World’s most aerodynamic suits - Interview with Deanna Panting, founder of Qwixskinz.

Free Speed: how properly designed clothing can make Olympic athletes faster.

Wind tunnels are one of the most important tools in an aerodynamicist’s armoury. They bridge the gap between real world aerodynamic testing and CFD simulation, allowing aerodynamicists to test physical parts in a more controlled environment. But how do wind tunnels work and what techniques should you use to achieve reliable and accurate results?

- ###### [A brief history](#windtunnelhistory)
- ###### [How does a wind tunnel work?](#howwindtunnelswork)
- ###### [Moving the air](#windtunnelmovingair)
- ###### [Cooling the air](#windtunnelcooling)
- ###### [Conditioning the flow](#windtunnelconditioning)
- ###### [Accelerating the flow](#windtunnelacceleratingflow)
- ###### [The test section](#windtunneltestsection)
- ###### [Scale models](#windtunnelscalemodels)

---

## A brief history <a name="windtunnelhistory"></a>

During the 19th century, research into aerodynamics was largely aimed at manned flight. The physical testing at this time involved a whirling arm, often gravity powered, which rotated a test object through the air. It was eventually realised that this rotation generated a circular wake that affected the accuracy of the results, so a new testing approach was required.

In response, Frank Wenham built the world’s first wind tunnel in 1871 [1] which was a 12ft (3.7m) long tube with a fan blowing air along its length. As aircraft and ground vehicles continued to develop, so did the capabilities of wind tunnel testing and by 1916, the Wright Brothers were utilising a larger tunnel with improved flow velocity and quality.

<ImageWithCaption
 src="/assets/images/wright-brothers-wind-tunnel.jpeg"
 alt="Side view of a long red, thick tube acting as a scale wind tunnel"
 caption={
 
 The 1916 Wright Brothers wind tunnel. CREDIT:{' '}
 <ExternalLink href="https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/196756/wright-brothers-1916-wind-tunnel/">
 National Museum of the United States Airforce
 </ExternalLink>
 
 }
 width="700"
 height="408"
 positionCenter={true}
/>

Aeronautical engineering continued to progress quickly and by 1920, NACA were building a 5ft (1.5m) wind tunnel based on a British Royal Aircraft Establishment (RAE) tunnel [2]. However, the need for better measurement techniques to match scientific understanding meant these tunnels quickly became obsolete. 

Over the last 100 years, aircraft have developed into faster and larger vehicles, so to conduct accurate testing, wind tunnels have had to evolve too. The <Link href="/blog/nasa-full-size-wind-tunnels">largest wind tunnel in the world</Link> is currently NASA’s Ames Research Center which can test aeroplanes with wing spans of up to 100ft (30.5m) [3]. While the accuracy of results now means that wind tunnels can be used to test anything from space shuttles to athletes. 

<ImageWithCaption
 src="/assets/images/nasa_space_shuttle_wind_tunnel_test.jpeg"
 alt="Silver space shuttle scale model in the test section of a scale wind tunnel"
 caption={
 
 A space shuttle under test in a NASA wind tunnel. CREDIT:{' '}
 <ExternalLink href="https://www.nasa.gov/mission_pages/shuttle/flyout/unflyable.html">
 NASA
 </ExternalLink>
 
 }
 width="700"
 height="394"
 positionCenter={true}
/>

---

## How does a wind tunnel work? <a name="howwindtunnelswork"></a>

Whether it is a plane or a racecar, fundamentally it is an object moving through air. However, to replicate the behaviour of the airflow around an object, wind tunnels flip this around and move air over a fixed object. 

Wind tunnels come in a range of layouts, sizes and speeds to cater for the many different applications of wind tunnel testing. The most common is the closed loop wind tunnel, variations of which form the basis of most aeronautical and automotive facilities. To understand the basics of wind tunnels, let's consider a subsonic, pressurised wind tunnel such as the <ExternalLink href="https://www.onera.fr/en/windtunnel/testing-capabilities">ONERA wind tunnel in France</ExternalLink>. 

