Originally Posted by packardpursuit

Finally, questions: Do you think the DFF was added primarily for aerodynamic effect, or was it structural? Was it simply added fin area or was it an external gusset?

With my 38+ years of being an aero/mechanical engineer for a major US aerospace firm, it does seem like there is a case for both reasons, though did one follow the other?

Take the case of the DFF on the B/C. I do not remember reading anywhere that the aircraft lacked in directional stability even with the increase in HP and the change from the 3 blade to the 4 blade prop. A turning prop is a destabilizing yawing force. Though the diameter did not change much between the Allison/3 blade and the Merlin 4 blade, the added blade should have reduced the directional stability. Maybe it was so slight, it was not noticed by the pilots. Therefore, the DFF seems to be structural in this instance.

Now.... Since early model "D's" did NOT have the DFF and IF there was a structural deficiency in the B/C, that would probably only been worse in the cut down rear deck of the bubble D. Is this a case of one factory not talking to the other? paper trail problem? or do all the accounts of pilots in the early D models that complained about the tail "wagging" on long flights to Germany and back force the issue to put the DFF on at the field? If the DFF was truly structural, field mods were probably not sufficient to add any real "strength" to the rear fuselage. Any vertical area aft of the canopy would surely bring back some of the directional stability lost from the cut-down rear deck, not to mention any flow distortion wake effects from the bubble.

Just my thoughts.

Tom

I need to thank The Pipe for making me re-examine the issue presented by the addition of the 85 gallon fuselage fuel tank. I have not previously seen it as a single entity, as a cause for addition of DFF, but rather as part of a whole structural issue associated with horizontal tail.

At this point, it seem that tail failures occurred before the addition of fuel tank, in British based P-51B's/ Mustang III's. So I don't think it is just a tank problem. On the other hand, common sense would indicate that it didn't or couldn't help! Will have to think on this some more. Dates are important and will have to do more research. BTW there is a list of all known Mustang related T.O's found over on the P-51 SIG sight, referenced earlier .

I think most people are of the belief that the DFF is just a minor sheet metal shell, to augment fin area. However, the internal structure makes it rather more than a shell. Its central beam structure is attached with bolts, to two fuselage frames, plus the skin attachment with screws. As for area itself, note the P-51D is noticelbly larger than the B/C version. Latter is lower and yet, also extends way farther forward . One could argue that B/C need less area because of the higher spine fuselage, but why does it jut forward of Fuse. Sta 248, while D stops short?

We can talk about that pesky "rearward CG" problem next time. More QUESTIONS!!!

Dear packardpursuit:

The PIPE Here again...sadly, over last weekend, I found out that my #1 CAD mentor, Howie Cohen, had passed on earlier this month of November.

But with "getting to the thank-you" for the reason for the dorsal fin strake on P-51s fitted with the extra 85 gallon/322 liter capacity fuselage fuel tank, I'll easily admit that NO ONE wants to have to deal with a TAIL-heavy aircraft of any size, at any time...not aeromodelers, nor full-scale pilots...and for full-scale aircraft test pilots, when one is going to be IN what they find out to be a tail-heavy aircraft, there's ALWAYS going to be that extra measure of caution!

It's always best to "balance twice before flying ONCE" with such matters...as if no balance checks ever get done before a maiden flight, you'd REALLY likely to be flying ONLY once with that sort of airframe... !!

Yours Sincerely,

The PIPE....!!

I thought this post on the SIG might be relevant to the DFF discusion.

http://p51sig.com/viewtopic.php?f=17...tart=30#p12253

Thought I would include a few pictures of our TF dorsal while on the topic, as it happens to be removed at the moment. You can see the internal rib structure, and profile shape.

Mike

Thanks mligosh

These pictures suggest that the DFF was NOT a structural enhancement as its construction (and mounting with plate nuts) is not typical load bearing design.

a quote from a gentlemen (who apparently owns a Mustang) I found on this website:
http://www.mustangsmustangs.com/theh...hp?topic=388.0

The dorsal fin on my -25 Mustang is made up of soft aluminium. There is relatively no structural strength increase with the installation of the DFF. Also, in re-reading the USAAF text a few times (I need to!), there is no description that states the vertical fin alone was subject to failure. The failures repeatedly described in the text, are the horizontal stab. AND vertical stab. failures. These are concluded to have been caused by certain aircraft maneuvering, not by basic aircraft instability. In re-reading the text, the solution seems to be a combination of increased rivet sizes, the installation of the DFF and the reverse rudder boost trim tab.
I may need to re-read this to verify my interpretation! However, the DFF is not a structural piece on the airplane. It is too flimsy for any real loading enhancement.
(I had the pleasure flying a very unstable Mustang last summer - one WITH the DFF installed. During high speed passes along a runway, the Mustang would visibly yaw during gentle banking flybys. Ground obsevers could easily see this flight attitude of the aircraft. The cause was due to the installation of the 100 gallon teardrop external fuel tanks. Once removed, it flew like an airplane again.)

