Did you know that only one fifth of the world’s ocean floor has been successfully mapped? Although this is a substantial improvement from 2017, where 6% was mapped, there is still an abundance of unchartered waters to explore [1]. Fortunately, the recent surge of development around Autonomous Underwater Vehicles (UAVs) has transformed these vessels into advanced research tools that are helping oceanographers understand the depths of our oceans and how to protect these oceanic habitats from climate change.

- ###### [What is an AUV?](#whatisanauv)
- ###### [The importance of hydrodynamic efficiency](#importanceofhydrodynamicefficiency)
- ###### [Case study: Using CFD to reduce AUV drag](#uavreducingdragcasestudy)

---

## What is an AUV? <a name="whatisanauv"></a>

A AUV is a torpedo shaped vessel that can manoeuvre itself close to the sea floor. Some of the latest designs are 1.5-5.5m (5-18ft) long and weigh between 20-1,400kg (45-3,000Ibs) [2]. They utilise sonar technology such as multibeam and side scanning systems that can map the seabed in 3D at much higher resolutions than could previously be achieved with other submarines. AUVs are also instrumented with fluorometers, turbulence sensors, magnetometers and water-quality sensors and all the data collected is stored onboard until the AUV reaches the surface.

<ImageWithCaption
 src="/assets/images/ecomapper-auv-diagram.png"
 alt="AUVs are often equipeed with a variety of sensors and measurement equipment to maximise the amount of data that can be colected during a mission"
 caption={
 
 The layout and features of the Ecomapper AUV. CREDIT:{' '}
 <ExternalLink href="https://www.usgs.gov/media/images/ecomapper-autonomous-underwater-vehicle-auv">
 USGS
 </ExternalLink>
 
 }
 width="700"
 height="190"
 positionCenter={true}
/>

AUVs travel through water via propellors or fixed hydrofoils, depending on the design. Propellor AUVs are typically used for seabed surveys whereas hydrofoil AUVs are more suited for offshore studies. The former uses spinning angled blades to create an area of low pressure above the blades and an area of high pressure below the blades. This pressure difference generates forward thrust.

A hydrofoil AUV uses variable buoyancy propulsion instead. This is where a pump moves internal weights back and forth to alter the vessel’s buoyancy. Its hydrofoils generate lift and glide forward as it rises due to its buoyancy and falls due to gravity.

---

## The importance of hydrodynamic efficiency <a name="importanceofhydrodynamicefficiency"></a>

AUVs are typically powered by onboard batteries and therefore, like any electric vehicle, the available battery capacity defines the speed at which the AUV can travel and the range of the mission. Smaller AUVs are only capable of deploying for several hours, whereas AUVs with larger batteries can survey for several days with survey speeds around 1-2m/s (2-4mph) [2].

<ImageWithCaption
 src="/assets/images/auv-battery.jpg"
 alt="A semicircular tray of battery packs and wiring of a typical UAV"
 caption={
 
 The layout of a typical AUV battery. CREDIT:{' '}
 <ExternalLink href="https://www2.whoi.edu/staff/nforrester/project-auv-li-ion-battery-systems/">
 Woods Hole Oceanographic Institution
 </ExternalLink>
 
 }
 width="700"
 height="351"
 positionCenter={true}
/>

To increase the range of the battery and therefore the potential duration of missions, engineers are focusing on maximising the efficiency of UAV design. One of the key techniques to achieving this is to optimise the shape of the vessel’s body to minimise drag. This means there is less drag for the motor to overcome, demanding less energy from the battery and enabling the AUV to travel for longer. Extending the length of missions will allow researchers to explore more remote places such as underneath ice formations.

- <Link href="/blog/electric-vehicle-aerodynamics-improving-range">
 How can aerodynamics improve the range of electric vehicles?
 </Link>

---

## Case study: Using CFD to reduce AUV drag <a name="uavreducingdragcasestudy"></a>

<ExternalLink href="https://www.airflowsciences.com/resources/case-studies">
 Airflow Sciences Corporation
</ExternalLink>
, a USA-based fluid dynamics engineering firm, were tasked with optimising the hydrodynamics
of a new AUV concept. The main goal was to optimise the shape of the hull to generate
the least amount of drag. To achieve this, a number of CFD tools were used to evaluate
different designs and establish the one with the highest performance.

The first stage involved simulating the flow of a variety of hull shapes in 2D. This allowed a quick comparison between different designs and visually showed the behaviour of surface pressure and where the flow separates. As expected, the shape with the more gradual curvature showed much less flow separation, as shown below.

<ImageWithCaption
 src="/assets/images/surface-pressure-distribution-auv-concepts.png"
 alt="Side view of the surface pressure distribution of two different AUV hull designs in 2D"
 caption={
 
 The surface pressure distribution of two different AUV hull concepts in 2D
 
 }
 width="700"
 height="470"
 positionCenter={true}
/>

The hull design with the most potential was then developed into a 3D model and simulated in CFD using the web-based app <Link href="/pricing">AirShaper</Link>. This software automatically creates the mesh, runs the CFD simulation and consolidates the results into a PDF report which includes a range of plots such as total pressure, surface pressure, and velocity streamlines.

<YouTubeEmbed videoId="O_DNDxCtI_I" />

However, as this AUV design was a new concept and therefore there was no experimental data to use as a benchmark, Airflow Sciences decided to use several CFD solvers to analyse the results.

‘We felt AirShaper was a cost-effective way to obtain a comprehensive analysis of the OpenFOAM k-omega turbulence model,’ explains Robert Mudry, President and Airflow Sciences Corporation. ‘We wanted to validate the AirShaper results, but with no data available to compare against, we decided to run the same mesh, boundary conditions and turbulence model in two other independent CFD solvers; <ExternalLink href="https://www.azorecfd.com">Azore</ExternalLink> and Fluent.’

<ImageWithCaption
 src="/assets/images/auv-airflow-sciences-drag-coefficient-diagram.png"
 alt="Diagram showing the AirShaper drag coefficient of the Airflow Sciences AUV design on a scale with other notable objects"
 caption={
 
 AirShaper calculated the drag coefficient of the Airflow Sciences AUV
 design to be 0.133
 
 }
 width="700"
 height="176"
 positionCenter={true}
/>

The results from the AirShaper simulation are shown below and the predicted drag coefficient of the optimised hull design was 0.133 in AirShaper, 0.128 in Azore and 0.143 in Fluent. ‘There was minimal difference in drag forces between the different CFD solvers and the variance of the drag coefficient figures was within an acceptable margin of error. This gave us confidence in the AirShaper software as well as in our hull design.’

<ImageWithCaption
 src="/assets/images/auv-airflow-sciences-surface-pressure-velocity-streamlines.png"
 alt="CFD screenshots showing a coloured surface pressure contour plot on the left and coloured velocity streamlines on the right"
 caption={
 
 The surface pressure contour plot (left) and velocity streamline plot
 (right) from the AirShaper report
 
 }
 width="700"
 height="197"
 positionCenter={true}
/>

Furthermore, as AirShaper also provided the numerical simulation files, Airflow Sciences could then conduct more detailed analysis as well as create custom graphics and videos using Azore.

‘We appreciate the opportunity to collaborate with Wouter Remmerie and the AirShaper team,’ concludes Mudry. ‘The simulation process using the AirShaper interface was straight-forward, the results were obtained quickly, and the report complete. We also liked the ability to import the OpenFOAM CFD results into the Azore post-processing software to create additional results in our corporate format. We are looking forward to more Airflow-AirShaper joint projects in the future.’

<YouTubeEmbed videoId="u3DXTrkba9g" title="Animated pathlines of an AUV" />

---

### Interesting links:

- <ExternalLink href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/autonomous-underwater-vehicle">
 An overview of Autonomous Underwater Vehicles
 </ExternalLink>

- <ExternalLink href="https://www.youtube.com/watch?v=4aaPZ80Yo4s&ab_channel=TNO">
 The benefits of Autonomous Underwater Vehicles
 </ExternalLink>

---

### References

[1] 2020. <ExternalLink href="https://www.unesco.org/en/articles/nearly-fifth-worlds-ocean-floor-now-mapped">Nearly a fifth of world's ocean floor now mapped</ExternalLink>. [Online]. Unesco.

[2] <ExternalLink href="https://www.vims.edu/research/units/legacy/cornwallis/auv/index.php">What is an AUV?</ExternalLink>. [Online]. VMS Virginia Institute of Marine Science.

