Original Message

The rotor disc at 20 knots headwind v tailwind in a hover is experiencing generally the same amount of benefit (ETL). Only when you start moving forward with a tailwind, you run into your dirty air, thereby requiring more power until you are through that column of dirty air with a positive relative airflow on the nose at approx 40 knots ground speed in your scenario. With the headwind scenario, the dirty air stays behind you so you remain in positive airflow off the nose. In a stationary position, 20 knots is 20 knots with some variation due to airframe design. The disc alone does not care.