Which forces in the lift equation can the pilot control?

• A. Density and velocity
• B. Coefficient of lift and velocity
• C. Temperature and humidity
• D. Pressure and humidity

Answer: (B)

The lift you produce is set by L = 0.5 ρ V^2 S CL, so you can influence two of the factors directly. The coefficient of lift, CL, changes with the angle of attack or blade pitch. By raising or lowering the collective (and using cyclic for the rotor disk attitude), you change the rotor blade pitch, which alters CL and thus lift at a given airspeed. The velocity term, V, can be affected by how you fly: using cyclic to propel the helicopter forward or sideways changes the relative wind over the rotor and increases V, while rotor RPM (within limits) also changes the speed at which air moves through the rotor disk.

Air density, ρ, is determined by environmental conditions like altitude, temperature, and humidity, and you can’t control those midflight. Pressure and humidity affect density, but they’re not controllable by the pilot, so they aren’t the factors you actively manipulate.