# What is Bernoulli Lift

## Swimming technique: propulsion through resistance or stroke?

The question of whether the propulsion in crawl swimming is generated by buoyancy or drag forces seems to have been conclusively answered in the early 1970s. Before, the trainers believed that the best way to propel yourself forward in the water was to pull your arm in a straight line from front to back - that is, by using the water resistance. The forces of resistance were just opposite to the movement of the hand. It was therefore imagined that the palm of the hand must be held perpendicular to the direction of movement. The trainers instructed the swimmers to pull straight from front to back with their palms at right angles to the direction of pull. The issue of whether propulsion in freestyle swimming is due primarily to lift or drag appeared to have been settled in the early 1970s. Prior to that time coaches believed that the best way to propel the body forward was to pull the hand directly backwards; that is, to use drag forces. The drag force produced is opposite the direction of hand motion. It was thought that the hand plane should be almost square to the direction of motion. Coaches applied this idea by teaching swimmers to pull directly backwards with the hand at right angles to the pulling direction. Observations on elite crawlers by Brown and Counsilman (1971) and Counsilman (1971) showed that the hands describe a curve instead of a straight arm stroke and led to the model that good swimmers perform paddling movements with palms up to generate buoyancy (hydrodynamic lift) for propulsion to take advantage of. By definition, lifting forces are perpendicular to the direction of flow around the hand. Assuming that the hand is always guided into "calm" water, this means that the lifting forces also act perpendicular to the direction of movement of the hand. Originally it was thought that the hydrodynamic stroke would be generated according to the Bernoulli principle: When wing-like objects are moved through the liquid at high speed and - relative to the flow - with a small angle of attack, high lifting forces are created with minimal resistance. According to Bernoulli, the lift arises from the pressure difference as a result of the higher flow velocity over the curved upper side of the wing compared to the flat lower side. According to this idea, the swimmer's hand is used like a wing: the back of the hand is more arched than the palm. In order to generate stroke with Bernoulli, the swimmer must describe a paddling movement with a small angle of attack. This mainly generates lifting forces and only low resistance forces. Counsilman's discovery sparked a revolution in swimming technology. The principle of the hydrodynamic stroke was practically universally accepted. In some teaching materials, the hand has been described as a "wing" or "propeller". Following observations that the hands of champion freestyle swimmers scribed curved paths during the pull phase of the stroke, Brown and Counsilman (1971) and Counsilman (1971) promoted the idea that good swimmers use sculling actions with their hands pitched to utilize lift forces as the dominant means of propulsion. By definition, lift forces are perpendicular to the flow relative to the hand. Assuming that the hand moves into "still" water, this means that the lift forces are also perpendicular to the line of motion of the hand. Initially, the lift in freestyle swimming was thought to be generated in accordance with Bernoulli's Principle: when "foil-like" objects move through a fluid at high speed and small angles to the flow large lift forces are generated and the drag forces are comparatively small . The lift forces arise from a difference in pressure as the fluid travels further and faster around the more curved side of the foil than the less curved side. Thus, a swimmer's hand could act as a foil because the back of the hand is more curved than the front. To generate lift by the Bernoulli Principle the hand should be sculled so that the angle between the hand plane and line of motion of the hand is small. This generates forces which are mostly lift rather than drag. A revolution in coaching practices followed Counsilman's work. Coaches taught swimmers to "sweep" with the hands. Lift as the main source of propulsive force in freestyle swimming was almost universally accepted. Some swimming texts depicted the hand as a "foil" or as a "propeller".Image 1 / Figure 1. James E. Counsilman was the first to propose the theory of hydrodynamic lift as the basic principle of propulsion in swimming based on kinematic studies. His idea was groundbreaking and fundamentally influenced swimming. Based on kinematographic studies James E. Counsilman was the first who proposed the theory of hydrodynamic lift as the main contributor to propulsion in swimming. His findings were the beginning of a revolution in swimming technique.
Quelle / Source: James E. Counsilman: Competitive Swimming Manual. Counsilman Co., Inc. Bloomington, Indiana (1977).
Fig. 2 / Figure 2. Similarities between the hand movement of a swimmer in a crawl and a rotating propeller blade. In the upper picture the arm pull consists of an inward-up-backward movement, in the lower picture it consists of an outward-upward-backward movement. Similarities between the hand of a swimmer during the front crawl stroke and the rotating blades of a propeller. Above: The swimmer sweeps her hand in, up, and back. Below: She sweeps her hand out, up, and back.
Quelle / Source: Klaus Reischle: Biomechanics of swimming. Fahnemann, 1988.
As the example shows, many of us were quickly ready to accept a model concept before sufficient fundamentals were available. It is probably still rash to simply claim that crawl propulsion is based purely on drag forces, but the general idea that this is due to hydrodynamic lift is likely based on false facts.In this example, many of us were quick to accept theory as fact before sufficient evidence was available. It may still be too early to state that freestyle propulsion is dominated by drag, but the commonly held belief that it is dominated by lift may be ill-founded and incorrect.
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