I picked this post because there are some good comments to point to the 'why' the NAA/NACAA 45-100 was revolutionary for a Production wing.
First, the progression of stages of airflow needs to be clarified.
Laminar flow. It exists at very low speed for a Mean Aerodynamic Chord of almost 6ft for the Mustang, specifically in the Reynolds Number Range of 600,000, equivalent to slow taxi speed for the P-51. It is characterized by a 'shallow/thin' Boundary Layer attached to the surface of the wing close to the leading edge. Indepepenent of the 'modern airfoils' such as NACA 23xxx or the 45-100, that region near the leading edge is small.
Transitional Turbulent Flow. It exists, exhibits some chaotic internal 'wakes' and disruptions to attached flow,but the flow remains attached with progressively thick boundary layers to 'regions' experience the pressure gradients from the full blown chaotic Turbulent flow state downstream, then
Turbulent Flow with chaotic locations/regions of attached flow but dominantly chaotic with low energy when compared to the free stream passing over it. This 'region' also co-incided with the highest level of math that I took in grad school. Calculus of Variations was a walk in the park compared to attempts to combine Navier Stokes with Chaos theory.
Lednicer, aka aeroweenie please step in.
So, the design of the 45-100 was designed by selecting a perferred pressure distribution, not as high (magnitude) as NACA 23xxx from LE to ~ 30% T/C, but more even through 50-60% before relatively sharp degradation. The region between say 30% and 50% had a better behaved Transitional TURBULENT flow. Not Laminar. Consequently the Center of Pressure for this airfoil was also farther aft so when Mcr was reached near T/C max it was significantly farther downstream than NACA 23xxx. The combination of loss of lift aft of the shockwave and increase of Moment of pitch was mitigated in comparison.
So, that is the 'theoretical' wing. In practice, the wing achieved purty damn gud results to smooth model wing despite camo paint, etc of a production P-51B-1 tested at Langley along with P-63, F4U, F6F, P-40, F4F and P-38. Despite the extraordinary values of the sevice condition vs 'sealed condition' it was a VERY conservative outcome because there was zero Meredith effect to mitigate the unsealed scoop. It effectively added a flat plate inside the cooling duct intake.
So,what?
The wing's extraordinary theoretical design was abedded by the extraordinary manufacturing process to include best practices in tooling, sheet metal fabrication, assy with care to surface imperfections via flush rivets, tight butt joints, etc - resulting in a clean wing at this stage. But the next process of puttying surface gaps, dimples/protrusions - then sanding, priming and painting the leading 40% of the wing ensured consistent results so long as the leading edge was not banged up enough to cause sections of the paint/filler to chip - which would introduce early 'boundary layer trips'.
Now - disclaimer. Certain media experts such as Greq opine that if the Spitfire had the same care in building a wing, that it would be as fast as a Mustang with same power.
I would opine thusly:
If a Spitfire performed elimination of all bumps/protrusions, began with completely covered landing gear, eliminated protruding armaments, etc - collectively would have been be a better design to shave drag.
But with a wing aready 20% thinner, it already had a wing with about the same profile/form drag of the fatter P-51 wing and the friction contributions of the features discussed were not so important as the basic airfoil section.
Greg also kinda forgot about the underwing radiators, the oil cooler, etc when opining about production quality
That said - a liitle more about the Meredith Effect - The REAL advantage, aerodynamically.
I would have humbly beseeched the master internet aerodynamicist, that most of the supriority of the Mustang over Spitfire, Focke Wulf, Messerschmitt, etc was the design and LOCATION of the cooling system. The wing was the icing on the cake, along with an early introduction in fuselage design excellence applying Projective Geometry.
To the 'doubters' of the value of the jet effect of minimum opening exhaust exit scoop, look to the comment in paragraph 3. below.
My calculations yielded about a delta of 180-200 THP loss when exit gate wide open.
Source, detailed flght test performance report, found on
P-51B Performance Test
MEMORANDUM REPORT ON
Pursuit Single Engine P-51B-1-NA, AAF No. 43-12093
May 18, 1943
| 2. High speeds obtained with the oil cooler flap and coolant flap set for automatic operation since there were no provisions on this airplane for selective operation and no time was available for a test installation of a selective control. |
True
Airspeed
M.P.H. | R.P.M. | Man.
Press.
" Hg. | BHP
From
Power
Chart | Altitude
Ft. | Coolant Flap
Position
Inches open
From Flush | Oil Flap
Position
Inches open
From Flush |
| (a) Low blower Operation | | | | | | |
363 | 3,000 | 60.5 | 1,450 | 5,000 | 6.0 | W.O (5) |
394 | 3,000 | 60.5 | 1,485 | 10,000 | 5.0 | 3.5 |
425 | 3,000 | 60.5 | 1,530 | 16,800 | 1.5 | 1.0 |
422 | 3,000 | 49.0 | 1,270 | 23,200 | 1.0 | Flush |
| (b) High blower Operation | | | | | | |
422 | 3,000 | 60.5 | 1,270 | 23,200 | 1.0 | .5 |
441 | 3,000 | 60.5 | 1,275 | 29,800 | 1.0 | Flush |
421 | 3,000 | 48.0 | 985 | 35,000 | .5 | Flush |
403 | 3,000 | 40.7 | 815 | 38,000 | .5 | Flush |
| 3. Opening coolant flap wide open from flush position slowed the airplane from 349 M.P.H. I.A.S. to 325 M.P.H. at 18,000 Ft.; opening the oil cooler flap decreased the speed an additional 10 M.P.H. I.A.S. |