• 29 Jun 2025 07:04 AM
• 0

VFS Downwash and Outwash Workshop

By Mike Hirschberg (VFS) and Dr. Tony Gardner (DLR)
Vertiflite, July/August 2025

The Vertical Flight Society held a two-hour Downwash and Outwash (DWOW) workshop on May 19 as part of Forum 81 (see “Forum 81 — Ideas Awaiting Orders,” Vertiflite, Jul/Aug 2025), organized by VFS Director of Strategy Mike Hirschberg. The workshop discussed the challenges of DWOW in vertical flight, emphasizing the need for improved testing methodology and computational fluid dynamics (CFD) modeling. Presentations from VFS, the UK Civil Aviation Authority (CAA), NASA and CFD developers highlighted the complexities of measuring and predicting downwash effects for helicopters, with even greater importance for multirotor rotorcraft, i.e., tandem helicopters and multi-thruster electric vertical takeoff and landing (eVTOL) aircraft. The US Federal Aviation Administration (FAA) has an initiative in DWOW (see “Washington Report,” Vertiflite, March/April 2025) for helicopters and eVTOL aircraft. However, due to the recent travel and engagement restrictions on US federal employees, no one from the FAA was approved to speak at the event, though some FAA employees were online. A recording of the workshop is posted on the VFS YouTube Channel. 

Detailed Discussions

Hirschberg introduced the workshop, stating that DWOW is an important safety issue that has been largely ignored by the operational community in the past. He proffered that there are four complementary deficiencies that exacerbate the issue:

• Helicopter pilots generally aren’t trained to understand the DWOW problem and how to mitigate its effects.
• Due to a lack of coordinated incident reporting, it’s not known how many other rotorcraft industry workers or bystanders are injured every year around the world. Injuries and damages are usually settled out of court, so there are no public records of DWOW incidents.
• There are limited databases of rotorwash test data for hover, approach and departure.
• CFD models have been inadequate, although there are some important advances underway.

This last point was the primary focus of the workshop.

Rex Alexander, the VFS Infrastructure Advisor, presented videos and data on DWOW incidents, as well as his past work with the National EMS Pilots Association, where safety campaigns based on Occupational Safety and Health Administration (OSHA) and American National Standards Institute (ANSI) standards were developed. Importantly, he highlighted the lack of a standardized approach to calculating DWOW, as well as the challenges of defining a stabilized approach for helicopters.

Dr. Mike Pryce, Emerging Policy Specialist for the UK CAA, highlighted the issue and the importance for safety. In 2022, a bystander was killed and others injured when a helicopter was landing near a parking area at a hospital in Plymouth, England.

Michael Radotich of NASA Ames Research Center outlined their testing plans using a mobile instrumentation cart and hot-film rake to measure outwash velocities of multiple rotors. The NASA test stand used fixed rotors on towers, avoiding the effect of aircraft trim on the result. The potential for new optical techniques to unobtrusively measure flowfields was discussed.

Jan Goericke of Advanced Rotorcraft Technology (ART) described their research and the capabilities of ART’s Viscous Vortex Particle Method (VVPM), as well as their FlightLab and Rotorcraft Comprehensive Analysis System (RCAS) codes. Dan Wachspress of Continuum Dynamics, Inc. (CDI) covered the prior history of DWOW validation and real-time simulation capabilities using the Comprehensive Hierarchical Aeromechanics Rotorcraft Model (CHARM) software. Prof. Juergen Rauleder of Georgia Tech presented their Lattice-Boltzmann simulations of ship deck landings and multirotor interactions, showing the complex unsteady phenomena that need to be considered in downwash modeling. Dr. Richard Brown of Sophrodyne highlighted the challenges of measuring and predicting downwash effects, particularly for multirotor eVTOL aircraft, with emphasis on the need for consistent measurement methods and validation against experimental data.

In the discussion that followed, downwash expert Sam Ferguson (calling in remotely) emphasized the importance of capturing unsteady data and the need to consider real-world interactions with obstacles and buildings. Prof. Marilyn Smith of Georgia Tech noted that historical data and lessons learned from past tests should be evaluated and integrated into the proposed and current research efforts of the group. Ferguson noted that 40–50 years of research and lessons learned are documented in the literature. The group discussed the importance of the type of sensor utilized to collect DWOW data, the need for improved measurement techniques, with laser and optical systems being recommended over traditional mechanical sensors.

Takeaways

The discussion highlighted the safety challenges associated with DWOW and its impact on helicopters and future advanced air mobility (AAM) aircraft. The consensus of the group was to improve computational methods of varying capabilities to be able to address the problem for current helicopters, rotorcraft and future VTOL aircraft configurations.

The group also highlighted the need for collaboration with government agencies, research institutions, CFD developers and industry stakeholders to address these issues, particularly regarding communications and information sharing between the different constituencies. VFS suggested taking a leadership role in helping to coordinate future research and data sharing. For instance, VFS has established a DWOW page (www.vtol.org/downwash) that also includes a library of research literature in this area.

A working group is being formed among the VFS technical committees to further engage the technical community. Contact Dr. Tony Gardner, the Chair of the VFS Aerodynamics Technical Committee, to get involved or for assistance in contacting an expert. Members can find contact information for other members and join the DWOW discussion group at www.vtol.org/hover.

For those who are looking for a starting point for downwash and outwash, the standard work is Sam Ferguson's 1994 “Rotorwash Analysis Handbook,” DOT/FAA/RD-93/31. This can be downloaded from the VFS DWOW Library at www.vtol.org/downwash-library.