitude increases
• D. Hover power is not affected by density altitude

Answer: (C)

Density altitude rising means thinner air, so the rotor has less air mass to push downward for the same blade motion. To hover a given weight you must produce the same thrust, but the reduced air density makes that thrust harder to achieve, so more induced power is required. The induced power for hover increases as density decreases (P_i ∝ T^(3/2)/sqrt(rho A)); as rho drops with higher density altitude, P_i goes up. In other words, hover power increases and rotor efficiency declines with increasing density altitude, so a helicopter needs more power to hover in hot, high conditions.

Density altitude rising means thinner air, so the rotor has less air mass to push downward for the same blade motion. To hover a given weight you must produce the same thrust, but the reduced air density makes that thrust harder to achieve, so more induced power is required. The induced power for hover increases as density decreases (P_i ∝ T^(3/2)/sqrt(rho A)); as rho drops with higher density altitude, P_i goes up. In other words, hover power increases and rotor efficiency declines with increasing density altitude, so a helicopter needs more power to hover in hot, high conditions.