How does aerodynamics affect speed?

How does aerodynamics affect speed?
Geoff Dymott
Geoff Dymott
1/17/2023

Aerodynamics is one of the most important tactics for improving performance, which in most cases is to go faster. So what is the relationship between aerodynamics and speed, and how do different industries optimise aerodynamics to increase speed? This article provides a detailed explanation of the theory behind these questions as well as an insight into how the aerodynamics of runners, cyclists, motorcycles, cars and aircraft are all improved.


Aerodynamic drag and speed

The Bugatti Veyron is one of the fastest cars ever made. The original 987bhp (1,001ps) version clocked a top speed of 407kph (253mph) in 2006 [1]. While the revised 2010 Super Sport increased power output to 1,183bhp (1,200ps), boosting the top speed to 434kph (270mph) [2]. So in this case, it required nearly 200bhp to gain 17mph.

A black and red Bugatti Veyron Super Sport
The Bugatti Veyron Super Sport achieved a Cd of 0.348 while the original version had a Cd of 0.355 [1], [2]. CREDIT: Bugatti

The similarity of the underlying cars demonstrates nicely the problem of aerodynamic drag in the hunt for speed. The standard force and power equations show that drag has a squared relationship with speed, whilst power has a cubed relationship.

The force and power drag equations written in algebra
The formulas for calculating the force and power due to aerodynamic drag

Where: Fd = drag force (N), Pd = power (W), ρ = density (kg/m³), v = velocity (m/s), Cd = drag coefficient, A = frontal area (m²).

Whether power comes from internal combustion, electricity or a person, it is difficult to find the increases needed to make small gains in speed. Instead, engineers look to the other three variables in these equations: air density, drag coefficient and frontal area.


Engineering drag reduction

Drag coefficient and frontal area are physical aspects of design that are well understood. Wind tunnels and CFD allow engineers to work with athletes and design vehicles to produce forms that reduce the force required to overcome aerodynamic drag.

Air density is a less obvious variable that can be used to enhance designs and optimise performance. Cyclists breaking the world hour record will choose temperature controlled velodromes at altitude to reduce the density of the air they are pushing through. Lower air density also means less oxygen which reduces the available power, so there is an important compromise to be found.


How aerodynamics affects the speed of runners

Human athletes are a good starting point to investigate how aerodynamics are used to increase speed. Analysis suggests that around 90% of the energy that Usain Bolt expended during his 9.58s record 100m run was required to overcome drag [3].

As a result, sportswear has been developed to be lighter and minimise drag. The original Nike Swift Suit worn by Cathy Freeman in 2000 has now evolved into arm and leg sleeves with 3D moulded blades that aim to reduce skin friction and wake size.

Design sketches showing how the air flows around the seams of the Nike Swift Suit
The original design sketches for the Nike Swift Suit for the 2000 Olympics. CREDIT: Edward Harber

A moving human being is a difficult subject to model. Individual shape, size and running style produce different air flows and wakes. Elite athletes will have bespoke mannequins for wind tunnel testing to precisely locate drag reducing features.

Different sizes of black drag reduction panels that can be applied to runner's legs
All athletes can now benefit from off the shelf drag reduction panels. CREDIT: Women’s Running

International competitions have strict clothing regulations to limit the impact of technology. Despite this, teams will go to impressive lengths to push the boundaries of performance. This was proven by the INEOS 159 Challenge where every available technology was exploited to help Eliud Kipchoge run the first sub-two-hour marathon. The team optimised the formation of the pace runners using wind tunnels and CFD, while an electric pacecar calculated the precise speed (to within 0.1kph, 0.06mph) required to guide the runners to the sub-two hour target.

Coloured CFD simulation showing the airflow around the pace runners in formation
CFD studies were used to select the aerodynamically optimum formation for the pace runners. CREDIT: INEOS 150 Challenge

How aerodynamics affects the speed of cyclists

Cycling has perhaps benefitted the most from recent aerodynamic advances. In part this is due to the capabilities of the latest Aero bikes and aerodynamic clothing, but also because it lends itself well to wind tunnel and CFD analysis. The relatively fixed rider postures can be easily modelled, analysed and optimised to reduce drag.

