In forest aerial spraying, how can spray concentration vary with slope when using helicopters?

• A. The uphill side can be more concentrated when spraying uphill, and the downhill side when spraying downhill
• B. The downhill side is always more concentrated
• C. Concentration is uniform regardless of slope
• D. Spray is not affected by slope

Answer: (A)

Spray distribution on slopes is affected by how the rotor downwash interacts with the terrain. On a hillside, gravity and the angled surface cause droplets to settle differently depending on the direction you’re spraying.

If you’re spraying uphill, the downward-moving air and the slope steer droplets toward the uphill edge, so that side ends up more concentrated. If you’re spraying downhill, the same forces push droplets toward the downhill edge, increasing concentration there. This creates a pattern where the uphill side is more concentrated when spraying uphill, and the downhill side is more concentrated when spraying downhill.

This contrasts with ideas of uniform distribution or no slope effect, which don’t account for how terrain alters droplet movement and deposition. In practice, this means planning the flight path and swath width with slope direction in mind to achieve even coverage.