A question about wing attachment 3

[enginesrus]

I found a post on some site about wing attachment on the old DC-3 airplanes. Is there anyone that has ever done an
FEA on that kind of wing attachment? Does anyone know what other planes used that same attachment method?
How well does that attachment method perform, in aerobatic maneuvers?

 

[verymadmac]

The North American T6 (Harvard) uses the same type of Wing Connection.

The Back to back angle that forms the wing connection on the DC-3 I seem to recall is life limited and prone to corrosion (and possibly the T6 as well). It could be difficult to get such an angle formed reliably these days. Its is likely to be stiffness limited, if it can't be demonstrated to be sufficient by hand, your margins probably too low.

 

[Andries]

A few years ago a T6 lost a wing, I think during aerobatics. As verymadmac mentioned, the T6 wing is attached by means of back-to-back angles with many fasteners. Fatigue cracks can cause such a fastener row to "unzip". Regular inspections are therefore essential. Although an FEA may be done,a hand stress calculation should be OK (that is how the aircraft was stressed way back in any case).

Andries

 

[enginesrus]

I have a difficult time believing this system works and lasts like it did. It just does not seem very strong. I can see where airflow around the wing holds it from any sort of bending, or up and down motion, and then the stress is just the shear on the attachment bolts. But when the load on to wing is in either the up or down bending, then the huge pry bar would really stress
that flange and the angles that make the flange.
How about the other Douglas planes like the DC-6, DC-7, did they use some other method on those?

 

[verymadmac]

I see the T6 inspection interval is every 200 hours.

One wonders how much improvement one could get out of the joint by using Lockbolts or some other fasteners with a better control tension than a bolt. I guess the washer specification is probably critical for this type of joint.

Just to illustrate the joint.
Link

 

[enginesrus]

And there is no wing spar in that area. Just 2 flanges bolted together. I call it very scary.
Look at it this way, if there was a structure on a 10,000 foot shear mountain side, and a nice chair with a safety belt was was bolted
about 3 feet in from the wing tip, and the wing was attached just like it is on the plane sticking out from the side of the mountain, would you be comfortable strapped in and sitting out there?

 

[WKTaylor]

E-us...

From my experiences with old airframes... aircraft designed/built thru the 1940's would have 'low service-life expectations' had poor fatigue and corrosion design-consideration and protection schemes.

The joints worked! I suspect that the real problem with structural integrity of these angle-to-angle-bolted joints is stress-corrosion cracking [SCC] and exfoliation corrosion [EXCO] that compromises the older generation alloys these angles were made from [like bare 2014-T3 and 2024-T3]. This situation was aggravated by typical extruded with 'standard surface quality' of that era... which allowed lengthwise micro-scoring lines and a degree of roughness that would be unacceptable today. Also, hole quality/tolerances/alignment on the original production... and early post-production repair/overhaul/mod... holes thru the angles would only aggravate deterioration of the angle-joints in-service.

The following video of B-29 production showed how little concern there was for 'refined workmanship' and corrosion protection since these bombers had a very short service life expectation. Clad sheet aluminum parts [flat or formed] were usually left uncoated and 'bare aluminum' extruded parts and machined fittings usually had Type II anodize or alodine... with/without a topcoat of zinc-chromate primer.

https://www.youtube.com/watch?v=ybGGmjm9JZ4

NOTE.
MANY next-generation past WWII aircraft have theme and variation of this wing-joint method... but with aerodynamic cleanliness in-mind. C-130 and CKC-135 join inner-to-outer wing structurally with rib-to-rib internal [aero-flush] recessed tension bolts/nuts along/under the rib-flanges... and still maintain integral tank integrity.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[enginesrus]

https://www.youtube.com/watch?v=b5aK_TDU4P8

Good video of DC-3 wing flange area.

 

[Sparweb]

I have a difficult time believing this system works and lasts like it did. It just does not seem very strong.

The problem with the joint isn't "strength", really. You can make that joint as strong as you like-
[ul]
[li]by making it much thicker and heavier[/li]
[li]by accepting the eccentric prying moments[/li]
[li]by accepting the moisture ingress[/li]
[li]by accepting the fatigue sensitivity[/li]
[li]by accepting the "zipper" failure mode[/li]
[/ul]

The problem is that it is not "stiff" or "robust".
Lots of compound deflections occur as the primary load is applied. Nothing stopping the joint from deforming (prying) either.

So as long as you don't care that it won't last very long, it's a great joint.

Very little was understood about fatigue or stress-corrosion-cracking at the time of the DC-3 design. There have been wing failures, but not enough to scrap the fleet. Nobody would design a wing joint like that today, though.

And yet...
There are dozens of DC-3s and Dakotas still flying around today. One parked right outside my hanger yesterday, in fact. So with all our high and mighty theoretical principles, the real world can muddle on with "good enough - barely" for a long time.

http://www.sparweb.ca

 

[enginesrus]

So then SparWeb you agree with me, on the DC-3 wing attachment being a scary deal, and not so trust worthy? I wonder if the turbo conversion folks are making this a better interface?

 

[rb1957]

oh come on. the DC3 has surely stood the test of time !?

Turbo DC3 (BT67) has unchanged wing/fuse interface.


another day in paradise, or is paradise one day closer ?

 

[crackman]

I guess I will put my two cents in.

The DC-3 wing attachment design was very typical not only of that time period but was used all the way up to present day. The primary intent of the design was to facilitate switching wings out. Other aircraft that come to mind: Curtiss C-46, T-6 Texan, P2V, C-130, etc. Many other more modern aircraft used similar joints at other locations in the wing, for example 707 uses this type of joint at the WS733 production break.

As for being a safe, as with anything, if treated properly it will function properly. DC-3 was designed in an early period where we barely understood fatigue. This doesnt mean douglas engineers didnt know what they were doing. I have see the wing joint tests where all bolts were actually instrumented so they knew exactly what load was being transferred. However, in those days OEMs were not required to develop inspection programs, that was the responsibility of the operator. So, I believe the earliest AD on the DC-3 was issued in 1939 specifically for this joint because it was not being inspected. Also, there was no mandatory torque requirement on the bolts.

As far as I know, there are three documented wing fatigue failures of the DC-3 and I have copies of the lab reports for two of them. If interested, back in 2017 at the airworthiness assurance conference I presented a paper which details one of the accidents, should be able to located it on the web, its entitled Airframe Structural Fatigue - Past, Present and Future. Anyways, I can tell you the primary culprit was lack of proper inspection. The inspections missed the crack and therefore the wing failed. Similarly in 2005 a T-6 texan crashed in florida from a wing failure of the wing joint. Again, lack of inspection. Note both the DC-3 and the T-6 had been full scale fatigue tested but prior to rules requiring DTA and ICAs. I have seen these tests and they predicted the in-service cracking. Today the DC-3 has a SSID with an FAA AD on it which mandates inspections. T-6 and other ex-military aircraft typically do not.

Anyways, this type of joint can be made to work and last if properly taken care of and respected. Unfortunately, these joints get worked pretty hard and many operators incorporate several repairs, service bulletins and/or major modifications which make it difficult to inspect and also reduce the fatigue life. They dont realize that adding major straps for static strength can have a huge impact on the fatigue life of this type of joint. Then, the design is blamed for not being safe when in fact it is the lack of proper attention to detail. In my experience, very few perform thorough evaluations on these old birds and rarely revise the inspection procedures.