From further reading on the subject, it is entirely possible that the addition of this non-structural "fix" kept the aircraft from getting into the flight regime that could cause the structural failure. Sort of like fences on wings. They themselves are not "structural", but make the wing stall more predictably.

Tom

This is a great discussion guys! I personally feel DFF is meant to be more structural than usually given cred tfor and two thers feel differently. It's all OK. Historically speaking there is some indication that the rate of tail failures in combat remained unchanged, so after all the effort expended, was the fix needed? Could it have been a show for pilot's peace of mind?

It has been reported that squadron CO's had final word on weather/if DFF'd were fitted to their aircraft, despite the T.O's. It is known that at least one D-5-NA was still on escort ops as late as feb.45, not having been modified. It's not inconceivable that these types did so til VE day.

Since I'm most concerned about understanding and recording correct shapes, I sure appreciate the bottom views. Thanks, again, mligosh.

Tom,
not to belittle your general observations, 6 small "fences" appearing on the Mustang's wing, are in fact, structural. They are specifically called Wing Skin Stiffeners. I believe they were added to strengthen the skin over the bays, just in front of ailerons.

USAAF pilots who were flying both B/C & D's didn't seem to notice any lack of directional stability inherent to Mustangs, either prior to, or after DFF inclusion. At least their complaints, if any, don't seem to have been recorded . However, they had plenty of negative to say right after DFF retrofit! many seemed to miss the yaw available particularly on ground straffing runs. RAF, however was a bit more vocal on the issue and seems to have been a hot topic to them, going back to Mustang L They even experiementley added sq. footage to existing fin, but not really a resounding success. Back here in US, Grumman test pilot, Corky Meyer also thought Alison Mustang seriously needed more fin. Yet, NAA did not see a need to make a change until the onset of the tail failures.
I think how one is taught to fly, can affect how an individual feels toward such a question. IIRC, Kermit Weeks complained upon flying his P-51C without it's DFF, for the first few times. He reported he felt " the airplane wanted to switch ends, all the time!". I wonder if he still feels that way, now?

Yes I am aware of the little stiffeners that are on the P51 upper wing around the aileron.
I also know the stiffeners that are found on the top of a Spitfire wing over the wheel well and that's exactly what they are ....stiffeners .....not fences.

Fences are tall objects like you see on the 1950s and 60s Jets predominantly on swept-wing Russian aircraft which are not structural stiffeners, they are for aerodynamic flow control. Even mother Grumman had to resort to them on the A6 and ea-6b.

tom,
I would agree, the so called fences on P-51 wing are not fences, at all. However it goes to show how much of these discussions are dominated by so much popular perception, rather than purely historical or even technical.

Dorsal Fin Fillet is how the item in question appears on drawing title block. Is it important? I think so. Within the NAA parts and assembly drawings there are two different indexes, A numerical index and an alphabetical index. In the numerical, one must first know the part/drawing number. That will tell you exactly where in one of the 26 different rolls (a-z) of microfilm, down to the frame or frames, the drawing you need can be found. In the alphabetical index, one must know exactly what NAA called the part you seek. Common usage of even well known aircraft components(such as rudder, aileron, etc.,) are not listed in any recognizable popular context, rendering the alphabetical index nearly useless, unless one also has an illustrated parts catalog or structural repair manual.

IMHO, recognizing the correct historical names of parts and features is important to the true, or at least truer, and often in our particular interest, a "more scale" understanding. Popular terms, such as "skegg", "strake", etc are not going to be found in any associate NAA or USAAF literature. Same can be said for "razorback" "cranked leading edge, "gull", etc. In all honesty, the latter are often later, well intentioned, attempts at descriptive terms, offered to identify salient features, but more often than not, tend to divert the story away from factual and become outright mythical. I doubt "razorback" will ever be totally erased from P-51 usage, even if there is no Mustang feature that even remotely reflects a sharp spine, as is a quite noticeable feature on earlier P-47's. Of course there are those that contend the spine shape is not the dominant issue at all... But I , personally, cannot see it any other way...

