MolaKule
Staff member
What is a Frame and what is its function in a fuselage barrel?
See Cujets picture below of a fuselage structure.
See Cujets picture below of a fuselage structure.
Q&A on Aircraft Structures II
- Thread starter MolaKule
- Start date
Rigidity?
Just a try from a driver, not an engineer, no googling:
Frames are vertical structural components that are placed at periodic intervals along the longitudinal axis of the airplane's fuselage. I would think the numerous frames are needed in pressurized airplanes especially to keep the skin attached without cracking as the skin expands/contract during pressurization, which is the primary life limiting factor for pressurized aircraft. That's in addition to, I'd imagine, reinforcing the the strutctural rigidity of the stringers especially during G loading events like turbulence, hard landings, or upsets. The barrel shape serves as a strong pressure vessel and the round shape of the frames facilitates that. The de Havilland Comet showed what can happen when you combine too many square corners with pressurization. My personal airplane is tube and fabric, unpressurized and doesn't really have frames, I guess, just steel tube crossmembers that connect the longerons and other appliances like wings, stabilizers, landing gear, seats, and the engine together.
Frames are vertical structural components that are placed at periodic intervals along the longitudinal axis of the airplane's fuselage. I would think the numerous frames are needed in pressurized airplanes especially to keep the skin attached without cracking as the skin expands/contract during pressurization, which is the primary life limiting factor for pressurized aircraft. That's in addition to, I'd imagine, reinforcing the the strutctural rigidity of the stringers especially during G loading events like turbulence, hard landings, or upsets. The barrel shape serves as a strong pressure vessel and the round shape of the frames facilitates that. The de Havilland Comet showed what can happen when you combine too many square corners with pressurization. My personal airplane is tube and fabric, unpressurized and doesn't really have frames, I guess, just steel tube crossmembers that connect the longerons and other appliances like wings, stabilizers, landing gear, seats, and the engine together.
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MolaKule
Staff member
2.7 gave an overall description, and Boomer gave us rigidity—all good.
Frames are like ribs in that they help to maintain shape, in this case, the shape of fuselage barrel. Stringers or Longerons run through or along the frames and are bonded to the frames.
The curved Frames, usually of a U-shape, are formed in a "hydroforming" press for commercial aircraft or, in the case of extremely high stress areas for military aircraft, are forged.
In the Boeing 747 main fuselage of 11 feet in radius, for example, the Frames are 6.3 inches wide, are spaced every 20 inches, 0.071 inches thick, with the stringers spaced about 8 inches apart.
The skins, or skin panels, are generally attached to the stringers, depending on the design. The skin panels are either riveted or chemically bonded to the stringers, and are generally .063 inches thick.
The skins, frames, and stringers all give rigidity to the fuselage barrel but still allow for some flexing. In certain areas of the fuselage, reinforcing parts called "fail-safe" straps or panels are used to strengthen areas under high stress, such as around window frames.
Frames are like ribs in that they help to maintain shape, in this case, the shape of fuselage barrel. Stringers or Longerons run through or along the frames and are bonded to the frames.
The curved Frames, usually of a U-shape, are formed in a "hydroforming" press for commercial aircraft or, in the case of extremely high stress areas for military aircraft, are forged.
In the Boeing 747 main fuselage of 11 feet in radius, for example, the Frames are 6.3 inches wide, are spaced every 20 inches, 0.071 inches thick, with the stringers spaced about 8 inches apart.
The skins, or skin panels, are generally attached to the stringers, depending on the design. The skin panels are either riveted or chemically bonded to the stringers, and are generally .063 inches thick.
The skins, frames, and stringers all give rigidity to the fuselage barrel but still allow for some flexing. In certain areas of the fuselage, reinforcing parts called "fail-safe" straps or panels are used to strengthen areas under high stress, such as around window frames.
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MolaKule
Staff member
One last exercise in the Aviation Forum.
Fill in the missing words:
A ------- is usually a light-weight, non-structural component and is usually made of ----------.
Fill in the missing words:
A ------- is usually a light-weight, non-structural component and is usually made of ----------.
fairing, fiberglass?
MolaKule
Staff member
You got it.
Fairings provide a transition to reduce drag where various structures meet and where one or more structures may have an abrupt leading or training edge. For example, you will see a fairing where the wing root meets the fuselage.
Fairings can be made of composites or fiberglass.
The 747 fuselage is about 23 feet in diameter.
Wow that's thin.
The logger and heavy equipment operator in me is like... may as well make it 3/8" ir 1/2" plate, just in case.
I worked on the floors for the 747 back in 99. I know they are way bigger than 11ft....... my mind was thinking 20 ft plus......
You guys are way over my head with all this engineering talk. ha ha
MolaKule
Staff member
Good catch on the typo folks. 
