English is not my native language. What is this author basically saying?
Quote:
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.
Basically, and without getting into the physics and equations, there are two theories describing the propulsive forces in swimming invovling the arm stroke (and to some degree the legs). The writer is describing some testing in a hydraulics lab to determine which of the two theories is more dominant in describing the propulsive forces.
One theory invovles the use of newtonian physics and "drag" - especailly the third law - for every action there is an equal and opposite reaction - if you push water back with your hand and arm, it pushes you forward (however, water is not a solid like the ground and the push back is not 100% - water moves out of the way and slips by). The writer explains that the maximum push back force is generated when your arm is at 90 degrees to flow. Thus the concept of Early Vertical Forearm (EVF) in swimming - get your arm and hand vertical to maxiimize propulsion. Use of the term "drag" to describe this newtonian theory is a little confusing, because "drag" is also sometimes used to describe the resistance to movement (frictional forces)
The second theory involves the use of the bernoulli equation and the generation of "lift". In this theory your arm and hand are like the wing of a plane, parafoil, or propeller blade. As your arm and hand (and body) pass through the water, lift forces act on them pushing you up and forward. The author explains that these forces are maximum at an attack angle of 45 degrees. The use of an S shaped stroke was once thought (and taught) to take maximum advantage of lift.
However, the author apparently found that the maximum propulsive forces in swimming were gernerated at an attack angle of the hand and arm between 50 and 60 degrees, thus concludiing that drag forces were making a larger contribution than lift. He concludes that diagrams depicting only the lift forces in swimming are not accurate.
The two theories have been espoused and argued extensively for years. Although the lift theory dominated some years back, I think the drag and newtonian theory are now thought to play a more dominant role.
There are many other forces at work (bouyancy and frictional drag for instance). Here's a good basic website expaining the forces and physics of swimming: www.east-buc.k12.ia.us/.../tp.htm
Use of the term "drag" to describe this newtonian theory is a little confusing, because "drag" is also sometimes used to describe the resistance to movement (frictional forces)
I don't think it's confusing if you define drag as any force which is directed opposite to the direction of travel.
I don't think it's confusing if you define drag as any force which is directed opposite to the direction of travel.
Good point- my thinking/use of drag has been more commonly (and perhaps not correctly) to describe just the forces of friction. Thanks.
I'm also not sure if the paragraph's conclusions follow from the authors arguments regarding maximum propulsion and angle of attack. Since there is only one paragraph, it's hard to tell. Personally, i've always felt that "drag" was the bigger force at work and lift significanlty smaller, but that's just my opinion.
Personally, i've always felt that "drag" was the bigger force at work and lift significanlty smaller, but that's just my opinion.
I agree. I really don't see much evidence that lift is a big factor. On the other hand it might be one of those little things that makes the best swimmers the best swimmers.
When i started swimming again last year (and joined usms), i ditched my old S stroke in favor of "over the barrel" emphasizing adduction to try to take more advantage of the drag concept. But, im slow compared to the best swimmers. That said, when i sprint, if sense body lift as body position gets higher in the water. Some of the best swimmers may be like hydrofoils, and im surprised you can't see through to the next lane under them.:D
Former Member
Thanks, that makes it better to understand.
Former Member
Evidently, Newton's 3rd Law of motion was not repealed, and is popular again with the EVF stroke. However, "lift" is still a contributor to propulsion, but not in the Bernoulli style. It's the "Newtonian (Dynamic) Lift" at the front of the stroke, coupled with "EVF," that enables us to "pivot pass our hands" as we swim. Remember, forces work in pairs.
