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Grand_Armee
07-14-2007, 06:10 PM
I've been looking at German and American aircraft of the late war period comparing horsepower ratings and weight mainly. What I'm seeing...or think I'm seeing... is that the German aircraft, some of which were significantly lighter than their Yankee counterparts, had a lot more horsepower but rarely a large speed advantage if any advantage at all. Some were even slower!

Without getting into things like drag co-efficients, I'm wondering if this American ability to get more speed with less HP is completely the result of the exhaust-driven-supercharger.

Grand_Armee
07-14-2007, 06:10 PM
I've been looking at German and American aircraft of the late war period comparing horsepower ratings and weight mainly. What I'm seeing...or think I'm seeing... is that the German aircraft, some of which were significantly lighter than their Yankee counterparts, had a lot more horsepower but rarely a large speed advantage if any advantage at all. Some were even slower!

Without getting into things like drag co-efficients, I'm wondering if this American ability to get more speed with less HP is completely the result of the exhaust-driven-supercharger.

horseback
07-14-2007, 07:44 PM
One word: Turtlewax.

Two words: Shadetree mechanics.

Three words: Increased Manifold Pressure.

cheers

horseback

Ratsack
07-14-2007, 10:09 PM
Firstly, the two American fighters powered by turbo supercharged engines were the P-47 Thunderbolt, and the P-38 Lightning. Both were much heavier than their German contemporaries, and both disposed of a lot more power than their opposite numbers. The R-2800-59 engine in the P-47 D developed about 2,480 hp with water methanol injection, which is quite a bit more than the Bf 109 G or Fw 190 A. The two Allison V-1710-89/91 engines in the P-38 J-25 developed 1,600 hp each, for a total of 3,200 hp for the fighter.

So, I suppose I'm saying there's a false premise in your question.

In relation to the Mustang, I think you're on firmer ground with the observation about similar power. However, the Mustang never had an engine with a turbo supercharger.

In my view, the answer is simple: drag. If you compare the Spitfire MkI with the Bf 109 E, you find similar power ratings and similar top speeds. If, however, you compare the Mustang I in RAF service (i.e., the Allison powered P-51) with the Spitfire MkVb, you find the Mustang considerably faster at 13,000 feet with similar power. The difference? Drag.

The comparison is underlined when you compare the Spitfire MkIX, powered by the Merlin 61/63/66 with the P-51 B, powered by the Packard Merlin V-1650-3. Top speed of the Spitfire IX was about 400-410 mph at 20,000 feet, where the P-51 B managed 440-450 mph at 26-28,000 feet. The difference? Drag.

We have to remember that the comparison with the Spitfire and Bf 109 is not entirely fair, given that these two aircraft were first-generation assays of this type of fighter. The Mustang was very much a second-generation fighter (perhaps third, if we differentiate between the early Allison-powered and the later Merlin-power examples). However, when you compare the Mustang to the other second-generation fighters, the difference is less striking.

For example, at 20,000 feet, the Tempest and Mustang would have much the same top speed. At low altitude, the Tempest should be faster. The long-nosed Focke-Wulf, the Fw 190 D-9 powered by the Jumo 213 A motor, had performance much closer to that of the P-51 D than its predecessor. This was, again, largely a matter of drag. The Dora was a cleaner design than the Anton.

It's drag, in my view.

Cheers,
Ratsack