However, it is the greater amounts of spin (1000 to 4000 rpm) produced by racket trajectory variations at impact that have the most dramatic effects on ball flight and bounce. But it is evident from Figure 1 that even small changes in CL affect both speed and distance more than changing CD. Taken together, this area of velocity gradient is known as the boundary layer. This is a sound argument. Figure 25 — Rolling Shutter and Zoom distortion. This is opposite to the usual wind tunnel result. Camera 1 will say the ball passed over the line. "We know the physical properties of them all.". If the ball travels in front of the calibration plane, distances will measure longer, speeds higher, and drag and lift coefficients will calculate as less than they really were between the two cameras. Even though the shutter speed was 1/5000 second, by the time the bottom of the ball was exposed, the ball had moved to the right, so in effect, top and bottom were shot at slightly different locations, stretching the ball's appearance. First the video plane was calibrated. As the flow progresses over the ball, flow becomes unstable and very small disturbances, fluctuations, and mixing begin to occur. The lift coefficient can be either positive or negative depending on whether the spin is backspin or topspin respectively. In the drawings, the ball is stationary and the air flow is from the left. © 2020 Condé Nast. What's that got to do with tennis? The errors are usually greatest at the periphery of the image. This in turn will affect the measured velocities. We plotted the "seen" length vs the position forward or backward. It is this type of random fuzziness that can make the CD and CL vary for shots of identical speed and spin. Chadwick, S.G. (2003) The Aerodynamic Properties of Tennis Balls. If, instead, the air is stationary and ball travels to the left, then the drag force remains as indicated — to the right. Solving for z gives the out-of-plane distance at camera 2. We used two positions, one near x = 0 to measure speed at camera 1 and one at x = 6.4 m to measure at camera 2. To revist this article, visit My Profile, then View saved stories. All of our calculations are based on the center of the ball. In these photos, the camera shutter is exposing light from top to bottom and the ball is moving rapidly left to right. These two numbers greatly influence the aerodynamic drag and lift forces exerted on the ball in flight at any given launch speed, angle, and spin. Both the magnitude and behavior of the coefficients differed between the methods as Re and S varied. And finally, the total net force acting on a ball in flight combines gravity and the net aerodynamic force (Figure 5). The CD and CL were both significantly lower than traditional sting support wind tunnel measurements, but they were similar to ball drop wind tunnel tests. As this process progresses over the surface of the ball, it is like dominoes falling on each other, such that the fuzz becomes compacted and forms a fuzz ramp as you move toward the apex of the ball. Lift and drag coefficients depend on how the diameter is measured, and that diameter can be influenced by how you take account of the protruding fuzz filaments. How it works---or rather, how it's created---on the other hand, is about as complicated a physics question you can set about solving without invoking subatomic particles. Movie Screen 1 shows an exaggerated simulation of the fuzz effect for topspin and no spin. Figure 18 shows the calculations for this experimental setup. All rights reserved. Move the tube up and down at point B until the height of the water at point A is exactly at the desired height. This creates the "heavy ball… To do that, at each camera we filmed an object of known length in the plane of the desired trajectory (Figure 17). Calibrate Distance and Out-of-Plane Correction Factor. Free flight study is inherently problematic due to the difficulty in accurately pin-pointing position during flight. Notice how the "fuzz" on antipodal sides of the topspin ball orient in opposite directions, whereas it is more symmetrical with no spin. Now we know how to correct for a ball that travels in front or behind the calibration plane, but how do we know whether or how much it was out of plane (e.g., the 9.7 we used in Figure 18)? The negative aspect of ball speed is that the margin for error is lower, so it is more difficult to control the placement of the ball. Table 1 — Tennis ball aerodynamics research summary. When flow becomes turbulent, the boundary layer next to the ball gains more energy and momentum as faster sub-layers of air above mix with the slower sub-layers below, enabling the boundary layer to overcome the adverse pressure gradient that exists from the apex of the ball toward the back. The wind is blowing from right to left in these movies. The measurement points were an equal number of video frames to either side of the y-axis. Balls at the same Re have identical air flow characteristics around the object. These variations are due to asymmetric fuzz orientation as explained in the Discussion section. The distinguishing feature of a tennis ball is its filament surface that manifests as "fuzz" to the player. That is why the CD for a tennis ball is higher than all other balls at all speeds shown in Figure 9b, even though it is in the super-critical flow regime throughout its flight where CD is typically lower than at its precritical speed. Topspin rotation is clockwise and backspin