Scale Effects on the Zero Yaw Drag Coefficient of Small Caliber Projectiles at Mach 2

The flow over ×1, ×1.5 and ×2 scaled NATO 5.56 mm M855 projectiles at Mach 2 and zero angle of attack was investigated numerically and experimentally. A 5 × 6 cm wind tunnel was used to measure surface pressure distributions across the three scales, to excellent agreement with CFD. The normalised pressure distributions were found to collapse on a single curve for all cases. The zero-yaw drag coefficients were calculated numerically and shown to decrease with increasing scale, however doing so before the effect of shock impingement on the projectile wake would be expected to be significant. Flow area blockage was shown to play an important role even for the smallest projectile scale, where the drag coefficient was still underpredicted relative to live range data from the literature. The numerical boundary layer profiles along the projectile length are analysed, and again, good agreement is observed between the three projectile sizes with some small discrepancies around the stagnation point.