Aerodynamics Testing

Researchers from the Sports Medicine department of the Georg August University, Göttingen, proposed to improve the evaluation process of Paralympic Alpine Skiing athletes. The assessment for this study involved an investigation of air loads on the human body with the purpose of determining

the drag force and drag coefficient of the DPS athletes (Brownlie et al., 2010). The aerodynamics testing was conducted using the large low-speed, subsonic and atmospheric wind tunnel at the Technical University of Hamburg-Harburg. Drag refers to the component of aerodynamic force which acts on a solid object in the direction of the relative flow velocity or wind. Tests were performed to investigate the relationship between the posture and velocity of the skiers, who were of varying sizes and adopted a range of different torso posture angles, whilst in sitting or standing positions. The objective of this experiment was to correlate the basic drag data provided by the wind tunnel experiment with the posture performance for each skier as part of the overall evaluation process of the DPS athletes. The data obtained should have use as a means of predicting the impact of skier body position on their performance during alpine skiing events.

Relationship between Wind Velocity and Drag Force

This study, which was inspired by a previous experiment from the 1970's (Bendig, 1975), looks at drag and lift areas as applied to the human body (Schmitt, 1964) and these should, in theory, remain constant for all velocities provided that the posture of a person is precisely reproduced (Watanabe, 1977). Equation 1 demonstrates that a quadratic relationship exists between drag force and relative velocity, if air density and drag area are constant. It is quite difficult to select an ideal reference area for the human body since the body varies significantly in size and proportions (Hoerner, 1965). In this experiment, drag coefficient (equation 2) refers to the use of the non-dimensional dynamic pressure and a fixed reference area, an approach that is in line with former DLR practice (DFVLR). In the 1^st campaign this fixed reference area was set at 0.5m², but for the 2^nd campaign this reference area was changed to 1m².

D = ½ρ . AD . V² (1)

Where: D – Drag; ρ – Density; AD - Drag area; V- Relative Velocity CD = D/q S = D/0.5 ρ V² S (2)

Where: CD - Drag coefficient; D/q – Drag Area; S – Area; D – Drag; ρ – Density; V- Relative Velocity In terms of drag, the so-called Drag Area is useful for those cases where an area of reference is not obvious, or where several component parts are combined in some way (Hoerner, 1965), such as in the case of a skier. It was decided to also calculate the Drag Area (equation 3) in this study because the drag force is predominantly a function of the projected frontal area of the skier.

D/q = CD . S (3)

Where: D/q – Drag Area; CD - Drag coefficient; S – Area Effect of Trunk Position upon Drag

The best performances in Alpine Ski competitions come as a result of many factors (equipment, technique, and especially velocity). There are also other individual factors that make a significant difference between one athlete and another, for example, body mass, drag area, mass and quality of equipment, physiological training conditions, psychological training conditions, and technique and posture. The latter two variables are the focus of this study.

It is difficult for alpine skiers to maintain the ideal posture due to the natural characteristics of the terrain over which they will ski and compete. An investigation of the relationship between the posture and velocity of a skier could lead to an improvement in the performance of the athlete (Watanabe, 1978). From a mechanical standpoint, there are only three major forces acting on a skier travelling down a slope and these are: gravity; friction between the ski and the snow; air resistance of the moving body (Barelle, 2004). While gravity causes downward acceleration and consequently acts as a speed-increasing factor, the other two oppose motion and act as speed-reducing factors.

Therefore, it is through changing both air resistance and frictional forces that postural changes may influence velocity. However, the role of these factors in Paralympic Alpine Skiing has not yet been investigated experimentally. The immediate concern is with the air resistance presented by the skier as posture changes can have more of a marked and varied influence on this factor than on frictional forces. The present study was designed to measure the actual value of air resistance presented by the alpine skiers in a wind tunnel, and to study its relationship with posture.

For the purposes of this experiment we were interested in some specific postures of the Paralympic alpine skiers, such as the manner in which they held their upper body (head, trunk and limbs). This was especially for the sit skiers, because the upper body represents the biggest movable area that the athlete can change. The head can be considered almost as a “rudder” because it leads the movement of the body. Although, in terms of dimension, the head is a small part of the body in comparison to the trunk, legs and arms, the head is relatively heavy and to keep it held in a good position requires the athletes to develop the specific musculature needed to maintain good trunk posture. The arms are also an important area to be considered as they are involved in the control of balance and the directional path. There is less that can be done to improve the drag area related to the legs of the sit-skiers as the legs are held firmly and securely within a protective shell.

Another interesting aspect associated with trunk posture is the fact that the ability to maintain the correct posture (in terms of quality and duration of movement) and to keep the body balance for each phase of the discipline being performed is very dependent on the level of spinal cord injury. For this reason the tests carried out in the wind tunnel proved interesting as an observation could be made of the coordinative reaction of each of the skiers when exposed to the air flow and to the subsequent changes in air flow speed, and their ability to maintain stable positions.

General feedback applied to Field/ Competition Conditions

In the artificial environment found in the wind tunnel, it is only necessary for the alpine skiers to concentrate on themselves and their body posture. However, during real training or competition situations their attention would be divided by a number of factors related to the environment itself and potential dangers, keeping to the set course of the race, speed control, coordination of arms movements, balance, posture and breathing.

As previously suggested in the 1970’s (Watanabe, 1977), although on that occasion related to a non-disabled alpine skier, it is the ability of the skier to ski with arms maintained tightly against the

body trunk that determines their performance. If we look at the drag force generated as a function of air resistance on a runner, it can be said that this does not represent a significant percentage difference to the performance. However, if we look at the drag force generated as a function of air resistance on an alpine skier, although the relative wind speed measured could be the same, the drag generated by the body area may represent a significant difference to the outcome. Therefore, it is evident that in this specific sport of Alpine Skiing, any small modifications in posture can significantly influence the speed, distance and time travelled by the skier, and as a consequence the performance of the athlete.

5.2 Exercise Physiology Laboratories