CFD pressure streamlines around a cyclist
The results from AirShaper, such as the pressure streamlines shown above, have helped cyclists adjust the position of their helmet, upper arms and body to minimise the wake

As with most innovations in sport, regulation changes are not far behind. Textured arm and leg sleeves can be used to reduce surface drag and control leg shape. In cycling, short lengths are controlled and oversocks banned, so teams wear long socks instead. While regulations continue to limit sock length, teams often turn to new innovations, which we saw at the Tokyo 2020 Olympics where the Danish cycling pursuit team applied kinesiology tape to their shins.

The importance of drag to cycling performance is perhaps best demonstrated by the typical speeds achieved in different disciplines. A Tour de France rider cruises at 48kph (30mph) sharing drag in a peloton. The world hour record average speed by a single rider in a velodrome is 56kph (35mph). The drag-assisted world record on a custom bicycle is 295kph (184mph) [4].

 

Denise Mueller-Korenek sets the bike land speed world record


How aerodynamics affects the speed of motorcycles

The exposed nature of a motorcycle leads to turbulent flows, reverse flows and separation which increases the total drag force the bike has to overcome. So, despite motorcycles having a frontal area approximately two thirds smaller than an average car, its drag coefficient can range from 0.5 to 1.0 which is almost double that of a modern car [5].

Similar to cycling, rider position and bike design are key to minimising drag. However, a recent trend has been to increase downforce by adding fairings and winglets to the front of motorcycles to increase the load on the front axle and optimise load transfer. This was one of the tactics employed by Ducati in MotoGP over recent years.

In terms of aerodynamic innovation, one of the most interesting motorcycle designs is the Voxan Wattman. This electric motorcycle set the world record in 2021, achieving a speed of 283mph (455kph) with Max Biaggi [6]. The removal of the requirement to corner results in a very different bike design and low drag riding position.

For example, three sets of bodywork were used to ensure maximum performance regardless of the wind conditions. This bodywork included a streamlined full fairing, a fairing with an exposed tail and a naked version. Furthermore, the seat was raised by 65mm which helped flatten Biaggi’s back and reduce drag.

Max Biaggi riding the Voxan Wattman at an airfield
A bespoke fairing and rider position helped reduce the drag of the Voxan Wattman. CREDIT: Visordown

How aerodynamics affects the speed of cars

Top speed has long been a priority for road and racing cars. Before wind tunnels and CFD, streamliner bodywork was handcrafted to minimise drag. The 1930s Mercedes versus Auto Union battles saw innovative bodywork fitted to usually open-wheel cars.

Modern supercars overcome the problem of low drag and high speed instability with careful aerodynamic design in wind tunnels and CFD. The Lamborghini Rayo project used AirShaper to optimise the drag and downforce of the front splitter, underfloor and rear diffuser. Air curtains were also added to shield the front wheels to minimise drag. This optimised bespoke bodywork delivered a 300mph (482kph) version of the Lamborghini Huracan.

 

The Bugatti Veyron was a game changer for top speed and introduced the concept of active aerodynamics to road cars. Moveable wings and variable cooling exits allow cars to perform well in all conditions, generating high downforce during cornering whilst still achieving fast straight-line speeds.

The technology seen on these ground-breaking hypercars often filters down to more conventional applications. A common feature for high-speed configurations is to deploy a small spoiler. This is a drag reduction device that generates downforce to retain stability. In fact, Audi had to add a spoiler to its Audi TT to solve some high speed stability issues.

Illustration showing the air brake deployed at 250km/h on a Bugatti Veyron
Braking above 155mph (250kph), the spoiler acts as an air brake causing another 0.6G of deceleration. CREDIT: Bugatti

The Veyron was originally considered impossible to improve because it was such a difficult engineering exercise. Bugatti however, once again took on the challenge and developed the Chiron Super Sport which, at the time of writing, was the world’s fastest car, clocking 304mph (489kph) [7].

