WWII single seat fighters - size comparison
Discussion
The P-47 was a big beast by the standards of pretty much any other WW2 single-seat fighter. It was basically an industrial HVAC system with wings and eight machine guns.
That's the complete power unit of a P-47 laid out as fitted to the fuselage - all that ducting to and from the engine, the rear-mounted turbocharger, the air-to-air intercooler and so on is why it's so bulky.
The Bf109 was the result of Willy Messerschmitt's philosophy of the bigger possible engine in the smallest, lightest airframe with the least frontal area and the tightest cowlings possible. Hence why the later 109s had to grow all sorts of bulges and fairings in their cowlings and wing surfaces to accommodate later additions of weaponry or equipment because there was so little 'spare room'. The 109 was also designed to use the minimal amount of structural elements, originally for the sake of lightness but later that proved to be highly suitable to dispersed mass production (for instance the forward wing mounting, undercarriage bracket and rear engine bearer are all combined into one single forging. But it did leave the 109 as a relatively flimsy aircraft - there was a saying in the Luftwaffe ground crews that "when an Fw190 crashes, they take the wreckage back to Focke-Wulf and Kurt Tank strengthens all the bits that broke...when a Bf109 crashes, they take it back to Messerschmitt and Willy weakens all the parts that are still intact."
That's the complete power unit of a P-47 laid out as fitted to the fuselage - all that ducting to and from the engine, the rear-mounted turbocharger, the air-to-air intercooler and so on is why it's so bulky.
The Bf109 was the result of Willy Messerschmitt's philosophy of the bigger possible engine in the smallest, lightest airframe with the least frontal area and the tightest cowlings possible. Hence why the later 109s had to grow all sorts of bulges and fairings in their cowlings and wing surfaces to accommodate later additions of weaponry or equipment because there was so little 'spare room'. The 109 was also designed to use the minimal amount of structural elements, originally for the sake of lightness but later that proved to be highly suitable to dispersed mass production (for instance the forward wing mounting, undercarriage bracket and rear engine bearer are all combined into one single forging. But it did leave the 109 as a relatively flimsy aircraft - there was a saying in the Luftwaffe ground crews that "when an Fw190 crashes, they take the wreckage back to Focke-Wulf and Kurt Tank strengthens all the bits that broke...when a Bf109 crashes, they take it back to Messerschmitt and Willy weakens all the parts that are still intact."
LukeBrown66 said:
What is similar is how similar the Mustang and Spitfire are, oh wait!
Seriously though I heard a proper 109 recently and it was utterly beautiful to listen to, for me arguably nicer than a Merlin!!
But that 47 was just a truck was it not!
The Mustang was quite a bit bulkier than the Spitfire with larger internal fuel tanks (and hence, greater range).Seriously though I heard a proper 109 recently and it was utterly beautiful to listen to, for me arguably nicer than a Merlin!!
But that 47 was just a truck was it not!
LukeBrown66 said:
Seriously though I heard a proper 109 recently and it was utterly beautiful to listen to, for me arguably nicer than a Merlin!!
I agree, but then its a full 10 litres bigger capacity than a Merlin, being same size as the later RR Griffon. The the first time I heard Black 6 at Duxford on its public display 31 years ago now 
I thought, what a fantastic noise. That supercharger whine 
Black 6 also retained its original hand-crank starter as well, which had a noise all of its own, especially when the big DB catches and fires into life.
2xChevrons said:
The P-47 was a big beast by the standards of pretty much any other WW2 single-seat fighter. It was basically an industrial HVAC system with wings and eight machine guns.
That's the complete power unit of a P-47 laid out as fitted to the fuselage - all that ducting to and from the engine, the rear-mounted turbocharger, the air-to-air intercooler and so on is why it's so bulky.
Blimey. I didn't appreciate that. That's the complete power unit of a P-47 laid out as fitted to the fuselage - all that ducting to and from the engine, the rear-mounted turbocharger, the air-to-air intercooler and so on is why it's so bulky.
Look at the size of that turbocharger!
