It was a while before large one-piece blown clear parts became common. The
early B-17s had framed nose pieces and didn't get a one-piece nose until the
the B-17F model, which came out in about 1942.
It was a matter of aerodinamycs. "Razorbacks" induce noticably less drag than "bubbletops".
If memory serves me well, the max.speed difference between P-51B and P-51D is about 20 km/h (if not more) in favor of the "razorback" one, for P-47s the difference is even larger.
With the P-51B, top speed is usually listed as 440 mph, while it's 437
for the P-51D, or about a 5 kph difference. It might have been more
severe with the P-47, but I don't have the data to compare the P-47D-23
and D-25. The first front-line fighters with all-round vision canopies
were the Westland Whirlwind, A6M Zero and Ki. 27, but these were all
more-or-less heavily framed (as were the nose cones of all B-17s up
through the B-17E). The first clear-blown canopy on an operational
fighter was probably the Typhoon (the first P-47 with a bubble canopy
used the Typhoon canopy). The technology to create canopies like this
was perfected during the war, and probably coincided with the
realization that the pilot required increased visibility. There may also
have been an issue with turnover protection for pilots which militated
against the shift.
Most any a/c with the straight back will be faster than it's
non-straight-backed brother. Even holds true for Cessnas. The older
straight-backed 150s, 172s & 182s are a bit faster than their later
versions with the rear windows.
Some of it was probably just keep on doing like we always have. Many early
open cockpits planes had a neck protection or roll bar to protect the pilot.
Boeing P-12E and P-26 for example.
When the started covering the cockpit it was probably a question on closing
what was already there. Look at the P-51 all up to the D, Corsair Birdcage,
Hellcat, Warhawk and Thunderbolt early versions, they all had "rear side
windows" or quarter windows to improve visibility. Only the two navy planes
really stayed the same except these, the only Corsair with Bubbletop was the
Goodyear FG-2. The rest seems to have had a higher priority for allround
view. Actually was used long after the war, just look how many early MIG
jets had it, or the swedish SAAB fighters.
Different priorities and some technique.
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