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Vol 2 All the Spitfire questions here


Sean_M

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Silly question, but why did the Spitfire Mk.21 have a high back? Seems like a bit of a design regression given bubble tops were already proven in several earlier Marks and used exclusively in later ones. Were they still clinging to some perceived advantages of the high back?

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1 minute ago, Graham Boak said:

Simply because the Mk.21 was delayed getting into service.  However the high back does have advantages: lower drag and greater stability.

 

Interesting, I didn't think the teardrop canopy would be that draggy and there's less surface area on the low back. Stability you mean mainly in yaw due to greater surface area aft of the CoG?

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The 21 had the high back because it was a limited number at a small factory (South Marston) to get the basic design going.  Don't be fooled by mark numbers- the "rear view" mod did not come into production until late '44 (by Supermarine on XIVs), Feb/Mar '45 by Castle Bromwich on XVIs (and eventually some IXs) and also on 22s (which had some other differences compared to 21s, too, or at least were intended to).  Mk.21s very slowly began to be built earlier in '44, but then took a while to get cleared for service use.

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9 minutes ago, Vlad said:

 

Interesting, I didn't think the teardrop canopy would be that draggy and there's less surface area on the low back. Stability you mean mainly in yaw due to greater surface area aft of the CoG?

Its not just about cross sectional area when it comes to drag, the transition between areas is important, the bubble conversely sticks out like a sore thumb. Not often mentioned but is why the original canopied and malcom hooded mustangs were about 15-20mph faster then the teardrop ones 

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5 minutes ago, PhantomBigStu said:

Its not just about cross sectional area when it comes to drag, the transition between areas is important, the bubble conversely sticks out like a sore thumb. Not often mentioned but is why the original canopied and malcom hooded mustangs were about 15-20mph faster then the teardrop ones 

Exactly, and also why the P-51D got a dorsal fin fillet to offset the instability brought about by the loss of keel area and turbulent airflow caused by the bubble canopy.

mike

 

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12 minutes ago, gingerbob said:

The 21 had the high back because it was a limited number at a small factory (South Marston) to get the basic design going.  Don't be fooled by mark numbers- the "rear view" mod did not come into production until late '44 (by Supermarine on XIVs), Feb/Mar '45 by Castle Bromwich on XVIs (and eventually some IXs) and also on 22s (which had some other differences compared to 21s, too, or at least were intended to).  Mk.21s very slowly began to be built earlier in '44, but then took a while to get cleared for service use.

 

Ah, that old chestnut. Makes sense then.

 

1 minute ago, PhantomBigStu said:

Its not just about cross sectional area when it comes to drag, the transition between areas is important, the bubble conversely sticks out like a sore thumb. Not often mentioned but is why the original canopied and malcom hooded mustangs were about 15-20mph faster then the teardrop ones 

I knew the B/C Mustangs were slightly faster than the D, I assumed it was because they were lighter, but it's interesting to know how much difference the high back makes aerodynamically. In the end the advantages of visibility seem to have won out, at least until designing for supersonic flight became an issue (then bubbles re-emerge another couple of decades later, nice design priority cycle).

 

At risk of digressing even further, didn't the British push their Mustang IIIs chasing V-1s to higher power settings than those approved for the later the P-51Ds?

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Just now, 72modeler said:

Exactly, and also why the P-51D got a dorsal fin fillet to offset the instability brought about by the loss of keel area and turbulent airflow caused by the bubble canopy.

mike

 

Indeed and it’s also why the hump on a 747 is so long, and indeed why it got longer on 747-8 when the fuselage was lengthened. 

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Be careful, the P-51B/C was faster at higher altitudes than the D because of the gearing on the engine.  Drop down only a few thousand feet and the D was faster than the B/C, and remained that way until sea level.  This was generally better in the actual combat altitudes.  The overall difference had more to do with this than the canopy change.  

