Influence of air rifle pellet geometry on aerodynamic drag

Abstract

Air rifles and air pistols find widespread use in formal and recreational sports events. Despite their widespread use in sport, they have rarely been studied scientifically. The influence of air rifle pellet geometry on aerodynamic drag was investigated experimentally and theoretically at Mach number of 0.58 (approximately 200 m/s) and Reynolds number of 54,000 using a low-turbulence open wind tunnel. Measurements were made of surface pressure and aerodynamic drag distribution for five pellets having different geometries. Pellet overall drag was also measured with a load cell system. Theoretical analysis based on two-dimensional potential flow theory was used to study the relationship between nose shape and drag. Results indicate that the overall drag of non-spherical pellets was dominated by the drag on their front face, with the face contributing approximately 65% of the overall pellet drag, while base drag contributed almost all of the remaining 35%. The net drag contribution of the pellet side-slopes was close to zero. The geometry of the front face had a weak influence on the drag acting on the pellet base. This influence was exercised through the behaviour of the free shear layer separating from the pellet head rim. It was apparent that the presence of the tail in a dome-head pellet enabled flow reattachment and a rise in base pressure, which reduced the base drag. In contrast, at Re ~54,000, flow reattachment on the rear surface of the spherical pellet was not possible. For this reason, its base drag was higher than that for the dome-head pellet. Flat-, cone-, and cavity-head pellets had higher overall drag coefficient values than a spherical pellet. The higher overall coefficients were due to the higher face drag than the spherical pellet, which was not compensated sufficiently by their lower base drag.