The effect of an aft-sting mount on the aerodynamic characteristics of a missile configuration in wind-tunnel tests was investigated. The focus of this research was mainly on the effects of an axisymmetrical Backwards Facing Step (BFS), representing the aft-sting, on the longitudinal stability of the missile and the near wake flow characteristics. Tests were conducted on a generic missile configuration model comprising of a slender cylindrical body and four cruciform aft aero-surfaces (stabilizers). A series of aft-stings with increasing diameter ratios d^*=d_s/d_b, between the aft-sting and the body base diameters (d^*=0,0.3,0.4,0.6,0.8,1), were mounted on the base of the model. Three flow topologies downstream of the base plane of the body were investigated: wake flow in the case of d^*=0 (“no-sting”), shear flow in the case of 0<d^*<1 (“aft-sting”) and boundary layer in the case of d^*=1 (“virtual sting”).

The longitudinal stability margin was measured in both subsonic flow conditions (U=70,90 m/s) and transonic Mach numbers (M=0.7,0.9), using an aft-mounted six component balance. When testing the missile configuration with small stabilizers, the longitudinal stability margin was found to be insensitive to the diameter ratio of the BFS in case of shear layer flow (from d^*=0.3 and up to d^*=0.8). Whilst the elimination of the flow separation downstream of the base plane of the model, in case of boundary layer flow (d^*=1) was found to have an unstabilizing effect on the longitudinal stability of the missile. Additionally, this effect grows with Mach number, in the transonic flow regime. The effect of changes in the base flow on the longitudinal stability margin decreases as result of increasing the stabilizer's span.

In addition, hot-wire measurements were conducted for evaluating the mean and instantaneous velocity field downstream of the base plane of the model. These measurements were carried out at a free stream velocity of U=10 m/s and Reynolds number of Re_d=49,000 (based on the body diameter). The mean velocity profiles measured show expected behavior for both the wake flow (d^*=0) and the shear layer flow (d^*=0.4) in terms of the mean velocity field, the recirculation zone geometry and flow velocities and the reattachment point location. A distinct peak in the velocity fluctuations at Strouhal number of St_d=0.21 (based on body diameter) was observed in case of wake flow (d^*=0). The presence of an aft-sting, while altering the flow to a shear layer topology (d^*=0.4), was shown to have a damping effect on the velocity fluctuations. Linear spectral stability analysis for inviscid small disturbances was carried out using Chebyshev collocation technique. The stability analysis shows similar trends to those measured. In the d^*=0 case (wake flow), the most unstable wave number calculated corresponds to the Strouhal number of St_d=0.25, but the instability is not restricted solely to this frequency. In the d^*=0.4 case, a weaker instability at a broadband low frequency was calculated.