Cool pix!

I chose my words carefully ‘aircraft type’ not aircraft...
Rare in terms of its use of an inline engine...
In terms of aircraft allied pilots encountered, it was relatively rare...

 

Yes, the Army Type 3 Fighter, aka Ki-61, 3,078 were produced. While not necessarily common, it was not rare either. As you said, aircraft type.

Yes, it was the only mass produced IJA fighter to use an inline engine. While in lines were experimented with, they proved to be increasingly temperamental in operation.

 

Nots "S" has had the field modification on its tail...the 51s tail was found to be too thin and weak and prone to breakage...a strengthening hack was created and eventually applied to all P-51s.

 

Swiping that one to my hard drive.

 

The dorsal fin modification was to correct yaw instability caused by airflow over the bubble canopy, not a strength issue, and was a factory mod

 

"Adding the dorsal fin solved the problem and factory production P-51-Ds obtained it starting with P-51d-10 series, while a field conversion kit was distributed to field units already using P-51D-5s"
They would take one out of the flight at a time, convert it while the rest flew their missions...that one would go back in and another chosen for the field conversion.

"caused problems with longitudinal stability due to reduced rear side surface area of the fuselage"

Was too thin.

Thankyou.

PS I also remember reading reports that the tail section could crease or tear during hard dog fighting (particularly in the dive) - Whilst I don't remember any aircraft being brought down by this, the aircraft nonetheless needed to be pulled out and patched/repaired to strengthen the area where the fin met the fuselage.
That would be a strength/design fault. The bar/spar installed effectively held the tail solid to the body of the aircraft.

 

Yes...It was for directional(longitudinal) stability, and was factory installed beginning with P-51D-10-NA (44-14254) onwards. "E2 S" (44-13926) is a P-51D-5-NA and is fitted with a field modification kit.

"Thin" is a matter of perspective. To me, it follows the length of the fuselage, so it is adding length not width. Further, airflow would be from the front and this addition is to the sides.