How does the fuselage handle the different loads?

Question

I have been trying to find out how the loads experienced by a fuselage are transferred between structural members (bulkheads, formers, frame assemblies, skin, longerons, stringers).

I understand that the following loads are typically experienced by the fuselage:

• empennage loads due to trim, manoeuvring, turbulence and gusts
• pressure loads due to cabin pressurisation
• landing gear loads due to landing impact, taxiing and ground manoeuvring
• loads due to the weight of passengers and cargo

What I don't understand is how are these loads handled by the fuselage structure. I would really appreciate if someone could help explain the concept to me.

Answer 1

Not sure this is what you're looking for, but if you're looking for a simple big picture analogy, try this:

Think of the fuselage as a very thin walled compressed air cylinder resting on an airbag at about it's mid point, filled with sandbags that are offset toward one end, enough so that the cylinder wants to tip over on that end. All the way at the other end you add more sandbags (to represent the tail pushing down) until the tube balances out. Your fuselage will be basically a tubular beam balancing on the air bag with the bottom half in compression and the top half in tension, and the largest compression load will be just on each side of the air bag, but mostly on the "tail" side because the moment arm is longer.

Almost all of the loads of the fuselage pass through the skin, the surface of the cylinder. The frames and stringers are mostly to keep the "paper thin" tube from buckling or collapsing.

As a compressed air cylinder, pressurization loads are also absorbed by the skin in tension, like a balloon. Usually at the aft end there is a spherical bulkhead to absorb the pressure while keeping the skins of the bulkhead (mostly) in tension. Sometimes this bulkhead is flat, usually when there are tail mounted engines that require a support beam running across, which can cause many headaches crack wise (on the RJs, a long sad story). The front end of the cylinder narrows down to a small flat bulkhead at the front of the cockpit, which being small isn't too much of a problem being flat.

The fuselage usually has a big cutout on bottom to accommodate the wing box and landing gear, which puts this great big notch in the structure right at the point in the cylinder where the compression buckling loads are highest. The wing box itself may form part of the fuselage structure there, but farther back the cutout usually continues, to make space for the landing gear. There is usually a massive beam, called a keel beam, to bridge the cutout aft of the wing box and absorb the compression loads along that section.

As a tube that is trying to sag forward and aft of the wing, there is also a force that wants to squash the tube flat at the peak bending point, resisted by the frames. There are usually several very heavy frames at this mid section to support the tube where this force is highest, and to tie the wing box into the fuselage structure.

The highest loads on the fuselage are usually compression buckling stress just aft of the rear spar wing attachments when in flight, or the gear attachments when on the ground, and are highest on landing touchdown. When aircraft have broken fuselages on hard landings (Boeing did it on their company 717 -formerly DC-9 - on hard landing testing and an Embraer 145 broke its back on landing in Brazil about 15 years ago - they did an excellent job keeping it out of the media) they fail in compression buckling right at that stress peak point.

I don't want to oversimplify, and there are lots of other stresses, like from the floor beams and such, but that's kind of the big picture.

Answer 2

This is a pretty broad topic and can encompass several college level textbooks on aerospace structural mechanics, but sufficed to say, loads are transferred between members at joints and fall into four basic categories:

• Axial loads, i.e., tension and compression loading.
• Shear loads where one member tries to slide past another at their contact points.
• Torsion loads where one member applies a twisting action to another member.
• Bending loads - where on member attempts to bend or flex another.

Aside from this I’m going to need specific places in a specific aircraft where one would like to do a structural analysis.