RicksAlfas said:
2xChevrons said:
The P-47 was a big beast by the standards of pretty much any other WW2 single-seat fighter. It was basically an industrial HVAC system with wings and eight machine guns.
That's the complete power unit of a P-47 laid out as fitted to the fuselage - all that ducting to and from the engine, the rear-mounted turbocharger, the air-to-air intercooler and so on is why it's so bulky.
Blimey. I didn't appreciate that. That's the complete power unit of a P-47 laid out as fitted to the fuselage - all that ducting to and from the engine, the rear-mounted turbocharger, the air-to-air intercooler and so on is why it's so bulky.
Look at the size of that turbocharger!
Edited by aeropilot on Friday 16th December 14:14
Simpo Two said:
Why did they go for the turbo/huge fuselage setup rather than the more conventional slender supercharged system. Is it because of the radial engine?
They didn't have a conventional slender supercharger system in the USA prior to their entry into WW2 (such as in Spitfire etc)I think all their turbo-supercharger systems were remote like this? P-38's Allison V12 were, and the radial in the bombers were as well.
A guy called Greg has done some very thorough analyses of the Jug on YouTube. Greg's planes and automobiles.
My understanding is that the turbo was chosen because it gave more consistent boost at high altitude without the need for complex multi speed, multi stage arrangements.
But that pic is impressive. No wonder the Jug was so big, and so expensive!
My understanding is that the turbo was chosen because it gave more consistent boost at high altitude without the need for complex multi speed, multi stage arrangements.
But that pic is impressive. No wonder the Jug was so big, and so expensive!
dontlookdown said:
A guy called Greg has done some very thorough analyses of the Jug on YouTube. Greg's planes and automobiles.
My understanding is that the turbo was chosen because it gave more consistent boost at high altitude without the need for complex multi speed, multi stage arrangements.
But that pic is impressive. No wonder the Jug was so big, and so expensive!
A turbocharger self-compensates for altitude. As the outside air reduces in density, the turbocharger will spin faster, thus compressing more of it in the same time. A supercharger, if geared directly to the engine crankshaft, will only work on a given volume of air, hence the need for a two-speed supercharger on later Spitfires. My understanding is that the turbo was chosen because it gave more consistent boost at high altitude without the need for complex multi speed, multi stage arrangements.
But that pic is impressive. No wonder the Jug was so big, and so expensive!
In addition, getting the turbocharger out of the engine area allows more room for intercooling and keeps the hot engine and hot turbocharger from heating each other.
Simpo Two said:
Makes you wonder what a turbocharged Spitfire would have ended up looking like 
Why did they go for the turbo/huge fuselage setup rather than the more conventional slender supercharged system. Is it because of the radial engine?
To get truly good combat performance at high (25,000+ft) altitudes you need a two-stage forced induction system - a single-stage system will either be too compromised (rob too much power at low altitudes or not give full potential at height), will have problems with the compressor (too large a rotor or the rotor spinning too fast so the edges go supersonic and start damaging themselves) or will run such high boost that you get detonation/backfire issues. You need to do your forced induction in multiple stages, cooling the charge between each stage and possible after the final one. Compromise can be reached with a two-speed supercharger (or one with a variable hydraulic drive like on the Daimler-Benz engines) but it doesn't give the same ultimate performance as a properly engineered multi-stage system. Why did they go for the turbo/huge fuselage setup rather than the more conventional slender supercharged system. Is it because of the radial engine?
The turbocharger is, on paper, the most ideal way of providing forced induction because it uses waste exhaust energy, can turn at various speeds entirely independently of the engine itself, can have its boost output varied 'on the fly' and to some extent is self-regulating as it provides boost in response to engine speed/load and throttle position. But turbos need extremely high-grade materials and lubricants to be reliable.
The Americans had a global advantage in being the only nation able to build serviceable turbos for aero-engines in the late 1930s...but only just. Early turbos were still pretty unreliable, fairly crude in operation and had various compromises forced on them. They could not reliably cope with exhaust gases at high temperatures straight from the cylinder head, so had to be mounted some distance 'downstream', and they had to be relatively large so they turned relatively slowly (keeping temperatures, balancing and lubricant demands down) while still delivering the required boost.
