D Mustangs with no fillet mod?

[51fixer]

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.

The tail cone on PE still has the original skins from when it left the factory on the sides and the top. It was a B tailcone and not a converted D on that part. The vertical is rebuillt out of a D.

[51fixer]

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.

We have discussed this on a few boards. We each have an opinion and thats ok.
My reasoning comes from the parts;

This is the forward fitting that attaches the vertical to the top rear of the horizontal. It is made from 7075 Alum, an alloy that approaches steel in strength properties. It is also massive in relative size compared to any kind of alum sheet material.


This is the same fitting with the .040 alum angle that forms the attaching point for the internal structure of the DFF in its relative location. This gives an indication of the relative forces that each part must carry. In fact the vertical structure acts as the mounting structure for the DFF, not the DFF reinforcing the vertical.


This shows the vertical attach fitting being inserted into the fwd vertical spar. The fitting is secured with many 1/4" dia alum rivets. 3 of the rivets also secure the alum angle used for the DFF attach. It is mounted where the yellow zinc primer is visible.


This is a view showing the leading edge of the vertical with the nutplates mounted in .032 2024 Alum skin. Only on the lower edge does the DFF mounting hardware go through more than a single layer of alum.


This shows the DFF's rear mounting positions. Again, alum parts made from sheet material. Also the external skin on the DFF is made from 3003 alum alloy. Very soft material which is why many of these are misshapen from people grabbing them to try to swing or push the tail around. The 3003 alloy is not used for structure but is used for fairings as it is easy to form. You could take a couple of fingers and easily bend this metal. The wedge shaped part that tapers away from the camera is the structural backbone of the DFF and is made of the stronger 2024 alum alloy. It is on the bottom of the rear half of the DFF and the flat portion is what attaches to the angle discussed above on the Vertical.


This shows the bottom view of the forward portion of the DFF. Just thin sheet for construction from the softer 3003 Alloy.

This is why I have the opinion that the vertical and the rear fuselage form the support for the mounting of the DFF. There isn't any way that the DFF forms any reinforcement for the vertical stab or the rear fuselage.
Rich

[visaliaaviation]

51fixer,

Thank you for taking the time, the photos, and making very astute observations. As I said, I will be looking at the engineering conclusions by NAA principals. It is my hope to explore this matter to a documentable conclusion and will be happy no matter how it turns out. I will gladly forward my findings to you.
thanks again.

[JDK]

An interesting Mustang thread!

Must have a lie down.

Thanks chaps.

[Jim MacDonald]

Thanks for taking the time to post the pics, Rich.

Mac

[drgondog]

As a former airframe structures engineer I would comment that the lateral loads on the rudder/vertical stabilzer would be transmitted as torsion, bending and shear loads to the fuselage.

The primary fuselage load path would be from the spar attached to the primary fitting via strong shear connections to the machined fitting shown.

I assume the spar/fitting combination is attached to a bulkhead close to at least one set of beam caps running fore and aft in the fuselage.

The torsion loads would be carried out by a bulkhead to outer rim and in turn would be distributed via fuselage shear panels (oute skin) between the beam caps.

The bending loads (both lateral and vertical would be carried as tension in one pair and compression in the other pair.

I can not imaging the fillet/fin acting as a load path or structural member for any purpose whatsoever.

From the documentation I have seen, the original fin was proposed ny NAA to improve Yaw stability at high speed - theoretically as result of losing turtleback. The fin was found to have been benificial to stability during sideslip and rolling manuever. These changes were incorporated about same time as reverse rudder boost tab -post June 1944 in production, kits for a/c produced earlier

Apparently the RAF had first discovered the issue and root solution at RAE.

I was under the impression that the primary reason was high speed Yaw stability but believe Charlie is correct and Gruenhagen's Mustang account supports the RAF contribution during acceptance of the Mustang III.

[Jim Beasley]

The only thing assuming any stress on these parts is ME since they're still a gazillion parts and not assembled into a plane!!!!!!!! aaaagh

My reasoning comes from the parts;

This is why I have the opinion that the vertical and the rear fuselage form the support for the mounting of the DFF. There isn't any way that the DFF forms any reinforcement for the vertical stab or the rear fuselage.
Rich

[dollar65]

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

In fact, this B model did have a dorsal fin:
http://www.mustangsmustangs.com/p-51/p51who/190.shtml

Laurent

[Locobuster]

I don't know if this adds anything to the discussion, but P-51C-10-NT, AAF serial #43-25054, "Hells Belle / Beaverhead Filly / Bertie's Bet", which soldiered on for over 700 combat hours with the 79th Ftr Sqdn., had one.

Also, in a discussion about his squadron-mate Woodrow Williams' P-51's breaking apart in mid air, Capt. Art Heiden told me: "Later examination of this, and other similar incidents where P-51's were coming apart in rough weather or during violent maneuvers, led to a dorsal fin improvement to later models the aircraft."

