## 1 Introduction  
Major League Baseball (MLB) is the third most popular sport in the United States, with 20 million viewers during the 2017 World Series alone [1]. The home run is an important aspect of the game and is often normalized as the number of home runs per batted ball (HR/BB). From 1960 to 2015, the HR/BB varied from 0.03 to 0.04, changing gradually from year to year with no apparent trend. The HR/BB increased abruptly starting in 2015, reaching 0.053 in 2017. The MLB commissioned a panel to study the unprecedented and dramatic change, which concluded that a small decrease in the ball's aerodynamic drag (3%) was responsible for the increased offense [2]. The high sensitivity of HR/BB to ball drag has motivated increased scrutiny of baseball drag and the methods used to measure it. The aerodynamic drag force on an object, \(F_{d}\), is often measured in a wind tunnel. The object is held stationary on calibrated force sensors as a uniform airflow is introduced. The effects of drag can be described through a non-dimensional drag coefficient, \(C_{d}\), while the aerodynamic effect of speed is often normalized by the Reynolds number, \(Re\)[3]. The drag coefficient is not constant but changes with \(Re\). The relationship between \(C_{d}\) and \(Re\) for smooth spheres was discovered independently by [PERSON] and [PERSON] [4]. The drag of a smooth sphere as a function of speed has three distinct regimes [5]. In the subcritical regime, (\(Re\leq 2\times 10^{5}\)) \(C_{d}\sim 0.5\), and it is nearly independent of \(Re\). The mostly laminar flow separates from the sphere relatively early, resulting in a large wake and the highest drag of the three flow regimes. In the critical regime (\(Re\sim 3\times 10^{5}\)), an abrupt decrease in drag occurs where \(C_{d}\sim 0.1\), and is known as the \"drag crisis\". This phenomenon is caused by the generation of laminar separation bubbles, whereby the laminar boundary layer separates, transitions to turbulence in the free shear layer, reattaches onto the sphere's surface and then separates again, resulting in a smaller wake and the lowest drag of the three regimes [6]. In the supercritical regime (_Re_ > 3 \(\times\) 10\({}^{5}\)) the boundary layer prior to separation is turbulent resulting in an intermediate drag in comparison to the other regimes. The drag coefficient increases gradually with _Re_ in the supercritical regime. Ball speeds representative of play are usually at the end of the drag crisis where flow is transitioning to fully turbulent. Air speed is not the only factor affecting flow separation. Ball spin will cause the flow separation points to move, which can affect the drag and cause lift by what is known as the [PERSON] effect [7]. The spin tends to move the separation points and wake in the direction of the ball rotation. The imbalance in pressure from asymmetric separation points causes a lift force. The distance the separation points move is roughly proportional to the spin rate [8,9]. The roughness, including seams on sports balls, increases the sensitivity to spin. Roughness may cause a transition in the boundary layer from laminar to turbulent flow, which reduces the speed for the onset of the critical regime while increasing drag in the supercritical regime. The dimples of golf balls are carefully designed to reduce drag over speeds used in play. The seams of a baseball also encourage turbulence at lower speeds and can alter the location of the flow separation points. The dependence of drag on ball orientation was studied in wind tunnels, as depicted in Figure 1, for 30 m/s (_Re_ = 1.5 \(\times\) 10\({}^{5}\)) [10]. Since the four-seam spin axis has two planes of symmetry, the drag dependence repeats four times per revolution. While wind tunnels are commonly used to measure the drag of sports balls, they have a potentially adverse interference effect. The support used to hold the ball stationary and measure aerodynamic forces can interrupt the airflow and alter the aerodynamic forces that are being measured.