None the less, the USA remained committed to the theoretical superiority of the turbocharger for high-altitude performance and this led to a string of specifications and prototypes which used two-stage turbo/supercharging - the turbo was the first stage, compressing the initial intake charge, which was then passed through an air-to-air intercooler to increase its density and reduce its temperature, then it was passed to a conventional engine-driven supercharger of a straightforward single-stage, single-speed design. That was the blueprint the USAAC wanted for its high-performance aircraft and a string of prototypes followed that plan in the late 1930s - the Curtiss XP-37, the Lockheed P-38, the Republic P-43 (which begat the P-47) and the Bell P-39.
All those except the Republic used the Allison liquid-cooled V12, the development of which was being followed on the assumption that it would have two-stage forced induction with a turbo. The XP-37 had the turbo and intercooler packaged between the engine and the cockpit, giving it an unfeasibly long nose, poor weight distribution and recurrent reliability problems. The prototype P-39 was covered in ducts, fillets, vents and gills for the turbo, intercooler, supercharger ram intake and so on, and all were so badly packaged and designed that tests in the NACA wind tunnel showed that the drag they inflicted undid all the power gains from the turbo at all but the highest altitudes. So Curtiss and Bell both removed the turbo from their designs to get something vaguely modern with the V12 into production - the P-40 and the P-39 Airacobra. The Allison engine still had the relatively crude supercharger design that was intended to serve as the second stage to a turbocharger, and hence suffered from restricted high altitude performance in comparison to the RR Merlin, the ongoing development of which centred around superchargers since RR did not have access to the knowledge or materials needed to make even the crudest turbos. Allison began work on a two-stage supercharger system for their engine, with the primary stage having a variable hydraulic drive, but they were now playing catch-up after years 'wasted' favouring turbos and were a small company also struggling to build and improve their existing product, so progress was slow.
The P-38 had more space in its twin-boom design to package everything effectively and did enter service with the turbo/supercharged Allison(s), but they were far from trouble-free and had particular problems with the wastegates freezing up at high altitudes.
The design that stuck was Republic's, mating the turbocharger to an air-cooled radial engine rather than the Allison V12. The conflict with the Allison was that the big aerodynamic advantage of the V12 - small frontal area and streamlined cowlings - only paid off if it was mated to a slimline fuselage (there's no point designing a perfectly contoured, tight-fitting cowling around an engine in the nose and then the fuselage ballooning out behind it to accommodate a turbo, an intercooler, and 20 feet of air ducting). With the broader profile of a radial engine, the P-43/P-47 already had an advantage in that the fuselage had more internal volume. The P-47 was also purpose-designed around the ideal packaging for such as system, with the turbocharger behind the cockpit, well away from the hottest parts of the exhaust tract and with lots of space to both fit the intercooler and arrange ducting in and out for it. In an opposite case to the original P-39, the P-47's induction setup, and its aerodynamic integration, was effective enough to increase power by such a margin that the P-47's sheer size wasn't really an issue - the size allowed its powerplant to produce so much more power that it easily compensated for its bluff shape and thick-set profile.
Dr Jekyll said:
Once you've cracked how to build a reliable turbocharger, you can't be far off building a reliable gas turbine. It seems odd that the US were ahead in turbocharging but not in gas turbines.
There's a recognised economic/industrial effect which I can't remember the name of, but it boils down to how it often doesn't pay to be a pioneer in a particular field, because you'll often find that your competitors will leapfrog you - cutting out the awkward development stage that you worked so hard on and focusing on the downstream developments that your work has unleashed.Ergo, the British couldn't build turbochargers so essentially ignored them and focused on supercharged piston engines. The Americans pioneered the difficult path of turbocharged piston engines, which unlocked a lot of the potential in gas turbines. The British saw no need to switch to turbos (the ultimate development of the old technology) but jumped straight into gas turbines while the American industrial and developmental effort was still heavily focused on turbos.
Of course that experience (GE was the main developer of American turbochargers for aero use) paid off when the Americans did start on their own gas turbines and they quickly regained momentum and parity.
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