---

Reducing the drag of an AUV with AirShaper

Hydrofoil Design - America's Cup Technology for Commercial Products

Safety is of paramount importance when travelling in the air. This makes certifying an aircraft a long and detailed process. However, over the last few decades, Computational Fluid Dynamics (CFD) has become instrumental in streamlining flight certification. The capabilities of high performance computing along with the flexibility of open source software and improvements in solver accuracy has made the latest CFD codes a viable alternative to expensive wind tunnel testing. 

We sat down with David Solar, Head of General Aviation (GA) and Vertical Take-off and Landing (VTOL) Department at EASA to hear his thoughts on the use of CFD in passing aircraft EASA regulations.

- ###### [What is certification?](#whatiscertification)
- ###### [The history of certifying aircraft](#certificationhistory)
- ###### [The dangers of test flying](#testflyingdangers)
- ###### [How are today's aircraft certified?](#certifyingtodaysaircraft)
- ###### [The structural integrity testing pyramid](#structuraltestingpyramid)
- ###### [Certifying the use of CFD](#certifyingcfd)
- ###### [Could the future of certification be simulation only?](#simulationonlycertification)
- ###### [Extreme configurations at the limit of separation](#extremeconfigurations)

---

## What is certification? <a name="whatiscertification"></a>

Certification is where governing bodies work with manufacturers and operators to ensure that an aircraft and its components are designed, manufactured and modified for safety throughout its life cycle. These governing bodies include the European Union Aviation Safety Agency (EASA), the UK’s Civil Aviation Authority (CAA) and the Federal Aviation Administration (FAA) in the United States. Each authority defines a set of regulations which a new aircraft or operation needs to comply with. So to prove the airworthiness of new developments, manufacturers have to conduct comprehensive testing programmes. 

---

## The history of certifying aircraft <a name="certificationhistory"></a>

Before Computer Aided Design (CAD) and Finite Element Analysis (FEA), flight certification had to rely on many physical flight tests. For example, the certification of the Anglo-French Concorde supersonic transport project in 1960 required six development airframes which flew for 5,127 hours [1]. Although, this project was arguably the most ambitious of it’s time.

<ImageWithCaption
 src="/assets/images/1960-concorde-101.jpg"
 alt="Side view of a white 1960s Concorde G-AXDN (101) on a runway"
 caption={
 
 Twenty Concorde were built of which six were used solely for development and certification. CREDIT:{' '}
 <ExternalLink href="https://www.heritageconcorde.com/airframe-detail">
 Heritage Concorde
 </ExternalLink>
 
 }
 width="700"
 height="421"
 positionCenter={true}
/>

Fast forward to 2014 and certification of the Airbus A350 was achieved with five aircraft and only 2,600 hours of flying [2]. Whilst the A350 may not have been the technical leap of Concorde, it arguably exists in a much more stringent regulatory era.

<ImageWithCaption
 src="/assets/images/airbus-a350.jpg"
 alt="A white Airbus A350 on a runway"
 caption={
 
 The Airbus A350 was first certified in 2014. CREDIT:{' '}
 <ExternalLink href="https://www.airbus.com/">
 Airbus
 </ExternalLink>
 
 }
 width="700"
 height="439"
 positionCenter={true}
/>

---

## The dangers of test flying <a name="testflyingdangers"></a>

Historically, test pilots have been revered due to the hazardous nature of their work. "Flight tests were only experimental and the risks pilots were taking was a big unknown at the time, which is why their life expectancy was short in some cases," explains Solar. 

"Now however, it has evolved significantly. Whilst there is always a risk to the first flight of an aircraft, simulations give us some confidence in how the aircraft will perform aerodynamically as well as it's landing qualities." 

- <Link href="/cases/optimization/cessna">Using CFD to optimise the efficiency of a Cessna winglet</Link> 

The risk of flight tests was demonstrated by the loss of the prototype BAC 1-11 and its seven crew members in 1963. The test was investigating the aircraft’s characteristics during stall and the high angle of attack lead to the wings blocking airflow to the control surfaces on the high tail. This resulted in deep stall, which is a specific risk to this type of aircraft. Sadly, this was not well understood at the time and was unrecoverable. 

<ImageWithCaption
 src="/assets/images/bac1-11-prototype.png"
 alt="Side view of the BAC 1-11 prototype in the air"
 caption={
 
 Understanding aircraft behaviour around stall with modern CFD and simulation might have been able to prevent the loss of the prototype BAC 1-11. CREDIT:{' '}
 <ExternalLink href="https://www.baesystems.com/en/heritage/bac1-11">
 BAE Systems
 </ExternalLink>
 
 }
 width="700"
 height="394"
 positionCenter={true}
/>

---

## How are today's aircraft certified? <a name="certifyingtodaysaircraft"></a>

The EASA type-certification process has four main steps [4]. How a manufacturer goes about completing these steps will vary, but is always after discussion and agreement with EASA. 

- Technical Familiarisation and Certification Basis

- Establishment of the Certification Programme

- Compliance Demonstration 

- Technical Closure and Issue of Approval 

The first two phases start before there is a completed design or a physical aircraft. CFD is used extensively to optimise the key performance characteristics of an aircraft, according to the specific requirements of the project. Presenting the concept and the CFD results to EASA, the two parties can come to an agreement about the exact nature of the certification process. This will be unique to each aircraft. 

Once the certification programme has been agreed, the next stage is Compliance Demonstration and Technical Closure and Issue of Approval. This is where the manufacturer needs to demonstrate that an aircraft meets the requirements of its type approval. Generally, this comes down to proving its structural integrity and illustrating it has suitable flight characteristics.

---

## The structural integrity testing pyramid <a name="structuraltestingpyramid"></a>

From a structural point of view, manufacturers are required to document every step of the design process from materials coupon testing to full aircraft flights. In modern aircraft, a blend of simulation and physical testing is used to make this process as efficient as possible.

<ImageWithCaption
 src="/assets/images/structural-integrity-testing-pyramid.png"
 alt="Infographic showing the importance of testing and simulation from level 1 materials to level 6 full scale aircraft"
 caption={
 
 Modern certification requires physical testing and simulation to work hand in hand. CREDIT:{' '}
 <ExternalLink href="https://www.appluslaboratories.com/global/en/what-we-do/service-sheet/aerospace-structural-testing">
 Applus Laboratories
 </ExternalLink>
 
 }
 width="700"
 height="442"
 positionCenter={true}
/>

CFD plays a key role in the process by generating flight load cases for the different sign-off criteria. Take-off, landing, cruise, stall and even taxiing will present different loads to different parts of the airframe. Even with a physical aircraft it is difficult to measure these loads, so CFD is used extensively to help engineers decide how to test each part.

- <Link href="/blog/airshaper-validation-study">Validating AirShaper with physical testing</Link> 

<ImageWithCaption
 src="/assets/images/aircraft-physical-testing.jpg"
 alt="Front view of a Boeing 787 Dreamliner during a physical rig test"
 caption={
 
 Physical confirmation testing of extreme load cases is based on CFD and FEA studies. CREDIT:{' '}
 <ExternalLink href="https://www.wired.com/2010/03/boeing-787-passes-incredible-wing-flex-test/">
 Wired
 </ExternalLink>
 
 }
 width="700"
 height="433"
 positionCenter={true}
/>

---

## Certifying the use of CFD <a name="certifyingcfd"></a>

Simulation tools are now used so frequently to support the certification process, regulatory bodies have had to specify the requirements of suitable CFD and FEA software packages. This ensures that only reliable and accurate codes are used by manufacturers to model scenarios and assess the flightworthiness of an aircraft. 

In 2020, EASA issued a <ExternalLink href="https://www.easa.europa.eu/en/proposed-cm-s-014-modelling-simulation-consultation">Notification of a Proposal to issue a Certification Memorandum</ExternalLink> on the subject of modelling and simulation. As well as demonstrating their aircraft is fit to fly, manufacturers also have to show that the analysis tools are accurate enough to calculate relevant load cases. 

<ImageWithCaption
 src="/assets/images/easa-cfd-regulations.png"
 alt="Screenshot of section B4 of EASA CM-S-014 regulations covering CFD"
 caption={
 
 EASA CM-S-014 highlights the potential sources of inaccuracy in a CFD simulation. CREDIT:{' '}
 <ExternalLink href="https://www.easa.europa.eu/en/proposed-cm-s-014-modelling-simulation-consultation">
 EASA
 </ExternalLink>
 
 }
 width="750"
 height="455"
 positionCenter={true}
/>

---

## Could the future of certification be simulation only? <a name="simulationonlycertification"></a>

Simulation and physical testing is the accepted way of design and certifying a modern aircraft. For the future, the likelihood of simulation completely replacing physical testing is a constant topic of discussion. 