The data given by Niu* says the radius for the double lobe cross
-section is 128 inches, or 256 inches in diameter or 21.3 feet, or a diameter of 6.08 meters.
*Airframe Structural Design, Michael C Niu (Lockheed Aeronautical Systems), page 389., 1991.
This should have read: "In the Boeing 747 main fuselage of 11 feet in radius, for example, the Frames are 6.3 inches wide, are spaced every 20 inches, 0.071 inches thick, with the stringers spaced about 8 inches apart."
The data given by Niu* says the radius for the double lobe cross
-section is 128 inches, or 256 inches in diameter or 21.3 feet, or a diameter of 6.08 meters.
*Airframe Structural Design, Michael C Niu (Lockheed Aeronautical Systems), page 389., 1991.
MolaKule
Staff member
Fuselage Question: Explain the function of the pressure bulkheads in a commercial transport aircraft.
Distribute internal air pressure stress in the longitudinal direction. I figure I would throw it out there........ I'm sure a better answer is coming though.
MolaKule
Staff member
A pretty good intro.
Any object with a volume has to have closed ends in order for pressure to be contained within it.
In order for aircraft to be pressurized for passenger comfort and to be able to fly above 10,000 feet, the fuselage has to have two closed ends, which are the aft pressure bulkhead and the forward pressure bulkhead.
The AFT pressure bulkhead (in the empennage) is in the shape of a dome. The forward pressure bulkhead ahead of the cockpit (and behind the radar's radome) may be either slightly curved, or flat. Another pressure bulkhead may be located below the passenger floor, which divides the passenger cabin from the cargo hold.
Here is some design and stress analysis of a pressure bulkhead using Finite Element Analysis (FEA):
https://www.icas.org/icas_archive/ICAS2020/data/papers/ICAS2020_0109_paper.pdf
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You are much too kind. I know I'm really a dummy in this stuff. Some interesting stuff in that analysis. I assume the Boeing 787 has composite pressure bulkheads?
Too bad the Ocean Gate CEO tried to invent the new wheel to see the Titanic. I think the ends were like a pressure bulkhead, made from titanium, I believe? But the main cylinder was composite. Which was not the favored material in that usage?
They say he was an aviation engineer. If anybody likes documentaries, it is a good one. On Netflix "Titan", the ocean gate submersible disaster.
MolaKule
Staff member
From what I can gather, the composite barrel was only sufficient for a finite number of dive cycles under those extreme compression events.
Yes finite amount of times. There was a lot of popping when the submersable was in use. Also lots of pops while testing it. Not good.
I'm still trying to figure out how to push something with a shoelace.
In other words, a shoelace is great in tension. But when placed in compression, is utterly useless. Titan was made of carbon fiber shoelace, placed in compression.
An aircraft structure is quite different. Much of the load carrying structure, from wing spars, to fuselage components and skins is in tension. A more appropriate application for carbon fiber than a barrel place under stunning compressive loads.
We can orient fiber strands as necessary for load path management. And there is a lot to like when it is done correctly. No fatigue limit, no corrosion risk, redundant load path configurations (such as the dual spar in a Extra 300L stunt plane, each of which can meet the 10G certification) and so on.
MolaKule
Staff member
Not quite as it depends on the compressive, torsion, or tension loads at a specific location.
For example, when rolling (due to aileron deflection), the fuselage experiences torsion (twisting) moments. When landing, the top of the fuselage structure experiences tension forces, whereas the bottom of the fuselage experiences compressive forces. On takeoff and cruise, these forces are reversed.
This is why we use longerons, plates, and frames to distribute those various forces throughout the structure.
My comment above was directed to the topic of fatigue analysis. Any structure, whether it be composed of metal or carbon fiber composites, has a finite fatigue life.
Material fatigue is the weakening of a material caused by cyclic loading, leading to progressive structural damage and eventual failure.
Stress and fatigue analysis, damage tolerance, and fail-safe design are aspects of the aerospace sciences that few ever graduate up to.
This is why aircraft companies have their own stress and fatigue analysis groups to make sure the structures can properly react to and survive all of those load forces.
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The S-N curve, (stress and number of cycles) for aerospace carbon can be better than aluminum. Both with regard to percentage of stress 70% ultimate load, for example, vs as low as 30% for aluminum. And for number of cycles.
CF can yield suddenly if over stressed, which is one reason it is sometimes blended with Kevlar.
In the end, I'm a fan of a riveted aluminum airframe and wing. Nearly always repairable.
6061 T6: (clearly even with very low loading it will eventually fail)
CF can yield suddenly if over stressed, which is one reason it is sometimes blended with Kevlar.
In the end, I'm a fan of a riveted aluminum airframe and wing. Nearly always repairable.
6061 T6: (clearly even with very low loading it will eventually fail)
