marine air wrote:
YOu guys have educated me on the differences in the fuselage of the A and the later D's. Was the fuselage enlarged and beefed up to support a larger radiator, or the fuselage tank, or a weight increase? Which fuselage design would be better for all out speed? Thanks
On the Allison-engined Mustangs, the lower longerons kinked upward a number of inches, from the location of the lower engine mount attach-points, allowing the wing to be mounted higher along the Z-axis, keeping as narrow of a side profile as possible. On these aircraft, the upper surface of the wing serves as the floor in the cockpit (just like in a P-40).
When the Merlin was introduced (P-51B/C's), the lower longerons were redesigned so that they carried mainly straight back from the lower engine mount attach-points, and then kinked upward aft of the wing/just before the radiator. This was so that the wing could be mounted lower, allowing for a clean line/airflow from the prop back over the bottom of the cowlings and wing through to the radiator intake. If the wing hadn't been mounted lower, the engine and carb intake trunk would have hung down below the bottom line of the wing (as seen in the early Merlin-engined prototype/test examples ahead of the production P-51B). The same lower longerons designed for the P-51B/C were used in the P-51D. Because the wing is mounted so much lower in the P-51B/C/D, a raised floor had to be added in the cockpit.
Here is a stock P-51A/A-36 fuselage (Yanks P-51A), where you can see the early lower longerons of the Allison-engine variants:

By comparison, here is a stock P-51B/C fuselage (CAF P-51C), where you can see the later lower longerons of the Merlin-engine variants:

This is the P-51B/C fuselage that was built in the very early stage of the P-51A project (discernible as a P-51B/C fuselage based on the line of the lower longerons):


And this is the P-51A fuselage built after deciding to go for a true P-51A (note the difference in the line of the lower longerons):

The lower longerons of the P-51D are identical to those of the P-51B/C:
