ADE is engaged with design & development of high precision laser guided bomb (LGB). It is
an air dropped munition that by help of its seeker guides itself towards the target. The associated
control law deflects the front canards to generate the desired trajectory. Figure 1 shows its overall
configuration. During its free flight, Air Weapon generates a roll which may be arising from
sources such as manufacturing asymmetry, small protrusions, separation dynamics or unsteady
atmospheric conditions. Though the roll is unintentional, it is generally not harmful, as small
amount of roll provides lateral stability to the flight. The control law is also such that it is able to
incorporate some amount roll while guidance is given to munition. The problem arises when roll
rate is more than 10 RPM which causes control law to be ineffective and it becomes difficult to
guide the munition.
In such instances, the canards are deflected so as to generate rolling moment for reducing the
RPM. It is observed that canards are not very effective in producing desired rolling moment due to
an “adverse roll” effect being generated from tail fin. It can be seen that the size of canards is
relatively large and when deflected, creates a large wake. The interaction of this wake with tail fins
is reason behind “adverse roll” effect. This phenomena leads to non linear rolling moment (w.r.t.
canard deflection). The non-linearity makes it very difficult to implement a roll control law in Air
Therefore, ADE has decided to modify the munition to a Free Rolling Tail configuration,
thereby, isolating the effect of tail roll on canard roll control effectiveness. However, whether the
free rolling tail will affect the overall aerodynamics of Air Weapon, is dictated by the roll rate of
tail, which is difficult to estimate.
ADE desires to investigate the “adverse roll” phenomena in Air Weapon by CFD-dynamics
studies with an objective to estimate the roll rate of tail fin under the influence of “induced” rolling