The CSBS Mk. IA, the first widely produced vector bombsight. The drift wires are visible on the right, the windage calculator on the left, and the altitude scale in the middle (vertical). The actual sights are the white rings near the top of the altitude slider and white dots mid-way along the drift wires. The drift wires are normally taut, this example is almost a century old.
English: The Course Setting Bomb Sight Mk IA is the first version of this bombsight design to see production, starting in 1917. This example is found in the RAF Museum collection. The operating principles of the "vector bombsight" concept can be explained using this image. Generally, the position and angle of various mechanical parts directly represent the vectors that would be drawn on a conventional navigational diagram or wind triangle. Turning the brass knob on the extreme left of the image (shaped like a plus-sign) pushes the square brass bar under the device just right of the middle of the image fore or aft, and thereby sets the airspeed vector. The current airspeed setting can be read off the scale embossed on the top of the bar, and the words "AIR SPEED" can just be made out on the main body of the bombsight on the left end of the bar. Rotating the top of the compass, using the knurled fitting, sets the direction of the wind pointer on the top of the compass. A gear train running through the center of the air speed bar causes this motion to also set the direction of the wind speed bar, the black and brass mechanism above the airspeed bar. Turning the wind speed knob, the small brass dial at the end of the wind speed bar, pushes the foresights back or forth along the drift bar above it to account for the portion of the wind that speeds up or slows down the progress of the aircraft. The same movement also pushes the entire drift bar assembly on the right to one side or the other, to account for the sideways movement of the aircraft due to the same wind. The result is the three vectors in the wind triangle being represented mechanically; the airspeed on the airspeed bar, the windspeed and direction on the wind speed bar, and the resulting aircraft course and ground speed on the main drift bar. Only the aircraft's heading is not directly represented, the CSBS calculates everything relative to the nose of the aircraft. In this case the wind has been set to a shallow angle, blowing on the tail slightly from the left. The indicator of the wind direction is the black arrow seen on the left side of the compass rose. The wind bar, under the drift bar on the right, can be seen to be matching this angle. As the operator turns the wind knob, the sights will move forward, as the groundspeed of the aircraft is being increased by the wind from the tail. This movement will cause the lower sights to move forward and increases the "range angle". Sighting through the rings, the bomb aimer would see the target pass through the sights, and thus drop the bombs, slightly earlier, which accounts for the higher ground speed due to the tailwind. This will also move the entire drift bar and wires very slightly to the right, indicating that the pilot should turn slightly left in order to compensate for the wind and fly directly over the target. The exact angle can be read off the arc-shaped scale in front of the compass. The bar rising vertically from the center is used to set the altitude by sliding the sights up or down. In this case it is set for relatively high altitude, near the maximum for this aircraft. The small bar in front of the ring sights locks them in position, pulling this rearward toward the bar unlocks it for movement. The brass knob behind the height bar is used to level the bombsight, in reference to the spirit levels seen on the left below the compass. This setting is not found on later models, where it was replaced by a smaller mechanism. The black disk under the compass is a compass corrector, which removes the influence of nearby magnetic fields in the aircraft.