The real-time measurement of football aerodynamic loads under spinning conditions

Aerodynamic effects play an important part in any sport where the ball experiences significant periods of free flight. This paper investigates the aerodynamic forces generated when a football is spinning quickly to generate swerve and more slowly to generate more erratic flight. The work reports on the application of an experimental method that measures the aerodynamic loads on a non-spinning, slowly spinning and fast spinning football, using a phase-locked technique so that orientation-dependent and steady ‘Magnus’ forces can both be determined. The results demonstrate that the orientation-dependent aerodynamic loads, widely seen in non-spinning data in the literature, surprisingly persist up to the highest spin rates reported. When predicting ball flight, it is generally assumed that at low spin rates a quasi-static assumption is acceptable, whereby forces measured on a non-spinning ball, as a function of ball orientation, apply for the spinning case. Above an arbitrary spin rate, the quasi-static assumption is replaced with the assumption of a steady ‘Magnus’ force that is a function of spin rate and ball speed. Using a flight model, the quasi-static assumption is shown to be only applicable for the lowest spin rates tested and the assumption of a steady ‘Magnus’ force only applicable at the highest spin rates. In the intermediate spin rates (20 -40 rpm), the persistence of the orientation effects is shown to have sufficient effect on the flight to be an important additional consideration

Design and control of a linear electromagnetic actuation system for active vehicle suspensions

Traditionally, automotive suspension designs have been a compromise between the three conflicting criteria of road holding, load carrying and passenger comfort. The Linear Electromagnetic Actuation System (LEA) design presented here offers an active solution with the potential to meet the requirements of all three conditions. Using a tubular permanent magnet brushless AC machine with rare earth magnets, thrust densities of over 6 x 105 N/m3 can be achieved with a power requirement of around 50W RMS, much less than equivalent hydraulic systems. The paper examines the performance of the system for both the quarter car and full vehicle simulation, considering high level control of vehicle ride and chassis roll, with the vehicle model being parameterized for a target Jaguar XJ test vehicle. Results demonstrate the ability for 100% roll cancellation with significant improvements in ride quality over the passive Jaguar system

Improvement of perceived vehicle performance through adaptive electronic throttle control

With the advent of production electronic throttle control there is scope for increased customer satisfaction through the optimization of the throttle pedal demand map to individual drivers. The aim of this study is to develop algorithms to identify, from variables measured in real time on a test vehicle, the requirement for and the direction of adaptation of throttle pedal progression. An on-line appraisal procedure has been developed to identify the individual 'ideal' progression (IIP) for any driver. During the appraisal the subject is exposed to a series of pedal progressions, and their verbal response to each change is used to converge to their optimal setting. Vehicle data acquired on these appraisal drives have been regressed against IIP in a full factorial study, and the most statistically significant driver model established. A preliminary implementation of the model is used to demonstrate that throttle progression adapts appropriately towards IIP, thereby matching vehicle performance feel to driver expectations

The identifying extended Kalman filter: parametric system identification of a vehicle handling model

This article considers a novel method for estimating parameters in a vehicle-handling dynamic model using a recursive filter. The well-known extended Kalman filter - which is widely used for real-time state estimation of vehicle dynamics - is used here in an unorthodox fashion; a model is prescribed for the sensors alone, and the state vector is replaced by a set of unknown model parameters. With the aid of two simple tuning parameters, the system self-regulates its estimates of parameter and sensor errors, and hence smoothly identifies optimal parameter choices. The method makes one contentious assumption that vehicle lateral velocity (or body sideslip angle) is available as a measurement, along with the more conventionally available yaw velocity state. However, the article demonstrates that by using the new generation of combined GPS/inertial body motion measurement systems, a suitable lateral velocity signal is indeed measurable. The system identification is thus demonstrated in simulation, and also proved by successful parametrization of a model, using test vehicle data. The identifying extended Kalman filter has applications in model validation - for example, acting as a reference between vehicle behaviour and higher-order multi-body models - and it could also be operated in a real-time capacity to adapt parameters in model-based vehicle control applications

Experimental studies of the aerodynamics of spinning and stationary footballs

The accurate discrimination of the aerodynamic parameters affecting the flight of sports balls is essential in the product development process. Aerodynamic studies reported to date have been limited, primarily because of the inherent difficulty of making accurate measurements on a moving or spinning ball. Manufacturers therefore generally rely on field trials to determine ball performance, but the approach is time-consuming and subject to considerable variability. The current paper describes the development of a method for mounting stationary and spinning footballs in a wind tunnel to enable accurate force data to be obtained. The technique is applied to a number of footballs with differing constructions and the results reported. Significant differences in performance are noted for both stationary and spinning balls and the importance of the ball orientation to the flow is highlighted. To put the aerodynamic data into context the results are applied in a flight model to predict the potential differences in the behaviour of each ball in the air. The aerodynamic differences are shown to have a considerable effect on the flight path and the effect of orientation is shown to be particularly significant when a ball is rotating slowly. Though the techniques reported here are applied to a football they are equally applicable to other ball types

Characterisation of football trajectories for assessing flight performance

Much discussion surrounds the flight of association footballs (soccer balls), particularly where the flight may be perceived as irregular. This is particularly prevalent in high profile competitions due to increased camera coverage and public scrutiny. Footballs do not all perform in an identical manner in-flight. This paper develops methods to characterise the important features of flight, enabling direct, quantitative comparisons between ball designs. The system used to generate the flight paths included collection of aerodynamic force coefficient data in a wind tunnel, which were input into a flight model across a wide range of realistic conditions. Parameters were derived from these trajectories to characterise the in-flight deviations across the range of flights from which the aerodynamic performance of different balls were statistically compared. The amount of lateral movement in-flight was determined by calculating the final lateral deviation from the initial shot vector. To quantify the overall shape of the flight, increasing orders of polynomial functions were fitted to the flight path until a good fit was obtained with a high order polynomial indicating a less consistent flight. The number of inflection points in each flight was also recorded to further define the flight path. The orientation dependency of a ball was assessed by comparing the true shot to a second flight path without considering orientation dependent forces. The difference between these flights isolated the effect of orientation dependent aerodynamic forces. The paper provides the means of quantitatively describing a ball’s aerodynamic behaviour in a defined and robust mathematical process. Conclusions were not drawn regarding which balls are good and bad; these are subjective terms and can only be analysed through comprehensive player perception studies

The application of simulation to the understanding of football flight

This paper demonstrates the value of using a flight model in the analysis of the flight of a football, and explores the complexity of the model required to produce useful results. Two specific aspects of the simulation are addressed: the need to include a model of spin decay and the requirement to include a full aerodynamic drag profile as a function of Reynolds number rather than a single indicative value. Both are aspects of the ball performance that are experimentally intensive to obtain. The simulated flights show that the inclusion of spin degradation is important if flight validation is the objective, but that it may be unnecessary in a comparative study. The simple analytical model of spin degradation is shown to overestimate the reduction in lateral deviation when compared to experimentally acquired data. Therefore, the experimental method is preferred. The analysis of the shape of the drag profile (drag coefficient against Reynolds number) is explored, and it is shown from the simulated flights that post-critical coefficients of drag have the greatest effect on trajectories, and an average drag value is sufficient for most modelled scenarios

The aerodynamic performance of a range of FIFA-approved footballs

Much discussion surrounds the flight of a football especially that perceived as irregular and is typically done so with little understanding of the aerodynamic effects or substantive evidence of the path taken. This work establishes that for a range of FIFA approved balls there is a significant variation in aerodynamic performance. This paper describes the methods used for mounting stationary and spinning footballs in a wind tunnel enabling accurate force data to be obtained, and the analysis techniques used. The approach has been to investigate a number of scenarios: Non-spinning Reynolds Sweep, Unsteady Loads, Orientation Sensitivity (Yaw Sweep) and Spinning Reynolds Sweep. The techniques are applied to a number of footballs with differing constructions and the results reported. To put the aerodynamic data into context the results are applied in a flight model to predict the potential differences in the behaviour of each ball in the air. The paper concludes that although the drag characteristics are different for the different balls tested the simulation suggests that this has only a limited effect on the flight of the ball. It is also shown that the unsteadiness of the aerodynamic loads is unlikely to be responsible for unpredictable behaviour. However, it is also shown that there are significant differences in the lateral aerodynamic forces for a range of FIFA approved match balls, and that these aerodynamic differences have a significant effect on the flight path for both spinning and for slowly rotating balls

Flight dynamics of ski jumping: Wind tunnel testing and numerical modelling to optimise flight position

Ski jumping is a highly competitive sport, where the distance jumped is greatly dependent on the aerodynamic effects of the athlete’s posture. In the current paper a wind tunnel experiment was conducted to generate a parametric database of the effects of posture on aerodynamics and to compute the aerodynamic force coefficients. The parameters considered were varied over ranges representative of modern ski jumping and included the ski incidence angle, the ski V-angle, the leg-to-ski angle, and the hip angle. Measured force data were then used to simulate ski jumping flightpaths via a numerical model, allowing for the effects of posture on jump distance to be investigated. The model was able to simulate single and multi-posture flights to suggest both static and dynamic optimums. It was found that the ski jumping system generated lift in previously unreported, non-linear methods, enabling the flow to stay attached at much larger incidences than traditional wings. When optimizing posture for distance, it was found that neither lift nor efficiency (lift-to-drag ratio) should be maximized, due to the reliance on both qualities for a successful jump. However, when considering multi-posture flight, it was found that the lift-to-drag ratio should be maximized immediately after take-off, to maintain horizontal velocity. Lift should be maximized as the athlete approaches landing, due to the highly curved flightpath reducing the negative impact of drag. Realistic recommendations have been made on the postures that athletes should utilize to improve their performance. This includes a single position optimum, a ‘safe’ optimum which allows some variation in an athlete’s ability to hold posture, and a further optimum should the athlete be skillful enough to dynamically alter posture as the jump progresses. </p

Aerodynamic drag reduction on a simple car-like shape with rear upper body taper

Various techniques to reduce the aerodynamic drag of bluff bodies through the mechanism of base pressure recovery have been investigated. These include, for example, boat-tailing, base cavities and base bleed. In this study a simple body representing a car shape is modified to include tapering of the rear upper body on both roof and sides. The effects of taper angle and taper length on drag and lift characteristics are investigated. It is shown that a significant drag reduction can be obtained with moderate taper angles. An unexpected feature is a drag rise at a particular taper length. Pressure data obtained on the rear surfaces and some wake flow visualisation using PIV are presented. © 2013 SAE International