 

The stability problems were largely to do with the lack of keel area aft of the canopy but also because the rounded rear of the canopy would tend to create alternating vortices (eddies) which would make the aircraft wiggle in yaw.  This is probably why there were so many variations in the D's canopy, to try and reduce this separation effect.   The Spitfire's teardop canopy was lower and possibly less affected, but to be fair, the Spitfire 21 came loaded with its own stability problems without adding another one.

 

Not just Mustangs but Spitfires and other types benefit from the 125/150 octane ratings, which I always thought was what was referred to by the code "Buster" but I gather Gingerbob has found things to be rather more involved.

 

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9 minutes ago, 72modeler said:

Exactly, and also why the P-51D got a dorsal fin fillet to offset the instability brought about by the loss of keel area and turbulent airflow caused by the bubble canopy.

mike

 

Some P-51Cs got the fillet before the D came in, seems there were some stability/control issues irrespective of the canopy/rear fuselage design.

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I’ll be damned @Graham Boak, had never heard about the gearing explanation, though I stand by the transitional explanation in terms of drag from very limited understanding of the physics having never formally studied that branch.  Also less my query gets buried in a interesting discussion, AB910 the bbmf mkv, armoured windscreen or not? 

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3 minutes ago, Vlad said:

Some P-51Cs got the fillet before the D came in, seems there were some stability/control issues irrespective of the canopy/rear fuselage design.

After the fin fillet was added at the factory beginning with the P-51D-10, it was also made available as a field kit  for installation on earlier blocks of P-51D's and also to many P-51C's  that had the 85-gallon fuselage tank to help offset instability, but the instability was caused not by the canopy or taller rear fuselage, but because the weight of the tank when full moved the cg too far aft; IIRC pilots were warned not to engage in any serious maneuvers until the tank had been emptied by half.

Mike

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17 minutes ago, Graham Boak said:

Be careful, the P-51B/C was faster at higher altitudes than the D because of the gearing on the engine. 

Mostly correct, I think. In the Mustangs/Mustangs website, the P-51B and C were listed as being powered by a V1650-3  Merlin, rated at 1,280hp. The P-51D's  were powered by a V1650-7 Merlin,  rated at 1,315hp, but which also had modified supercharger impellers to give higher rated power at altitude. The prop gearboxes on all three variants were identical. In a 1944 Wright Field performance test, the P-51B achieved 440 mph and a P-51D achieved 437 mph. Just my opinion, but the B/C's might also have been faster due to only having four guns versus the six of the D/K models. The increased hp of the -7 engine most likely offset the increased weight of the extra guns, mountings, feed chutes, and ammunition as compared to the earlier variants.

Mike

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Well, on second thoughts gearing was the wrong way of putting it.  The difference between the two engines was the same as that between the Merlin 61 in the Spitfire F. Mk.IX and the Merlin 66 in the LF. Mk.IX.  The supercharger was slightly cropped to give a lower maximum throttle height but this required less power to run the supercharger.   So there was a net gain up to the full throttle height for the Merlin 66 which then fell away above that,  The equivalent Mustang engines were the -3 and -7.  The overall engine remained much if not entirely the same

 

The aft tank introduced by the Long Range Programme did indeed cause instabilities that led to the instructions described above , but this was another matter.  This tank was introduced on the B and was normal practice by the time the D appeared.  Instability and increased drag were caused by the introduction of the high bubble canopy with cut-down rear fuselage, as on the Spitfire and indeed the P-47.  Presumably the Firebrand had so much drag anyway that no-one noticed a little bit more..  My point was not to deny this, but to highlight the more important factor of the engine.  Next tine anyone claims any fighter or variant is faster than another, ask them "At what altitude?"

 

PS  the above written before the posting #1364..

The weight of the guns etc., or any similar small weight increase, only has a very small effect on the maximum speed.  The additional muzzle and exit chute would create drag that would have slightly more.

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6 hours ago, PhantomBigStu said:

AB910 the bbmf mkv, armoured windscreen or not? 

Looking at this photo Stu, I'd say yes, the earlier externally armoured windscreen the Vbs had, afaik, the Vcs went to an internally armoured screen which was less obvious. No guarantee it is still fitted. The housing is still there but no telling if it is filled with a genuine armoured screen.