"Simulations are becoming more complex and we are pushing the boundaries with physical testing, but will we ever certify an aircraft using simulation only?" questions Solar. "I'm not sure it will ever happen, actually. I think we must always check the results in the real world to validate the models, maybe less extensively than before in some areas, but for the extreme cases and conditions this needs to be checked by physical testing."

<ImageWithCaption
 src="/assets/images/cfd-of-airplane-wing-engine-flap.jpg"
 alt="Colourmap CFD comparison of the surface flow of an aiplane wing, engine and flap"
 caption={
 
 Complex flow regimes like engine/wing/flap interactions are still too variable in CFD to fully sign-off. CREDIT:{' '}
 <ExternalLink href="https://arc.aiaa.org/doi/pdfplus/10.2514/1.J059869">
 AIAA
 </ExternalLink>
 
 }
 width="700"
 height="426"
 positionCenter={true}
/>

Physical testing also allows every detail of the design to be assessed because it requires real parts. Whereas CFD simulations often involve simplified models that may not capture all the intricacies of vortex generators and small gaps, which could affect the overall aerodynamics. 

---

## Extreme configurations at the limit of separation <a name="extremeconfigurations"></a>

Aircraft flight characteristics are of most interest to regulators at the extremes of performance, particularly in low-speed stall conditions and high speed transonic flow. 

<ImageWithCaption
 src="/assets/images/contrails-on-high-lift-airplane-wing.jpg"
 alt="Close up shot of contrails on a high lift airplane wing configuration in the air"
 caption={
 
 Contrails demonstrate vortices and heavily worked flow on a high lift wing configuration. CREDIT:{' '}
 <ExternalLink href="https://aerocorner.com/blog/how-flaps-work/">
 Aerocorner
 </ExternalLink>
 
 }
 width="700"
 height="466"
 positionCenter={true}
/>

"There are still some configurations which are very difficult to simulate," highlights Solar. "Low speed is when you have the most complex wing configuration with perhaps single slotted, double slotted, triple slotted flaps, some slots with different extensions and multiple transitions usually with a high angle of attack. We can run simulations on a very small portion of the wing, but to simulate the full aircraft accurately requires a high amount of computing power which is the limit at the moment."

Computing power will continue to increase, making that last goal of a full aircraft simulation possible in the future. For now, CFD is a key contributor to aircraft safety, fitting alongside the other real world testing capabilities of modern aircraft manufacturers.

---

### Interesting links:

- <ExternalLink href="https://ntrs.nasa.gov/api/citations/20210015404/downloads/NASA-CR-20210015404%20updated.pdf">NASA Guide for Air Certification by Analysis</ExternalLink>

- <ExternalLink href="https://ntrs.nasa.gov/api/citations/20200002394/downloads/20200002394.pdf">NASA Computational Study of a Generic T-Tail Aircraft</ExternalLink>

- <ExternalLink href="https://www.baesystems.com/en-uk/heritage/bac1-11">BAE Systems Heritage BAC1-11</ExternalLink>

- <ExternalLink href="https://flightsafety.org/asw-article/discoveries-on-ice/">NASA flight research into icing</ExternalLink>

- <ExternalLink href="https://www.easa.europa.eu/en/document-library/type-certificates">Database of EASA Type Certificate Datasheets</ExternalLink>

---

### References

[1] <ExternalLink href="https://www.heritageconcorde.com/airframe-detail">The Concorde Fleet - Breakdown of each air frame built</ExternalLink>. [Online]. Heritage Concorde. 

[2] <ExternalLink href="https://www.airbus.com/en/products-services/commercial-aircraft/the-life-cycle-of-an-aircraft/test-and-certification">Test and Cetification</ExternalLink>. [Online]. Airbus. 

[3] <ExternalLink href="https://www.easa.europa.eu/en/proposed-cm-s-014-modelling-simulation-consultation">Proposed CM-S-014 Modelling & Simulation – for consultation</ExternalLink>. [Online]. EASA. 

[4] <ExternalLink href="https://www.easa.europa.eu/en/domains/aircraft-products/aircraft-certification">Aircraft certification</ExternalLink>. [Online]. EASA.

[5] 2007. <ExternalLink href="https://www.easa.europa.eu/sites/default/files/dfu/CS-25_Amdt%203_19.09.07_Consolidated%20version.pdf">Certification Specifications for Large Aeroplanes CS-25</ExternalLink>. [Online]. EASA.

[6] 2016. <ExternalLink href="https://www.easa.europa.eu/en/document-library/product-certification-consultations/equivalent-safety-finding-cs-25251b-vibration">Equivalent Safety Finding CS 25.251(b) - “Vibration / buffeting”</ExternalLink>. [Online]. EASA.

---

The role of CFD in passing aircraft EASA regulations

As eVTOL projects move from the drawing board and simulation to physical components and flight, they need to be appropriately certified for commercial operation. Certification is a long and rigorous process, overseen by engineers like David Solar, Head of General Aviation and Vertical Take-Off and Landing at the <ExternalLink href="https://www.easa.europa.eu/en/regulations">European Union Aviation Safety Agency (EASA)</ExternalLink>. 

AirShaper was lucky enough to speak with David and discuss the challenges of certifying this new eVTOL breed of aircraft.

- ###### [What is an eVTOL?](#whatisanevtol)
- ###### [eVTOL certification](#evtolcertification)
- ###### [Certifying the powertrain](#certifyingthepowertrain)
- ###### [Certification differences between cargo and passenger](#certificationdifferences)
- ###### [Regulating vertiports](#regulatingvertiports)
- ###### [The future of eVTOLs](#futureevtols)

---

## What is an eVTOL? <a name="whatisanevtol"></a>

Urban air mobility is a developing concept that sees personal transportation take to the skies above our cities. Electric vertical take-off and landing (eVTOL) aircraft are the machines that will allow this to happen. 

- <ExternalLink href="https://verticalmag.com/news/airshaper-takes-fresh-approach-to-complex-field-of-computational-fluid-dynamics/">How AirShaper takes a fresh approach on eVTOL aerodynamics</ExternalLink>

An eVTOL essentially combines vertical and horizontal flight. They typically utilise propellors in the vertical position for take-off and landing, similar to a helicopter. Whereas for forward flight, some use the same propellors to generate lift and forward thrust, while others use horziontal propellors in combination with a wing. The simplest eVTOLs are small, light passenger aircraft that utilise electric motors to power the propellers with on board batteries to store energy. While the more complex eVTOL designs feature innovative propellor mechanisms and electric jet engines integrated into fixed wings. 

<ImageWithCaption
 src="/assets/images/archer-aviation-evtol-side-2.png"
 alt="Front view of a grey eVTOL that features vertical propellors and fixed wings"
 caption={
 
 The Midnight eVTOL from Archer Aviation combines propellors with fixed wings and is capable of flying up to 100 miles. CREDIT:{' '}
 <ExternalLink href="https://www.archer.com/midnight">
 Archer Aviation
 </ExternalLink>
 
 }
 width="700"
 height="378"
 positionCenter={true}
/>

---

## eVTOL certification <a name="evtolcertification"></a>

To certify an aircraft, governing bodies such as EASA, CAA (Civil Aviation Authority) and FAA (Federal Aviation Administration) work with manufacturers to establish a set of regulations that ensure the safety and airworthiness of an aircraft during design, manufacture and modification. The manufacturers then have to conduct comprehensive testing programmes to prove that their aircraft complies with these standards. 

- <Link href="/blog/cfd-in-passing-aircraft-easa-regulations#certifyingtodaysaircraft">How are today’s aircraft certified?</Link>

However, eVTOLs are a new type of aircraft with technologies and concepts that have never been certified before. Consequently, authorities are collaborating with manufacturers to write the rulebook on eVTOL certification.

<ImageWithCaption
 src="/assets/images/joby-aviation-evtol.png"
 alt="Side view of a stationery grey eVTOL from Joby Aviation"
 caption={
 
 Joby Aviation were the first eVTOL company to reach the second stage of FAA certification. CREDIT:{' '}
 <ExternalLink href="https://www.jobyaviation.com/news/joby-completes-second-stage-certification-process/">
 Joby Aviation
 </ExternalLink>
 
 }
 width="700"
 height="437"
 positionCenter={true}
/>

Although as David Solar explains, some standards can be carried over. “The certification of an eVTOL propeller is fairly similar to that of a conventional aircraft. But with eVTOLs you also have to consider the no single failure criteria and the application of the aircraft as the aerodynamic design is very much dependent on the aircraft’s objectives.” 

