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NASA’s Steve Yoon on the latest research to help shape the eVTOL industry

By Alex Scerri | September 1, 2023

Estimated reading time 17 minutes, 36 seconds.

NASA has always been a hotbed for all forms of aeronautical research, and it is no coincidence that the developing branch of aviation, eVTOL, would be studied as well. Located at Moffett Field in California, NASA Ames Research Center has been one of the locations where this work is still going on in earnest.

Alex Scerri (right) with Steve Yoon (left) outside the NASA Advanced Supercomputing Facility at NASA Ames Center. Steve Yoon Photo

With state-of-the-art facilities, such as the vertical motion simulator (VMS) and the National Full-Scale Aerodynamic Complex (NFAC), the world’s largest wind tunnel recently used by Joby for propeller testing, there is no doubt that the data garnered at this center will help shape eVTOL vehicles and the industry for years to come.

I recently had the opportunity to visit the center, where I spoke to Steve Yoon, chief of the computational physics branch, to get a better appreciation of NASA’s contribution to this emerging ecosystem. The visit would not have been possible without the co-ordination of NASA’s office of communications and VMS lab manager Duc Tran, research scientist and engineer Patricia Ventura Diaz, and aerospace engineer Carlos Malpica, who all took the time to provide some great insight on their work for background for this interview.

This interview has been edited for length and clarity.

Alex Scerri: Steve, can you share your path that brought you to work at NASA Ames?

Steve Yoon: I am an aerospace engineer with a specialty in studying airflow around aircraft with computer simulations. We call this computational fluid dynamics [CFD]. I started working on rotorcraft about 12 years ago by evaluating how to model the airflow around new kinds of rotorcraft, such as the Bell XV-15 tiltrotor (an aircraft that takes off like a helicopter and then flies like an airplane) and coaxial rotors (a configuration that has two rotors turning in opposite directions). Recently, consumer drones like quadcopters have become very popular. To understand the drone multirotor flow interactions, Patricia Ventura Diaz and I analyzed aerodynamics of various quadcopter configurations using high-fidelity CFD methods.

In the past, I conducted research for numerous aeronautics and space exploration vehicles and propulsion systems, including the space shuttle orbiter, space shuttle main engine, hypersonic aircraft, supersonic combustion ramjet engines, and planetary entry vehicles that used similar CFD analysis methods. My Ph.D. research at Princeton University was focused on airflow at high speeds, called transonic aerodynamics.

Alex Scerri: What are the eVTOL related activities at NASA Ames?

Steve Yoon: NASA’s advanced air mobility [AAM] research will transform our communities by providing for the movement of people and goods off the ground, on demand, and into the sky. This air transportation system of the future will include low-altitude passenger transport, cargo delivery, and public service capabilities. NASA is delivering data to guide the industry’s development of electric air taxis and drones and to assist the Federal Aviation Administration in safely integrating these vehicles into the national airspace. This will set the stage for a flourishing industry by 2030.

On the AAM vehicle side, NASA Ames supports the AAM mission by developing advanced tools for rotorcraft conceptual design, the state-of-the-art technologies for computational analysis, flight control evaluations, and wind tunnel test data for validation. The work is particularly focused on passenger-carrying eVTOL vehicles.

The National Full-Scale Aerodynamic Complex (NFAC) at NASA Ames Center. Alex Scerri Photo

Alex Scerri: How and when did this research start?

Steve Yoon: NASA Ames has a long history of working on advanced vertical flight vehicles such as the XV-15 tiltrotor program, which was launched in the 1970s. Complex problems of tiltrotors were studied, and sufficient test data was acquired for validation. The XV-15 was a precursor to the Bell Boeing V-22 Osprey, which is now in service with the Navy, Marines, and Air Force.

Our direct involvement in eVTOL research started with a sub-project called the Design Environment for Novel Vertical Lift Vehicles (DELIVER), managed by Dr. Colin Theodore in 2015. Small multirotor vehicles had often been designed using an approach that consists of “sketch, build, fly, and iterate.” In that approach, there was no systematic way to explore trade-offs or determine logical next steps for design improvements.

It was neither possible to account for multiple real-world constraints up front in design nor to know what the performance would be with a given design. The goal was to demonstrate whether it was possible to apply current conceptual design tools to small and novel vertical-lift vehicle designs, and to improve those tools with new technologies for usability, operability, and community acceptance. We improved our tools and technologies by modeling and design modifications of multiple commercial drones.

Recently, those technologies have been applied to larger vehicle designs, including NASA’s reference vehicles for quadrotor, side-by-side, quiet single-main rotor, and tiltwing conceptual vehicle designs. This work allows us to examine the airflow changes from small drones to larger vehicle concepts, and NASA can demonstrate how to use CFD tools to evaluate different configurations.

Alex Scerri: What is the significance and impact of NASA Ames being in Silicon Valley and so close to many AAM startups?

Steve Yoon: Silicon Valley has been the global center of innovation. We believe that technology drives innovation. NASA Ames Research Center’s proximity to high-tech companies and academia helps cross-sector communications and collaborations. Some of the employees in startup companies worked at Ames before joining the industry. We had a similar experience when SpaceX was a startup.

Alex Scerri: What is the NASA AAM National Campaign?

