Great explanation. ![Thumbs UP! :aok](http://bbs.hitechcreations.com/smf/Smileys/default/thumbsup.gif)
The early 51 D models had some stability issues because they had less side area due to the removal of the turtle deck. It was resolved with the addition of the dorsal fin.
![salute :salute](http://bbs.hitechcreations.com/smf/Smileys/default/salute.gif)
Sik
The 'reduced side area'due to installing the bubble canopy is a myth.
The RAF/R-R discovered the yaw instability immediately after Merlin installed in Mustang I. AL 963 was modified with both a dorsal fin,then increased chord fin/rudder combination. The issues and flight test results were reported to NAA in Feb 1943. NAA tested the dorsalfin o the P-61B-1 and NACA tested both he DF and a tall tail in fall 1943. InMarch 1944,both the DF for P-51B and P-51D were released to Service Group for kit production, but the P-51D production article was only released for P-51D-5 (both Inglewood and Dallas) at #651, and for last block P-51C-10-NT.
The additional benefit to streamlining flow during yaw, increasing effective fin/rudder area - was reduction of torsional loads on empennage in yaw due to prop vortex.