“For example, when transitioning from vertical to horizontal flight, depending on if it is a tilt or not a tilt, the propeller may stay in the airflow fixed at some point, so each propeller design is adapted to a specific aircraft configuration. Right now, there is no magic formula, manufacturers have to find the best compromise between performance, noise and aerodynamics.” 

However, with some of the latest eVTOL concepts utilising propellors for both vertical and forward flight, is the certification process different for a propellor used for take-off and landing vs a propellor providing horizontal thrust? 

“Yes, they each have different loading conditions which leads to different ways to not only design, but also to demonstrate that the propeller is fit for flight. There is not a 'one size fits all' solution, we evaluate the loading conditions as well as the fatigue tests and understand how these are used on the aircraft to arrive at the final certification," says David.

<ImageWithCaption
 src="/assets/images/joby-tilt-rotor-patent.png"
 alt="Diagram showing the mechanism of the Joby tilt rotor transitioning from vertical to horizontal"
 caption={
 
 Detail from the Joby tilt rotor patent shows a unique mechanism that requires bespoke certification. CREDIT:{' '}
 <ExternalLink href="https://www.atec-amt.org/uploads/1/0/7/5/10756256/atec_journal-fall20_112920.pdf">
 Aviation Technician Education Council
 </ExternalLink>
 
 }
 width="250"
 height="452"
 positionCenter={true}
/>

With urban air mobility such a novel industry, manufactures are constantly iterating their concepts to achieve the best performing designs. This has led to a huge variety of concepts, which make defining a set of standards that are suitable for all designs a real challenge. Furthermore, is there the danger that governing bodies could actually dictate the design direction through implementing certification standards? 

“Right now, everybody is coming with new ideas and at the end of the day the market will probably choose the ones that are most fit for purpose,” says David. “I think we will see some different designs according to the specific missions, but it won’t be hundreds of designs. Once we have visibility on the most popular concepts, we can then write specific guidelines for each type of eVTOL.” 

“We try to be as neutral as possible and not favour one design over another. That's why we have high level objective requirements that don’t consider whether a concept is a hovercraft or a plane because this prevents us introducing bias into the system. We don't know which eVTOL concept is going to be the best yet so we don't want to penalise a design by imposing requirements that may prevent some concepts achieving certification.”

---

## Certifying the powertrain <a name="certifyingthepowertrain"></a>

The eVTOL world has only ignited recently due to the capabilities of modern electric powertrains. High voltage batteries supply electrical energy to motors which power the rotors. However, vertical flight is an energy intensive process which requires a battery with a high energy density and therefore a high weight. This presents a conundrum for manufacturers: larger batteries provide the energy necessary to increase the range of an aircraft, but this additional weight requires more propulsion to keep in the air, requiring even larger batteries.

“It’s all dependent on the evolution of battery technology, but for now there is a trade-off between the payload and the range performance an eVTOL can achieve. This means that advances in the battery weight to power ratio will give the biggest increases in performance, although improvements in aerodynamic efficiency will still provide some gains, but these will be of much smaller magnitude," concludes David.

<ImageWithCaption
 src="/assets/images/lilium-evtol-battery-testing.jpeg"
 alt="Factory showing several battery testing machines at Lilium's headquarters"
 caption={
 
 eVTOL manufacturers are putting a lot of effort into researching suitable battery and cell technology. CREDIT:{' '}
 <ExternalLink href="https://lilium.com/newsroom-detail/liliums-battery-strategy">
 Lilium
 </ExternalLink>
 
 }
 width="700"
 height="394"
 positionCenter={true}
/>

---

## Certification differences between cargo and passenger <a name="certificationdifferences"></a>

The battery energy density vs payload question also depends on whether the aircraft is intended for transporting passengers or cargo, as each scenario demands very different energy requirements and thus affects the achievable range. 

“One of the big cost drivers at the moment is when manufacturers will change the battery,” says David. “The likes of Volocopter are willing to swap the battery, recharging them in a very low, steady controlled environment so that the battery is in perfect condition for charging. While others are looking at solid-state batteries that could have less of an impact on ageing.”

The different flight paths of cargo and passenger aircraft also means different certification requirements, as David explains. “If you are operating your cargo not over cities, then you can have lower security using the battery basic category that we have defined in our special condition window. There are differences in some of the redundancies especially if you go to the very low end of the basic category where you also have differences in terms of software development. It may be that you can have only an emergency landing requirement in the basic category because we consider that you will be flying over some non-congested, hostile environment so you could find a spot within a reasonable amount of time to land.”

“However, if you are operating passenger commercial flights, the level of safety in Europe, means manufacturers have to comply with an 'enhanced category' which is the highest of safety level we have today. This also applies to cargo if you are operating over what we call congested sites, over cities or environments. In these cases, we are requesting that you are carrying on the flight even with a failure to an alternate destination. These differences have a lot of design implications behind them.”

<ImageWithCaption
 src="/assets/images/charging-evtol.jpg"
 alt="Side view of a white eVTOL plugged in and charging on the runway"
 caption={
 
 Fast charging is good for aircraft turnaround times, but damaging for battery ageing. CREDIT:{' '}
 <ExternalLink href="https://aviationweek.com/business-aviation/beta-outlines-evtol-charging-considerations">
 Aviation Week
 </ExternalLink>
 
 }
 width="700"
 height="467"
 positionCenter={true}
/>

---

## Regulating vertiports <a name="regulatingvertiports"></a>

Flying across the skies of our cities does not only cause safety concerns, but EASA also have to consider the operational noise of busy vertiports or landing sites and the impact this will have on the people living there. With no eVTOL certified to fly commercially yet, EASA have had to conduct comprehensive research to understand the challenges of certifying vertiports. 

<ImageWithCaption
 src="/assets/images/evtol-melbourne-skyline.jpeg"
 alt="Graphic showing several eVTOL aircraft flying over the Melbourne city in Australia"
 caption={
 
 Busy airspace above cities will be a contentious issue for inhabitants. CREDIT:{' '}
 <ExternalLink href="https://www.aviationtoday.com/2022/09/06/experts-discuss-investment-opportunities-advanced-air-mobility-australia/">
 Avionics International
 </ExternalLink>
 
 }
 width="700"
 height="350"
 positionCenter={true}
/>

“There is no noise level which has been defined so far, but we are taking measurements. It's not like a helicopter or aircraft where you have ideal noise limits which are already set up. It is so new and we don't yet know in detail the noise profile of these machines, so we are learning by doing,” highlights David. 

“For sure they will be quieter than a helicopter and also companies know that noise is critical for public acceptance. I think the quieter the aircraft, the more successful it will be with the public. So there is a strong incentive for companies to minimise the noise footprint. We'll see later down the road if we have to put in some limits, maybe in cities or other environments, but so far, we are gathering data and taking the time to think about it.”

---

## The future of eVTOLs <a name="futureevtols"></a>

There are very few knowns about the way the urban air mobility concept will evolve over the next decade but organisations like EASA are at the forefront of defining how eVTOLs will become a part of our daily lives, safely. 

“It will depend on how the industry is adopted by consumers. It will be expensive at the beginning, so unlikely to be used by most individuals. The price of eVTOLs will likely exceed that of cars, because of the high battery costs and also the pilot will need to be trained as well as licenced. Longer term, costs may reduce if production can be scaled up and the operational requirements are lowered so you don’t need a pilot to fly the aircraft. I still think it will be challenging to reach the current automotive industry levels of production and cost.” 

<ImageWithCaption
 src="/assets/images/evtol-concepts-diagram.jpg"
 alt="Diagram showing the multirotor, lift and cruise, tilt rotor and ducted vector thrust eVTOL concepts"
 caption={
 
 Not all of the different eVTOL concepts will survive, but nobody knows which will succeed. CREDIT:{' '}
 <ExternalLink href="https://lilium.com/newsroom-detail/lilium-architecture-design-principles">
 Lilium
 </ExternalLink>
 
 }
 width="750"
 height="391"
 positionCenter={true}
/>

So when can we expect to take our first flight in an eVTOL? “That’s a difficult question!” smiles David. “It will ultimately depend on the first real flight tests of the final configurations as only then will we have a clear view on any problems that need to be addressed before certification. What I can say is that there is at least one OEM targeting the Paris Olympic games. After that, eVTOLs will still be in limited operation as there is still a lot of work to be done on the certification side. But I expect that the eVTOL industry will continue to progress from 2024 onwards.”