Steve Yoon: The National Campaign will promote public confidence and accelerate the realization of emerging aviation markets for passenger and cargo transportation in urban, suburban, rural, and regional environments. To help make AAM a reality for the United States, NASA began hosting a series of activities called the AAM National Campaign. The goals are to promote public confidence in AAM safety, give prospective vehicle manufacturers and operators, as well as prospective airspace service providers, insights into the evolving regulatory and operational environment, and facilitate community-wide learning while capturing the public’s imagination. The AAM National Campaign brings together aircraft manufacturers and airspace service providers to identify maturity levels for vehicle performance, safety assurance, airspace interoperability, etc., and to develop and demonstrate integrated solutions for civil use.

Alex Scerri: Can you briefly explain what the NASA Ames VMS is and how it is being used in AAM research?

Steve Yoon: The VMS facility complex provides researchers with exceptional tools to explore, define, and solve issues in both aircraft and spacecraft design. It offers fast and cost-effective solutions using real-time piloted simulation, realistic sensory cues, and the largest motion range of any flight simulator in the world. With the ability to travel up to 60 feet [18 meters] vertically and 40 ft. [12 m] laterally, the six degrees of freedom VMS provides the highest level of motion fidelity available in the simulation community.

Every U.S. aircraft has NASA-developed technology on board, and the VMS supports many of the country’s most sophisticated aerospace research and development programs. The system can be customized to simulate any vehicle — whether existing or in the design stage. Simulated vehicles include supersonic X-planes, airships, helicopters, vertical/short takeoff and landing aircraft, commercial airliners, spacecraft, and more.

The VMS allows NASA researchers to study the limits of what makes a comfortable air taxi ride. The out-the-window graphics, along with the motion of the VMS cockpit — up to 60 ft. [18 m] vertically and 40 ft. [12 m] horizontally inside a 10-story tower — contribute to the feeling of a real flight, including turbulence and wind gusts.

Alex Scerri: Is there anything you can share from NASA Ames studies of passenger ride experience in AAM vehicles? 

Steve Yoon: Airline passengers today expect a smooth ride with little turbulence. While turbulence can’t always be avoided, aircraft design considerations and designs limit what the passenger feels. In order to create a viable market for eVTOLs, designers will have to create a comfortable passenger experience.

NASA is researching ride quality to better understand how these aircraft should be designed for an ideal passenger experience. NASA’s research provides design guidance to industry manufacturers ensuring passengers have a smooth and safe ride. NASA is researching human physiological response to motion, vibration, and noise stimuli that the team expects eVTOL passengers to experience. This work is also happening at NASA’s Armstrong Flight Research Center in Edwards, California.

Alex Scerri: You must be generating a lot of incredibly useful data. How does this find its way to the end-users, such as the AAM original equipment manufacturers (OEM), regulators, etc.?

Steve Yoon: Any non-sensitive documents/information/data that are approved by NASA Form 1676 for public release may be available in the U.S. NASA publishes as much of its information as possible and strives to communicate findings through presentations at technical conferences, participation in Standards Development Organizations committees, and informational working groups, such as the AAM Ecosystem Working Groups.

Alex Scerri: Can you give us some information on NASA’s Revolutionary Vertical Lift Technology (RVLT) Project and the development of conceptual design tools to accurately evaluate the noise and performance of urban air mobility (UAM) vehicles, as well as the upcoming seminars?

Steve Yoon: The three-day in-person workshops have been used for training new NASA employees, academia, and industry partners in the use of NASA-developed software for the eVTOL research area. The workshops in 2022 and 2023 were open to U.S. participants.

Patricia Ventura Diaz is a research scientist and engineer at NASA. Alex Scerri Photo

Alex Scerri: How would an OEM need to proceed to collaborate with NASA Ames, and are the facilities available on a commercial basis?

Steve Yoon: Industry needs to apply for a partnership with NASA through the solicitations listed on the sam.gov website. If chosen, the partner can establish a Space Act agreement to work with NASA. Reimbursable Space Act agreements involve the use of NASA’s facilities, personnel, or equipment primarily for the benefit of the agreement partner. NASA undertakes reimbursable work when it has unique goods, services, or facilities which can be made available to another party in a manner that does not interfere with NASA mission requirements and is consistent with the agency’s mission.

Alex Scerri: What would be your advice to any student wishing to join NASA Ames in the future?

Steve Yoon: Every U.S. aircraft flying today and every U.S. air traffic control facility uses NASA-developed technology in some way. New engineers and researchers at NASA are expected to develop new technologies for next-generation aircraft and spacecraft. Dream big to turn our dream of urban air mobility into reality. Civil service positions at NASA are advertised on usajobs.gov. Students can also apply to intern at NASA Ames by visiting intern.nasa.gov, which can lead to potential career opportunities.

Alex Scerri: With your overview of so much work in the field, when do you think we could see these vehicles in regular service?

Steve Yoon: I think AAM is experiencing a renaissance in aviation. Cars changed how and where we live in the past hundred years. AAM may shape the future of our lives for the next hundred years. First movers in the eVTOL industry are working hard toward certification. However, it is crucial that we continue to make safety a top priority as we work toward this goal. Once the AAM community safely passes the post-certification phase, the AAM market has potential for exponential growth. Air taxis are coming, and they may arrive sooner than we think.

NASA’s supercomputer at NASA Ames. Alex Scerri Photo

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