Simulations for Designing Safe and Efficient Air Taxis

Overview

The advent of Urban Air Mobility (UAM) redefines the concept of transportation in large cities, and envisions a new era of aviation transportation with highly automated and safe vehicles that will transport passengers and cargo in urban and suburban areas.

NASA plays an important role in the development of the UAM ecosystem by identifying key research areas and establishing a fleet of conceptual vehicles for UAM applications. Different concept vehicles are being developed, including designs like tiltwing, quadcopter, ducted-rotor, tilt-rotor, and ducted-fan aircraft.

Project Details

Understanding the complex flow structures in rotary-wing aircraft is key to designing quieter, safer, and more efficient UAM vehicles. This is where high-fidelity computational fluid dynamics (CFD) comes into play: using NASA’s state-of-the-art supercomputers, we simulate the flow around different air taxi vehicle concepts. The vehicles and the surrounding air are modeled using hundreds of millions of grid points. The flow is solved using NASA’s OVERFLOW CFD code, a high-order accurate Navier-Stokes solver. OVERFLOW is loosely coupled with the comprehensive rotorcraft code CAMRADII. By using the two codes, the complete rotorcraft solution is calculated precisely, including aeroelastic effects and trim.

Results and Impact

Among the different vehicle configurations found across the UAM community, the tiltwing aircraft offers a compelling design. By tilting the wing to 90 degrees, the aircraft can hover, take off, and land vertically, just like a conventional helicopter. An efficient cruise flight or “airplane mode” can be achieved by tilting the wing back to a horizontal position. For this project, we analyze the performance of a tiltwing concept vehicle in hover and in cruise.

Most of the vehicle designs considered for UAM have multiple rotors, while the single-main rotor helicopter is often ignored, despite its importance due to its known path to Federal Aviation Administration certification. We studied the Quiet Single-Main Rotor (QSMR) helicopter concept, with different rotor geometries considered, and found that a drooped tip and reduced tip speed decreases the sound pressure level by 13 decibels when compared to the baseline rotor design. However, cruise performance is affected by these characteristics, and rotor efficiency drops by 60%.

The tiltwing and QSMR designs, part of NASA’s concept UAM vehicles, are intended to focus and guide the agency’ research activities in support of aircraft development for the emerging UAM market. The advanced CFD tools we are developing will assist in the rotorcraft design process for future UAM vehicles.

Why HPC Matters

The performance of UAM vehicles can only be calculated accurately by solving the fluid equations on overset grids, comprising hundreds of millions of grid points. Rotorcraft simulations are unsteady, as the blades are rotating relative to the airframe. The tiltwing vehicle comprises eight rotors with five blades each, and the QSMR needs a very fine grid near the tip to capture the details of the vortices. Only with the most powerful supercomputers can such complex simulations be solved in just a few days.

The simulations were carried out on NASA’s Pleiades, Electra, and Aitken supercomputers at the NASA Advanced Supercomputing (NAS) facility at Ames Research Center, using 1,200 to 3,000 Intel Xeon Broadwell, Ivy Bridge, Skylake, and Cascade Lake processors. In addition, NAS’s mass storage system is essential to support this work—the data stored for post-processing a single case can take up to 50 terabytes of memory.

What's Next

This work comprises a small part of the efforts conducted over the past few years to analyze the performance of NASA’s UAM concept vehicles. Different concepts have been simulated using high-fidelity CFD and the agency supercomputers, including a side-by-side rotorcraft, a quadcopter, the tiltwing, and the QSMR vehicles. The next steps will focus on studying the acoustics of these vehicles, with a clear goal in mind: ensuring the future of UAM by having safer, quieter and more efficient vehicles.

More Information

• Towards Urban Air Mobility
• Towards Urban Air Mobility: The Side-By-Side Air Taxi
• Towards Urban Air Mobility: NASA’s Quadcopter Air Taxi Concept
• High-Fidelity Simulations of a Quadrotor Vehicle for Urban Air Mobility
• Exploring Drone Aerodynamics with Computers

Researchers

• Patricia Ventura Diaz, NASA Ames Research Center
  
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• David Garcia, NASA Ames Research Center
  
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• Seokkwan Yoon, NASA Ames Research Center
  
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