Q&A on Aircraft Structures I

[MolaKule]

We're going to try a Q&A on aircraft structures that comprise various parts of an aircraft since Aviation and aircraft in general is a popular topic.

Our first question is, "What is a Stringer (also called a longeron), and what is its function for an aircraft fuselage barrel?

Some of the better on-line references for study are:

https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/aerospace-structures/

https://bluetunadocs.com/downloads/acsguide.pdf

 

[Cujet]

Just off the top of my head. In many sheet metal aircraft, a stringer is a relatively small A channel or C channel section of sheet metal that is affixed to the aircraft skin and runs fore-aft in the fuselage or along the inside of the wing from root to tip. It improves rigidity, overall strength, and in many cases, is also used to transfer loads from the sheet metal to more robust structures.

In the case of modern pressurized aircraft, stringers are designed and very well affixed to prevent a crack or perforation from spreading beyond the boundary boxed in by the framework, to other areas of the structure. Adding damage tolerance and a fail safe aspect to the design. This may be done by a combination of fasteners, varying skin thickness where the stringers attach and structural epoxy, or other methods.

Here you can see the stringers in a Gulfstream G600 jet.

 

[Cujet]

Also of note, in Gulfstream jets, there is no wing spar. The design uses ribs and stringers to prevent buckling, maintain shape, increase rigidity and so on. But the wing skin carries all the load. About 1/2 inch thick at the wing root, and tapering to about 1/4 inch (a bit less) thick at the wing tip. The vast majority of the wing's structure is external. Even the front and rear beams are reasonably thin.

The 4 beefy attach points are affixed externally. The many minor attach points are affixed internally.

The wing has nothing protruding above or below it, leading to an ultra smooth and aerodynamically clean, low drag shape. It is designed and configured to carry as much fuel as possible. So the entire wing is a fuel tank. Allowing the latest versions to go over 7,750 nautical miles (about 9,000 statute miles) range.

 

[wings&wheels]

I was always befuddled by frame vs. bulkhead; bulkheads don't have to be solid, which of the two actually attaches to a stringer and which does not. etc.....

 

[Cujet]

I was always befuddled by frame vs. bulkhead; bulkheads don't have to be solid, which of the two actually attaches to a stringer and which does not. etc.....

That brings up the point that these structures are quite standardized with US built aircraft, and have been for more than 80 years. But the nomenclature is variable, especially with foreign designed/built aircraft. It should come as no surprise that when a designer strays from convention, the failure mode is radically different.

Thinking of two different composite designs where few/no stringers were employed and complete sudden failure was the result.

Composite designs often have minimal stringers, always made from the same composite materials, and they are bonded to the skin. A crack propagates right through the structure as if it were one single part, which it really is. Unlike modern sheet metal designs, where cracks don't travel into the adjacent structure.

Just a wild guess, but if this tail had employed a conventional design, it would not have broken off in this case.

 

[MolaKule]

I was always befuddled by frame vs. bulkhead; bulkheads don't have to be solid, which of the two actually attaches to a stringer and which does not. etc.....

We're going to get to frames shortly.

@Cujet Thanks to Cujet for the great pics and the background info.

In a fuselage barrel, the Longerons or Stringers carry the "longitudinal" loads, such as the twisting and bending loads encountered in all phases of flight. The spacing of the stringers depend on the expected loads (forces). With the frames and skins, they give the structure its strength.

Longerons run through the Frames, next question.

 

[wings&wheels]

Stringers, frames and bulkheads and an ELT at the lower end of the aviation spectrum courtesy of a recent battery replacement.

 

[Cujet]

Stringers, frames and bulkheads and an ELT at the lower end of the aviation spectrum courtesy of a recent battery replacement.
View attachment 279932

Now that's more like it! A real world airplane! With a form of C channel stringers. My Cessna is the same.