---

### Interesting links:

- <ExternalLink href="https://aviationweek.com/air-transport/faa-releases-first-evtol-certification-basis-comment">FAA Releases First eVTOL Certification Basis For Comment</ExternalLink>

- <ExternalLink href="https://verticalmag.com/news/airshaper-takes-fresh-approach-to-complex-field-of-computational-fluid-dynamics/">AirShaper takes fresh approach to complex field of computational fluid dynamics</ExternalLink>

- <ExternalLink href="https://www.easa.europa.eu/en/light/topics/vertical-take-and-landing-vtol">Vertical Take-off and Landing (VTOL)</ExternalLink>

- <Link href="/blog/airspeeder">Airspeeder - The Electric Flying Car Racing Series</Link>

---

eVTOL certification interview with EASA

## Why validate software?

Computer Aided Engineering (CAE) is a constantly growing force in the modern world. The ability to simulate and experiment virtually can save significant resources, time and money. Consequently, many industries have shifted their focus to utilising Computational Fluid Dynamics (CFD) software which is allowing engineers to conduct more detailed aerodynamic analysis and is even replacing wind tunnel testing in some cases. 

However, as effective as these simulations are, the results must be validated to ensure they are accurate. Correlating results to real world data is a key part of this, but it is also important to benchmark software against existing techniques. This is what DARcorporation recently completed with AirShaper software.

---

## Using CFD in Aeronautical engineering

<ExternalLink href="https://www.darcorp.com/about-dar-corporation/">DARcorporation is an aeronautical engineering consultancy</ExternalLink> based in Lawrence, Kansas. Specialising in aircraft design, they use proprietary software in conjunction with tools from external suppliers.

<ImageWithCaption
 src="/assets/images/air-vy-400-commuter-aircraft.jpeg"
 alt="Air Vy 400 commuter aircraft concept in flight"
 caption={
 
 DARcorporation worked with Transcend on the Air Vy 400 commuter aircraft concept. CREDIT:{' '}
 <ExternalLink href="https://www.transcend.aero/">
 Transcend Air
 </ExternalLink>
 
 }
 width="700"
 height="394"
 positionCenter={true}
/>

Much of their work is in the conceptual and prototype stages of projects ranging from Unmanned Aerial Vehicles (UAVs) to multi-engine jet aircraft. Therefore, utilising accurate simulation tools is vital to the success of these projects as the majority of work takes place before any physical components or aircraft exist. Consequently, the engineers at DARcorporation use CFD to feed results into structural studies and control surface design.

---

## Benchmarking AirShaper

Alongside their own Advanced Aircraft Analysis (AAA) software, DARcorporation has extensive experience of a range of simulation tools including <ExternalLink href="https://www.darcorp.com/flightstream-aerodynamics-software/">FlightStream</ExternalLink>, <ExternalLink href="https://www.plm.automation.siemens.com/global/en/products/simcenter/STAR-CCM.html">STAR-CCM+</ExternalLink> and AirShaper. Flightstream is a surface vorticity-based simulation package that offers speed and convenience after meshing, but results in a less detailed simulation. STAR-CCM+ is a fully fledged CFD package which comes with the traditional overheads of meshing, long run times and post-processing requirements. Whereas, AirShaper achieves accurate CFD simulations quickly, whilst automating much of the set-up, so you don’t have to be an expert to benefit from using CFD.
<ImageWithCaption
 src="/assets/images/cessna-c210j-centurion-streamlines-side.jpeg"
 alt="CFD image of Cessna C210 Centurion showing coloured velocity streamlines and surface static pressure"
 caption={
 
 DARcorporation use a variety of simulation tools to carry out aircraft design studies. This {' '}
 <ExternalLink href="https://www.darcorp.com/projects/cessna-210j-centurion/">
 Cessna C210 Centurion CFD analysis
 </ExternalLink>
 shows velocity streamlines and surface static pressure. CREDIT:{' '}
 <ExternalLink href="https://www.darcorp.com/about-dar-corporation/">
 DARcorporation
 </ExternalLink>
 
 }
 width="700"
 height="258"
 positionCenter={true}
/>

DARcorporation wanted a CFD package that achieves high accuracy in short runtimes. This would allow them to generate aerodynamically efficient designs quicker, increasing the number of test cases they can accomplish. To identify the CFD software that best suited their needs, DARcorpoation conducted a validation study where they compared FlightStream, STAR-CMM+ and AirShaper against real-world wind tunnel data.

Real-world testing is expensive and slow, but benefits from using an actual aircraft or model. Whereas, simulations often have to be simplified which can lead to assumptions that ultimately affect accuracy. These errors cannot always be resolved, but a thorough knowledge of them means they can be accounted for in the design process.

It would be too impractical to conduct a bespoke wind tunnel test for this validation study, so another data source was needed instead. As discussed in our <Link href="/blog/best-aerodynamic-resources">Aerodynamic resources blog</Link>, NASA offers a wide range of open-source aerodynamic studies and data. One such study was carried out using a Cessna 210 Centurion light passenger aircraft in the Langley full scale wind tunnel. The Cessna has a relatively simple high-wing layout, common to many light aircraft, making it a suitable test case without being overly complicated. 

- <ExternalLink href="https://ntrs.nasa.gov/api/citations/19730016341/downloads/19730016341.pdf">Read the original NASA wind tunnel report for the Cessna 210</ExternalLink>

<ImageWithCaption
 src="/assets/images/cessna-c210j-centurion.jpeg"
 alt="A white and red Cessna C210J Centurion flying over countryside"
 caption={
 
 The high-wing layout of the Cessna 210 Centurion made it a suitable test case for the validation study. CREDIT:{' '}
 <ExternalLink href="https://jackfleetwood.com/">
 Jack Fleetwood
 </ExternalLink>
 
 }
 width="700"
 height="326"
 positionCenter={true}
/>

---

## The study

Flightstream and Star-CCM+ both require the user to mesh a 3D model and carry out a meshing convergence study. This ensures that the mesh itself does not unduly influence the results. AirShaper on the other hand, automates this process when a model is uploaded, saving time and effort. 

After running simulation sweeps at different airspeeds and angles of attack, the pressure or vortex (in the case of Flightstream) data is post-processed. This turns detailed node information into useful aerodynamic data. Three standard coefficients were chosen for analysis for the validation study: lift, pitching moment and drag.

---

## Lift coefficient results

As expected, the various simulation packages gave a spread of results. It’s also important to note that the wind tunnel results will differ slightly from reality due to the limitations of full-sized testing. 

<ImageWithCaption
 src="/assets/images/airshaper-validation-figure-2-1-lift-coefficient.png"
 alt="Line graph showing the lift coefficient for different angles of attack for the three simulation packages and the reference wind tunnel data"
 caption={
 
 Lift coefficient vs angle of attack results for each CFD software and the wind tunnel reference data. CREDIT:{' '}
 <ExternalLink href="https://www.darcorp.com/about-dar-corporation/">
 DARcorporation
 </ExternalLink>
 
 }
 width="700"
 height="431"
 positionCenter={true}
/>

For the purposes of this study, the range of angles of attack up to around 10 degrees are of the most interest. Analysing the lift coefficients, we can see that all three softwares match the lift gradient well, but have differing offsets when compared to the wind tunnel data. FlightStream and AirShaper agree quite closely but Star-CCM+ has a further offset. The specific reason for this difference requires further study. But overall, the results for AirShaper are in the range of the two software packages that DARcorporation knows well.

At higher angles of attack, the software packages predict an earlier onset of stall compared to the measurement data. Software inaccuracies could be a cause (but both StarCCM and AirShaper predict this). Another cause could be geometry which was present on the real airplane (small seams or vortex generators which can delay stall) but not on the 3D model. As the NASA data is quite old, there is unfortunately no way of finding out.

---

## Pitching moment coefficient results

The pitching moment coefficient results follow a similar form to the lift coefficients. The slopes of all three simulations match quite closely to the wind tunnel data, with minimal offsets.

