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This is a still from the film Dunkirk. I realise this is not a historical document, but I have also found a similar feature in photos of real Spitfires. What is the purpose of this?

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    $\begingroup$ Its for use at a drive-through window? $\endgroup$ – Criggie Jan 5 '18 at 22:08
  • $\begingroup$ Could it have been intended for a star navigation instrument? $\endgroup$ – KalleMP Jan 6 '18 at 17:41
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According to flight manuals, this allows the pilot to maintain a view on the outside if the windscreen becomes obscured:

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Source: Aircraft manual

This manual locates the panel on the right side. Online forums see this as a typographer mistake, but @TomMcW has found some aircraft had it indeed on the right side:

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Source

On the reason for mist apparition:

Note the punch-out panel on the canopy cover, an early solution to the new problem of canopy misting caused by the rapid altitude changes possible in the Spitfire.

Source: Flight Journal (pg 28)

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  • $\begingroup$ Seen this description elsewhere too supposedly reported from official documents, so it may well be right, but really it seems to be in entirely the wrong spot for that. In fact, the fact that it is in the pilots line of sight in general seems like a strange design decision. Also seen it reported as helping relieve pressure to facilitate bail-out at high speeds too... $\endgroup$ – Trevor_G Jan 5 '18 at 20:30
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    $\begingroup$ Cool. I suppose the pilot would side slip the plane, and lean to his left and look through this tiny window, to land with a wind screen obscured by oil or mist or something else. $\endgroup$ – avl_sweden Jan 5 '18 at 22:15
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    $\begingroup$ There are only theories? Wouldn't the manufacturer know for certain why they're adding a small window to the canopy? I agree these sound like good reasons, but just curious why you say "theory" and not, say, "reason". $\endgroup$ – BruceWayne Jan 5 '18 at 22:58
  • $\begingroup$ Could the pilot reach through there with a window cleaning tool? Would that dictate which side it was on so he could control the aircraft with dominant hand rather than use that for cleaning window? $\endgroup$ – KalleMP Jan 6 '18 at 17:38
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    $\begingroup$ It appears some models may have had them on the right $\endgroup$ – TomMcW Jan 6 '18 at 23:00
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Mins is right, but it's a more general thing: Glider planes have them too, or at least those built in the 80s that I flew in the 90s.

One of their effects is that they whistle in the (self-made) wind. Basically any deceleration is immediately noticeable in the note of the whistle going down (helps avoid stalling)... Relevant for a glider, but probably inaudible over a Spitfire's noise!

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    $\begingroup$ I've seen such windows on all gliders I've ever seen or flied in, but they could all be closed with a sliding part. $\endgroup$ – vsz Jan 5 '18 at 22:32
  • $\begingroup$ I recall ventilation being poor in small planes, not familiar with gliders except they might have no fans to cool you when waiting to taxi, could this be for fresh air when there is a lot of sun coming through a large clear canopy? $\endgroup$ – KalleMP Jan 6 '18 at 17:45

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