Although, it is the drive for efficiency of electric vehicles that is likely to offer the next batch of aerodynamic innovations. The Lotus Evija for example, takes advantage of packaging opportunities that an electric powertrain has to offer. A flat battery pack and in-wheel motors eliminate the engine, transmission, cooling system and fuel tank that normally make up much of the frontal area of a car. This offers more opportunity to exploit the underbody and through-body flows.

Close up image of the rear left of the Lotus Evija showing through channels
The Lotus Evija has through channels to reduce effective frontal area and produce downforce. CREDIT: Lotus Cars

How aerodynamics affects the speed of aircraft

The latest hypercars represent the height of development of high-performance ground vehicles, with the fastest now running into compressibility effects, which are well known to aerospace engineers. Aircraft design accounts for air density in multiple ways. Altitude is obviously a key variable, but speed itself starts to have a significant effect. This is what makes machines like the North American X-15, the Lockheed SR-71 Blackbird and Concorde such engineering achievements.

Concorde aircraft flying at high altitude
Concorde flew at very high altitude to make flight faster and more fuel efficient. CREDIT: Getty Images

At lower speeds, air is usually considered to be incompressible. As a result, air density is defined by altitude, temperature and humidity. As aircraft approach the speed of sound (generally referred to as Mach 1), compressibility becomes an issue.

In this transonic region, air molecules can no longer move quickly enough to move aside for the wings and fuselage. Instead, the air is locally compressed and becomes more dense. The rate of change of density rises rapidly above Mach 1, so much so that it increases with the square of the speed which in turn increases lift, but also drag. This higher drag means even more power is required to go faster. Together with the aerodynamic consequences of the shock waves produced at high speeds, this effect is called the sound barrier.

The sonic boom has been largely responsible for making supersonic airliners unworkable over land, diminishing the business case for a Concorde replacement. High-supersonic aircraft like the SR-71 and proposed hypersonic aircraft have problems with aerodynamic heating and engine performance that make them very difficult to design and operate.

A dark grey SR-71 flying over mountains
The SR-71 has a titanium body to overcome aerodynamic heating from skin friction drag. CREDIT: NASA

A common feature of supersonic aircraft of the 1970s is the swing wing, from the F1-11 to the Panavia Tornado. Like the active aerodynamic wing configurations on a Veyron, these are used to overcome the compromise between low landing speeds and high cruising speeds. At low speed, a straight wing is preferred to generate lift and minimise take-off and landing distances. At high speed, these generate too much lift and drag. Sweeping them back reduces drag for speed and fuel efficiency purposes.

Illustration of a Rockwell B-1 supersonic bomber in plan, front and side view
The swept wings of a Rockwell B-1 supersonic bomber are capable of Mach 2.2 at altitude. CREDIT: Plane Encyclopedia

These types of aircraft were largely a product of the Cold War where innovation in drag reduction was required to outrun other aircraft. In 2022, drag reduction is still very much a priority to maximise speeds whilst maintaining maximum fuel efficiency.

The next step for aircraft is manned hypersonic vehicles which create new challenges for aircraft design and propulsion. It will be interesting to see the aerodynamic solutions to these problems, but one thing is for sure, CFD simulations and wind tunnel testing will play an important role in solving them.


Interesting links:


References

[1] 2006. Technical Specifications Bugatti Veyron 16.4 [Online]. Bugatti.

[2] 2006. Technical Specifications Bugatti Veyron 16.4 Super Sport [Online]. Bugatti.

[3] 2013. Physics of running fast: Scientists model 'extraordinary' performance of Bolt [Online]. Institute of Physics.

[4] 2019. Fastest cyclist ever: Setting a World Record speed of 183mph on a bicycle [Online]. Cyclist.

[5] 2020. How to improve motorcycle aerodynamics [Online]. AirShaper.

[6] 2021. Voxan and World SBK Champ Max Biaggi smash Electric Motorcycle Speed Record [Online]. BikeSocial.

[7] Bugatti Chrion Super Sport 300+ [Online]. Bugatti.


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