[visaliaaviation]

Recent post in this thread have generated some semi-random questions and comments.

How strong does it have to be , too be viewed as a "practical" strengthening aid?

Could the Mustang's addition of DFF have been seen as a tortional damper to delay onset of vertical surface deformity, leading to fluttter and failure?

Could it's triangular configuration have been viewed as a gusset, dampening bending (up/dn) loads of aft fuselage?

Possibly a combination of both?

It is obviously strong enough to withstand the aerodnamic loads imposed by 450mph+ slipstream.

[Glenn Wegman]

It is obviously strong enough to withstand the aerodnamic loads imposed by 450mph+ slipstream.

The wing fillet fairings would fit that statement too. Would you say that the wing would depart without the fairings? The engine cowling would fit that statement too for that matter and it's fastened with Dzus fasteners. Does this mean that the cowling supports the engine assembly?

The first paragraph in the Dorsal Fin T.O. states:

"To correct the tendency, existing when certain manouvers are performed, for P-51D airplanes to assume a high angle of yaw which may overload the horizontal stabilizer, a dorsal fin will be installed."

If it was installed to enhance the structural integrity of the empennage it probably would have stated so.

Seems pretty black and white!

I believe Historically, any addition of a dorsal or ventral fin was done so to correct yaw instability.

Glenn

[Jim Beasley]

I can tell you that the B/C without the DFF is much more yaw unstable than the D with the DFF. While it's more fun to fly with it without the DFF, It's easy to see the practical benefit of a DFF; from a gunnery perspective, one would desire more straight-line stability.

Also don't forget that the "H" and "TF" had additional surface area added, and it wasn't for structural purposes. It was for yaw control.

jb

It is obviously strong enough to withstand the aerodnamic loads imposed by 450mph+ slipstream.

The wing fillet fairings would fit that statement too. Would you say that the wing would depart without the fairings? The engine cowling would fit that statement too for that matter and it's fastened with Dzus fasteners. Does this mean that the cowling supports the engine assembly?

The first paragraph in the Dorsal Fin T.O. states:

"To correct the tendency, existing when certain manouvers are performed, for P-51D airplanes to assume a high angle of yaw which may overload the horizontal stabilizer, a dorsal fin will be installed."

If it was installed to enhance the structural integrity of the empennage it probably would have stated so.

Seems pretty black and white!

I believe Historically, any addition of a dorsal or ventral fin was done so to correct yaw instability.

Glenn

[visaliaaviation]

Glenn,

you wrote:
"If it was installed to enhance the structural integrity of the empennage it probably would have stated so."

You are absolutely correct. the T.o.s are black and white, but to assume that additional area is the only consideration is not without danger of possibly reaching and promoting false conclusions. While ostensibly complete and not specifically wrong, some information in the TO may not be included.

To that,I would point out a statement concerning DFF inclusion, appears in an interim B/C/D pilot manual (no TO# but an official USAAF publication, non-theless), provides "increased stability in rolls and to strengthen the tail" . Also black and white, but definitely suggests an additional function and or reasoning. Which is more correct understandig???

Not to split hairs, but the wing to fuselage fillets, cowls panels, etc were not an after-thought to correct a percieved aerodynamiclly induced structural failure. The DFF definitely was. 51fixer felt the attach fasteners defined if the DFF is considered structural or not. Are any of its fasteners considered non-structural??

An aerodynamic reasoning could be bolstered by the extended nature of B/C ' DFF. This could indicate a desire to keep the added area equal between B/C and that of D. Or could it possibly have been a strenthening of B/C fuselage in the area where the upper longerons are non- existent in the 16" fwd of Fuse. Sta 248?. Admitedly a stretch, but can it be ignored out of hand?

The question is: Was the DFF fitted for structural or additional fin area considerations? I'm attempting to point out that the answers are not always obvious, simple, POPULAR, or even nullify the possiblity it could be Mustang's DFF's were a combination of both aerodynamic and structural refinements.

[Matt Gunsch]

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

In fact, this B model did have a dorsal fin:
http://www.mustangsmustangs.com/p-51/p51who/190.shtml

Laurent

I went and found the air comics issue about the restoration and the plane that shed it's tail for the the B model did have a fin, I stand corrected. put hey, it is about the first time I looked at that issue since 81.

[Glenn Wegman]

[quote="visaliaaviation"]
The question is: Was the DFF fitted for structural or additional fin area considerations? [quote]

Read my above post, and the T.O.

Usually structural components are not installed with non structural screws and dimple washers through enlarged holes. (such as the other empennage fairings and wing fairings.

The "D" manuals clearly show the incorrect routing of the oil system tubes from the engine to oil cooler and back to the reservoir. Many "new" guys have boiled the oil on the first takeoff by folowing the manual.

Would you like to despute/debate that also and claim that all of the flying Mustangs are plumbed incorrectly as the manuals state otherwise?

Glenn