Steve.

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11 hours ago, Graham Boak said:

Well, on second thoughts gearing was the wrong way of putting it.  The difference between the two engines was the same as that between the Merlin 61 in the Spitfire F. Mk.IX and the Merlin 66 in the LF. Mk.IX.  The supercharger was slightly cropped to give a lower maximum throttle height but this required less power to run the supercharger.   So there was a net gain up to the full throttle height for the Merlin 66 which then fell away above that,  The equivalent Mustang engines were the -3 and -7.  The overall engine remained much if not entirely the same

 

I did not know the 51D had the equivalent of the LF.IX engine. But I find the British definition of "low altitude" somewhat comical, given that the Merlin 66 LF.IX still had a full throttle height noticeably greater than the Bf 109G-6 and Fw 190D-9, neither of which are considered low altitude specialists. The P-51D still outperformed almost every version of Bf 109 it might have faced at 20,000 feet or higher even with, apparently, a "low altitude" supercharger.

 

11 hours ago, 72modeler said:

the weight of the tank when full moved the cg too far aft; IIRC pilots were warned not to engage in any serious maneuvers until the tank had been emptied by half.

 

I fly an online WWII combat flight sim that tries its best to be realistic, and this is something that new (and occasionally even veteran) virtual pilots need to be told when they yank the stick, unsurprisingly enter a flat spin, then curse that the Mustang is useless, impossible to fly in combat, and "how did this unstable heap of junk win the war?". 🤣

Edited by Vlad
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Greetings all. I don't think this question has been addressed before but my apologies if it has. I have noticed what appears to be at least two styles of Spitfire XIV spinners, based on my observation of the placement of the fasteners that apparently connect the spinner to the back plate. The style I have observed most often is shown in the below picture, where there are two fasteners between each prop blade positioned close to the back plate. Stencils, which I believe to show the "Locked" and "Unlocked" position of the fastener appear fore and aft of the fasteners.

 

Spitfire_Mk_XIV_MV259.jpg

 

And then there is this type, shown below, with what appears to be 5 fasteners located more forward near the seam for the outer cone.

 

u9q135jcgqa41.png

 

Can anyone shed more light on this difference and can it be determined which versions of the XIV got what?

 

I'm asking because the Airfix FR XIVe seems to have split the difference, offering just 5 stencils but showing them near the back plate. I don't think this is correct.

 

20200621_111504-jpg.585743 

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38 minutes ago, Crimea River said:

I'm asking because the Airfix FR XIVe seems to have split the difference, offering just 5 stencils but showing them near the back plate. I don't think this is correct.

I have to agree with you on this. I also think Airfix is not correct. I built mine with the Airfix stencilling but have not, as yet, found an image that is not the same as the two you have described. More images in Troy's post:

 

 Ray

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  • 2 weeks later...
1 hour ago, Vlad said:

Was there ever such a thing as a high back F Mk.XIV with clipped wings? Or were all the clipped ones FR bubble tops?

Here's one 

rNPjsEB.jpg

 

Here's another 

Spitfire08.jpg

 

Or by "ever" did you mean a specific timeframe? Either way, be advised that clipped versus full span on most Spitfires is something you can change any time you have a screwdriver and a spare hour,  so while generalisations are possible, they are only generalisations 

 

Just for fun here's a full-span bubbletop, MV268. 

27143879467_6eb3b1b99a_z-jpg.545124

 

 

mv257-jej-pinterest-jpg.545131

 

(Confusingly, the one currently masquerading as MV268 has clipped tips, and is actually MV293.)

pyozVqNS4pQC0oVpzUZ56Eku2tlJX-YzVmtkxVZE

 

and bubbletop XVIII SM845 with full span tips

 800px-Supermarine_Spitfire_XVIIIe_%E2%80

 

 

Edited by Work In Progress
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Are those restored ones representative of how those aircraft would have been historically? I guess what I mean is, were clipped wings used at squadron level for extended operations or just sporadically on some examples because it could be swapped easily?

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