Fin fillet on P-47 and P-51

[MiloMorai]

Why were these added when the fuselage was cut down for the bubble canopy?

[colmbo]

There was reduced yaw stability with the reduction in side area after the removal of the turtle deck.

[Guppy35]

Also retrofitted to many 51B/C as well

[streakeagle]

Early P-51Ds did not have it. In the case of the P-51, it always had a slight instability in the yaw axis for gunnery purposes. The fin extension just happened to be implemented after production of the P-51D had started, but was already needed for the earlier variants. So it is not true that it was solely added due to the cut-down deck for the bubble-canopy. Of course, the lost side area probably made the problem a bit worse, helping encourage North American to develop a solution. I would be interested in hearing first hand accounts from pilots who flew the various combinations of P-51B/C/D with and without the fin extension to see how much this really affected regular and combat flight as well as see if the B/C with the fin extension was even more stable than the D with the fin extension (i.e. how much of a difference did the lost side area really make with the bubble canopy installation).

[MiloMorai]

Thanks guys. Thought it was for yaw.

Did the razor back P-47s have the same problem as the P-51B/Cs?

[streakeagle]

I can't answer that one. I haven't read and re-read the P-47 history the way I have studied Mustangs. But surely it was to correct a yaw stability problem, and if no razorbacks ever got the extension, that would make it likely that it was added solely to compensate for the lost side area.

[streakeagle]

Here is a quote for the internet:

The early "bubble-canopy" Thunderbolts had suffered from some directional instability as a result of the loss
of aft keel area.  From the P-47D-30-RE production lots onward, a dorsal fin was fitted just ahead of the
rudder. This innovation successfully restored the stability.

I don't have enough memorized about the P-47 to judge its accuracy, but the web page with this quote looked really nice and informative:
http://www.368thfightergroup.com/P-47-2.html

[drgondog]

The inherent yaw instability when throttle was applied or reduced first became very noticeable after Merlin 61/four blade prop was installed on the Mustang X. The primary issue was the strengthened Vortex (and resultant lateral forces on Fin/Rudder and upwash on the left Horiz stab). It was discussed in detail with NAA reps in February 1943, post mods on AL 963 for which a DFF was added as well as lengthening chord of Fin/Rudder to increase area to compensate for the added torque/prop vortex of the Merlin 'System'.

It was installed on 43-12105 as well as 44-13257 (IIRC) to test as the production 104-25001 and 109-25001 was released for production in late March, 1944. Kits were in the field for P-51B/K in late April along with Reverse Boost Rudder tab system.

The elevator was upgraded along with DFF - which reduced empennage failures but only lessened the Yaw issue - not 'solve'. The closest modification to solution was the DFF combined with fin cap combined with extending the P-51H fuselage 13 inches and cutting 85 gallon fuse tank to 55 gallon.

The YAW issue had ZERO effect due to the bubble canopy, but there is evidence that the ballooning effect on elevator experienced during terminal dive tests of P-51B-1-NA 43-12102 and P-51D-NA 42-106539 during November 1943 led to the reduced ange of incidence of the H.Stab and introduction of the metal elevator kit.

[Vraciu]

D Mustangs without the fillet could snap roll violently within a certain speed/control input range and shed the tail.  The B was less susceptible to this.   There is some documentation on it out there that explains it more thoroughly. 

[Vraciu]

"Unless a dorsal fin is installed on the P-51B, P-51C and P-51D airplanes, a snap roll may result when attempting a slow roll. The horizontal stabilizer will not withstand the effects of a snap roll. To prevent recurrence, the stabilizer should be reinforced in accordance with T.O. 01-60J-18 dated 8 April 1944 and a dorsal fin should be installed. Dorsal fin kits are being made available to overseas activities"



----

A crash report example:

"Sections II and III of T.O. 01-60J-18 had not been accomplished. The stabilizer was approximately 20 percent below the strength of a completely reinforced stabilizer. It is believed that this type of failure will be completely eliminated after compliance with T.O 01-60J-18 and the installation of a Dorsal Fin and reverse rudder post tab."

A Supplement to the Basic Technical Order states:

"1. Due to horizontal stabilizer failures which are believed to have resulted form slow rolls, all P-51B, P-51C and P-51D airplanes will not perform slow rolls pending the installation of dorsal fin and rudder reverse trim tab, and compliance with T.O. No. 01-60J-18."

[Chalenge]

When a P-51B/C/D with dorsal fin in a slip configuration is made to stall the aircraft shows little yawing tendency that causes a skidding stall to develop into a spin. The same is not true in the absence of the dorsal fin.

[streakeagle]

Great information on the T.O. and fin kit. So many sources so wrong in blaming the cut down rear deck for the change.

[Puma44]

Interesting trivia on the D model.  The vertical stab/fillet is canted slightly to the left to aid torque/P factor.  If memory serves me correctly it is 2 or 3 degrees.  Not a dead on rear view but, the offset can be somewhat seen.

[colmbo]

Interesting trivia on the D model.  The vertical stab/fillet is canted slightly to the left to aid torque/P factor.  If memory serves me correctly it is 2 or 3 degrees.  Not a dead on rear view but, the offset can be somewhat seen.

As is the vertical stab/fillet on a Cessna 172.  IIRC the Corsair does not have the fin offset which is part of the reason for the big rudder deflection on takeoff.

[Vraciu]

And the 109 and Buchon have it despite opposite rotation on their respective engines/propellers.   YIKES.