Flight Testing Comes Down to Earth

| Aerospace Testing

eVTOL propeller undergoes wind tunnel testing in large chamber

Wind tunnel aerodynamic testing chambers provide vital insights into the performance of a new generation of electric VTOL aircraft

The focus of the aviation industry in making the transformation to electric propulsion has resulted in considerable levels of innovation, not only in power train and propulsion design, but also in aerodynamics.

One aspect of electric aviation that’s stretching the capabilities of aerodynamic testing is the new generation of electric vertical take-off and landing (eVTOL) aircraft, with companies making use of the most state-of-the-art wind tunnel chambers to characterise their new products.

NASA Provides Chamber Facility

NASA’s Ames Research Centre in California’s Silicon Valley is home to the National Full-Scale Aerodynamic Complex (NFAC), which is currently the world’s largest wind tunnel facility.

Managed by the US Air Force’s Arnold Engineering Development Complex, NFAC contains two large operational wind tunnels, which can handle some of the largest and most complex aircraft in the world. Data from propeller testing in the NFAC, which is widely considered to be the gold standard for aircraft aerodynamics and performance, was instrumental in the development of a range of iconic vehicles, including the space shuttle, the V-22 Osprey, the F-35 Joint Strike Fighter and a number of helicopters.

Despite being a military facility, NFAC is highly flexible and has the capability of adapting to emerging technology, something that’s encouraged by the US Air Force’s “AFWERX Prime” programme, the purpose of which is to facilitate innovation in the commercial non-military sectors by making facilities such as NFAC available to a wider set of users than would normally be the case for military establishments.

According to Lt Col Tom Meagher of the AFWERX Prime programme, a cornerstone of AFWERX Agility Prime is fostering interagency partnerships and collaboration to progress the advanced air mobility segment.

“The NFAC testing facility is a perfect example of being able to use unique government test resources and infrastructure critical to enabling industry progression,” he says.

eVTOL Test Campaign at NFAC

One company that’s making use of NFAC is electric passenger aircraft specialist, Joby Aviation, the first eVTOL company to test its propeller in the NFAC’s smaller 40-by-80 foot wind tunnel.

According to JoeBen Bevirt, Founder and CEO of Joby, testing is a critical part of the eVTOL aircraft programme and the opportunity to gather data on the performance of its propellers in one of the world’s largest wind tunnels is an exciting step toward commercialisation.

“This facility helped introduce historic aircraft to the world, and now it’s doing the same for the next generation of sustainable aviation,” he says.

The test campaign will cover all tilt angles and speeds through the expected flight envelope, providing Joby with consistent and high-fidelity data on the performance, loads and acoustics of its propeller systems in support of its certification programme with the Federal Aviation Administration (FAA).

Working in partnership with the US Air Force and NASA, Joby is installing a production-intent electric propulsion unit and propeller assembly in the wind tunnel mounted to a six-degree-of-freedom force and moment balance to capture performance data. The blades are instrumented to measure the loads experienced while rotating, and a representative wing section of the Joby aircraft allows careful analysis of aerodynamic interference effects.

The NFAC propeller test campaign is expected to produce data of unparalleled quality – exceeding what is captured during normal flight testing – due to superior instrumentation and precise control of variables. The full test campaign is expected to take several months to complete.

Wider Testing Programme

This isn’t the first excursion that Joby has taken in cooperating with NASA on aircraft testing campaigns. Previously, the two organisations were working together on a variety of projects exploring electric aircraft technology in general, including the design of the agency’s all-electric X-57 Maxwell prototype. NASA also completed a two-week acoustic testing programme with Joby in 2022 as part of its Advanced Air Mobility National Campaign.

Joby’s piloted, all-electric eVTOL aircraft that’s currently using the wind tunnel has already completed more than 1000 test flights. It is designed to offer a faster and quieter method of providing commercial aerial transport across cities and communities with zero operating emissions. Joby expects to launch commercial aerial ridesharing service in the United States in 2025.

Climatic Wind Tunnel

Further north across the Canadian border is the ACE Climatic Wind Tunnel (ACE CWT), a facility operated at Ontario Tech University.

This is designed to provide independent testing capabilities across a number of industries, including aviation, to validate prototype products under a wide range of climatic conditions.

Climatic simulation systems include solar capabilities, rain, freezing rain, light snow and blizzards – all with wind speeds capable of 300 km/h at temperatures ranging from -40˚C to +60˚C.

Ontario Tech University claims that the ACE CWT is unique due to its 7 to 14.5 m² variable nozzle, which enables testing of subjects ranging from very small to extremely large devices in a wide variety of wind and climatic combinations.

eVTOL Prototype Test at ACE CWT

ACE CWT is available for commercial users as well as academic research tasks. A recent user of the tunnel was Horizon Aircraft, a supplier of hybrid electric VTOL aerial vehicles, which has successfully completed initial transition flight testing of its “Cavorite X5” 50% scale eVTOL prototype aircraft.

Commenting on the success of the testing campaign at ACE CWT, Horizon Aircraft’s CEO, Brandon Robinson explained that the Cavorite X5 prototype’s performance exceeded expectations after exploring high forward speeds and measuring aerodynamic forces, control authority and mechanical system function with the wings open at varying fan speeds.

“We were particularly happy with pitch and roll stability and control throughout the entire transition envelope, as well as the embedded fan performance. Having real-world test results match our detailed predictions was further endorsement of the abilities of our engineering team in successfully modelling the aircraft. We can now use these results to refine our CFD models and further de-risk the outdoor transition flight test programme that is scheduled to begin soon.”

Hybrid eVTOL technology

Horizon’s hybrid eVTOL concept allows the aircraft to fly 98% of its mission in a very low-drag configuration like a traditional aircraft and is one of the only eVTOL aircraft currently able to do so. Horizon believes that flying most of the mission as a normal aircraft is safer, more efficient and will be easier to certify than radical new eVTOL designs.

The Cavorite X5 aircraft can also operate in Short Take-off and Landing (STOL) or Conventional Take-off and Landing (CTOL) modes. The full-scale aircraft will be powered by a hybrid electric architecture that can recharge the battery array in-flight while providing additional system redundancy and flexibility.

Now that the small prototype climatic wind tunnel testing is complete, Horizon Aircraft will move to outdoor transition flight testing next as the company continues the detailed design of its full-scale aircraft, anticipating final assembly for flight testing in 2025.

Jonathan Newell
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