The P-51's boundary-layer splitter was not in itself something new, nor was the so called "Meredith effect". Junkers discovered that thrust could be generated by radiator heat in the 1920s, and most radiator designs from the 1930s onwards made use of the Meredith effect and had boundary-layer splitters or ducts.
(Image removed from quote.)
The P-51 was so fast because it was a superbly aerodynamic design. Everything was shaped to minimize drag, including the wing profile at the expense of lift at high angle of attack. The only compromise was the bubble canopy starting with the D model. The P-51 cuts trough the air like no other WWII piston engine fighter. In comparison the Spitfire and 109 (especially the G-6) bludgeon their way through the air. The only planes that come close are the Russian Yak fighters; in Russia the need for streamlining was born not from the need for range, but from the need to get competitive speed with limited power.
From Leonard "Kit" Carson, P-51 ace and test pilot:
"The most prominent speed secret was the dramatic reduction of cooling
drag. Placing the airscoop on the belly just in front of the rear edge
of the wing removed it as far as was practicable from the turbulence of
the prop and placed it in a high pressure zone which augmented air
inflow. Tests in the wind tunnel with the initial flush mounted scoop
were disappointing. There was so much turbulence that cooling was
inadequate and some doubted that the belly scoop would work. The
breakthrough was to space the scoop away from the surface of the belly
out of the turbulent boundary layer of the fuselage. Further testing
showed that spacing it further out would increase cooling but at a cost
to overall drag. Various wind tunnel tests established the spacing at
the current distance which represents the best compromise between
spacing out from the turbulent flow of the fuselage, drag and airflow.
With the flow into the scoop now smooth and relatively nonturbulent,
the duct leading to the radiator/oil cooler/intercooler was carefully
shaped to slow the air down (the duct shape moves from narrow to wide,
in other words a plenum chamber) enough from the high external speeds
to speeds through the heat exchangers that allowed the flow to extract
maximum heat from the coolant. As the air passed through the radiators
and became heated, it expanded. The duct shape aft of the radiator
forced this heated and expanded air into a narrow passage which gave it
considerable thrust as it exited the exhaust port. The exhaust port
incorporated a movable hinged door that opened automatically depending
on engine temperature to augment the airflow. The thrust realised from
this "jet" of heated air was first postulated by a British
aerodynamicist in 1935. The realization of thrust from suitably
shaped air coolant passages is named after him and called the "Meredith
Effect". Some have said that at certain altitudes and at a particular
power setting the Meredith effect was strong enough to actually
overcome all cooling drag; this is not regarded as being accurate by
most aerodynamicists. It greatly contributed to overall efficiency of
the cooling system but never equaled or overcame cooling drag. "
As to that image of the 109's cooling system.... I love how the artist shows the boundary layer conforming to the upper surface of the duct... In his dreams perhaps, but not in reality. You can bet all you own that the boundary layer (already very turbulent) immediately separated when it reached that 60 degree bend. There is almost no plenum volume to speak of either.
Back in the 1980s, we experimented extensively with radiator duct design on our two March Formula 2 chassis and quickly learned that you required a carefully shaped splitter to keep the boundary layer away from the inlet. Otherwise, it created tremendous turbulence and backpressure in the plenum, greatly reducing flow through the core. That in turn would require a larger core, adding unacceptable weight.
If anyone seriously thinks that the 109's radiators were as efficient as that of the P-51, they are deluding themselves.
My regards,
Widewing
