Tag: aerodynamics

Ski Racing: Where Champions are Made on the Course and in the Lab

If you have ever watched the winter Olympics, you have probably watched in awe as the alpine ski racers flew down the course. Years of training to perfect technique and build strength are essential for any athlete trying to compete with the best, but in a sport where hundredths of a second can separate first and second place, racers are always looking for ways to shave time. Understanding the forces that slow them down and their relationship to body positioning gives these athletes a competitive advantage.

Photo of Mikaela Shiffrin, a world champion ski racer, in a low tuck position. She is turning around a blue gate racing in the 2022 winter Olympics.
Photo of Mikaela Shiffrin in a low tuck at the 2022 winter Olympics taken by Robert F. Bukaty.

Forces

The figure is a free body diagram of a skier. It shows a stick figure of the skier's body with a gravitational force acting down, a normal force acting perpendicular to the skis, a friction force acting parallel to the skis, and a drag force acting parallel to the path of the skier.
Free Body Diagram of the forces on a skier.

Several forces act on a skier during a race, including gravity pulling him or her down the hill, friction from the snow, and drag. Friction and drag decrease speed. Drag force occurs when an object moves through a fluid. It is a function of the fluid properties, the cross-sectional area of the object, and the object’s surface roughness. In ski racing, the fluid is air. Drag force increases as velocity and cross-sectional area increase.

A diagram of the air flow around a circle. The air flow is shown with arrows pointing in the direction of flow. One arrow strikes the circle at the center. The others bend to move around the circle, this is the attached flow. At the back of the circle, the flow separates and bends into spirals of turbulent wake, creating the low pressure zone.
Diagram of the air flow around blunt objects highlighting pressure drag.

There are two categories of drag. Viscous drag is caused by friction between the air and the body. Skiers decrease this force by wearing smooth suits. Pressure drag occurs because the air stream separates to move around the body. When the air strikes an object, particles build up and compress on the front surface. They are more spread out behind the object, creating a low-pressure zone that induces drag.

Body Position Basics

The drag force has the most significant impact on speed. Ski racing involves adopting a low tuck position to decrease cross-sectional area. In a low tuck, the athletes bend at the waist and knees while tucking their poles beneath their armpits.

A photo of a ski racer in a low tuck position. She is in a wind tunnel wearing full ski racing gear, including a helmet, goggles, speed suit, gloves, poles, boots, and skis.
A figure taken from Brownlie et al. of a ski racer in a wind tunnel.

Athletes also spend time in wind tunnels during the preseason to perfect their body positioning. This testing, however, is time consuming, expensive, and fails to highlight the distribution of flow around the skier’s body. To better understand these forces, several researchers, Elfmark, Asai, and Brownlie, used a mix of wind tunnel testing and computational fluid dynamics to estimate the drag force, visualize air flow, and find the position that minimizes drag.

Ideal Body Position

A figure of a digital skier's body with a plot overlaid. The plot has length along the athlete on the x-axis and drag coefficient on the y-axis. The drag coefficient spikes when moving over the head, at the elbows and knees, and when moving over the thigh.
Figure taken from Elfmark et al. showing how drag force increases along the length of the body.

The data showed that an upright, open body position will increase the drag force, hence the need for a low tuck. Specifically, the head, upper arms, thighs, and lower legs have the largest affect on drag. This is because these are blunt areas on the body that the air stream must separate to move around. The figure shows that the drag force spikes as the air strikes the head, upper arms, and thighs, and around the arm pits. Additionally, Elfmark’s team found that the ideal position was when the knees filled the gap beneath the arm pits. This decreases the need for the air to separate, facilitating a smooth air flow and thus less drag.

Although these results are interesting, they have significant limitations. The tests modeled a static position of the skier, which is unrealistic in a dynamic sport.

Staying airborne: How bird wings are built for aerodynamic and efficient flight

Flight is a concept that has, until relatively recently in history, eluded humanity. However, birds have been successfully flying for approximately 130 million years, proving themselves to be a physical marvel of the natural world. And while our means of flight have historically been crude in design and performance, nature provides an elegant, efficient solution to get creatures off of the ground. Rüppell’s griffon vultures have been recorded flying as high as 37,000 ft, while some species of shorebirds have been recorded flying as far as from Alaska to New Zealand over eight days without stopping. But how exactly do birds seem to effortlessly overcome gravity so effectively? And perhaps more importantly, how might we apply these answers to improve manmade aircraft?

Morphology

Obviously, the exact aerodynamics and physical characteristics of birds will vary from species to species, but there are still underlying similarities that enable birds to fly. A bird’s wing consists of a shoulder, elbow, and wrist joint which establish the wing’s basic shape and allow a range of motion. Covering the wing are structures called primary, secondary, and coverts, which are all groups of feathers that provide lift and stabilize flight. Feathers consist of flexible fibers attached to a center shaft, called the rachis. Overtime, the rachis will become damaged from fatigue and large instances of stress. As a result, birds will molt and regrow their feathers on a regular basis. 

A diagram of the structure of a bird wing
Picture by marcosbseguren on Wikimedia Commons

Generally, a bird’s body will be adapted to either gliding flight, in which the wings flap very infrequently, or active flight, in which the wings flap nearly constantly. For gliding birds, such as the ocean dwelling albatross, the wings will extend far away from the body, and prioritize both wing and feather surface area over flexibility. Additionally, these wings will have a thick leading edge, and will be much straighter. However for fast, agile birds, such as falcons, the opposite is true. Consequently, agility is sacrificed for energy efficiency. In both cases, the rachis will change shape and rigidity, becoming larger and stiffer for gliding flight and smaller and more flexible for agile flight. 

