I thought I had replied to this a couple of days ago..I believe all of difference in performance was due the difference paid to excrescence drag on the P-51, or the drag caused by details. At the beginning of the war, excrescence drag was around fifty percent of the base drag. Airplane performances were far worse than what was predicted by wind tunnel tests. Drag due to gaps, to mismatches, to forward facing steps, drag due to skin waviness, drag due to protruding shapes, fasteners, drag due to skin beat up by installing solid metal rivets, flush or otherwise, etcetera. Much of which could be avoided by paying attention to detail design, by tooling that eliminated built in bumps, by better assembly practices that didn’t beat up the exterior surfaces.
Now the Spitfire was an exceptionally clean design for its time, with a base drag coefficient that was nearly half the base drag coefficient of the P-38 Lightning (or the Bf109 for that matter). And much better than any of the early American fighters.
But as you might have discovered, the P-51’s base drag coefficient was even better.
My theory is that NA paid attention to the details in the P-51 manufacture. After all, there was no chance to achieve any improvement in laminar flow if the surfaces were lumpy and bumpy. And the P-51 development occured after NACA started studying sources of excrescence drag on American fighters.
The laminar wing, the Meredith effect cooling ducts also contributed to the P-51’s improved performance.
Interesting enough, the laminar flow wings didn’t help the P-51’s dive speed as its thicker wing meant the P-51 started seeing drag rise due to compressibility earlier than the Spitfires thinner wings.
While I agree with much of what you said, I would opine that your use of excresence drag is casting too wide a net. It is separable from friction drag. Excresence drag includes physical features such as bumps/lumps/stubs/masts, turrets, fairings, etc fit in that category. Smaller (magnitude) elements such as butt joint mis-match, pan head rivets, depressions in flush rivet joints, dents, grainy paint, etc that trip boundary layers or which features increase surface friction for turbulent flow with adverse pressure gradient. Friction drag also includes gaps such as uncovered wheel wells, control surfaces, etc.
To be clear, when you refer to 'base drag' - are you referring to all components of parasite drag including but not limited to zero lift parasite drag, parasite drag as f(CL), parasite drag of components immersed in slipstream which are not f(RN) such as bomb racks, masts, armament?
The only reference I have seen for Spit IX, in a obscure RAE paper, was a reference to 'base' or CDo in low speed wind tunnel. Obscure enough that I can't put my finger on it. That said, from memory the CD0 for 100mph (for wind tunnel smooth model) was in range of 0.023 for the referenced Spitfire. That value, nor any below, did not include induced drag.
For NACA - Langley tests in 1943, The equivalent (in RN = 6.3x10^6) for full scale P-38 was 0.022 for sealed condition, 0.029 for Service Condition; for P-51B = 0.0173 for sealed condition, 0.0209 for Service Condition at RN=6.19x10^6. Hoerner cites the Bf 109 at Cdt = 0.030 for full scale wind tunnel test at Chalais-Moudon at RN=2x10^7. For radial engine comparisons at Lagley, F6F = 0.021 for sealed, 0.0293 for Service condition. ALL tests - sealed/unsealed - were performed on random production article fighters, Conclusion?
Lots of Excresence Drag for everything but the P-51B. The drag of the sealed condition airframes was essentially the sum of the surface friction and profile items of the total wing/fuselage/empennage. Most of the unsealed components were ducts, gaps, ejection and link slots, exhaust stacks, mast replaced,etc.
Note; The P-51B values do not include Meridith effect thrust. The usealed 'underslung duct' value of Cdp was 0.0011, or about 6% above the total Service condition. Removed from the tables as a result of Net Zero Cooling Drag, the Service condition CDp lower to 0.0198
The Primary difference between the P-38, F6F and the Mustang was the very low drag of the wing of the P-51B.
Note: Unless extracting drag data from well documented report, in which each cited drag value for a specific feature is carefully defined, each 'baseline' references test environment (full scale prodction, 1/4 smooth model) so that RN for the test is better understood for comparisons; that HP, THP, Momentum Losses, exaust gas thrust, need to be carefully normalized with respect to units - to provide basis for comparisons.
The P-51 wing is commonly referred to a 'laminar flow' wing, but not at NAA. It was called Hgh Speed/Low Drag from the beginning. NAA knew that surface finish techniques and production standards of the 1940s could never yield the aerodynamicist dream of full laminar flow, although Horkey claimed results in the 25% range T/C for XP-51F/G/J and P-51H with NACA 65 and 66 series wings.
That said, the combination of moving the T/C max fom 25% area common to1930s designs, to 37.5% yielded two benefits, maybe three. The lower velocity gradient from LE to 37.5% T/C seemed to delay both B/L separation (turbulent not laminar) and movement of CP during transonic transition. The effect was a reduced 'frontal area' of pressure drag of wing and wake to the freestream as well as a reduced Pitching Moment Cm - which is why the drag rise even for a fat wing aproached that of the Spitfire, and why the Mustang did not require dive flaps like P-47 and P-38to recover quickly from terminal dives.