N.A. P-51 Mustang

[breadneck]

So for some very strange reason, i never really picked up on/registered this "variation" of the P-51 before. Willingly or unwillingly, IDK. While discussing P-47 types with a modelling colleague of mine, he mentioned the "ventral fillet" found on the P-47 N type and it being there to help stabilize the aircraft?! I did know that the P-47 N had this feature for whatever reason, but i never seem to have noticed this feature on the Mustang "pursuit" 51 type. So i`m all out a 1:72 scale nut, and i would like some input on what kits do/don`t have this ventral fillet. As i never noticed this before, i have no way of knowing what kit to look for. Some historical facts about this would be highly appreciated. And also if anyone care to enlighten me as to wheather or not the type designation were abbrevaitions for origins etc. or just plain letters (P-51 A,B,C,D...) over...

Edited February 17, 2013 by breadneck

[mawz]

P-51 designations are a bit of a rats nest, but can be summarized as follows:

Mustang Mk.1 - Original RAF production, Allison engine, 2x.50 in nose, 2.x50 + 4x.303 in wings, variable rad inlet. USAAF got a few as XP-51's

Mustang Mk.1A - RAF's improved version, no nose guns, 4x20mm wing guns. US P-51 and F-6A. Mk.1A's and F-6A's had cameras fitted

A-36 - USAAF's dive bomber version, only early/mid production Mustang not purchased by RAF (7 were operated though, 1 by test unit in UK, 6 by 1437 flight). 2x.50 nose guns, 4x.50 wing guns, dive brakes and bomb shackles, fixed rad inlet

P-51A - USAAF's first fighter Mustang, RAF designation Mk.II. Essentially an A-36 with the nose guns and dive brakes removed.

P-51B - First Merlin Mustang, Burbank built, retains P-51A armament. RAF's Mustang III, 4 blade Hamilton Standard Prop

P-51C - Dallas built P-51B, also Mk.III in RAF service

P-51D - low-back, 6 gun Mustang, built by Burbank and Dallas factories, Hamilton Standard prop. RAF Mk.IV

P-51K - Dallas built D with Aeroproducts prop, unofficially Mk.IVa in RAF service.

P-51H - 'Lightweight' Mustang, improved D, altered wing design and new fuselage, only production Mustang never operated by RAF.

So only the B/C are really origin-based designations, K sort of is as all K's were Dallas-built, but in fact designates the Aeroproduct prop, Dallas built ~1800 D's as well as the K's.

I'm not aware of a ventral fillet per se on the Mustang. The A-36 and P-51A did have something akin to one in front of the new radiator inlet, but that was dropped with the B/C as the radiator inlet was again reworked for the more demanding Merlin installation. But that was an airflow modification rather than for stability.

Edited February 17, 2013 by mawz

[breadneck]

I`m only quoting my friend on the "ventral fillet" i didn`t even know this word existed or for that matter what it is. What i am referring to is the contineous "ridge" slightly tapering downwards from the tail section and forwards (on the upper back of the fuselage) Seems to strengthen the tail section?! IDK..

THIS ONE HAS "IT" (the yellow section)

http://www.scalemates.com/products/product.php?id=142255

AND THIS ONE DOES NOT HAVE "IT"

http://www.scalemates.com/products/product.php?id=146811

Limey`s and their Oxford English...you just gotta love `em.

(i googled it and sure enough, ventral means underside and dorsal means topside. So basically something like a dorsal fillet???)

Edited February 17, 2013 by breadneck

[timc]

Breadneck,

To answer your questions: Any P-51D after the P-51D-5-NA would have the dorsal fin fillet. For complete accuracy, the fin fillet was introduced during the P-51-5-NA production run but to keep it simple, just use anything after the -5-NA. We won't get into the fact that there were two types of fin fillets for the P-51D's. Some P-51B's and C's had a dorsal fillet added but that was done, as far as I know, as a field modification but I may be wrong.

The P-47D-30 and -40 had the fin fillet added whether on the production line or in the field. The P-47M-1 also had the fin fillet added in the field. The P-47N had a different shaped fin fillet than the vast majority of the previous aircraft (I say vast majority becase in at least one instance, an "N" type fin fillet can be seen on a P-47M-1).