<ImageWithCaption
 src="/assets/images/onera-wind-tunnel.png"
 alt="Schematic showing the closed loop layout of the F1 wind tunnel at ONERA in plan view"
 caption={
 
 The layout of the subsonic continuous pressurized closed-loop wind tunnel at ONERA. CREDIT:{' '}
 <ExternalLink href="https://www.onera.fr/fr/windtunnel/f1-subsonic-wind-tunnel-pressurized-high-reynolds-number-high-efficiency">
 ONERA
 </ExternalLink>
 
 }
 width="600"
 height="565"
 positionCenter={true}
/>

This type of tunnel circulates a mass of air around a closed-loop circuit and flows high speed air over the object in the test section. This object could either be a scale model or full-size vehicle. This wind tunnel loop can be broken down into four main areas:
- Fan
- Cooling
- Flow conditioning
- Test section

<YouTubeEmbed
 videoId="L1AYo9Mk1EI"
/>

---

## Moving the air <a name="windtunnelmovingair"></a>

At the heart of the wind tunnel is the fan, which needs to be extremely powerful to achieve the flow velocity required at the test section. This results in large, multi-bladed fans with variable speed and sometimes variable pitch propellors. They can be over 15m (49ft) in diameter and are driven by electric motors rated up to 10MW. The larger the fan, the higher its efficiency but the more difficult it is to build and house.

<ImageWithCaption
 src="/assets/images/volkswagen-wind-tunnel-main-fan.jpeg"
 alt="Front view of the main fan in the Volkswagen wind tunnel with an engineer crouching infront for scale"
 caption={
 
 The main fan of the Volkswagen wind tunnel in Wolfsburg. CREDIT:{' '}
 <ExternalLink href="https://uk.motor1.com/news/185851/volkswagen-wind-tunnel/">
 Motor1
 </ExternalLink>
 
 }
 width="700"
 height="394"
 positionCenter={true}
/>

---

## Cooling the air <a name="windtunnelcooling"></a>

As the air moves around the wind tunnel, it generates friction with the walls. This, together with the pressurised nature of the majority of wind tunnels causes the air to increase in temperature. To make sensitive measurements, temperature control is very important for both the air flow and the instrumentation. This is why tunnels of this nature have very powerful cooling systems, usually liquid based, and a radiator matrix through which the air flows.

---

## Conditioning the flow <a name="windtunnelconditioning"></a>

To maximise the flow at the test section, the air needs to travel around the circuit as efficiently as possible and minimise any losses. To help the air turn round corners efficiently, a series of aerofoil-section vanes are fitted to each corner to help bend the flow through 90 degrees with minimal loss in velocity. 

<ImageWithCaption
 src="/assets/images/icing-research-tunnel-turning-vanes-2.png"
 alt="Close up photo of large aerofoil-shaped turning vanes at the corner of a the Icing Research Wind Tunnel"
 caption={
 
 The main fan of the Volkswagen wind tunnel in Wolfsburg. CREDIT:{' '}
 <ExternalLink href="https://www.nasa.gov/specials/irt360/">
 Icing Research Tunnel
 </ExternalLink>
 
 }
 width="500"
 height="365"
 positionCenter={true}
/>

After the air has turned the final corner, it enters the settling chamber where the flow is conditioned to be as straight and turbulent free as possible. This is achieved using a honeycomb structure where tubes force the flow to be parallel and minimise swirl. The air then passes through a series of mesh screens which help to reduce variations in longitudinal flow velocity and break up an eddy currents. 

<ImageWithCaption
 src="/assets/images/wind-tunnel-flow-conditioning.png"
 alt="Diagrams with arrows showing the direction of airflow as it moves through honeycomb structures and mesh screens"
 caption={
 
 Diagrams showing how the flow is conditioned through honeycomb (left) and mesh screens (middle and right)
 
 }
 width="900"
 height="233"
 positionCenter={true}
/>

---

## Accelerating the flow <a name="windtunnelacceleratingflow"></a>

The last stage the air has to go through before entering the test section is a Venturi. Shrinking the diameter of the wind tunnel down to the cross sectional area of the test section, accelerates the flow to the required test speed. Downstream of the test section, the cross section is progressively increased through a diffuser expanding the flow and slowing it down again.

---

## The test section <a name="windtunneltestsection"></a>

The test object is mounted in the <Link href="/videos/wind-tunnel-test-section/L1AYo9Mk1EI">wind tunnel test section</Link>. If the object is an aircraft, it will be mounted via a sting in some manner, typically from the rear. The sting is used to run instrumentation connections to and from the model and to move it through different test attitudes whilst measuring loads.

