Rotor Downwash

Definitions

Downwash is the downward flow of air through the rotor disk, and is the same as the induced flow through the rotor disk.

Objective(s)

Understand how downwash is produced, what conditions affect the strength of the downwash, and operational considerations to minimize the effect of helicopter downwash on aircraft, personnel, and objects on the vicinity of the helicopter.

Description

1. Main rotor downwash and behavior
2. Different flight configurations and the strength of rotor downwash
3. Safe operating procedures for managing the effects of rotor downwash

Instructional aids

None specified

Content

How downwash is produced

• In a hover, rotor downwash is equal to the net upward rotor thrust. Air is drawn from above the disk and accelerated through the disk.
• Downwash 3-2-1
  • Spreads outward in all directions to a distance approximately 3 times the rotor diameter.
  • The velocity of the downwash reaches a maximum about 2 rotor diameters below the disc.
  • When hovering out of ground effect at greater than 1 rotor diameter, rotor downwash dissipates into the surrounding air without appreciable surface interference.
  • Below 0.75 times the rotor diameter, the surface interferes with the downwash, producing ground effect.
• Downwash is present below the helicopter, and spreads outward in all directions.

Flight configuration and downwash

• Disc loading and air density determine the strength of the rotor downwash
  • Stronger downwash is produced at high gross weight and low air density (eg, high density altitude)
• Downwash is strongest when the helicopter is in a hover or slow hover taxi.
• Downwash is still produced in forward flight
  • Two trailing vortices are produced above ETL
  • These vortices behave similarly to wingtip vortices produced by airplanes
• Wind can blow rotor downwash downwind from the helicopter

Safe operating procedures

• Avoid operating within 3 rotor diameters of small aircraft when hovering or in a hover taxi.
• Avoid crossing below and behind landing or departing helicopters.
• When landing off-airport, consider the surface: downwash can produce whiteouts or brownouts, and loss of the ground and horizon references near the ground.
• ATC expects helicopter pilots to determine whether their downwash will cause undue hazard to persons or property on the surface (AIM 4-3-17).
• Wheeled helicopters can tax on the surface to minimize the amount of downwash in their vicinity.
• Hover taxiing produces the most downwash, and should not be used if the downwash could damage parked aircraft in the vicinity, or if it will blow dust, snow, or debris.
• Air taxiing is the preferred method for repositioning helicopters on an airport, since this minimizes downwash (and is more efficient).
• Helicopters should not overfly other aircraft, vehicles, or personnel when air taxiing.

Real-life advice and experience

Anecdotal accounts, and more from PPRUNE’s Rotorheads forum

Additional resources

httpv://www.youtube.com/watch?v=KQMyWDHmba8

httpv://www.youtube.com/watch?v=v8HZAC8LF7Y

httpv://www.youtube.com/watch?v=3rUZdHtlSRk

httpv://www.youtube.com/watch?v=AIQY1O2wXts

httpv://www.youtube.com/watch?v=0f3YDL273XM