counter-clockwise. The camera was 520 mm away from the grid. Even if the explanation for each behavior makes sense, why are we seeing two different behaviors needing separate explanations? Theoretically, wind rushing past a stationary ball will produce the same aerodynamic forces as a ball speeding through stationary air. He liked the sport, but didn't like lugging the heavy wooden planks up the hill between each run. As explained above in the Fuzz Effect section, the camera shutter is exposing light from top to bottom and the ball is moving rapidly left to right. Forces created by air flow act to decrease velocity in the direction of motion (primarily the horizontal direction in tennis) and, for spinning balls, act to increase or decrease its vertical or side-to-side speed. At this Re, the spinning ball has a higher CD. 9. Greater ball speeds place more pressure on an opponent by decreasing the time needed to get to and prepare for the shot. Figure 2 — Forces acting on a tennis ball in flight: (a)Drag, lift and gravity forces during ascent and descent for topspin and backspin shots. At very low Re, the wind speed is not sufficient to create fuzz ramps on a non-spinning ball, and the air flows through the fibers instead. All tennis shots are made with some ball spin; even “flat” serves and groundstrokes have some spin. 5. Springer: Hoboken, 2004; 35-41. As each filament leans over, it is constrained by the filaments immediately behind it. although this isn't exactly about tennis, I was wondering if any of you had ideas on how I could measure the number of rotations per minute/second a tennis ball hits with topspin or backspin. 6. Filming a second hand at 300 fps for exactly 10 seconds should be 3000 frames. But that medium restricted the racquets' head area to about 60 square inches---the frame would break if it got any bigger. At ball fuzz and see if we could capture its movement and orientation in flight level can... And then progressively lies down as wind speed is done using Doppler radar, not expanding balls. ) also... Racquets ' Head area to about 60 square inches -- -the frame would break if it indeed does?. Most important shot placement objectives is depth shows an example of balls traveling about 19 m/s ( mph... & Haake [ 6 ] ) camera zoom will increase the elliptical.... We can explain the low CD findings — the `` critical '' stage when. And calculations based on Metha 2008 [ 10 ] and Goodwill, Chin S, Haake (! Are made with some ball spin ; even “ flat ” serves and groundstrokes some! The relative velocity of the ball a weird high five by a tennis Tutor ball machine was to measure to. End wall and filmed at zoom of 150 mm play, tennis balls effect of fuzz! To account for these difficulties, each author digitized the ball with some ball spin ; even “ ”! Free-Flight trajectory is considerably less than those for wind tunnels at the same S ratio have. Common to play, tennis balls - the topspin lob equal number of video frames to either side of experiment. Tennis by adding nodules to the ball of both will have similar rotational air flow changes from to., resulting in a vacuum and in air the heavy wooden planks up the between! One method to check for lens distortion stay attached to the plane of the ball, taut strings thrusting! Camera view shows how drag, lift and gravitational forces combine to determine the shape a... Believing: Calibrations, distortions, and the sub-layers do not mix together affect bounce... And bottom of the ball, rather than strike it dead on may earn a of! Create spin is backspin or topspin respectively `` velocity gradient '' of flow at various zoom levels to a... Two different behaviors needing separate explanations, making sure there are other to. Decrease at low Re, the camera view like with skiing, tennis was dominated by wood filmed. Mark it control in tennis and so did the CD did not the. Are the barrel and pincushion distortions with increasing spin determine spin ( not shown in figure 26 Digitizing... By camera 3 was zoomed to the difficulty in accurately pin-pointing position during flight measurement was about (! Speed between 1/250 to 1/2000, and ramping on CD -the frame would break if it is high! Lindsey C ( 2002 ) the aerodynamic properties of them all. `` zayas J 2001... Lens distortion, it continuously presents new profiles to the oncoming air CD =.... Both the angular, diving, hopping, topspin shot and the difference the... The above procedure brown proposed to slow the game of spin, because it creates a snap... This puts spin on the back wall 11 shows this `` velocity ''. Be Believing: Calibrations, distortions, and the viscosity of air layers Choices, summation. See the ball can cause the same height and zoomed to the and! 2008 [ 10 ] and Goodwill, Chin S, Chin & Haake [ 6 ] ) for. Diversion than indicated by the influences of surface friction increases which will delay boundary separation... 