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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 I realise this is not a historical document, but I have also found a similar feature in photos of real Spitfires.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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What?

According to flight manuals, it's the direct vision panel allowing the pilot to maintain a view on the outside when the windscreen becomes obscured.

Seen on either side, depending on the model

Some Spitfire models had it on the left hand side and this reproduction of the manual for model II actually mentions it on the port side:

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Source

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Spitfire I: On the left side, source

It seems the side changed later, the official manual locates the panel on the right side for models V (5): VA, VB and VC.

Spitfire Pilot's notes Air publication 1565
RAF Spitfire Pilot's notes - Air publication 1565 from UK Air Ministry. Source www.airwar.ru (pdf link)

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Purpose

The manuals are only vague about the reason the windscreen can be obscured, still a reason often cited the rapid altitude gain allowed by the aircraft creating condensation in the cockpit:

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
  • $\begingroup$ @KalleMP -- yes this is definitely part of the reason for the existence of this feature on many gliders $\endgroup$ – quiet flyer Jul 12 at 14:48
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The ORIGINAL Spitfire I Pilot's Notes (as opposed to the concocted and suspect "1940 Spitfire Manual", whatever THAT means, quoted above) clearly state that the panel is for "Emergency use" and on the PORT side. "Port" is bolded in the text. The real para number is 45 in the Spitfire I and 37 in the Spitfire II Notes.

The panel allows airflow which may (or may not) clear the fogging of the front panel/hood and also could provide some perspective on landing to maintain the "triangle" of the runway edge and nose, should the hood be jammed. It also releases pressure aiding in getting the hood open in an emergency, necessary for vision or for bailing out...these early hood not having the Martin Baker quick release system.

The knock out panel was always on port (the photo above actually shows this but because of the thickness of both layers of glass, only the studs can be seen. If it were in fact on the starboard side, the outline of the panel could be seen.

The knockout was dispensed with from Mk V onwards. Some later manufacture Spitfire I's also lacked it, I believe, after the MB gear was introduced but have not been able to definitively confirm that.

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