Design Analysis of the Bell Airacobra from Cannon to Tail (1943)

[thunderbird]

i wonder why there is disparagement of Merlin based P-39s? Is it based on the Merlin XX engines installed in P-40s? What the P-39 needed was the same thing the P-51 needed: the two-stage after-cooled Merlin 61. Thus engined, the P-39 would have been on par with the Spitfire Mk IXs, maybe a touch faster, and would have had the altitude performance they needed.

And as for not needing low altitude fighter performance, Supermarine built many LF spitfires, with cropped wings and engine superchargers set to produce more power at low altitude.

Still don't understand why Allison didn't examine a Packard Merlin mustang engine and develop a simlar supercharger system for the Allison. Except of course, for Allison being a GM company not interested in spending any of its own money on anything.

Perhaps one thing to note, Supermarine and Rolls Royce were quick to respond to the needs of the RAF during the war with lots of modifications and customizations. Bell and Allison, not so much.

 

[FLYBOYJ]

i wonder why there is disparagement of Merlin based P-39s? Is it based on the Merlin XX engines installed in P-40s? What the P-39 needed was the same thing the P-51 needed: the two-stage after-cooled Merlin 61. Thus engined, the P-39 would have been on par with the Spitfire Mk IXs, maybe a touch faster, and would have had the altitude performance they needed.

That's your assumption. First tell us how it would fit into the P-39 and then why do this when the P-51 with the Merlin became available? If I was in the British purchasing commission I wouldn't trust Bell based on their deliveries of P-39s and then finding out they didn't perform as promised.

And as for not needing low altitude fighter performance, Supermarine built many LF spitfires, with cropped wings and engine superchargers set to produce more power at low altitude.

Still don't understand why Allison didn't examine a Packard Merlin mustang engine and develop a simlar supercharger system for the Allison. Except of course, for Allison being a GM company not interested in spending any of its own money on anything.

You answered your own question. The same reason why North American developed the P-51 rather than settling for building P-40s under license.

Perhaps one thing to note, Supermarine and Rolls Royce were quick to respond to the needs of the RAF during the war with lots of modifications and customizations. Bell and Allison, not so much.

A contractor is not going to offer anything unless the customer (in this case the US government) is going to pay for it. At the same time I would bet that both Bell and Allison offered improvements to their products that were turned down by the AAF

 

[pbehn]

Perhaps one thing to note, Supermarine and Rolls Royce were quick to respond to the needs of the RAF during the war with lots of modifications and customizations. Bell and Allison, not so much.

NAA also had a long standing and mutually respectful relationship with the British, who were purchasing the T-6 Texan and were involved in discussions on the P-509 which led to the Mustang MkI /P-51. The British requested all sorts of stuff and NAA got on with it, this was a great help in making the P-51B what it became, things like wing strong points and plumbing for external tanks. NAA got on with fitting armour and SS tanks while to this day people claim the British requests were to get out of the P-39 contract. A two stage Merlin doesnt readily fit in a P39, you need a whole new airframe, and once in the next issue is cooling it and its intercoolers.

 

[SaparotRob]

I picture a P-39 with a huge funny car supercharger intake sticking out of the side.

 

[MIflyer]

Allison DID take a Merlin two stage supercharger and stick it onto a V-1710 and found it worked fine - for a little while. Note that unlike the Allison supercharger, the Merlin supercharger did not come off the engine without resorting to a saw and it turns out it did not run well when not part of an integral crankcase, failing pretty quickly. As to why Allison did not at least add a second speed to its single stage supercharger, I have no idea. It would have required a new rear accessory case but I think that would have been easily possible without a problem shoehorning into the P-39, P-40, or P-51 airframe.

And note that for the P-63 and F-82 Allison did develop an auxiliary stage supercharger to replace the turbo and couple it to the original accessory case via the starter interface. That would not have fitted onto a P-39, P-40, or P-51 without significant airframe changes, since it added quite a bit of length to the whole engine package.

Fact is, a V-1710 with a GE turbo was a very capable package and in general turbocharged engines gave better performance than mechanically supercharged equivalents. And as a someone who took too much thermodynamics in college I'll never understand why no one but Stanley Hooker thought up using a liquid cooler aftercooler, which solves all kinds of problems compared to an air cooled equivalent. The fact that Sir Hooker was a theoretical aerodynamicist and "not much of an engineer" but came up with that brilliant solution while literally legions of mechanical engineers did not either develop or copy it merely emphasizes the man's brilliance. It was that aftercooler more than anything that restored the Spitfire to viability and enabled the P-51 to show up over every enemy capital. The P-38 suffered from various intercooler problems and a liquid intercooler could have fixed those with minimal, but never underestimate the power of stupid people in large groups.

And for the F-82 at Air Force orders NAA abandoned the proven Merlin installation of the P-82 and went with a boosted V-1710 WITHOUT an intercooler and as a result built an airplane that almost had to be abandoned. Note that SECAF Symington was a former GM executive.

 

[GrauGeist]

Just as a refresher: the XP-39 was designed for, and installed with, an Allison V-1710-17 - which was a turbosupercharged version.

So it literally started out equipped with a system everybody keeps asking "but what if they installed (insert engine/supplemental power system here)?"

 

[Thumpalumpacus]

Crap! I thought he was dead.

Nah, he's still hanging around.

 

[MIflyer]

Just as a refresher: the XP-39 was designed for, and installed with, an Allison V-1710-17 - which was a turbosupercharged version.

Yes, and they pulled the turbo because the drag associated with it made the airplane slower.

 

[FLYBOYJ]

P-39 Turbocharged Prototype?

I'm interested in learning more about the XP-39 prototype that used a turbocharger. I'm told that the additional equipment (intercoolers and exhaust manifolds I guess) created too much drag. Does anyone have any more information on this? Any pictures and diagrams that show the system in...

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