twin boom designs

[joeblogs]

The fact is that the Aircorps would not pay Allison to develop a two speed or two stage supercharger for their engine. They were told to put a one speed, single stage supercharger on the engine and the GE turbo would do the rest. This is essentially how the installation in the B-17 worked as well.

Unlike the turbosupercharger, gear driven superchargers must be designed with the exact engine in mind - so it is the job of the engine manufacturer to design a good one for their motor. The best example is what Hooker did with the two stage unit on the late model Merlins. The ones that made the Spit IX and the P51D so fantastic at high altitudes.

P&W were just about the first to have a two stage geared supercharger on a high output engine - the Twin Wasp installation on the F4f. When it worked, it was just what the plane needed, but the supercharger was tempermental. P&W went on to put an effective 2 stage unit on the R2800, used in the F6f and F4u. But developing these units took many years and lots of money.

Allison can be criticized for a lot of things (for example, Doolittle complained how dirty their factories were early in the war), but it is not their fault for following the contractor's orders.

-Blogs

Originally posted by Captain Virgil Hilts
...There are any number of reasons that the Allisons were never equipped with a two speed crank driven supercharger, although none of them are particularly good reasons. Mostly the reasons given have to do with simplicity, expediency, expense, and related excuses. The Allison could have been equipped with a two speed supercharger, it just wasn't. ---.

[Angus]

Originally posted by Captain Virgil Hilts
Even the Merlins tuned for high altitude performance couldn't match the turbocharged Allisons for power above about 28000 feet. The problem is that the higher the altitude, the faster you have to turn a supercharger to create a given pressure. But the faster you turn a supercharger, the more power it takes to turn it. At some point, it takes more power to turn the supercharger than can be gained from the pressure generated. Also, above a certain speed, a supercharger will cavitate, and actually produce less pressure. And finally, the faster the supercharger runs, the more it heats the air, making the air less dense, even though it has high pressure.

Now the same principles do apply to a turbocharger, but to a far lesser degree. Both the P-47 and P-38 used the same basic GE turbocharger, and both were capable of overspeeding the turbocharger at high altitudes.

The P-38 was the only Allison equipped fighter with a turbocharger. The P-39 originally had a turbocharger. However, in the interest of streamlining for drag reduction, among other reasons, the USAAF/USAAC had Bell remove it. The P-39 COULD have had 1725 to 1750HP at 29000 feet, but they took away the turbocharger before the plane went into production. Just imagine the small, lightweight P-39, equipped with the -30 series 1725HP Allison, four 50's in the wings and a 20MM cannon shooting through the hub of a Hamilton Standard paddle prop.

There are any number of reasons that the Allisons were never equipped with a two speed crank driven supercharger, although none of them are particularly good reasons. Mostly the reasons given have to do with simplicity, expediency, expense, and related excuses. The Allison could have been equipped with a two speed supercharger, it just wasn't. Same thing applies to props. Curtiss could have been tasked with building hydrostatic paddle props, instead of the crappy electric thin blade props, but they weren't. The Curtiss prop most commonly fitted to Allison equipped planes was inefficient, and unreliable. Pilots who flew planes equipped with Curtiss props continually complained of burned out generators and blown fuses or popped circuit breakers.

Although Bodie spoke highly of the idea of a Merlin powered P-38 in his book, he also said the conversion was never attempted, and no one knew if it would actually work as well as was thought. Bodie, at least when I was swapping emails with him, seemed to think the F-15 powered P-38K with Hamilton Standard props was better than any Merlin conversion could have hoped to be. And remember, the P-38K had the early 3 blade prop, not the later four blade. You would only need to look into the dramatic improvement seen in the P-47 when Republic swapped from the Curtiss Electric prop it shared with the P-38 to the Hamilton Standard paddle prop to see how much the P-38 would have gained with the same prop, even without the F-15 Allisons.

What alt Bands are you talking about there? In real life, Merlin powered Spitfire Mk IX's (Merlin 61) could cruise at 43K, the only limiting factor being the Pilot, - and that is already in 1942! There were further improvements after that!
Merlin 45, Merlin 66, Merlin 70, CCC's and NOT, are not the same things.
So, I have two questions.
Why could the Allison only be made for high alt work in the P38 (Not P40, P39 and P52), and secondly, how does it rack up against a Merlin 61 or 70?
(NOT 45 or older)