I must admit I fly with the cockpit open because it gives a better rear view. I know its not realistic at high speed but its there and others will be using it, well on refllection not the 109 pilots! Johnny Johnson often wrote that he cleared his 6 by looking in his mirror and using 'coarse rudder' to swing the a/c left and right. Now if we had a decent mirror at decent resolution........ At the moment our ability to see a 109 on our 6 is too limited by the lack of a good mirror so I don't have any qualms about having the canopy open.
What's needed is a good mirror,
then perhaps serious head buffet if beyond the limit of the canopy (or perhaps head restriction to cockpit-closed range?) above certain airspeeds. Say 150mph? Higher? It should be possible to open the cockpit and look around at lower speeds.
btw I have sat in a Spifire cockpit and it is possible to see about half of the tailplane by stretching the neck. Also there was a straps adjustment lever allowing the pilot more room to move (but you wouldn't in combat) and described here including a picture of a pilot with his head out of the cockpit but harness (loosely?) fastened:
http://spitfiresite.com/2010/04/the-...-spitfire.html
The seat belts used on the Spitfire and other British aircraft of the era are often being referred to as the Sutton Harness.
The Sutton harness commonly used during the initial to mid-war period consisted of four straps about 2” wide that had a row of grommets spaced about 1and 1/2 ” apart.
The shoulder straps of the harness were attached at a single point to a transverse cable which was attached by brackets bolted to the ends of the fuselage longerons behind the pilot’s head/shoulders. The length of the cable provided a degree of shock absorption upon impact. Also, the wire could be slackened by means of a lever in the cockpit to permit the pilot to lean forwards.