Warbird Information Exchange

Saville is quite correct. There is no historical basis for the Dorsal Fin Fillet fitted to P-51D "because of loss of side area" with the new canopy arrangment. As he has shown, ALL Merlin Mustangs were required to have them retrofitted at the same time and for exactly the same reason. Also, new aircraft were being fitted in production.

AFAIK, P-51D-5-NA had them fitted late in production and are the only flillet-less D variants to appear. ALL subsequent D blocks at both Inglewood and Dallas had the Fillet added as a production item. According to note appearing on DFF drawing (P-51C) last 400 C's had them installed at factory. No Mustang of Dallas D/K variants were produced without the DFF.

DFF apparently came in two D styles, from both factoies. Early "swaybacked" (modern term), so called because of is curved long top edge, was the type and subject of the retrofit and first production type. The later D DFF is striaght on its long top edge. Both have the same part number.

The DFF as retreofitted to P-51B and C was different than either, as it did resemble the earlier D type, but extended well forward of the fuselage transport joint. Both D types began aft of the joint.

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Charles Neely

To illustrate the B/C portion of the of conversation



Shay
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Semper Fortis

Good view Shay. Thanks.

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Charles Neely

While we're on the subject, here are some close up shots of the recreated B/C model fin fillet addition, on Impatient Virgin. The photos show how the fin extends past the joint, as mentioned, and how it was mounted. In the first shot you can actually see the difference between the stock D model fillet and the field mod fillet for the B/C models.

Here is one without.

The gentleman who I spoke to (who said he had restored the aircraft) said that this particular fillet was a genuine North American part.

Rich

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Rich Kolasa
www.crystalgraphix.com

Princess Elizabeth has the nutplates still installed for the fillet but they were filled in.
P-51s had a problem with the horizontal failing during certain maneuvers.
There are a few TOs that address fixes that were incorporated to improve these faults.
One problem was the rudder forces were light and it was easy to over control with the rudder. They changed the rudder trim tab control linkage so that the tab would travel faster than the rudder and in the same direction as the rudder. This acts to increase the pedal force needed to push the rudder as the rudder moves. This limits excessive rudder inputs and the forces on the tail.
Another TO added the Fillet and there are a number of reinforcements added to ribs where the attach points are for the rudder and elevator. Later model horizontals have outboard ribs made from .063 alum where earlier had .040 ribs. The fillet is a fairing and doesn't add any strength just surface area.
Rich

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Rich Palmer

Remember an Injured Youth
benstear.org
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51fixer wrote:"The fillet is a fairing and doesn't add any strength just surface area."

Interesting assumption, and I don't know for sure. However, there is a bit of a controversy regarding weather the DFF did or didn't "add strength".

At least one pilot's manual says specifically that it did. I will be looking at a large cashe of war time Mustang engineering data in the very near future with the hope of finding a specific answer.

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Charles Neely

My dad talked about this change coming in to the design while at NAA Dallas. He was in the lofting department when the change first came down and they had many a meeting about weather to make it swayback or straight. The swayback was put into the add on fillet because it added a bit of strength to the design. The production, straight version, was engineered in to the airframe so that it needed no sway.

He recalled the change being introduced because of the need to strengthen the tail to prevent the horizontal from failing. Whether there were actual failures or not I don't know.

Just my 2 bits...


Here's a pic out of the company magazine showing a fillet on an Allison powered wind tunnel model...

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Zane Adams
There I was at 20,000 ft, upside down and out of ammunition.
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Is the mod necessary for non combat operations today?

So is Audrey the only one?

The addition of the various DFFs is all very interesting because there seems to have been quite a lot of NAA experimentation early on (as the model shows above and it is not the only one). For whatever reason, and despite comments and reccomendations from the British to add more vertical area, NAA saw no reason to incorporate the addition until the structural problems became manifest in early 1944. Long held popular perceptions simply do not hold up when compared to what was going on at the time, nor to documented facts (the T.O's) which required the DFF addition.

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Charles Neely

Is there any evidence that shows the DFF field modification was carried out on the P-51A's and A-36's still in service throughout the war?

Shay
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Semper Fortis

It is very simple to determine if the dorasl fin added strenght is how it is attached. If the fin is attached with structural screw, rivits, or bolts, it is structural. If it attached with machine or sheet metal screws, it is aerodynamic.


Princess is not a good example to use as it has a B tail, but was built using D fuselage parts, and unless you asked Pete Regina, you have no idea what was B or D model. The B model that the tail came from did not have a dorsal fin

That airframe has been significantly rebuilt twice since it left Regina's hands.

My understanding is that all of the "D model" has been built out of it during the course of those subsequent trips to the shop.

It is a good bet that the skins there are D model, other wise, someone went thru alot of work to make new skins, rivit them in place, then install a butt load of nut plates that were not going to be used, then have to go back and plug them, because they were not going to be used.