What technical features account for the P-51B/D's low drag relative to other fighters of the day (e.g, Spitfire Mk IX)?

Question

I am really sorry to dredge a popular topic up again. Unfortunately, after reading through some papers on the P-51 as well as all the past forum threads I could find on this topic, I have found myself very confused about the source of the P-51(B or D)'s drag advantages over the Spitfire Mk IX.

First, let's establish some facts.

1. In flight tests with equivalent engines & power-plants, the P-51B/D was 25-30 MPH faster than the Spitfire in level flight. Drawing from David Lednicer's (a professional aerodynamicist's) comparison of WW2 fighters (Source: http://www.wwiiaircraftperformance.org/mustang/Lednicer_Fighter_Aerodynamics.pdf), he states "with the same version of the Rolls-Royce Merlin engine and propeller installed, the Mustang X was measured to be 23 mph faster than the Spitfire IX....The P-51B is even faster than the Spitfire."

2. The Mustang had lower total drag than the Spitfre. Again, showing a table from Lednicer's article, which uses drag estimates from flight tests and wind tunnels of the era. (Note: he notes in the article that he has been conservatively high with the P-51's drag).

So we know that the Mustang was faster than the Spitfire and had lower total drag.

However, I have been confused out of my mind as to what technical features account for this lower drag. Can this community come to the rescue?

Now, the most obvious answer to this question would be that the P-51B/D incorporated an airfoil section that was designed to maintain larger regions of laminar flow on it, lowering the skin-friction drag of the wing. However, to quote Lednicer again: "in service the aircraft was unlikely to have a substantial laminar flow on the wing and transition occurred in the first 15% of the chord".

Additionally, to quote John Anderson:

So it's unlikely that significantly reduced skin friction drag on the wing accounted for the lower drag unless someone has information that shows that whatever <15% extent laminar flow on the P-51B/D wings was markedly higher than similar extent of laminar flow on the wings of the Spitfire IX or other fighters of the era.

Answer 1

The P-51 was one of the first, if not the first, airplane design in which the aerodynamics of its cooling system were focused on- and in which the airflow through the radiator and its ducting were given the same sort of attention as the airflow around the fuselage.

Indeed, the origins of the P-51 design were originally investigated in the context of an aerodynamic cleanup of the P-40- in particular, reducing the drag associated with the P-40's radiator placement and cooling air flow through it.

This led to the fundamental idea of burying the entire radiator within the fuselage profile to reduce drag and using an advanced scoop design to bring airflow up into it, and using variable exit doors to control the airflow through it.

Answer 2

The above answer by Niels is on the money. According to an article I read recently by one of the actual designers of the P51, the reduction in cooling drag was the main reason for the better speed. It is late and I don’t have time to search for it, or the man’s name but I am sure he was quite clear on that and had data to back up what he said. The laminar flow wing had a lot less to do with it than I was led to previously believe according to him. The flow over the wing was not as ‘laminar’ as the designers thought it would be, but the radiator design was very, very good.