<ImageWithCaption
 src="/assets/images/aeroplane-scale-model-z-sting-mount.jpeg"
 alt="Silver scale model of an aeroplane mounted in the wind tunnel test section via a Z sting mount"
 caption={
 
 Scale model aircraft mounted in the wind tunnel test section via a Z sting mount. CREDIT:{' '}
 <ExternalLink href="https://www.onera.fr/en/windtunnel/model-support">
 ONERA
 </ExternalLink>
 
 }
 width="700"
 height="500"
 positionCenter={true}
/>

If the object is a full scale ground vehicle, it will usually sit on load measuring pads or a rolling road. The latter is more accurate as it allows the wheels to rotate at speeds that match the tunnel speed. Whereas, scale model road vehicles are often suspended just above the ground. In either case, the interaction with the ground plane is essential to model as the vehicle operates in a boundary layer in the real world that must be replicated in the wind tunnel. 

<ImageWithCaption
 src="/assets/images/daimler-truck-wind-tunnel-smoke.jpg"
 alt="Perspective view of a red lorry cabin in an open section wind tunnel with oncoming air visualised with smoke"
 caption={
 
 Smoke testing a full-size lorry cabin which is sitting on load measuring pads. CREDIT:{' '}
 <ExternalLink href="https://media.daimlertruck.com/marsMediaSite/en/instance/ko/Product-Pictures.xhtml?oid=45626421">
 Daimler Truck
 </ExternalLink>
 
 }
 width="700"
 height="421"
 positionCenter={true}
/>

---

## Scale models <a name="windtunnelscalemodels"></a>

It is relatively unusual to test at full scale as the costs associated with a large enough facility are prohibitive. Therefore, companies often opt for <Link href="/videos/scale-model-wind-tunnel-testing/qqYiz1eo-F4">scale model wind tunnel testing</Link>. This allows aerodynamicists to make quick iterations during test programmes, the parts are faster and cheaper to manufacture and scale models are overall easier to work on. 

Scale models are typically 10-50% of full size, depending on the application. Large aircraft models are at around 10%, while road cars are often at 50% scale. But regardless of the scale, wind tunnel models are highly accurate with refined external geometries and a high standard of build quality to minimise the effect of any discontinuities on the results.

Internally, the models are full of instrumentation. This could include the load balance, which is responsible for measuring the lift and drag forces as well as the yaw, pitch and roll moments. Pressure tappings will be implemented on the surface of the model to measure static pressure, all of which have to be connected via rubber tubing to a pressure scanner.

<ImageWithCaption
 src="/assets/images/scale-model-instrumentation.jpeg"
 alt="Silver scale model of an aeroplane with the external geometry removed so that the instrumentation and pressure tubing cables are visible"
 caption={
 
 An example of the instrumentation inside a scale model of an aeroplane. CREDIT:{' '}
 <ExternalLink href="https://www.nlr.org/article/high-tech-wind-tunnel-models-creating-competitive-edge/">
 Netherlands Aerospace Centre (NLR)
 </ExternalLink>
 
 }
 width="700"
 height="359"
 positionCenter={true}
/>

The wind tunnel itself will also have a range of measurement techniques available. The forces on the model may be measured through the floor or the sting, rather than inside the model. While there are a a number of flow visualisation techniques such as <Link href="/videos/how-wind-tunnels-work-measurement-techniques-piv-pressure-taps-force-balance-pressure-probe/yA9FdqJdiKk">Particle Image Velocimetry (PIV)</Link>, smoke jets and flow visualisation paint. These help aerodynamicists visualise the behaviour of the flow to further their understanding.

---

### Interesting links:

- <ExternalLink href="https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-are-wind-tunnels-58.html">What Are Wind Tunnels?</ExternalLink>

- <ExternalLink href="https://www.youtube.com/watch?v=KC0E0wU6inU&ab_channel=SauberMotorsport">How Wind Tunnels work - F1 explained - Sauber F1 Team</ExternalLink>

---

### References

[1] <ExternalLink href="https://history.nasa.gov/SP-440/ch1-3.htm">Wind Tunnels of NASA. Chapter 1 - Whirling Arms and the First Wind Tunnels</ExternalLink>
[Online]. NASA. 

[2] Smith, Y. 2015. <ExternalLink href="https://www.nasa.gov/the-nacas-first-wind-tunnel">The NACA's First Wind Tunnel</ExternalLink>
[Online]. NASA. 

[3] 1992. <ExternalLink href="https://www.nasa.gov/centers/langley/news/factsheets/WindTunnel.html">NASA's Wind Tunnels - IS-1992-05-002-LaRC</ExternalLink>
[Online]. NASA. 

---

How does a Wind Tunnel work?

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

Technology - Can it improve lives?

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>

---

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