Nomak:
You wrote...
Test data means nothing to a pilot in combat. Isnt true combat performance what we are trying to compare here? At least I am.
In comparying anything you need to define the scope and the criteria by which you will compare. What set of benchmarks are you using for comparing the P-51 and the 109's "true combat performance"? There is some value to pilot accounts in comparing relative performance differences between aircraft when appropriately taken as just a part of the whole of the analysis. However common sense tells me that relying heavily or solely on pilot stories to compare relative performance differences is fraught with problems that prevent accurate peformance benchmarking.
Based on just the basic technical data comparing the P-51D to a Bf-109K-4/G-10, I'd have to say that the 109K-4 matched head-to-head against the P-51D would have been a tough opponent. That's not even running through any calculations.
P-51D
Powerplant: 1490HP, 1650HP WEP
Normal Loaded Weight: 9500 lbs
Max Level Speed: 437mph @ 25,000 ft
Wing Area: 233 sq ft
Bf109K-4
Powerplant: 1800HP, 2000HP WEP
Normal Loaded Weight: 7438 lbs
Max Level Speed: 452mph @ 19,700 ft
Wing Area: 174 sq ft
The K-4 has a lower weight, better thrust-to-weight ratio, and higher top speed below 20,000 ft. Not sure what the difference in wing area would have played in terms of energy retention and bleed without doing the calculations but my guess is that the K-4's T/W ratio balance out the induced drag in maneuver due to smaller wings vs. the P-51D.
The P-51D would probably have been outclassed against a K-4 below 20,000 ft in terms of both speed and sustained maneuvering. Above 20,000 things equalize more as you go up in altitude but the K-4 would have still been an extremely dangerous opponent.
And this comes from a lover of the P-51

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Of course I've limited the scope of the analysis to basic technical data as a benchmark. Things change as you broaden or change the scope of what you're comparing. For instance the virtues of the P-51 show up at a more strategic level of comparison- e.g. it's range and endurance, it's flexibility in roles, etc.
Tango, XO
412th FS Braunco Mustangs