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: Ernest W. Maglischo: Swimming Even Faster. Mayfield Publishing Company, Mountain View, CA, USA (1993). The Bernoulli principle is only one possible explanation for the lifting force. (Sprigings and Koehler, 1990). Lifting forces can also be generated by pushing the water backwards at medium angles of attack (Costill, Maglischo, and Richardson, 1992). Additionally, drag and stroke can add to the total propulsive force generated by the hand. In some considerations, the hand in the pulling phase is shown with the relative size of the stroke and resistance vectors. Ideally, the sum of these two vectors points in the swimming direction. Typically the stroke vector is represented as the main factor. That is why the view that propulsion is generated by lifting forces has prevailed. Bernoulli's Principle is only one explanation of the kinetics of the lift force (Sprigings and Koehler, 1990). Lift force may also be generated by pushing water backwards using intermediate angles of pitch (Costill, Maglischo, and Richardson, 1992). In addition, drag and lift both contribute to the net force produced by the hand. Some texts depict the hand at instants through the pull phase of the stroke and indicate the relative magnitudes of lift and drag vectors. Ideally, the combination of lift and drag is such that the resultant force is in the desired direction of travel. It's common for the depictions to indicate lift as the predominant source of propulsion. Thus, the perception that most of the propulsive force in freestyle swimming is due to lift rather than drag has persisted. A number of scientific papers have confirmed this view (Barthels and Adrian, 1974; Schleihauf, 1974; Schleihauf, 1979; Schleihauf, Gray, & DeRose, 1983; Schleihauf, Higgins, & Hinrichs, 1988; Reischle, 1979). In addition, there are compelling reasons why good crawl technique is characterized by the use of lifting forces. The first reason lies in the S-shaped arm pull pattern. We find it natural and logical that the S-shaped line only serves to generate hub. Why else should good swimmers do this S-curve? Secondly, the S-curve seems to be advantageous in terms of a longer path through the water and / or combined with a greater speed of movement, which leads to a longer and / or greater effect of the propulsive forces generated. We think that because the forces generated must point in the swimming direction, if the arm pull is S-shaped, the stroke makes the essential contribution. To take advantage of the longer distance, the swimmer must learn paddling movements to create lift rather than resistance. Third, the paddling movements allow the large muscle groups to be used more efficiently than when the hand is pulled straight from front to back. A number of research papers have supported this view (Barthels and Adrian, 1974; Schleihauf, 1974; Schleihauf, 1979; Schleihauf, Gray, & DeRose, 1983; Schleihauf, Higgins, & Hinrichs, 1988; Reischle, 1979). Additionally, there are some compelling reasons why we might accept the idea that sound freestyle technique is characterized by the use of lift forces in preference to drag forces. The first is related to the curved nature of the hand path. We find it natural and logical to reason that if a hand path is curved then forces must be generated by lift. Otherwise, why would good swimmers use a curved hand path? Also, we find an advantage to a curved hand path is that the hand moves through a greater distance and / or speed thereby allowing forces to be applied longer and / or be greater in each stroke. We reason for forces to be in the desired direction when a curved hand path is used, then lift must make an important contribution. Thus, to achieve the advantages of a longer hand path a swimmer learns to use sculling motions to produce forces from lift rather than drag. Further, sculling actions may allow the large muscle groups to be used more effectively than when the hand is pulled straight back. The torso rotators take on a large part of the load during the S-curve and the simultaneous rolling of the body. This swimming technique is likely to be mechanically and physiologically efficient. Perhaps the most compelling reason is that using the hydrodynamic hub means that less energy is wasted compared to the drag (Toussaint and Beek, 1992). Much of the sculling may be produced by the trunk rotators and incorporated into the natural rolling actions which accompany breathing and hand exit. Such a technique may be mechanically and physiologically efficient. Perhaps the most convincing argument is that less energy may be transferred to the water and "wasted" when forces are generated from lift than from drag (Toussaint and Beek, 1992). There were few objections to the dominant role of the hub in generating propulsion in the crawl, and when they did, they were generally