Which of the following is NOT a condition that affects rotor downwash?

The condition that does not affect rotor downwash is indeed the altitude of the helicopter. Rotor downwash refers to the downward flow of air produced by the rotor system of a helicopter as it generates lift.

The weight of the helicopter impacts the amount of lift that needs to be generated, thereby influencing the amount of downwash produced – heavier helicopters with higher weight require more lift and thus create a stronger downwash. Similarly, air density has a direct impact on rotor performance. As air density decreases (like at higher altitudes), the rotor's ability to generate lift is diminished, which can affect the nature and intensity of downwash. Lastly, forward speed influences downwash as increased speed modifies the airflow patterns around the rotor, augmenting the velocity and distribution of downwash.

In contrast, while altitude can have effects on overall performance due to changes in air density, it does not inherently change the characteristics of rotor downwash itself at a given weight and loading condition. Therefore, altitude is not a direct factor that varies the downwash produced by the rotor system.