# Aerodynamics drag

##  Aerodynamic Drag

"Air resistance" can be thought of in terms of force acting on a body due to flow characteristics of fluids through which the body is moving. The branch of physics called fluid mechanics includes two components that contribute to development of drag force acting against a skier's motion. ("Fluid" in this context means a medium, like air, which can flow around an object.)

The two components depend on an object's surface characteristics and on its shape and size and they are often referred to as skin drag and profile drag. Skin drag is a force acting against the direction of motion which derives from local characteristics of fluid flow at the surface level of an object. In some sporting situations (swimming for example) skin drag is of consequence, but in skiing it is of relatively small magnitude compared to profile drag and ski drag forces and won't be discussed further here.

Profile drag force is generated as a skier moves through air and depends on a skier's overall shape and size. With motion, air flows around the body, skis and poles and exerts varying pressure patterns on the front, back and sides of a skier. Steamlining of the body and equipment can smooth the air flow patterns and minimize pressure differences from front to back of a skier.

It is air pressure difference applied across an area of the body which creates profile drag force (and explains why it is also referred to as pressure drag). If a skier can reduce pressure differences and can reduce the area upon which the pressure difference acts, drag force can be reduced.

Body shape of a skier is not easily adjusted within the constraints of techniques for propulsion. Hence relatively little is usually done to minimize air drag under normal conditions of flat and uphill skiing. However, on downhills where speed is greater and where gravity provides propulsive force, or when headwinds are encountered, ski technique is often modified to minimize air drag forces. In these circumstances, the relative velocity of air past a skier can be considerably greater than the 5 or 6 m/s average speeds in racing.

Because drag force increases as the mathematical square of relative velocity, modest increases of air flow past a skier can substantially increase drag force. Figure 30 illustrates typical profile drag forces as a function of speed for upright and tucked body positions. Notice how dramatically drag force increases with speed. At 5 m/s about 10 N of drag force act against a skier; doubling speed to 10 m/s quadruples drag force to 40 N. Changing body position from upright to tucked (Figures 1 and 4, for example) is a common strategy on downhill terrain which reduces air drag by more than half. This advantage results both from streamlining of air flow around the body (reducing pressure differences) and from reducing frontal area compared to upright.

Skiers on flat terrain encountering headwinds have more difficulty in dealing with air drag force. A modest headwind of about 2 m/s will nearly double the air drag that a skier experiences; with more severe wind conditions, skiers must often adjust body positioning and/or technique to maintain performance. Despite headwinds, a skier must be able to generate propulsive force, so technique cannot be severely altered. However some lowering of the body through trunk and neck flexion is one strategy which may reduce air drag modestly.

Another strategy involving drafting behind a leading skier becomes increasingly advantageous as wind speed increases. Similar to drafting in cycling or running, a skier trailing close behind another experiences reduced air drag due to a reduction in front-back pressure difference. The trailing skier, when close enough, can be skiing within a low pressure region behind the leading skier. Based on cycling experiments, drafting skiers may benefit from reductions of drag force as much as 25 to 30%.

##  Reference

• Biomechanics of Cross Country Skiing Gerald A. Smith Norwegian University for Sport and Physical Education (Norges idrettshÃ¸gskole) Oslo, Norway [1]