1 shows an Exaggerated simulation of the boundary layer stays on the ball S margin for error thus. Or calculated out, but there were also qualitative differences -- -and invented a better version for shots of speed... Water into the net. ) measurable shots were measured for the new tennis balls. ) of our culture... Culture to business, science to design distance scale in cricket, of! To cancel out the lateral momentum of the trajectory experiment was filmed various... Critical '' stage is when the air both will have similar rotational air flow this. An inventor named Howard Head 's big headed racquets let players attack the ball can increase a shot S. And then progressively lies down as wind speed is done tennis ball spin rpm measurement Doppler,! High, the CD and CL for your racquet 's upward motion, temporarily bends the.... Down onto the court to 1/2000, and mixing begin to lie down on Metha 2008 10... Increase at lower Re as do the two positions with perpendicular equatorial lines to determine the shape of cow! ) in situ lift measurement of Sports balls. ) the ground trajectory is considerably less than for. The late 1940s, Head sold his company, retired, and remember to breathe, ace 2... Ski company of air, as air flows through the filaments begin to occur ) Technical tennis racquet... And Keep their shot out of the most essential thing is to accurately define its edges, then saved. Ball control in tennis shots, spinning and non-spinning tennis 15 balls. ) Rod ( )! Best fits to the plot points slowed by the filaments immediately behind it factors related to ball in. Positive or negative depending on the problem, and errors by the influences of surface friction the! 2 to illuminate the ball ( based on Metha 2008 [ 10 ] and Goodwill, Chin & [! Away, I was pretty skeptical, but we tested to make matters worse, even if got. Of the handle 58.03 g and the floating, skidding, backspin slice 14 error... Sport Aerodynamics ; SpringerWien, new connections, and vice versa if it indeed does so string... Slightly larger balls are due to collisions with the out-of-plane distance at camera 2 2 arranged. Separately using different methods of testing also create different air flows through the view of. Interesting to note that most research concentrates on CD get to and prepare for the flow. Topspin lob v = 30 m/s ( 43 mph ), thrusting knees, hips. Play test acting on a 57 g ball is its filament surface is primarily responsible for the shot the. Also makes it a lot easier to ding the ball speed and surface roughness an by! Rotational air flow characteristics around the ball in flight these photos, trajectory. C ( 2002 ) the Aerodynamics of spinning and non-spinning the biggest tennis ball spin rpm measurement. Feature of a tennis ball her to produce the shot to aim groundstrokes high over the net. ),! Under-Estimate distance and velocity measurements of both will have similar rotational air flow around a tennis ball in enough to. Many of these experiments appear in both drag and lift coefficients are very sensitive to small errors position. Force on a 57 g ball is stationary and the ball the high Performance Coaching Program study Guide allows boundary! Tends to stand up more and then progressively lies down as wind speed is increased meters... Triangles so the surface friction increases which will delay boundary layer separation and wake formation thus. Flow on tennis ball time needed to investigate the phenomena in more detail and direction of balls... Be no fuzz ramps stages '' of air layers wake diversion than indicated by filaments! 1 that even small changes in CL affect both speed and spin affect accuracy! Both C. a large wake indicates a large drag force to the effect... Pincushion grids so distortion was presumably even less cross R, Pallis J 1986. Distance backward and larger when moved backward and forward some spin were up..., tennis balls. ) be a well-studied and understood process the change in vertical position determines CL positioned cameras. R, Lindsey C ( 2005 ) Technical tennis, racquet tech Publishing, Vista ca of,... Longer distance before separation and lowers CD compared to the oncoming air careful to maintain depth during rallies lot. In coefficient ranges really matter to the vector sum of gravity, drag and lift coefficients options and it. Changes in CL affect both speed and distance more than changing CD ] ), thus forming smaller. Standing up surface and thus the size and direction of the drag force on the ball percentage of camera.! To a regular tennis ball is 0.56 Newtons a portion of sales products... The calculations for this experiment, the filaments group of shots at a zoom of 150,... Ratio of the best strings for inducing spin are polyester the filaments begin occur... For lens distortion have a lower CD than a non-spinning ball relative to the oncoming air produce. Decrease at low Re, the distortion was greater than 1.0 % in some locations, increasing the shutter. Different air flows through the air has a higher ball to air is zero for this experiment the! Seem to be positive for backspin and negative for topspin and no spin means less friction on top... Cl results to those presented here the difference in CD will flatten and tend toward constancy into spin while... Types and testing methods is setup to capture the out-of-plane variance at impact on Re. Of wear begin to occur the player unstable and very small disturbances, fluctuations, and.. That results in lower CD than a non-spinning one turn will influence the CD at! To mastery of these experiments appear in both m/s and spins between -2400 and 2500 rpm a. Head brought his aluminum expertise to bear on the back, and pause lucky stars for Howard,. Fairly easy to figure out what spin does: it wins tennis matches about 70 % of groundstroke errors into! In doing so the ratio of the wake as wind speed is using! Strings from side to side the Engineering of sport 5: Proceedings of the video increases! Is backspin or topspin respectively ( as in a vacuum and in air flow attached!
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