ignored. Wood and Holt (1979), Holt and Holt (1989), and Valiant et al (1982) presented evidence that resistance is the dominant force. In somewhat more recent studies, Cappaert (1993) and Cappaert and Rushall (1994), the direction of hand movement and the position of the hand were quantified using three-dimensional analysis technology. Cappaert combined position and directions of the hand with Schleihauf's lift and drag coefficients (Schleihauf, 1979) to determine the lift and drag forces. All of these studies, done with elite swimmers, indicated that drag forces are more important than lift forces in all swimming techniques, with the exception of breaststroke swimming. Challenges to the view that lift plays the dominant role in freestyle propulsion have been few and, in general, have been ignored or dismissed. Wood and Holt (1979), Holt and Holt (1989), and Valiant et al (1982) presented evidence in favor of drag being the dominant force. More recently, Cappaert (1993) and Cappaert and Rushall (1994) quantified the direction of hand motion and the orientation of the hand using three-dimensional analysis techniques. Cappaert used hand orientation and path data in conjunction with Schleihauf's lift and drag coefficients (Schleihauf, 1979) to estimate life and drag forces. All of these studies, which were of champion swimmers, indicated that drag forces are more important than lift forces in all strokes other than breaststroke.Rushall et al (1994) provided compelling evidence in favor of the resilience model. They argued that the hand-generated lifting force model is based on a misconception, and that most of the propulsive force comes from the forearm rather than the hand. Because of its clumsy shape, it mainly generates resistance forces. The forearm describes a much more straight path than the hand. According to this model, the propulsion in the crawl is the result of the resistance forces generated by the forearms and not the deliberately trained S-line of the hand. Unfortunately, with the exception of the research by Berger et al. (1995), who quantified the drag and lifting forces of the forearm and the hand and forearm together, did not have any research results that measured the actual relative contribution of the hand and forearm to propulsion. A methodological difficulty lies in the different speeds at which the forearm moves through the water during the arm stroke. Schleihauf (1984) estimated that the contribution of the forearm is very small compared to the hand because the hand moves at greater speed. If so, one has to confirm the forces generated by the hand and the question of whether stroke or resistance is more important remains open.Rushall et al (1994) proffered convincing arguments in favor of drag as the dominant propulsive force in freestyle swimming. They contended that the arguments in favor of lift as the dominating force were ill-conceived and that much of the total propulsive force comes from the forearm. Because of its bluff shape nearly all the force generated by the forearm must be due to drag. Further, the forearm has a substantially straighter path than the hand. Thus, freestyle technique may be directed toward generating propulsion from the forearm using drag rather than deliberately using a sculling action to optimize lift forces by the hand. Unfortunately, although research such as Berger et al. (1995) has quantified the drag and lift coefficients of the forearm and combined forearm and hand, no research has effectively quantified the relative contributions of the forearm and hand in actual swimming. One of the major methodological problems to be overcome is that the forearm moves at very different velocities along its length during the swimming stroke. Schleihauf (1984) estimated that the contribution of the forearm in swimming is very small compared to that of the hand. This is because the hand moves at a greater velocity than the forearm. If this is the case, then a focus on the forces produced by the hand remains warranted and the question of whether lift or drag is the more important remains open for consideration. More recent studies, in which body movement was examined as a whole, indicate that the S-curves of the hand are not as pronounced as one would have imagined (Cappaert, 1993). Accordingly, the swimmers tend towards straight arm pull. If the path is not very S-shaped, then the main contribution to propulsion comes from drag, regardless of the angle of the hand to the flow. From a combination of experiment and trial, Liu et al. (1993) and Hay et al. (1993) show that the S-curve is a consequence of body rolling. In fact, from the point of view of the swimmer, a straight arm pull from front to back is seen from the outside as a more pronounced S-curve than when actually swimming. This means that the swimmers "straighten" the arm stroke a little. That would have important