# Question Regarding Dorsal-Root Extensions



## Zipper730 (Sep 19, 2020)

Looking at the designs of aircraft that have extensions to the leading-edge root of the tailfins such as the P-51D, and P-47N. While it appears both aircraft had them to provide additional side-area to deal with the rear-fuselage cut-back to make room for a bubble-canopy, it also seemed that, for the P-51D, that it was to deal with fin-stalls.

I'm curious if the reasons they dealt with fin-stall was to use a highly swept-structure to...

To create a vortex that would be directed up and across the leading edge of the surface to remove low energy air and swap in high energy air
To create a larger up-wash in front of the surface, making stalls occur more gradually
Or some mix of both? If the former is the case, how come people didn't realize they could put that on aircraft and produce designs that had enormously increased AoA over existing designs?



 Airframes
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 drgondog
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 FLYBOYJ
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S
 Shortround6
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W
 wuzak


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## Mustangtmg (Sep 24, 2020)

*I'm no aerodynamicist or aeronautical/aero-space engineer, but those guys who ARE, state, that in the case of the P-51 the loss of the rear "razorback" (or "built-up") fuselage had little or NOTHING to do with a need for the Dorsal Fin, as it was called in the 1940s. It's since been called the "Dorsal Fin Fillet" because it's a fillet and not a structural part.*

*Consider this...the P-51B and the P-51C were having the same issues with their empennage (they either twisted and deformed or came OFF completely!) of the"early" P-51Ds (we're talking "before the addition of the DFF late in the P-51D-5-NA batch - S/N 44-13903 was the first production P-51D with a DFF).*

*The problem was traced to essentially two things that were brought on by the "upgrade" of the Mustang from the Allison V1710 to the Packard Merlin V-1650-3 and later the V1650-7, coupled with the larger diameter 4 blade Hamilton Standard Cuffed Prop that turned the HP into thrust. The power of the Merlin and the increased "propwash" from that bigger prop with another blade was having an impact on the empennage. The DFF, along with some other "fixes" mandated by USAAF Technical Orders (TOs) pretty much DID fix the situation. There was a T.O. issued for P-51Bs and P-51Cs (by serial number ranges) that mandated the installation of NAA-provided DFF install kits, along with the hardware to change the rudder trim tab to a REVERSE rudder tab (it made it HARDER to yaw the plane when the rudder pedals were pushed). The T.O.s gave specific instructions on how to do the mods to existing aircraft. Other "fixes" were mandated for all B/C/D/K Models and came in a separate T.O. Note that the LAST 400 P-51Cs to roll out the back door at NAA Dallas were built with DFFs and the reverse rudder tab so, their serial numbers do NOT appear on the T.O.s.*

*OK...sorry this is long, but rest assured, "cutting down the rear fuselage" wasn't the cause for a need for the DFF-install.*

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## pbehn (Sep 24, 2020)

In addition to cutting down fuselages they also started fitting rear fuselage tanks and long range external tanks.


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## Mustangtmg (Sep 24, 2020)

I wrote an article on the DFF that was going to be in _*Mustangs International Magazine *_5 yrs ago, but Michael O'Leary, after having me edit it and re-edit it for about 6 months, decided to not use it.

In retrospect, I've learned SOOO much about not just the DFF, but the whole Mustang since then. I helped James William Marshall every-so-little with his new book "P-51B North American's Bastard Stepchild That Saved the 8th Air Force." I've read all except for the heavy-duty engineering stuff (I'm a retired pharmacist and an ex-GA pilot but NOT an engineer like Bill!) going back to manuscript copies over a year ago.

Back to your point about the "rear fuel tank," the 85 gal fuselage tank began being installed in the P-51Bs and Cs at their respective factories rather early in the War, and in the P-51Ds in both factories from Day One. When that tank was full, it put the center of gravity too far to the rear of the "envelope" and made flying it very "squirrely" until they burned off about 60 gal of it on takeoff and climbout. I am "in the corner" with the guys who say that the 85 gal fuel tank, when full, NEEDED a DFF for the plane to stay stable.


I love everything about the Mustang family ... all 15,000 of them!

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## pbehn (Sep 24, 2020)

Mustangtmg said:


> I wrote an article on the DFF that was going to be in _*Mustangs International Magazine *_5 yrs ago, but Michael O'Leary, after having me edit it and re-edit it for about 6 months, decided to not use it.
> 
> In retrospect, I've learned SOOO much about not just the DFF, but the whole Mustang since then. I helped James William Marshall every-so-little with his new book "P-51B North American's Bastard Stepchild That Saved the 8th Air Force." I've read all except for the heavy-duty engineering stuff (I'm a retired pharmacist and an ex-GA pilot but NOT an engineer like Bill!) going back to manuscript copies over a year ago.
> 
> ...


My point was that cutting down the rear fuselage wasn't the only modification, neither was putting in a Merlin. The P-51B/C also had DFF fitted and they didn't have the fuselage cut back but did have internal and external tanks fitted as well as that Merlin "thang". In keeping with forum humour, it was probably all the fault of a Malcolm hood or a relation to temperamental British gunsight.


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## swampyankee (Sep 24, 2020)

Otto C. Koppen, the designer of the Helio Stallion, among other aircraft, was (reportedly) quoted as saying that no aircraft every built had too much vertical fin area.

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## pbehn (Sep 24, 2020)

swampyankee said:


> Otto C. Koppen, the designer of the Helio Stallion, among other aircraft, was (reportedly) quoted as saying that no aircraft every built had too much vertical fin area.


By that do you mean "It's complicated"?


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## Zipper730 (Oct 29, 2020)

Mustangtmg said:


> *I'm no aerodynamicist or aeronautical/aero-space engineer, but those guys who ARE, state, that in the case of the P-51 the loss of the rear "razorback" (or "built-up") fuselage had little or NOTHING to do with a need for the Dorsal Fin, as it was called in the 1940s. It's since been called the "Dorsal Fin Fillet" because it's a fillet and not a structural part.*
> 
> *Consider this...the P-51B and the P-51C were having the same issues with their empennage (they either twisted and deformed or came OFF completely!) of the"early" P-51Ds (we're talking "before the addition of the DFF late in the P-51D-5-NA batch - S/N 44-13903 was the first production P-51D with a DFF).*


So, the problem was predominantly related to the propeller design? That said, with the fillets often added to P-51B/C's, it would appear that they were smaller in size than the P-51D's, which indicates that the cutback made things worse...

With all that said: The design of the dorsal-root fillet produce a vortex?


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