| 摘要 |
The ventilation grille (10) has a series of fixed, parallel bars fixed in an opening. The bars are V-shaped strips (12) arranged as chevrons which form the ballistic screen. The following proportions must be respected: P is less than or equal to 2cosalpha/3 (L+E) where L is the length of the arms of the metal strips, P is the distance between two adjacent strips, alpha is the half angle of the bars, and E is the width of the arms. A projectile encounters at least two arms (20) on every rectilinear trajectory, what ever the angle of the shot. The value of alpha is 45 degrees. The strips are fixed obliquely in the opening in relation to the ground.
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