Air Resistance and Sports

©1993 NAOC TM NAOC 7-056

Air resisance and speed

You may have experience of sticking your face out of the car window.
The faster the car is running, the stronger wind pressure you feel. Inside the car, you feel no wind but the car itself is running against the wind pressure.

Imagine how much wind pressure a ski jumper gets when running down the slope at the speed of 90 kilometers per an hour. To increase the velocity the skier has to fight against the wind pressure.

Athlete fights against the air

Even in the 100 meters sprint running, the time gets worse if the wind blows against the runner.
In the sports with faster movement such as speed skating and Alpine skiing, the air resistance is much greater than in the sprint running. Minimizing the air resistance is very important.
Ski jumping, though not the speed competition, the speed at the take off is necessary to fly far.
Do you know what is the velocity of various sports?

How to minimize the air resistance?

To make wind resistance small, you know that you must crouch and make your body small.
But what is the best position of your arms? Should they be kept in front of your body, or behind the body? What is the best inclination of your head and body?
The answers have been found in the researches using the wind tunnel experiments.

Even a little difference of the arm position

The air resistance in various posture were measured in the wind tunnel experiment.
How much difference are there in the actual Alpine skiing between the crouching posture like the photo and the standing posture?
The skier in the crouching egg shape posture running in the velocity of 100 kilometers per an hour may lose the velocity down to one third if he just stands up with his arms hanging down at the body sides.
The value of air resistance relates to the arms and hands position too.

The shape and the air flow

A big man gets greater air resistance than a small man.
The larger the cross section of the body, the greater the air resistance the body gets. But the cross section is not the only factor.

Air flows in a similar way to the water does.
The pictures below are the stream observed in the river, swimming pool, even in the washbowl. The air and water hitting against the object detours around it. Since they have velocity, they can not flow into just behind the object. That causes vacancy in the backside. The high air pressure in front of the object pushes it back into the low pressure. The pushing force, shown as the red arrow, causes the air resistance.

Opening the door and strange air resistance

You can try an experiment. Turn on the ventilation fan of the room.
After a while, open the room door from outside. What happens?
You may feel unusual resistance when you open the door. The ventilation fan made low pressure air inside the room. And the low pressure pulls the door inward causing the air resistance of the door.

Swirling air

The air behind the object flows into the vacancy and makes swirly stream. The swirls are the evidence of unnecessary air resistance.
The F1 car racer flollowing the rival car uses the slipstream which is the air flow into the vacancy behind the rival car.

Though the size and the cross section is similar, some types of shapes can minimize the swirls.
The two shapes in the figure are similar when you look at them from the front side. The cross sectional area is the same. Is the air resistance the same?
No. The secret is not in the front part but in the rear part.
You know that the rear part of the airplanes and ships have the shapes like this. These shapes prohibit the vacancy and swirls and successfully decrease the air resistance.

The shape is seen not only in the artificial things like airplanes and the ships but also in the natural things. The birds flying in the air and the fish swimming in the water which we often see in our daily life have this kind of shape that has scientific reasons.

Bumpy surface of human body

Unfortunately our body shape is not like birds or fish.
It is difficult to make a posture which doesn't make swirls. Some competition needs body action and speed at a time. The competitors are not free to concentrate on the air resistance only. However the researchers have been finding the way to minimize the swirls and decrease the air resistance. The findings have been applied to the actual competition.

Some shape may look strange or grotesque, but there are some scientific reasons.