Your friend was right when he told you that the fin fillet was there for stability. When the respective manufacturers cut down the fuselage to accomodate the bubble top (P-51D and P-47D-25) a good chunk of fuselage was lost in the process. To regain the directional stablity that was lost with the cut down fuselage, they added the fin fillet. The fin fillet serves no other purpose.

Mawz is correct on the letter designations regarding the P-51's . Since when are people from Toronto, Ontario, Canada referred to as "Limey"? Down here in the lower 48, we call em Cannucks or just Canadians - eh. I think the term Limey is seen as less than endearing.

Edited February 17, 2013 by timc

[breadneck]

Breadneck,

To answer your questions: Any P-51D after the P-51D-5-NA would have the dorsal fin fillet. For complete accuracy, the fin fillet was introduced during the P-51-5-NA production run but to keep it simple, just use anything after the -5-NA. We won't get into the fact that there were two types of fin fillets for the P-51D's. Some P-51B's and C's had a dorsal fillet added but that was done, as far as I know, as a field modification but I may be wrong.

The P-47D-30 and -40 had the fin fillet added whether on the production line or in the field. The P-47M-1 also had the fin fillet added in the field. The P-47N had a different shaped fin fillet than the vast majority of the previous aircraft (I say vast majority becase in at least one instance, an "N" type fin fillet can be seen on a P-47M-1).

Your friend was right when he told you that the fin fillet was there for stability. When the respective manufacturers cut down the fuselage to accomodate the bubble top (P-51D and P-47D-25) a good chunk of fuselage was lost in the process. To regain the directional stablity that was lost with the cut down fuselage, they added the fin fillet. The fin fillet serves no other purpose.

Mawz is correct on the letter designations regarding the P-51's . Since when are people from Toronto, Ontario, Canada referred to as "Limey"? Down here in the lower 48, we call em Cannucks or just Canadians - eh. I think the term Limey is seen as less than endearing.

My friend i quoted earlier is the limey here (a brit) not mawz! Don`t think he`s even on this forum..I thought you guys would get that.

Back to the Mustang. Some very interesting facts on the dorsal fin fillet and why it appeared. Somehow i didn`t make that "connection" Seems perfectly logical to add something like that after removing the razorback/humpback/highback etc etc..

Gonna have a closer look at my P-51 D kits. Hopefully i have both variants. My limey friend did mention that the newer Academy P-51 D has the fin fillet separate from the rest of the fuselage. As i do not posess this particular kit, i have no way of knowing this for sure. Don`t know about the new Airfix kit either. Will have to check that out. This sure did open a new dimension for my interest in the P-51 Mustang :D

Also got the Tamigawa covered so there`s just bound to be a winner somewhere in there..out.

Edited February 17, 2013 by breadneck

[timc]

My friend i quoted earlier is the limey here (a brit) not mawz!

Ah, my error. By reading your text, it seemed as though you were applying that word to mawz. It's tough to convey intent and meaning in a typewritten response. No worries!

[breadneck]

So basically the spineless P-51 D`s weren`t excactly abundant? Or were they all retrofitted with the dorsal fin fillets? Am i making any sense at all here?

[feddawg]

Most P-51D's will have the fillet. Perhaps the most notable aircraft without the fillet is George Preddy's Cripes a mighty 3rd. Best advice is to work from a photo or serial number. Most 72nd scale kits are molded with the fillet but it is fairly easy to remove, Obscuro makes a resin conversion for the Tamiya kit.

Edited February 18, 2013 by feddawg

[agboak]

I think Passion Waggon lacked the fillet as well. Personally I find that one more notable than Preddy's. Or do I mean noticeable?

The new Airfix kit has the fillet moulded integrally with the fuselage. It is no great trouble to remove in 1/72.

[breadneck]

Well obscuro i didn`t see too much of online. Seems pretty obscure. So some D types were infact not retrofitted with this feature? Maybe they didn`t really need them..I am not seeing the full connection between the P-51 B/C and the P-51 D. Did they phase out the early B/C types and retrofit these to be D types with the fin fillet or what? Confused..

