Sorry to dig this old thing up, but I'd like to add my vote for having canopies that open. I was gonna ask for this, but did a search first and found this
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It was, in fact, common practice to fly with the canopy open, not only for CV ops but also just when cruising at low to medium altitudes. Especially in warm seasons and climates. Look at how many pics you see of USN planes especially cruising along with the top down.
Plexiglass canopies act just like plexiglass greenhouses. Sunlight comes through the plexiglass from above and heats the cockpit interior (which is usually a fairly dark color that collects heat). However, the IR heat radiated by the warmed interior can't escape through the plexiglass. Thus, the cockpit interior temperature rises. Another contributing factor is heat coming back from the engine, especially on air-cooled engines. With liquid-cooled engines, the heat gets channeled away from the cockpit to the radiators, but with the radial engines, the cockpit is actually in the heated airflow coming off the cylinders, especially if there's a cowl flap on the top. This is perhaps why pics of open cockpits are more common for USN planes than USAAF.
WW2 planes didn't have air conditioners, so the only way to cool the cockpit was to either go high enough that the cold outside air sucked the heat out, or let in fresh air from outside. Most planes had small fresh air vents, but at low to medium alts on hot days, especially with a radial engine, these didn't do enough. Therefore, the pilots often opened the canopy, which not only let in way more air, but removed the greenhouse effect, which was the main cause of the problem to begin with.
In addition, of course, pilots could see better with the canopy open, so (unless they were way up high or it was cold outside), they often had the lid open to have a better chance of spotting the nme. Then they'd close it when they spotted the nme. You often see this sequence in the close-ups of old war movies
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As to closed canopies shattering during a dive, I can see that being possible even on unpressurized planes. Canopies almost always had rubber seals around the edges to keep out rain and keep in warmth at high alt, as well as to reduce drag. These of course weren't perfect seals, so air pressure was the same both inside and out while cruising along at high alt. However, the seals were good enough to really restrict airflow. Thus, in dive, the pressure outside would rise faster than the air could get inside through the seals, keeping the inside at a lower pressure. If the dive was steep enough, and went far enough, I can see it possibly causing a big enough pressure difference to crush the canopy, although I'm sure different planes behaved differently in this regard.
So, I think having canopies we can open at will is quite realistic. At least for those with sliding canopies or windows, so not on 109s, for example. But I agree, we need to consider all the game effects.
On the plus side, you get better visibility, cockpit temperature not being a game factor. However, there should be some restriction on opening the canopy at high alt, because the pilot would freeze. Maybe have a pilot wound-type fainting spell kick in above a certain alt, but which isn't permanent and just lasts until you close the canopy?
Of course, there's the drag issue, too. And perhaps some canopies would break off at very high speeds if not latched down closed?
As to pilot damage, on most planes there was no armor in the canopy itself, at least the part that moved, although there were exceptions. I guess that would have to be looked at plane-by-plane. But in the absence of moving the armor, I can't see there being much difference as to pilot vulnerability. In fact, it might make the pilot a bit safer. For instance, German 20mm had such a sensitive fuze that it would explode on plexiglass, resulting in the pilot eating beaucoup shrapnel from the shell and the canopy. OTOH, if the canopy was open, maybe that shell would go by right under his nose without hitting anything.
