Would it have been cheaper to....

[tomo pauk]

...

The big advantage of the P-51 was that its designers probably did the best design of an aircraft cooling system that was practical for a combat aircraft of the era. They also did a their other detail design very well: most single-engined, piston-powered (radial or V-12) fighters had zero-lift drag coefficients of about 0.02 to 0.023. The P-51 was an outlier at about 0.017 (the Bf109 was, at least from some reports I've seen, an outlier in the other direction: 0.029).

For the Bf-109, it very much depends on what variant we talk about. The 'clean' ones, like the F series, have had the Cd0=0.023. The lumps bumps on the Emils and later Gustavs were bound to add significantly on the Cd values, though.
The data card for the Bf-109F-4 (here). The Cd0 value is the 'Schnellflug Cw', listed near to the bottom of the card.

 

[tomo pauk]

...

The big advantage of the P-51 was that its designers probably did the best design of an aircraft cooling system that was practical for a combat aircraft of the era. They also did a their other detail design very well: most single-engined, piston-powered (radial or V-12) fighters had zero-lift drag coefficients of about 0.02 to 0.023. The P-51 was an outlier at about 0.017 (the Bf109 was, at least from some reports I've seen, an outlier in the other direction: 0.029).

For the Bf-109, it very much depends on what variant we talk about. The 'clean' ones, like the F series, have had the Cd0=0.023. The lumps bumps on the Emils and later Gustavs were bound to add significantly on the Cd values, though.
The data card for the Bf-109F-4 (here). The Cd0 value is the 'Schnellflug Cw', listed near to the bottom of the card.

 

[OldSkeptic]

For the Bf-109, it very much depends on what variant we talk about. The 'clean' ones, like the F series, have had the Cd0=0.023. The lumps bumps on the Emils and later Gustavs were bound to add significantly on the Cd values, though.
The data card for the Bf-109F-4 (here). The Cd0 value is the 'Schnellflug Cw', listed near to the bottom of the card.

True, there was lot of variation with the 'F' being the best. Both the 109 and Spit had'1st generation' meredith radiators, which probably offset radiator drag by about 50%, the Mustang (and Mossie to a slightly lesser extent) were 2nd generation, offsetting by 90% for a Mustang. Hawker claimed that one of their Tempest prototype versions actually got a net gain in thrust (forgot which one).

Positioning was a significant factor in this (as well a depth and shape). The under wing design was not the best position (turbulence, wing movement, etc, etc), a wing inset design or under the fuselage were the best.

Interestingly the Spit III prototype probably had significantly better radiator drag, due to a more advanced design but also the positioning, if you look at the pictures it was further forward than a std Spit, more into clean air.

 

[tomo pauk]

The Spitfire III have had the fully enclosed undercarriage, both main and tail wheels, that alone would earn it 10 mph? The wing spand and area were also a bit reduced, and, correct me if I'm wrong, the prototype Mk.III carried no guns, or at least no cannon - all together another 10 mph worth?

 

[wuzak]

True, there was lot of variation with the 'F' being the best. Both the 109 and Spit had'1st generation' meredith radiators, which probably offset radiator drag by about 50%, the Mustang (and Mossie to a slightly lesser extent) were 2nd generation, offsetting by 90% for a Mustang. Hawker claimed that one of their Tempest prototype versions actually got a net gain in thrust (forgot which one).

Most likely the Tempest I with LE radiators, or the Tempest VI with the annular radiator.

 

[wuzak]

The Spitfire III have had the fully enclosed undercarriage, both main and tail wheels, that alone would earn it 10 mph? The wing spand and area were also a bit reduced, and, correct me if I'm wrong, the prototype Mk.III carried no guns, or at least no cannon - all together another 10 mph worth?

The prototype had fully enclosed undercarriage doors, but problems with the doors meant they were taken off - with no appreciable change in performance.

EDIT: I see now that you included the retractable tail wheel, which would give an improvement. Also note that at the time the III was being developed normal Spitfires didn't have the cannon either.

 

[tomo pauk]

OldSkeptic is right to point to the thing that Spit III incorporated a redesigned radiator, or, actually two different designs, at least when going by Morgan Shackledy.
Another change was the internal BP glass (should earn a few MPH vs. external). The wing area and span were decreased via removal of the wing tip.

Re. armament carried - looks like the Spit III didn't carry any? M S are not clear what kind of performance was achieved with or without guns, at least I cannot pinpoint that in the book.

 

[Greyman]

according to British documents:

Vb wing = minus 6.75 mph (approx) - two cannon, small wing bulges
Vc wing = minus 10 mph (approx) - two cannon, two cannon stubs, large wing bulges

I'm not sure about projecting cannon ejector chutes, these might take off another 1.25 mph from each of the above.