<ImageWithCaption
 src="/assets/images/airshaper-validation-figure-2-2-pitching-moment-coefficient.png"
 alt="Line graph showing the pitching moment coefficient for different angles of attack for the three simulation packages and the reference wind tunnel data"
 caption={
 
 Pitching moment coefficient vs angle of attack results for each CFD software and the wind tunnel reference data. CREDIT:{' '}
 <ExternalLink href="https://www.darcorp.com/about-dar-corporation/">
 DARcorporation
 </ExternalLink>
 
 }
 width="700"
 height="423"
 positionCenter={true}
/>

---

## Drag coefficient results

The drag results show a larger set of offsets between simulation and the wind tunnel, with all three CFD packages resulting in less drag than the wind tunnel data. This is not unexpected as the model in the wind tunnel has more imperfections and limitations compared to a perfect 3D CAD model. However, for this coefficient, AirShaper and Star-CCM+ are a close match.

<ImageWithCaption
 src="/assets/images/airshaper-validation-figure-2-3-drag-coefficient.png"
 alt="Line graph showing the drag coefficient for different angles of attack for the three simulation packages and the reference wind tunnel data"
 caption={
 
 Drag coefficient vs angle of attack results for each CFD software and the wind tunnel reference data. CREDIT:{' '}
 <ExternalLink href="https://www.darcorp.com/about-dar-corporation/">
 DARcorporation
 </ExternalLink>
 
 }
 width="700"
 height="424"
 positionCenter={true}
/>

---

## Reaching a conclusion

Experience tells us that it is not unusual to have four different answers for the same test. Quantifying and explaining these differences further are the next step in such an exercise. However, for DARcorporation, the benchmarking process proved that AirShaper produces results within the range they would expect from their normal tools, whilst being easier to use and set-up. 

‘AirShaper takes a lot of effort out of the hands of an engineer,’ concludes Dennis van Dommelen, of DARcorporation. ‘It is simple and very straightforward to use. The uploaded aircraft does not require any modifications and after setting the right angles and airspeed, it is ready to run. Elimination of the mesh generation step is a major time saving and the accuracy of the standard run settings is comparable to other tools that are available.’

---

##### Interesting links:

 

<ExternalLink href="https://www.darcorp.com/statement-of-work-aircraft/">
 DARcorporation’s work flow for aircraft conceptual design
</ExternalLink>

 

<Link href="/blog/avl-racing-benchmarking-airshaper">
 AirShaper validation study with AVL Racing
</Link>

 

<Link href="/cases/optimization/cessna">
 Optimizing the aerodynamics of the Cessna 210 winglet
</Link>

 

<ExternalLink href="https://www.aviationconsumer.com/aircraftreviews/aircraftstepups/cessna-210-centurion-2/">
 A review of a Cessna 210 Centurion
</ExternalLink>

 

<ExternalLink href="https://www.darcorp.com/projects/transcend-air-corporation/">
 The design and analysis of the Transcend Air Vy 400 VTOL aircraft project
</ExternalLink>

---

A validation study of AirShaper software

How does aerodynamics affect speed?

Drones are everywhere these days. As with many technologies, the advancements made in energy storage with lithium-ion batteries has been a game changer. The drone industry is estimated to be worth $43 billion in 2025. They are no longer toys for hobbyists, but serious pieces of hardware used throughout the world and have even flown on <ExternalLink href="https://mars.nasa.gov/news/8923/nasas-ingenuity-mars-helicopter-succeeds-in-historic-first-flight/">Mars!</ExternalLink>

<ImageWithCaption
 src="/assets/images/quadcopter-drone.jpeg"
 alt="Quadcopter drones are used for everything from filming movies to supporting search and rescue operations. CREDIT: www.london-fire.gov.uk"
 caption={
 
 Quadcopter drones are used for everything from filming movies to
 supporting search and rescue operations. CREDIT:{' '}
 <ExternalLink href="http://www.london-fire.gov.uk">
 www.london-fire.gov.uk
 </ExternalLink>
 
 }
 width="700"
 height="341"
 positionCenter={true}
/>

---

## Quadcopters

The first type of drone that comes to mind are quadcopters, which use multiple rotors to provide lift and propulsion. These are small and easy to use and therefore perfect to replace helicopters in a range of tasks where hovering is a key element.

<ImageWithCaption
 src="/assets/images/drone-thermal-camera.jpg"
 alt="Emergency services use drones equipped with infra-red cameras to capture thermal images. CREDIT: www.dronedj.com"
 width="700"
 height="350"
 positionCenter={true}
/>

However, as the applications for drones has increased in areas such as agriculture, mining, defence, mapping and surveying, the range and flight time of quadcopters makes them less ideal for these types of tasks. For example, if you want to map a vast area of agricultural land using advances in drone automation, battery life becomes a major issue. A quick look at a typical manufacturer’s website shows that flying time and range are among the first things that need to be considered.

<ImageWithCaption
 src="/assets/images/drone-field-survey.jpeg"
 alt="Field surveys for agriculture can be automated with drone technology. CREDIT: www.pix4d.com"
 width="700"
 height="394"
 positionCenter={true}
/>

---

## Fixed wing VTOL drones

This has given rise to fixed wing VTOL drones. These are hybrid aircraft with lift producing bodies and wings that also have the ability to hover using rotors. Once airborne, the wings are able to support the drone and the only power requirement is for horizontal motion. This is especially important for drones with heavy camera payloads which are very energy hungry. They also give operational benefits where there isn’t space to land a standard winged drone.

<ImageWithCaption
 src="/assets/images/vtol-drone-example.jpeg"
 alt="VTOL drones combine wings and vertical rotors for short and vertical take-offs. CREDIT: www.deltaquad.com"
 width="700"
 height="266"
 positionCenter={true}
/>

---

## Optimizing fixed wing VTOL drones

The combination of aerodynamics and rotor dynamics makes a fixed wing VTOL drone a more complicated vehicle to design than a quadcopter. From a purely aerodynamic perspective, the basic design of the body can be taken as a standard 3D modelling and CFD problem. AirShaper has a number of tools to make this design process as simple as possible.

We have created a video which explains some of the <Link href="/videos/airfoil-design-for-a-drone/kAXN3MlQxxc">design considerations for drone airfoils</Link> and another video which shows a <Link href="/videos/drone-design-2-3d-flow-analysis/FZHYcqRcDJM">3D flow analysis of blended bodies</Link> using AirShaper. The capability of our software was fully tested during our project with <ExternalLink href="https://www.neuralconcept.com/post/sensefly-airshaper-and-neural-concept-designing-the-ultimate-drone">Sensefly and Neural Concept</ExternalLink>. Using our cloud simulation capabilities as their CFD engine, they optimised the design of a blended wing drone.

- <Link href="/research/deep-learning-automated-drone-design">
 Check out our technical paper on using Neural Networks to optimize a
 fixed-wing drone
 </Link>

<ImageWithCaption
 src="/assets/images/blended-body-drone-optimization_2.png"
 alt="The blended body of the original design (left) was optimized (right) using AirShaper and neural networks to increase overall lift"
 width="800"
 height="298"
 positionCenter={true}
/>

---

## Propulsion is important to all aircraft

Once there is an optimized basic vehicle that meets the various design requirements, the next stage is to integrate the power source. That requires aerodynamic design input in two areas: the propeller itself and the way it interacts with the rest of the design.

In isolation, propeller designs are a complicated field of aerodynamic study in their own right. Studies of propeller thrust and efficiency have been carried out from the original Wright Flyer to the current day. Drones share many similarities with full size aircraft and so their propulsion systems have been researched for a surprisingly long time.

---

## Integrating propellers into aircraft design

Putting the intricacies of rotor design aside, adding one or more to an aircraft design can be seen as a problem. The complex flows can dramatically affect an otherwise well-designed shape. This doesn’t have to be the case with careful design and modern CFD.

In the world of full-size aircraft, <Link href="/videos/electric-aviation-nasa-interview-on-maxwell-x57-part-2-propeller-design/UDZBqDQGU8o">NASA’s X-57 Maxwell aircraft</Link> studies have shown how electric propulsion can be used to revolutionise wing design. We covered this in a series of videos where we interviewed NASA engineers and discussed that there is minimal difference between a drone and a full-size aircraft.

“They are all going to have to deal with some of these trade-offs,' highlights Nick Borer, Performance and sizing lead at Armstrong and Langley. "How does the flow interact with the wing, are you using it for efficient thrust production or are you using it to efficiently generate lift in edgewise flight?”. These are just some of the challenges that face drone designers when integrating propellers.