..i kinda llike the one without the fillet though. It looks more sleak ;-)

[agboak]

The B/C were phased out when worn out (or possibly just when due for major overhaul) but not converted to Ds - that would have required a complete rebuild of the rear fuselage and it simply wasn't worth it. There were those pilots who preferred the earlier variant because it was slightly faster and performed better at higher altitudes. The D had a different version of the engine with a slightly lower full throttle height.

Not all of the early Ds were fitted with the fillet. I wouldn't care to quote a proportion. However, the photographs of those in sevice without fillets may simply have been taken before they were modified.

[Jennings]

Only part of the first block of P-51Ds (the D-5 block) lacked any fillet at all. And the majority of those had the fillet retrofitted in the field. All late D-5s and later blocks had the fillet fitted on the production line. There were two major and several minor variations of the D/K fin fillet. The initial type, and the type often retrofitted is sometimes referred to as the "swayback" fillet because its upper line is slightly curved. The later type was perfectly straight. Note on this drawing the subtle differences in the panel lines. Once you notice the difference it's pretty easy to spot in photos.

[feddawg]

North American produced about 1000 P-51D-5's between Inglewood and Dallas. As Jennings said the change occurred during the D-5 production run so a number less than 1000 were delivered without the fillet.

Here is a link to the Obscuro site:

http://www.obscureco.com/products.html

Chris does nice stuff but it is a minor mod, I'd cut and sand it myself.

A couple of other notable P-51D-5's, Frenesi and Lou IV.

[anotherP51nut]

Your friend was right when he told you that the fin fillet was there for stability. When the respective manufacturers cut down the fuselage to accomodate the bubble top (P-51D and P-47D-25) a good chunk of fuselage was lost in the process. To regain the directional stablity that was lost with the cut down fuselage, they added the fin fillet. The fin fillet serves no other purpose.

This is one of the more enduring myths concerning the introduction of the bubble canopy on the Mustang and the "need" to add a dorsal fin fillet to compensate for the cut-down fuselage. The fillet was indeed added for stability reasons, but not for anything to do with a "loss of keel area" from earlier models...in fact the Technical Orders also intended that the dorsal fillet was to be added to B/C models. There is a long discussion about this on the P-51 Special Interest Group website. Basically it was determined that in order to improve the flying characteristics of the horizontal stabilizers ( there were problems with "inadvertent' snap rolls in certain flight regimes )the dorsal fillet was to be added to P-51B, C and D aircraft (the K hadn't yet gone into production at the time of the document) to improve directional stability. The last 400 C models had the fillet added in production, and the fillet was also added as a field modification to some Bs and Cs. Certainly in the case of the RAF the fillets were added very late 1944/early 1945.

To add to the confusion, in addition ot the two fillets Jennings has illustrated, the version for the B/C was slightly longer and shallower.

Richard

Edited February 18, 2013 by anotherP51nut

[rav_pl]

I think Passion Waggon lacked the fillet as well.

Yes, but later she got the fillet.

The new Airfix kit has the fillet moulded integrally with the fuselage. It is no great trouble to remove in 1/72.

No problem at all. I did it about week ago on my "Passion Wagon" (soon now in the gallery). The Airfix fillet is full of plastic, so you can remove it without making a hole. I cut the fillet before I joined fuselage halves (for easier compare to drawings), but you can also do it afterwards. Then you just need to create correct shape (I did it with sand paper), fill some panel lines on the fin and create two new lines on each side.

Edited March 20, 2013 by rav_pl

[agboak]

It may be true that the fillet was discussed earlier than the D variant, but it is no myth that the bubble canopy would have reduced the directional stability. If there had been reason to fault the B/C then it would have been more important on the D, which is perhaps why the D received priority when it came to refits and the fillet is comparatively uncommon on the B/C.

[mawz]

It may be true that the fillet was discussed earlier than the D variant, but it is no myth that the bubble canopy would have reduced the directional stability. If there had been reason to fault the B/C then it would have been more important on the D, which is perhaps why the D received priority when it came to refits and the fillet is comparatively uncommon on the B/C.