Aerodynamics

One of the most unique aerodynamic characteristics of birds is that nearly all of their lift and thrust is exclusively generated by their wings, as opposed to aircraft that implement both wings and engines. This provides, among other things, near instantaneous control of both flight direction and speed. In other words, this gives birds an advantage when hunting, escaping from predators, and maneuvering through a landscape. 

To aid in the generation of thrust and lift during flight, birds will change their wing shape through a process called active morphing. During flight, the wing will be bent inwards and twisted up during the upstroke, and extended and straightened during the downstroke. As a result, this minimizes drag while maximizing thrust and, consequently, energy efficiency. This can aid in anything from traveling farther distances to hunting prey.

An osprey folding its wings in while catching a fish
Photo by Paul VanDerWerf on Wikimedia Commons

Applications

Initially, these principles may seem difficult to realistically utilize in aircraft. After all, we are limited by the materials available and the size that aircraft must reach. However, small steps could be taken to improve the energy efficiency and responsiveness of aircraft. For example, wing shape, material flexibility, surface finish, and moving joints could all be explored. In fact, research at MIT is currently being conducted on flexible wings made of scale-like modular structures. If experiments like this are successful, it could show that aircraft designs inspired by nature may be the future of the world of aeronautics.

Which is more stable, washing machines or birds? The answer might surprise you

What do birds and washing machines have in common? Shockingly, it’s not the ability to wash clothes. Rather, most birds and washing machines are great examples of vibration isolation systems.

Now that’s cool and all – but what is a vibration isolation system?

Better known as a mass-spring-damper system, vibration isolators are generally a mechanical or industrial mechanism that can reduce the amount of vibrational energy produced by a system. Vibration isolators are incredibly important; studies show “undesirable vibrations” can shorten a machine’s service life and even permanently damage the machine and those using it. Considering this, engineers are constantly improving upon current vibration control systems, and are now looking to birds for inspiration.

But why birds? Well, to understand this, let’s consider a bird as a simple mass-spring-damper system.

Avian vibration isolation system represented as mass-spring-damper-system
Simple approximation of avian vibration isolation system as mass-spring-damper system. Taken from the 2015 study: ‘The role of passive avian head stabilization in flapping flight.”

First, visualize vibrations as an oscillating force stemming from the bird’s body moving back-and-forth. Vibrational forces can be generated by the flapping of wings, unexpected gusts, and/or movement of legs. Now, if we continue up from the body to the neck, we can see where avian skeletal and muscular structure really begins to “show off its feathers.”

Characterized as a multi-layered structure, the avian neck contains many sections of “hollow” bones, connected by surrounding muscles. The structural units (muscles and bones) of the avian neck have properties of both springs and dampers, optimizing them for vibration isolation.

Simplified representation of multi-layered neck as spring-damper structure

For starters, we see the muscles largely act as springs. Springs have the unique ability to move a body with its vibrations. This behavior is present in the muscles connected to the bone segments, in that they are capable of instantaneously compressing, elongating and twisting in response to rapid changes in the body’s movement. This elastic response prevents not only the head, but the whole bird, from shaking when bombarded when vibrations from any form of movement.

Simplified visualization of multi-layered spring-damper structure. The transparent grey portion represents the hollow bone, which is connected by the black lines, or strong spring-like muscles. The empty space between each unit would consist of the softer, damper-like muscle. Taken from the 2021 study: “A novel dynamics stabilization and vibration isolation structure inspired by the role of avian neck.”

Alternatively, the muscles, primarily those not connected to bone, can act as dampers. Effective dampers are similarly identified by the ability to move with vibrations; however, they can dissipate some of the vibrational energy as heat, or store energy until relaxed. The interior muscles are capable of slowly deforming (changing shape) if exposed to steady vibrations, allowing for dissipation of excessive vibrational energy.

But hey, what about those bones?

The avian neck has nearly three times the number of bone sections than most mammals, on top of muscles entirely surrounding the neck. This drastically increases the bird’s flexibility, helping it maneuver through sharp positional changes, thereby further limiting the effect of vibrational forces.

Finally, what makes the avian vibration isolator truly superior is its passive activation. As engineers at Shanghai Jiao Tong University point out, manmade passive vibration isolators fall short because they require sensors and input energy to adjust for “shocks and random vibrations.” As previously explained, the multi-layered neck is well equipped to handle random oscillations, yet, more importantly, the bird’s neck muscles can passively change position to brace for incoming vibrations.

A recent study from Stanford University proved this concept by recording a whooper swan’s reaction to different strength gusts. They found that swan’s neck adjusted to protect the head, and that even when the flapping doubled, the movement of the head reduced by a quarter. Finally, it is important to note that passive activation is not limited to the sky; researchers have found that mainly terrestrial birds like chickens and pigeons have a similar neck structure and system for maintaining stability and clear vision.

Overall, continuing to study the avian vibration isolation system could prove very beneficial for many different applications. For a more in-depth look at the current work out, check-out the studies referenced throughout the article. Otherwise, enjoy watching this chicken work its body control magic!

Mercedes-Benz “Chicken” Magic Body Control Advertisement, highlighting the chicken’s amazing head stabilization ability.