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A bubble canopy is a canopy made without bracing, which attempts to provide 360° vision to the pilot.
Bubble canopies have been in use since before World War II, with some experimental bubble canopy designs in the World War I era. The British had already developed the "Malcolm hood", which was a bulged canopy, but the British Miles M.20 was one of the first aircraft designs to feature a true bubble canopy. Although it never went into production the concept of the bubble canopy was utilised on other British aircraft, such as the Hawker Typhoon and Tempest. It was also fitted to the P-51 Mustang and P-47 Thunderbolt amongst others.
The purpose of a bubble canopy is to give a pilot a much wider field-of-view than flush canopies, such as those seen on early models of the F4U, P-51 and P-47, which left a conspicuous blind spot behind the pilot that enemy pilots could take advantage of to sneak up on an aircraft.
The first combat aircraft had narrow fuselages, which often were not tall enough to block visibility to the rear. As planes became larger, heavier and faster, designs had to be made stronger, which often meant a taller rear fuselage, but designers tried to maintain the narrow fuselage for visibility.
However, as speed continued to increase, it became necessary to enclose cockpits -- and this, in turn, streamlined aircraft so that they were faster. Increased "G-loading" during maneuvers forced pilots to wear tight, restrictive shoulder harnesses, and armor plating began to be installed to protect pilots from projectiles coming from behind.
Unfortunately, these changes denied a pilot the ability to twist around and look directly behind (known as "Checking Six," or looking at the "Six O'clock" position directly to the rear). Mirrors offered some help, but had a narrow field of view.
Prior to bubble canopies, some aircraft, such as the P-40 Warhawk, featured a hybrid flush canopy design, combining a narrow rear fuselage with a glass enclosure conforming to the shape of a full-width fuselage. This provided increased visibility while still allowing a pilot to keep the canopy closed for greater performance.
Another hybrid was the Malcolm hood. While not offering as much visibility to the rear as the P-40 enjoyed, it allowed a pilot more visibility than a flush canopy would.
The true bubble canopy offers the most visibility to the rear, by allowing a low rear fuselage while still providing the enclosed cockpit. This increases the pilot's field of view, but at the expense of rear armor of the cockpit, making the pilot more susceptible to enemy gun fire from the rear. This is less of a concern in modern jet fighters as most aerial combats are settled with air to air missiles from long distances, and nearly all have some kind of radar to catch those sneaking up from behind.
Today's jets use a compromise between the true bubble and the hybrid. While the canopy is large, the rear fuselage on a modern fighter is not as low as on a true bubble design. However, the higher rear fuselage doesn't reduce the aircrews' practical visibility, as the modern combat aviator is strapped in tightly, wears a helmet, and sits in an armored ejection seat -- all of which make it more difficult than ever to turn and look behind.