Exactly, the fillet compensated for both the elevator issue on the B/C/D/K and the reduced keel on the D/K as well as reducing the problems associated with the rear fuselage fuel cell. Conversion of high-back designs to a bubble canopy caused directional stability issues every time. Both the P-47D and the Spitfire had issues with this conversion, it was one of the reasons for the adoption of the Spiteful tail on Griffon Spitfires (admittedly the Spitfire had a whole host of reasons for needing a larger tail and rudder).

The Mustang was particularly sensitive to this due to the stability issues associated with the fuel cell behind the cockpit on the Merlin Mustangs, which also caused directional stability issues when in use. Of course the reduction in long-range missions allowed that cell to go unused, I suspect that's why we didn't see more B/C's with the upgrade since it would be a lesser issue when not using that fuel tank and they were not usually needing the extra range by the point that conversions became available.

Edited February 18, 2013 by mawz

[breadneck]

Well that sure settled that

[tourist]

Richard (anotherP51nut) gave the correct explanation for the dorsal fin fillet on Mustangs.

The first P-51D-5's were retrofitted with the DFF but early production blocks were replaced fairly quickly with later ones so the D-5's didn't stay very long in service.

[anotherP51nut]

Thanks Christian for chiming in. I might have overstated the case, but what I was trying to say was that the dorsal fin fillet was the solution to an instability problem that was already identified in the P-51B/Mustang III by both the British and the Americans, and not a result of the loss of surface area due to the cut-down rear fuselage of the D. I'm not saying that the aerodynamic effect of that loss of keel area was a myth, or that the fillet would not have helped to compensate. I was addressing the misconception that the dorsal fin was the response to the redesign of the rear fuselage for the bubble canopy.

The tech order which required the addition of the dorsal fin fillet to the early P-51D-5-NA, also ordered that the fillet was required to be added to the P-51B/C's. The reasons governing this decision were exactly the same: to prevent yawing conditions, at high speed, which were known to cause catastrophic failure of the horizontal stabilizer on all Merlin-powered Mustangs then in service. In addition to the fin fillet, the rudder trim tab linkage was required to be changed to an anti-boost tab. Basically this was to prevent the pilot from overcontrolling the aircraft, loading the tail.

There were a number of causes for this instability in the Merlin Mustang; the increase in power of the Merlin, the four-bladed prop, and possibly the fuselage fuel tank ( although issues caused by the tank were addressed more by the introduction of metal elevators and an inertia bob weight). This is the report from the Royal Aircraft Establishment:

"The introduction of a more powerful engine and a four-bladed propeller to the Mustang aircraft (the resulting type being known as the Mustang III or P-51B) gave rise to serious changes in directional trim and stability as compared with the original Mk. I or P51-A; in particular the forces to produce sideslip were reduced to very small values especially at high speed, where dangerously large angles of sideslip could be induced for small pedal forces. Several accidents involving structural failure of Mk.III tailplanes were attributed to the asymmetric tailplane loading when the aircraft had been inadvertently yawed due to these small pedal forces. Consequently N.A.A. introduced a retrospective modification consisting of a small dorsal fin and geared antibalance tab on the rudder. Flight tests of this installation were undertaken both in America, by N.A.A., and in this country, by R.A.E."

The fact that it may have also helped to compensate for the loss of surface area of the D compared to the B/C was a bonus, not the reason for it's introduction.

I know you already knew all this Christian, but thought I would just try to explain myself here more clearly.

Thanks,

Richard

Edited February 21, 2013 by anotherP51nut

[anotherP51nut]

Of course the reduction in long-range missions allowed that cell to go unused, I suspect that's why we didn't see more B/C's with the upgrade since it would be a lesser issue when not using that fuel tank and they were not usually needing the extra range by the point that conversions became available.

