So much for answering the original question. Let me try to address the points more individually:
Why is it that the P51 is so much faster than the Spit 16, but the Spit 16 holds E better and is so much better in the vertical? If the P51 is so much more aerodynamically "cleaner" than the Spit, then shouldn't it hold E as well? If the Spit is better in the vertical because it is lighter, shouldn't it also have a higher top speed (power to weight)?
You need to understand a few concepts which are, more or less, in opposite pairs:
- parasite drag vs. induced drag
Parasite drag is comparable to holding your arm out of a car window. The faster you go, the more drag your arm creates which you can feel from the force of the wind on your skin. One of the big contributing factors to parasite drag on an aircraft are the wings. Big wings that create lots of lift incur more parasite drag than small wings that create less lift.
Induced drag is caused by the force required to create lift. The faster you go, the less induced drag you incur. At low speeds, induced drag increases, but a large wing will incur less induced drag at low speed than a small wing.
The P-51 wing incurs high induced drag at low speed, but little parasite drag at high seed. The Spitfire is the opposite. Because the P-51 incurs little parasite drag compared to the Spitfire, it has a better thrust/drag ratio, and so it is faster.
- Powerloading vs. energy retention
Powerloading is the ratio of an aircraft's weight to its horsepower. Better powerloading means better acceleration and better climbrate. Energy retention is comparable to kinetic energy. The heavier aircraft retains energy better than the lighter aircraft because of its higher kinetic energy state. The Spitfire may be slower than the P-51 because of their thrust/drag ratios, but it accelerates and climbs better because it is lighter. That the spitfire doesn't slow down much in a turn is due to its low induced drag, not energy retention. An aircraft with superior energy retention will lose speed more slowly after diving to 500mph and then leveling off compared to one that is lighter (assuming similar drag coefficients), so you see that what the spitfire can do is not an example of superior energy retention.
What I have said is a simplification, but I hope it helps to tease out the most important concepts.
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