- <ExternalLink href="https://ntrs.nasa.gov/api/citations/20170005883/downloads/20170005883.pdf">
 Find out more about the NASA X-57 high lift wing propeller system{' '}
 </ExternalLink>

---

## Adding rotating elements to an AirShaper CFD project

To understand all the airflow interactions caused by the propellers, modelling the effect of the propellers and rotating elements in CFD is essential. We have recently introduced this to the AirShaper platform and explain how it works in this <Link href="/videos/how-to-model-rotating-elements-in-cfd-computational-fluid-dynamics-using-mrf-ami-and-more/zFSXtRGk_H8">rotating elements video</Link>.

<ImageWithCaption
 src="/assets/images/airshaper-propeller-cfd.png"
 alt="Drone rotor simulation in AirShaper allows the use of multiple propellers"
 width="700"
 height="343"
 positionCenter={true}
/>

Simulations can now be run with and without a rotor implementation to establish the effects on the design. For a VTOL drone design, it may also be important to look at the effect of rotating and non-rotating propellers to ensure flight characteristics remain unchanged if they are not in use. Drones have come a long way in the last 10 years and we look forward to continuing developing the capabilities of AirShaper to help simulate these fascinating types of aircraft!

---

##### Interesting links:

 

<ExternalLink href="https://airshaper.com/cases/sense-fly-increasing-drone-flight-time">
 Increasing flight time of drones
</ExternalLink>

 

<ExternalLink href="https://airshaper.com/videos/how-to-model-rotating-elements-in-cfd-computational-fluid-dynamics-using-mrf-ami-and-more/zFSXtRGk_H8">
 Learn more about modelling rotating elements in CFD using MRF, AMI and more
</ExternalLink>

---

How CFD can optimize VTOL drone design

The Audi TT concept was first unveiled at the 1995 Frankfurt Motor Show. This two door coupe featured four wheel drive and was powered by a 1.8 litre inline four cylinder turbocharged engine. The clean lines of the <ExternalLink href="https://www.tate.org.uk/art/art-terms/b/bauhaus">Bauhaus-inspired styling</ExternalLink> wowed the Frankfurt audience and the wider media and was deemed a revelation for the previously conservative Audi brand.

The production car sold well initially, but some high-profile incidents in Germany made it clear that the car had some problems with high-speed stability. Audi had to solve these issues fast and developed a new version which incorporated a small rear spoiler and revised suspension settings.

<ImageWithCaption
 src="/assets/images/audi-tt-spoiler-side.jpeg"
 alt="Audi TT rear spoiler side view"
 caption={
 
 The comparatively small rear spoiler was rejected by stylists initially
 but made a huge difference to the high-speed handling of the revised car.
 CREDIT:{' '}
 <ExternalLink href="https://www.caranddriver.com/reviews/a15140613/we-take-audis-tamed-tt-for-a-spin-first-drive-review/">
 Car And Driver
 </ExternalLink>
 
 }
 width="700"
 height="283"
 positionCenter={true}
/>

---

## Modelling the Audi TT in CFD

The changes Audi made to the TT were a success and the car has now become something of a classic. But how much difference did that rear spoiler actually make to the aerodynamics and overall performance of the Audi TT? To find out, we decided to use AirShaper software to conduct an aerodynamic study which compared the original version of the Audi TT without the spoiler against a revised version with the spoiler.

This was fairly easy to do: we purchased a public 3D model of the Audi TT with a spoiler, and then created a copy without spoiler. Further detail could be added by automatically incorporating a rotating wheel simulation. This identifies the wheels from the base geometry and is a valuable tool when investigating automotive aerodynamics.

- <Link href="/videos/how-to-model-rotating-elements-in-cfd-computational-fluid-dynamics-using-mrf-ami-and-more/zFSXtRGk_H8">
 Learn more about modelling rotating elements in CFD using MRF, AMI and more
 </Link>

---

## CFD results

AirShaper allows simulations to run in the cloud and produces a number of different ways to visualise the results, all of which are detailed in an automatically generated PDF report. However, one of the simplest methods to compare results is to look at key numbers. The drag and lift coefficients for both the front and rear are shown in the table below.

| | Without spoiler | With spoiler |
| --------------------------------------- | --------------- | ------------ |
| Drag Coefficient, Cd | 0.305 | 0.265 |
| Total Lift Coefficient, Cl | 0.098 | -0.018 |
| Front Lift Coefficient, Cl,f | -0.063 | -0.072 |
| Rear Lift Coefficient, Cl,r | 0.161 | 0.053 |

As this is a public 3D model, don't focus on the absolute numbers, but rather on the relative differences. The most important change for the Audi TT is the reduction in rear lift coefficient from 0.161 to 0.053. This clearly indicates how a small feature like a spoiler can have a strong impact on the rear lift of the car.

---

## Correlating results with reality

Fortunately for us, after the Audi TT had been revised, the changes were covered in various automotive magazines. <ExternalLink href="https://www.caranddriver.com/reviews/a15140613/we-take-audis-tamed-tt-for-a-spin-first-drive-review/">Car And Driver</ExternalLink> in the US conducted a correlation study between the original Audi TT, the revised version with the spoiler and several competitors at 125mph. The rear lift values of this study are detailed in the table below.

| | Original TT | Revised TT | BMW Z3 | Mercedes SLK | Porsche Boxster |
| --------------- | ----------- | ---------- | ------ | ------------ | --------------- |
| Rear Lift (Ibf) | 148 | 53 | 64 | 104 | 68 |

Although absolute values cannot be compared (due to the lack of detail on the 3D model), both simulation and test feature a comparable 65% drop in lift.

---

## Visualising the results

The power of CFD is that it gives detailed visualisations of the results. For example, surface pressure and surface friction can be displayed as coloured maps, while velocity streamlines show how the air behaves as it flows over a vehicle. These visualisations help us to really understand the interactions between airflows and the differences between the two configurations with and without a spoiler.

---

## Streamlines

<ImageWithCaption
 src="/assets/images/audi-tt-spoiler-comparison-top-view.png"
 alt="CFD streamlines of the wake with and without the spoiler on the Audi TT"
 caption={
 
 Streamlines show the wake with the spoiler (top) vs the wake without the
 spoiler (bottom) in{' '}
 <ExternalLink href="https://airshaper.com/">AirShaper</ExternalLink>
 
 }
 width="800"
 height="596"
 positionCenter={true}
/>

In its original form, the Audi TT acted similar to an aircraft wing. The flow over the top surface has to speed up to meet the under-car flow at the rear of the car. As Bernoulli’s equation sets out, this higher velocity means the static pressure is lower above the car than below, otherwise known as lift. The air follows the body shape all the way to the rear bumper, creating a strong downwash to the wake.

The spoiler works by disrupting this flow and slowing it down. A pocket of recirculating air is created ahead of the spoiler. This slow-moving air means a higher static pressure, eliminating the lift. The flow also separates from the rear window much earlier, creating a recirculation behind the car and a much larger wake that no longer downwashes.

<ImageWithCaption
 src="/assets/images/audi-tt-spoiler-comparison-side-view.png"
 alt="Streamlines showing that with the spoiler the flow seperates earlier creating a large wake. Without the spoiler, the flow remains attached and creates a strong downwash"
 caption={
 
 The results from{' '}
 <ExternalLink href="https://airshaper.com/">AirShaper</ExternalLink> show
 that with the spoiler (top) the flow seperates much earlier creating a
 large wake. Without the spoiler (bottom) the flow follows the shape of the
 rear bumper and creates a strong downwash
 
 }
 width="800"
 height="374"
 positionCenter={true}
/>

---

## Pressure maps

The effect of this is illustrated in the pressure maps shown below. The increased pressure on the spoiler affects the surrounding areas of the car as well. Hot colours show where pressure rises on the rear window, the top of the boot and even on the areas downstream of the spoiler. Lift is decreased by the higher pressures on horizontal surfaces and drag reduced by the increases on rearward-facing areas of the car.

<ImageWithCaption
 src="/assets/images/audi-tt-spoiler-comparison-pressure-3d-view-2.png"
 alt="Two pressure maps showing that the spoiler increases the surface pressure on the rear window and boot compared to without the spoiler"
 caption={
 
 Comparing the two pressure maps in{' '}
 <ExternalLink href="https://airshaper.com/">AirShaper</ExternalLink>, we
 can see that the spoiler (right) increases the surface pressure on the
 rear window and boot compared to the original version without the spoiler
 (left)
 
 }
 width="600"
 height="341"
 positionCenter={true}
/>

---

## A simple solution to a big problem

The Audi TT demonstrates how a big problem can be solved with a relatively small solution. The spoiler added here is not a rear wing element generating downforce in its own right. It is simply a trip to interrupt the airflow, which changes the aerodynamic behaviour of the whole car. Audi made changes to the rear suspension too, but it is likely that the spoiler had the most significant effect on improving high-speed handling.

