Free Lords of the Sky: Fighter Pilots and Air Combat, From the Red Baron to the F-16 by Dan Hampton

point, and shoot at fairly easily. One degree of this arc (referred to as a mil ) equals 1 foot at 1,000 feet. * If you then extend your 1-degree cone out to 2,000 feet, the same mil is 2 feet wide.
    If you have a known reference, such as the size of your gunsight, then you can estimate the range to your enemy by how much of him fills up the sight. For instance, the Albatros D-I had a 28-foot wingspan. Understanding mils now, we know that at 1,000 feet an aircraft with a 28-foot span would fill a 28-mil aiming reticule. Of course, 1,000 feet is much too far to shoot with your Vickers gun, so we double the aiming reticle to 56 mils, and the target’s wingspan would fill it at 500 feet. (See Appendix A, “Anatomy of a Dogfight.”)
    One of the first aerial sights was called a frame or gate sight . This was a little metal rectangle just like a picture frame, and the idea was to put the target within the frame and shoot away. Adapted from a naval gun-laying system, it worked well enough against non-maneuvering aircraft such as bombers and reconnaissance planes. The early Fokkers flown by Boelcke and Immelmann had gate sights because there was no alternative. But it only really accounted for deflection if the target was moving horizontally across your line of sight, and once aerial fighting evolved to aggressive, three-dimensional maneuvering, another solution was needed.
    A ring sight was constructed of concentric metal circles and was mounted on the back of the gun. Up near the muzzle was a vertical post with a small red painted bead on the tip. If you lined up the tip of the post within the inner circle of the ring, then the bullets would go where you aimed them at zero degrees of deflection. However, by using a ring instead of a frame, you could now also judge lead angles for deflection shots no matter how your target was maneuvering. Guns were typically bore-sighted for 200 yards and the rings were sized to assume an enemy traveling at 100 mph was crossing perpendicular to you at a full 90-degree deflection angle. This meant a bullet time of flight of two and a half seconds against an enemy who would travel about 38 feet during that time. Another part of the mental gymnastics you had to do while flying and fighting was to compensate for the bullet drop (against gravity), since none of the sights did. However, it was known that a Mark VII shell fell 14.4 inches over 200 yards, so the pilot just had to wing it and aim a bit high. It took a lot of practice.
    Single-seat pilots had to do this alone and rarely had the time that gunners and observers enjoyed in multiseat aircraft. They needed some way to accurately and immediately point and fire without aligning rings and beads. In 1915 an optical sight was proposed by the Aldis brothers of Birmingham, England. Like a modern rifle scope, the lenses provided several key advantages. It showed a prealigned aiming point that appeared over the target image. Unlike iron sights, the viewer’s eye distance from the sight makes no difference, since the lenses are collimated, or focused on infinity; as a result, the pilot doesn’t have to hunch forward to shoot.
    The prototype had a 3X magnification, which would be fine for a stationary rifleman but proved extremely disorienting to a pilot moving at 100 mph, so the magnification feature was discontinued. The outer ring was sized to provide a full deflection shot against a 100-mph target at a range of 200 yards. All four lenses were hermetically sealed, and the tube was filled with an inert gas to prevent fogging. Because it was mounted on the cowling, the sight could be obscured by leaking engine oil, so an anti-fouling flap was installed that the pilot could close or open as needed. Later production sights protruded through a hole cut in the windscreen.
    British fighters began flying with operational Aldis sights by mid-1916, and they became standard equipment for the last two years of the war. Both the French and the Germans copied the