This isn't supported by the evidence. If you look at 8th Air Force targets for the last six months of the war the destinations were still very deep into enemy territory, especially when they went after oil and transport: Berlin, Jena, Nuremburg, Zwickau, Salzburg, Prague, Ulm, Augsburg, Cottbus, Dresden, Chemnitz, Swinemunde and Stettin (the last two in what is now Poland), to name just a few. A couple of the 8th Fighter Groups moved to the continent temporarily in late 1944, but the rest still flew from England which didn't move any closer to the continent ;) ( although I'm sure there were times when aircrew wished it had! )

I suspect that it was more a case of the American Bs and Cs being replaced by Ds and Ks through the last half of 1944 as they came off the assembly lines in large numbers. No point in putting the hours into converting the tails of older aircraft when the fighter groups were being re-equipped with brand new Ds and Ks which were readily available. It was a different case with the RAF which kept the Mustang III in service much longer than the Americans, past the end of the war in fact, and you will see the fin fillet on these more often.

Richard

Edited February 20, 2013 by anotherP51nut

[agboak]

Thanks for the full description of the problems leading to the fin fillet, I appreciate it.

It might be interesting to note that some fighters did not seem to have problems attributable to cutting down the rear fuselage: Typhoon and the Yak series come to mind. The Lavochkins had a very deep rear fuselage anyway. The Ki.61/100 may not have been around long enough for any problems to be determined, and "needs must" anyway. The P-40Q had too many other changes, perhaps. Although the Spitfire XX-series did indeed grow a larger fin and tailplane, the bubble canopy on late Mk.IXs/XVIs or even Mk.XIVs do not seem to have lead to any specific redesign, unless you think that the specific change in the fin/rudder on the Mk.XVIII is credited to that rather than a more general improvement. Either way, nothing was done to the Merlin variants, and the initial trials go back far enough to have identified any problems. The aft tank(s) however did lead to considerable complaints over the loss of stability.

[dogsbody]

The Academy 1/72 F-51D has a separate two-piece fillet as does the Heller P-51D. There may be others but this is all I have info on.

Chris

[anotherP51nut]

Thanks for the full description of the problems leading to the fin fillet, I appreciate it.

It might be interesting to note that some fighters did not seem to have problems attributable to cutting down the rear fuselage: Typhoon and the Yak series come to mind. The Lavochkins had a very deep rear fuselage anyway. The Ki.61/100 may not have been around long enough for any problems to be determined, and "needs must" anyway. The P-40Q had too many other changes, perhaps. Although the Spitfire XX-series did indeed grow a larger fin and tailplane, the bubble canopy on late Mk.IXs/XVIs or even Mk.XIVs do not seem to have lead to any specific redesign, unless you think that the specific change in the fin/rudder on the Mk.XVIII is credited to that rather than a more general improvement. Either way, nothing was done to the Merlin variants, and the initial trials go back far enough to have identified any problems. The aft tank(s) however did lead to considerable complaints over the loss of stability.

You're welcome Graham, it was probably a bit of a fine distinction I was trying to make. I always respect your input. The fin fillet likely did help to compensate for the loss of surface area, but that wasn't the problem it was designed and introduced to cure. I think the myth grew out of the timing...the fillet appeared very early in the production of the D so it would look like it was a response to the cut-down fuselage compared to the B/C. The fact that relatively few USAAF Bs and Cs had them would add to the impression it was only a problem on the D/Ks. Also, if I understand correctly, the handling problems caused by the fuselage tank were addressed more by the introduction of metal elevators and inertia bob weights ( as well as restrictions to the flying limits in the pilot's notes ) rather than the fillet. None of this helps me to build better models but the research is as much fun as building for me. The Mustang is something I am drawn to in particular...sort of like the Spitfire and Halifax for you I imagine.

Interesting comparison points you raise too, especially regarding the Spitfire and the changes it went through. I have the impression, which could be wrong, that the enormous vertical tail and rudder of the 22/24 and the Seafire 47 were more to counter the enormous increases in power of the Griffon than anything to do with the cut-down rear fuselage? I don't know the Spitfire as well but what always astonishes me was its capacity to be developed and adapt while still retaining a certain "Spitfire-ness" (!?). One of my plans (or dreams more like) would be to have a display with a 1/32 early Spitfire I with two-bladed prop, flat canopy, etc, beside a fully loaded Seafire 47...sort of a first and last sort of thing. What a beast that last Seafire was!!

Richard

Edited February 20, 2013 by anotherP51nut

[Barneydhc82]

To set the record straight eh, we spell CANUCK with one "N" eh!

Them durn Southern folk never culd spell.

Barney