From talking with Chris Fahey, and some full scale 51 jocks, the Fin Fillet was a non-structural fix for an oscillation that would cause stab/fin failure in certain flight regimes. It only weights 4 to 5 lbs, very light.
Fillets were also field retrofitted to A,B, and C models. I've been told by several 51 pilots that the old wivestale about it replacing "lost side area" when they went to the D is just that, a tale.
There are many small details people get wrong, or completely miss...such as the slight added area at the LE wing root on the D when compared to the A ,B, C.
It's all very interesting stuff....

"From further reading on the subject, it is entirely possible that the addition of this non-structural "fix" kept the aircraft from getting into the flight regime that could cause the structural failure. Sort of like fences on wings. They themselves are not "structural",...."

Exactly.

That portion of the wing that juts forward at the root was called the 'Expanded Leading Edge', by NAA. It was probably coined during early NA73X development, and still called that in P-51B& D engineering reports, '43-'44. When it was enlarged for p-51D, it saw the addition of an auxillary spar located forward of the wheel well and attached at its inboard end to the fire wall. Light weight and not structural? or is it?

The only time an airplane part, exposed to the slip stream is no longer structural, is the instant when its come off your airplane! That recent racing Mustang, that lost its elevator trim tab, killed it's pilot and spectators, wasn't that the result of a structural failure? Does the estimated weight and strength of several pilot's who question the strength value of a similarly exposed part negate the words quoted directly from a period P-51B/C Pilots Handbook, which states " a dorsal fin fillet has been added to strengthen the tail".? Now, I'm not disregarding or refuting whatever those august gentlemen might opine about the DFN, but also can't deny what the official view was, at the time the part was ordered to be added. Does anyone think NAA engineers hadn't worked out an EXACTING figure for how much strength was being added by the addition of the DFF? Weather near actual zero contribution, or some "good to so many G's estimate", including side loads, THEY KNEW.

ok, let the myths deepen. From some text in one of my many P-51 books I remember that the extended leading edge in the "D" was a result of some additional forward rake in the main gear due to some complaints of the B being to easy to nose up onto the prop on unprepared surfaces. Yes, all "additions" to any aircraft have to be able to take the air loads, and be designed in such a away that the new "loads" to not flow into older nearby existing parts and cause them to fail. If not careful, the addition of the DFF on the mustang could have actually made matters worse by imposing NEW loads into the structure surrounding the tail-tail-fuselage joint.

now.. structural or non- structural. the design of the DFF as shown at LEAST in the pictures presented recently, as I already mentioned is NOT a structural enhancement (even for the 1940's!) No self-respecting airframe structural engineer or his supporting stress analyst would let him use plate nuts to transmit load! Structural enhancements that truly dissipate unexpected loads are done internally (if accessible) or externally with doubler plates and interference type fasteners (either non removable like rivets or removable like Hi-loks (or whatever they called them back in the 40's).

I have not seen any before and after drawings of the in-production DFF, so I cannot comment if any "thickening" or "doubling" of any buried parts on the horizontal or vertical tails were done at the same time.

Tom

OK Tom, I surrender and will incorporate a different view, from this point on! Thanks.

On the question about the larger Expanded L.E. onP-51 D/K, here is what I gleaned from the NAA landing gear assembly drawings, 1983 & 84' P-51B/C shown left, while P-51D/K shown right. B/C is same geometry for all variants prior, going back to Mustang I.

Note general dimensions and retraction geometry appear to be identical. Note too, center of the retracted D/K wheel is a tad closer to the 25% (chord) wing ref. line. Both share 11 deg. forward rake when extended. I suspect that the new LE allows the gear to swing and stop slightly higher in the wheel well, thus resting farther back. While the gear retraction geometry is, for all intents and purposes, identical, the cast magnesium trunion block (large bearing in which the gear pivots) has also carries a pair of small specifically angled 'fork' tabs, upon which the leg doors pivot.P-51D doors, while similar in general shape will not fit earlier gear openings at either leg or central positions and the leg doors retraction geometry is different enough that B/C trunion will not work in a D/K and visa versa.

After having to shape doors to fit my 60% model( https://www.rcgroups.com/forums/show...0&postcount=27)), I'm fairly convinced a significant reason for the larger E.L.E. of P-51D, was to eliminate the complexities of several blending shapes, from fuselage and wing, all converging in a very small area, right in front of the radiator intake. B/C and earlier, have an undercamber feature. at W.Sta 17.5 (about where the leg door is overlapped by the central door). P-51D central doors appear to be of flat wrap skinning on their finished exteriors and benefitted from easier production and cost savings. Possibly eliminating some surface drag, too.

My guys say it was to make room.

Then theres the two different D model DFF depending on Dallas or Inglewood plant ��
One is straight, one has a curve...