Experiments were conducted to estimate the effects of ballistic damage on the aerodynamics of helicopter rotor airfoil sections. The lift, pitching moment, and drag were measured on nominally twodimensional blade specimens with representative prescribed and actual ballistic damage. The measurements were made at chord Reynolds numbers between 1 3 10 6 and 3 3 10 6 and Mach numbers up to 0.28. Force balance measurements were complemented by chordwise and spanwise pressure measurements to assess the three-dimensional nature of damage on the aerodynamics. The quantitative data were supplemented by surface oil-e ow visualization. Generally, it was found that ballistic damage degraded the aerodynamic performance of the blade specimens, with a reduction in lift accompanied by signie cant increase in drag and change in the center of pressure. However, in some cases signie cant damage produced surprisingly mild effects. The results are useful in helping to model and assess the overall vulnerability of helicopters to ballistic damage.