It is interesting to see how a problem like this caught out a large automotive company like Audi in 1998. Now, modern computing power and tools like AirShaper allow us to carry out analysis like this from a home PC.

---

##### Interesting links:

 

- <ExternalLink href="https://www.tate.org.uk/art/art-terms/b/bauhaus">
 Learn about some design history whilst studying aerodynamics
 </ExternalLink>

 

- <ExternalLink href="https://www.caranddriver.com/reviews/a15140613/we-take-audis-tamed-tt-for-a-spin-first-drive-review/">
 Read the Car And Driver road test review of the revised Audi TT
 </ExternalLink>

---

A CFD analysis of the Audi TT spoiler

Few road cars are capable of achieving top speeds above 250mph (402kph) - the realm of the hypercar. Cars like the multi-million-dollar Bugatti Chiron Super Sport 300+ or the McLaren Speedtail are a couple of examples. But what if you had a supercar and wanted to achieve this hypercar level of performance?

This was the goal of <ExternalLink href="https://7x.design/gallery-1">7X Design</ExternalLink>, who converted a Lamborghini Huracan from a 600bhp, 200mph (320kph) production supercar into a 2000bhp, 300mph (480kph) coach-built Lamborghini Rayo hypercar.

---

## 21st century coachbuilding

Coachbuilding is the engineering art of taking a base vehicle chassis and making it into something unique. In the 1920s, companies like HJ Mulliner created one-off Rolls Royce bodies for their customers. More recently, Pininfarina developed the Ferrari Enzo-based P4/5, also known as the 'Glickenhaus', which generated more downforce and less drag than the original Enzo. This year, 7X Design took a Lamborghini Huracan and created the Lamborghini Rayo, which featured a bespoke carbon fibre body as well as extreme engine tuning.

---

## The Lamborghini Rayo brief

The project was instigated by David Gomez, founder of 7X Design and the owner of a tuned Huracan. He approached <ExternalLink href="https://www.adrianoraeli.com/7x-rayo">Adriano Raeli Design</ExternalLink> to see what was possible. 'The brief was pretty straightforward,' reveals Adriano Raeli, Design Specialist at Adriano Raeli Design. 'We had to re-engineer the aerodynamics of a car with around triple its original horsepower to create a matching aero body that would allow it to achieve its best top speed.'

'There were some practical limitations as we wanted to maintain the original road-going capabilities of the car,' continues Raeli. 'For example, the McLaren Speedtail is a special vehicle with an elegant shape, but it is compromised on the road in terms of ramp angles and overall length. We wanted to avoid this. We also couldn't change the roof and wide windshield because they were part of the structure. The A-pillar in particular generated problematic drag-inducing vortices.'

<ImageWithCaption
 src="/assets/images/7x-design-lamborghini-rayo-rear.jpeg"
 alt="The starting point: a 1,900bhp twin-turbo modified Lamborghini Huracan. CREDIT: www.undergroundracing.com"
 width="700"
 height="390"
 positionCenter={true}
/>

---

## Inspiration to reality

Automotive styling is a unique mixture of engineering and design. The most elegant of lines often create unworkable aerodynamics. Despite the different aerodynamic requirements and <Link href="/videos/the-relation-between-turbulence-reynolds-and-wind-tunnel-testing/sV8l8xd4MEQ">Reynolds numbers</Link>, the project was specifically inspired by figher aircraft.

<ImageWithCaption
 src="/assets/images/f35-fighter-jet.jpg"
 alt="Fighter aircraft provide some of the inspiration for Adriano Raeli's high performance automoitve projects. CREDIT: www.lockheedmartin.com"
 width="800"
 height="294"
 positionCenter={true}
/>

'They are one of the fastest and highly technological vehicles on the planet, short of spacecraft,' observes Raeli. 'When David saw the initial sketch he said, 'this is it!' That was the starting point of the design process. We then went from those quick couple of lines to a 3D model and then evolved the aerodynamic design of that model in simulation.'

<ImageWithCaption
 src="/assets/images/7x-design-lamborghini-rayo-initial-sketch.png"
 alt="The elegant original napkin sketch required engineering input to turn into a workable package. CREDIT: www.adrianoraeli.com"
 width="800"
 height="209"
 positionCenter={true}
/>

---

## CFD simulation with AirShaper

There were three aerodynamic aims for the CFD programme:

1. Reduce the drag to allow the car to achieve the 300mph (480kph) target

2. Create balanced downforce to ensure the car remains stable at speed

3. Limit the total amount of downforce so the car doesn't get squashed at high speeds

To help achieve this, Raeli experimented with the design of the rear wing and in the end replaced it with two winglets. 'For some projects, a rear wing is a must, there's no other way to achieve that kind of downforce without a wing,' explains Raeli. 'To reduce drag, we opted for winglets instead, which are located in cleaner air. So I designed a general aerofoil profile and tested it in CFD. There was no specific math behind it initially, just general design and aerodynamic principles. AirShaper’s morphing tool was instrumental to evolving the design and it's helped us achieve better speeds and downforce without a rear wing.'

<ImageWithCaption
 src="/assets/images/7x-design-lamborghini-rayo-rear-winglets.png"
 alt="Rear winglets replace the full width spoiler and are key aerodynamic elements of the design. CREDIT: www.adrianoraeli.com"
 width="800"
 height="302"
 positionCenter={true}
/>

AirShaper's aerodynamic shape optimization tool also helped with the design of the underbody and the rear diffuser, which are essential as they generate significant downforce with minimal drag. The software showed that instead of a fairly square cross section, a more rounded and larger central tunnel with smaller side tunnels performed better.

A lot of effort went into the front splitter, to optimize the way the air is accelerated underneath the car. Additionally, large air curtains were added to shield the front wheels and reduce their drag. Other areas of improvement included the height and length of the tail, the rear window cover, side air intakes, front wheel well ventilation and more.

---

## The benefits of CFD

AirShaper is not just designed for aerodynamicists, but also designers as well. The intuitive and easy to use software allows designers to make CFD a key element in their design process. 'You are able to test your design changes and see how they affect the aerodynamic performance of the car,' highlights Raeli. 'If we test a specific detail or a volume change in the nose for example, within a few simulations we can understand how it is affected, positively or negatively. We can then adjust the design for the next round of optimizations whilst keeping in mind the proportions we wanted to maintain.'

---

## The results

Through the rounds of development, the drag coefficient was reduced by 28.5% to a Cd of just 0.279 whilst doubling the downforce (thanks to the optimized underfloor aerodynamics and top winglets).

<ImageWithCaption
 src="/assets/images/7x-design-lamborghini-rayo-pressure-clouds.png"
 alt="The pressure clouds of the final design show a signifcant drag reduction which is vital for the top speed attempt"
 width="800"
 height="315"
 positionCenter={true}
/>

The car was taken to the <ExternalLink href="https://www.applusidiada.com/global/en/what-we-do/services/spain-proving-ground">IDIADA test track</ExternalLink> in Spain for initial testing of the new body, rendered in carbon fibre. The results were better than the team had hoped for, despite the limitations on the available straights. The car achieved 278mph (447kph) with more to come from further powertrain and aerodynamic development. Later this year, the car will be taken to a more suitable location to try to reach the 300mph (480kph) target.

<ImageWithCaption
 src="/assets/images/7x-design-lamborghini-rayo-testing-2.png"
 alt="The finished car was tested at the banked turns and long straights of IDIADA's proving ground. CREDIT: img1.wsimg.com"
 width="800"
 height="448"
 positionCenter={true}
/>

'What we have here is a great achievement because, if you think about it, how many companies attempt something like this?' questions Raeli. 'They usually have to invest millions of dollars in R&D and a lot of time. Whereas our small team, together with AirShaper's expertise and the knowledge of the people who built the car, allowed us to upgrade the Lamborghini Huracan to achieve those ambitious speeds. I think that's a pretty big accomplishment.'

---

##### Interesting links:

 

<ExternalLink href="https://www.instagram.com/7xdesign/">
 Check out 7x Design's Instagram
</ExternalLink>

 

<ExternalLink href="https://www.applusidiada.com/global/en/what-we-do/services/spain-proving-ground">
 The IDIADA proving ground
</ExternalLink>

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