The Supermarine Spitfire: an early example of area ruling?

[alan1000]

The Supermarine Spitfire, although designed in the mid-1930s, is said to have enjoyed the highest mach rating of any propellor-driven fighter in World War 2. Even the Mustang, with its state-of-the-art flush-rivetted laminar-flow wing, could not catch the Spitfire in the dive.

From examining photos of Spitfires it occurs to me that it had a somewhat area-ruled fuselage. If you look at a picture taken from slightly above, you can see that just behind the cylinder banks, roughly in line with the wing leading edge, there is a noticeable reduction in the fuselage cross-section. This continues for some distance, then, behind the point of maximum chord thickness, the cockpit canopy causes cross sectional area to increase once more.

I assume this "area-ruling" was not designed as such, because the concept hadn't been thought of then (as far as I know). But I wonder if it was nonetheless effective in delaying the onset of compressibility effects?

 

[rb1957]

which mark of Spit are you tlaking about ? i suspect it one of the later ones, XIV + ??, so it not really an "apples and oranges" comparison ... for example, according to wiki (gosh, i shiver at that ...) P51D max speed 437mph, Spit Vb 387 mph, which have similar engines (1500hp Allison, but then the supercharger has an important impact on effective power at altitude). On that, Neville Shute's autobiography has a nice section on the work he did to optimise the thermodynamic efficiency of the supercharger.

part of my point (as you note, they didn't know about area ruling at the time) is that a later model spit had a larger engine installed, so a wider nose would be expected. I don't remember any "waisting" of the fuselage of the earlier marks.

btw, Love Michael Palin's commentry on the story of the Spitfire, when he gets buzzed by one of the BoB flight planes ... brillant !

 

[rikrollinga]

I don't think it was effective.

The spitfire was flying at about M=0.5 if I remember well, so compresibility is, though noticable, not important enough to have any benefits from a not too good area rule that resulted by chance.

 

[KENAT]

alan, you're correct that there were a lot of claims about Spits going nearly supersonic in a dive, coming back with popped rivets, bent wings etc. I'm led to believe that this may have been exaggerated though.

That said, my understanding is that most of the supposed superiority of the Spit in high speed dives was down to the aerofoil section. The Mustangs section was more optimized to planned speed range, such that it performed less well outside that range as I understand it.

rb1957 makes a good point that late model spits were no longer Merlin Engined but used the larger Griffin which would have caused some of the bulge. Heck, different marks of Merlin may have required some re-contouring.

Also some lat marks had bubble canopys which changed the rear fuselage a lot.

It's an interesting question though, and off the top of my head I can't say for sure it wasn't a factor but I'm skeptical.

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[rikrollinga]

Shouldn't all of this be under the "aircraft engineering" topic?

 

[btrueblood]

What Kenat said. Mustangs and later Mark Spitfires and its variants that used laminar-flow airfoils had better cruise performance, but traded lower critical mach number for the airfoils, with subsequent lower dive speeds. Highest dive speeds recorded for Spits were in the mid 0.8 mach no's., highest of any production aircraft of the time, and were recorded for the Spit Mk. XI, which had two-stage supercharged Merlin but kept the original elliptical wing and NACA 22xx foil sections.

I can't see any real area-rule variation in any Spitfire mark, as it would be in the area of wing-fuselage join to have any noticeable effect. All Spit marks have a lovely blended root fillet that enlarges along the chord length, you'd expect to see a wasp waist near the center chord position for area ruling.

 

[KENAT]

rikrollinga, as it's a question about aerodynamic phenomena this seems as good a place as any.

Here's some profile images of a spit.

http://www.socalvalue.com/airace/spitfire/spitfire.htm

I suspect the cockpit is slightly too far forward to achieve much area rule effect - though I understand what you're saying.

You're thinking of it acting like kauchman carrots, rather than pinching the waste deliberately swelling at the trailing edge.

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[alanjmasterman]

Thank you for your replies, gentlemen! A very informative and interesting discussion. By the way, as a matter of interest, I was looking at a photo of a Mk IX when I first noticed this feature of the Spitfire's waist. In the earlier marks it does not seem so pronounced. The Merlin fitted in the Mk IX was a different beast to the earlier versions, of course, and I wonder whether the changes to the cooling system, which were needed to accommodate the new supercharging system, may have dictated more prominent cylinder banks? I'm only speculating, of course...

 

[Sparweb]

...On that, Neville Shute's autobiography has a nice section on the work he did to optimise the thermodynamic efficiency of the supercharger.

I thought it was Sir Stanley Hooker?

His autobiography is the only one I've seen with engine cycle charts in the back.


Steven Fahey, CET