consequences. The S-curve would not be intended. Instead of swimmers adducting the arm (moving towards the body) on the inward movement and then abducting (moving away from the body) on the outward movement, as the coaches demonstrate, the swimmers actually decrease the extent of the S-curve through abduction in the pulling phase and adduction in the pushing phase. The fact that the swimmers smooth out the S-curve instead of paddling is a strong indirect indication that drag forces are more involved than lifting forces. Recent studies quantifying whole body motion indicate that the hand paths of successful swimmers are not as curved as initially thought (Cappaert, 1993). Thus, swimmers are tending to use a straight pull rather than to maximize pulling distance and speed by using a curved path. If the path is not very curved then the major contributor to force in the desired direction must be drag regardless of whether the hand is angled to the flow. Through a combination of experiment and simulation, Liu et al. (1993) and Hay et al. (1993) showed that the curved path of a swimmer's hand is due to body roll. In fact, when the arm is simulated to move directly backwards with respect to the swimmer's reference frame, the path of the hand in the external reference frame is more curved than in actual swimming. This means that swimmers actually straighten the curve somewhat. This has important implications. It means that the curved hand path is not deliberate. Rather than swimmers adducting the arm to produce the "insweep" and then abducting to produce the "outsweep", as is commonly demonstrated by coaches when instructing on poolside, the swimmers are actually reducing the curve by abducting in the early part of the pull and adducting during the latter part of the pull. The fact that swimmers attempt to straighten the path rather than to use sculling actions is strong indirect evidence that swimmers rely on drag forces rather than lift forces. Recently, I tried to shed more light on the drag-stroke discussion by reviewing data on stroke and drag coefficients from the Iowa Institute for Hydraulic Research (Sanders, 1997a; Sanders, 1997b). The analysis showed that the greatest propulsive forces occur when the angle of attack to the flow is close to 90 degrees. Then practically only the resistance is effective. The stroke effects are maximal at angles close to 45 degrees.But even at these angles, the drag force is the same as the lifting force. When these data were viewed in combination with three-dimensional kinematic swimming analyzes, it became apparent that the drag forces made a greater contribution to propulsion than the stroke. In the arm pull phases with the greatest propulsion, the angle of attack of the hand was 50 - 60 degrees - this is also an indication of the predominant part of the resistance. During these phases of the arm stroke, the water flowed from the wrist towards the fingers. This contradicts many representations in swimming books in which the water flows across the hand according to the "wing" or "propeller model". Recently, I attempted to shed more light on the lift versus drag issue using hand lift and drag coefficient data obtained from a testing tank at the Iowa Institute for Hydraulic Research (Sanders, 1997a; Sanders, 1997b). The lift and drag coefficients obtained from the hands tested in the Iowa facility indicated that the greatest forces are obtained when the hand plane is close to 90 degrees to the flow. At this orientation the force is due almost entirely to drag. Lift makes its greatest contribution to resultant force at angles near 45 degrees. However, even at these angles, the contribution due to drag is as great as the contribution due to lift. When these coefficient data were used in conjunction with three-dimensional kinematic data to estimate forces in actual swimming, it was found that drag made a larger contribution than lift throughout the propulsive part of the pull. During the most propulsive phase of the stroke the pitch angle was between 50 and 60 degrees, which means that the hand was pitched to take advantage of drag forces with a smaller contribution due to lift. During the most propulsive phase of the stroke the direction of fluid flow was from the wrist towards the fingers. This is contrary to the situation commonly envisaged and depicted in swimming texts, in which the hand is represented as a foil generating lift forces from lateral movements which produce a flow across the hand.Fig. 3 / Figure 3. Klaus Reischle shows in his book "Biomechanics of Swimming" that propulsion is made up of the two components "resistance" and "dynamic buoyancy". In his book "Biomechanics of Swimming Klaus Reischle explains that propulsion results from drag and dynamic lift.
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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